Monitoring Volcanoes: Techniques and 

Strategies Used by the Staff of the 

Cascades Volcano Observatory, 1980-90 

U.S. GEOLOGICAL SURVEY BULLETIN 1966 
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Monitoring Volcanoes: Techniques and 

Strategies Used by the Staff of the 

Cascades Volcano Observatory, 1980-90 

JOHN W. EWERT and DONALD A. SWANSON, Editors 

U.S. GEOLOGICAL SURVEY BULLETIN 1966 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1992 

For sale by 

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Library of Congress Cataloging in Publication Data 

Monitoring volcanoes :techniques and strategies used by the staff of the Cascades 

Volcano Observatory, 1980-90 I john W. Ewert and Donald A. Swanson, 

editor. 

p. cm.-(U.S. Geological Survey bulletin ; 1966) 

Includes bibliographical references. 

Supt. of Docs. no.: 119.3:1966 

1. Volcanism-Research-United States-Methodology. 2. Cascades 

Volcano Observatory. I. Ewert, John W. II. Swanson, Donald A. Ill. Series. 

QE75.B9 no. 1966 

[QE522] 

557.3 s-dc20 

[551.2'1 '072073] 92-31158 

CIP 
CONTENTS 

Introduction 1 

John W. Ewert and Donald A. Swanson, editors 

1. A Real-Time Seismic-Amplitude Measurement System (RSAM) 5 

Thomas L. Murray and ElliotT. Endo 

2. A Low-Data-Rate Digital Telemetry System 11 

Thomas L. Murray 

3. Operating Low-Power Telemetry Networks in Severe Environments 25 

Andrew B. Lockhart, Thomas L. Murray, and Bruce T. Furukawa 

4. A System for Acquiring, Storing, and Analyzing Low-Frequency Time-Series Data in 

Near-Real Time 37 

Thomas L. Murray 

5. Seismic Data-Acquisition Systems at the Cascades Volcano Observatory 45 

ElliotT. Endo and Gloria Smith 

6. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount 

St. Helens, Washington 53 

Eugene Y. lwatsubo and Donald A. Swanson 

7. Electronic Tiltmeters for Volcano Monitoring: Lessons from Mount St. Helens 69 

Daniel Dzurisin 

8. Slope-Distance Measurements to the Flanks of Mount St. Helens, Late 1980 

through 1989 85 

Eugene Y. Iwatsubo, Lyn Topinka, and Donald A. Swanson 

9. Monitoring Radial Crack Deformation by Displacement Meters 95 

Eugene Y. Iwatsubo, John W. Ewert, and Thomas L. Murray 

10. Trilateration and Distance-Measuring Techniques Used at Cascades and Other 

Volcanoes 103 

Eugene Y. Iwatsubo and Donald A. Swanson 

11. Installation of Bench Marks and Permanent Reflectors for Geodetic Deformation 

Networks 115 

Michael P. Doukas and John W. Ewert 

12. Geodetic Leveling as a Tool for Studying Restless Volcanoes 125 

Daniel Dzurisin 

13. Using First-Order Class II Geodetic Leveling Procedures to Monitor Vertical 

Displacement 135 

Kenneth M. Yamashita and William P. Kaiser 

Contents Ill 
IV Contents 

14. Single-Setup Leveling Used to Monitor Vertical Displacement (Tilt) on Cascades 

Volcanoes 143 

Kenneth M. Yamashita 

15. A Single-Setup Trigonometric Leveling Method for Monitoring Ground-Tilt 

Changes 151 

John W. Ewert 

16. Lake-Level Monitoring as a Tool for Studies of Crustal Deformation 159 

Jack W. Kleinman and Peter M. Otway 

17. Techniques for Continuous Monitoring of Surface Water at Active Volcanoes: 

Examples From Loowit Drainage, Mount St. Helens, and Kelut Crater Lake, 

Indonesia 17 5 

Kenneth A. McGee, Andrew J. Sutton, David E. Wieprecht, and Mark E. Iven 

18. Fundamental Volcanic-Gas-Study Techniques: An Integrated Approach to 

Monitoring 181 

A. Jefferson Sutton, Kenneth A. McGee, Thomas J. Casadevall, and J. Barry Stokes 

19. Video Surveillance of Active Volcanoes Using Slow-Scan Television 189 

Bruce T. Furukawa, Thomas L. Murray, and Kenneth A. McGee 

20. Basic Photography at Mount St. Helens and Other Cascades Volcanoes 195 

Lyn Topinka 

21. The Importance of Field Observations for Monitoring Volcanoes, and the Approach 

of "Keeping Monitoring as Simple as Practical" 219 

Donald A. Swanson 
Monitoring Volcanoes: Techniques 

and Strategies Used by the Staff of the 

Cascades Volcano Observatory, 1980-1990 

INTRODUCTION 

By John W. Ewert and Donald A. Swanson, Editors 

This bulletin marks the tenth anniversary of the 

May 18, 1980, eruption of Mount St. Helens by 

describing many of the monitoring techniques used since 

then to track the volcano's ongoing activity. All the 

chapters in this volume were written by technicians and 

scientists who are now or were at some time in the past 

10 years members of the Cascades Volcano Observatory 

(CVO), which was established in Vancouver, 

Washington, in the summer of 1980 in response to the 

eruption of nearby Mount Sl Helens. The chapters in the 

bulletin describe the techniques and strategies employed 

by CVO's staff to collect data used to monitor volcanic 

activity and predict eruptive activity of Mount St. 

Helens, as well as other volcanoes in the United States, 

Latin America, and New Zealand. The chapters focus on 

the methods for acquiring and managing data, not on the 

interpretations resulting from the data. We anticipate that 

the audience of the bulletin will be those scientists and 

technicians, particularly in developing countries, who 

themselves are, or will be, monitoring volcanoes. The 

bulletin is not intended to be a comprehensive text on 

volcano monitoring but instead a distillation of how 

CVO's staff responded both to the need to monitor an 

active composite volcano and to the dynamic 

technological developments during the 1980's. Further 

reading in volcano-monitoring techniques can be found 

in UNESCO (1972), Civetta and others (1974), Tazieff 

and Sabroux (1983), and Tilling (1989). 

Much of the seismic monitoring of Mount St. 

Helens during the decade was conducted by S. D. 

Malone and associates in the University of Washington's 

Geophysics Program, not by CVO staff members. 

Techniques used by Malone's group are described in 

unpublished in-house manuals (Malone and Zollweg, 

1985; Malone, 1989) available upon request from 

Malone (Geophysics Program AK-50, University of 

Washington, Seattle, WA 98195). The methods used by 

Malone's . group are appropriate for large seismic 

networks that employ mainframe computers to acquire 

and process data. Examples of how these techniques are 

applied to interpret volcanic activity are papers by 

Malone and others (1981), Malone (1983), Malone and 

others (1983), Hofstetter and Malone (1986), and 

Swanson and others (1985). 

Several of the methods used to monitor volcanoes 

have changed greatly since 1980 as a result of 

technological advances. Most important of these is the 

revolution in the field of electronics, particularly 

microcomputers. Smaller, more powerful electronic 

distance meters (EDM's), low-power digital telemetry, 

PC-based seismic-data acquisition and analysis systems, 

and laptop-computer controllers and interfaces are just 

some of the results of this revolution that have allowed 

us to improve established monitoring techniques or 

develop new ones. 

When Mount St. Helens began showing signs of 

unrest in March 1980, most of the U.S. Geological 

Survey personnel dispatched to the volcano had previous 

experience monitoring Kilauea while stationed at the 

Hawaiian Volcano Observatory (HVO). Although most 

of the monitoring techniques used in 1980 were 

developed and (or) used at HVO, it became apparent that 

the different eruptive style of composite volcanoes in 

general, and of Mount St. Helens in particular, required 

modification of some of these monitoring practices. The 

chapters in this bulletin illustrate the degree to which 

modification was required. 

The chapters fall into several logical groupings: 

telemetry, acquisition of seismic data, data management, 

ground deformation, geochemistry, and imaging. The 

first three groupings represent the areas in which the 

most marked technological progress has been made. 

Rapidly changing conditions prior to eruptive activity 

require that data be available to volcanologists in as 

Introduction 1 
close to real time as possible, in order to make 

reasonable assessments of the future and to aid in 

hazard-mitigation efforts. To be most effective, the data 

must be available in a format in which the different 

measured parameters can be easily compared and rapidly 

analyzed. 

The real-time seismic-amplitude monitor (RSAM) 

(Murray and Endo, chapter 1 of this bulletin) allows 

seismic data to be quantified and utilized at the time 

such data are most needed-when the seismic activity at 

a volcano is saturating all other seismic-data acquisition 

and analysis systems. Inexpensive, low-power, 

radio-telemetry systems (Murray, chapter 2; Lockhart 

and others, chapter 3) allow almost any sensor to send 

back data from the volcano in near real time. The 

microcomputer-based volcano-monitoring system using 

the program BOB (Murray, chapter 4) permits all data 

gathered by a monitoring team to be recorded, processed, 

and displayed on a common time base. Development of 

this system, together with that of the PC-AT and Sun 

workstation seismic systems (Endo and Smith, chapter 

5), more than anything else represents the most dramatic 

new contribution to volcano-monitoring methods and 

techniques made by the CVO group. These PC-based 

systems are powerful, portable, and very inexpensive 

compared to what was only marginally available at the 

beginning of the 1980's. 

Fully half of the chapters cover techniques for 

monitoring ground deformation, using both telemetered 

and nontelemetered methods. This emphasis in part 

reflects the success of the CVO staff in predicting 

eruptive activity at Mount St. Helens using these 

methods (Swanson and others, 1983, 1985). Four of the 

chapters deal directly with monitoring at Mount St. 

Helens itself and provide suggestions for field procedures 

gained from a decade of frrst-hand experience: those by 

Iwatsubo and Swanson (chapter 6), Dzurisin (chapter 7), 

Iwatsubo, Topinka, and Swanson (chapter 8), and 

Iwatsubo, Ewert, and Murray (chapter 9). Two of the 

chapters cover aspects of EDM techniques and strategies 

employed by CVO personnel at volcanoes other than 

Mount St. Helens (Iwatsubo and Swanson, chapter 10; 

Doukas and Ewert, chapter 11). The rest of the chapters 

on deformation cover various aspects of leveling, 

including geodetic leveling (Dzurisin, chapter 12; 

Yamashita and Kaiser, chapter 13), single-setup leveling 

using classical (Yamashita, chapter 14) and trigonometric 

(Ewert, chapter 15) methods, and lake leveling 

(Kleinman and Otway, chapter 16) that draws from 

experience gained in the United States and New Zealand. 

In the chapters on leveling we introduce the term 

single-setup leveling to replace the widely used but 

inexpressive and slang term dry tilt. The new term also 

replaces tilt leveling, which mixes a method (leveling) 

with a goal (to determine tilt). Single-setup leveling 

implies that the techniques of leveling are used with only 

one instrument setup-an implication that aptly describes 

the method. We hope that this more expressive term is 

used in the future by other workers. 

The two chapters on geochemistry describe a 

technique for continuous monitoring of springs and lakes 

(McGee and others, chapter 17) and give an overview of 

an integrated approach to geochemical monitoring 

(Sutton and others, chapter 18). The geochemistry 

chapters reflect the perceived need of CVO's 

geochemists to obtain data in close to real time. 

Continuous geochemical monitoring of a variety of 

chemical species will no doubt provide important 

monitoring data that can be integrated with other 

monitoring data in the years to come. 

The chapters on imaging describe the 

slow-scan-television surveillance system used to view the 

crater from 8.5 km away (Furukawa and others, chapter 

19) and the photodocumentation efforts (Topinka, 

chapter 20) used to trace the development of the eruption 

and the evolution of the crater, dome, and adjacent 

landscape. The bulletin's final chapter (21), by Swanson, 

points out the value of on-site observations and 

measurements in monitoring volcanoes, stresses that such 

techniques constitute important components of any 

monitoring program, and puts forth the view that 

monitoring should be done in as simple a way as is 

practical. 

The progress in monitoring techniques displayed in 

this bulletin is notable on several counts. Available 

technology has been adapted to monitor active volcanoes 

by a very small group of dedicated and innovative 

people. Most of the development of new procedures, and 

the modification of old ones, was conducted with the 

guiding philosophy of keeping the techniques as 

inexpensive as practical using easily accessible 

technology. This philosophy results in systems that are 

both suitable for, and within the budgetary reach of, 

developing nations that have a need to evaluate, monitor, 

and mitigate volcanic hazards. In addition, personal, 

hard-won, trial-and-error experience gained from ten 

years of monitoring Mount St. Helens and other 

volcanoes contributes significantly to the improved 

procedures. 

Most of the chapters were written by the technical 

personnel of CVO, who were largely responsible for the 

development and testing of the monitoring technologies. 

The new and improved capabilities resulted from the 

stimulating, give-and-take environment fostered by an 

observatory setting in which scientific and technical staff 

interact and learn with each other on a daily basis. This 

sort of environment encourages people to ask themselves 

"how can we do this better" and gives them the 

2 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
opportunity and freedom to try to do better. We believe 

that the chapters in this bulletin demonstrate the wisdom 

of promoting and maintaining such an environment 

Nothing is permanent except change. All of the 

techniques described in the bulletin can and will be 

improved with time. Certainly modifications will need to 

be made on a case-by-case basis for each volcano that 

requires monitoring. The chapters in the bulletin should 

provide well-grounded starting points for such 

improvements. We particularly hope that scientists and 

technicians in developing countries-our primary 

audience-will find the bulletin useful and will be able 

to adapt the methodologies to their particular problems. 

REFERENCES CITED 

Civetta, L., Gasparini, P., Luongo, G., and Rapolla, A., eds., 

1974, Physical volcanology: Amsterdam, Elsevier, 333 p. 

(Developments in Solid Earth Geophysics 6). 

Hofstetter, A., and Malone, S.D., 1986, Observation of volcanic 

tremor at Mount St. Helens in April and May 1980: 

Bulletin of the Seismological Society of America, v. 76, p. 

923-938. 

Malone, S.D., 1983, Volcanic earthquakes: Examples from Mt. 

St. Helens, in Earthquakes: Observations, theory, and 

interpretation: Amsterdam, North Holland, p.436-455. 

__ 1989, The HAWK real-time seismic recorder users 

manual: University of Washington Geophysics Program 

in-house publication. 

Malone, S.D., and Zollweg, Jim, 1985, University of 

Washington Seismic Processing System: University of 

Washington Geophysics Program in-house publication. 

Malone, S.D., Endo, E.T., Weaver, C.S., and Ramey, J.W., 

1981, Seismic monitoring for eruption prediction, in 

Lipman, P.W., and Mullineaux, D.R., eds., The 1980 

eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 803-813. 

Malone, S.D., Boyko, C., and Weaver, C.S., Seismic precursors 

to the Mount St. Helens eruptions in 1981 and 1982: 

Science, v. 221, p. 1376-1378. 

Swanson, D.A., Casadevall, T.J., Dzurisin, D., Malone, S.D., 

Newhall, C.G., and Weaver, C.S., 1983, Predicting 

eruptions at Mount St. Helens, June 1980 through 

December 1982: Science, v.221, no. 4618, p. 1369-1376. 

Swanson, D.A., Casadevall, T.J., Dzurisin, Daniel, Newhall, 

C.G., Malone, S.D., and Weaver, C.S., 1985, Forecasts and 

predictions of eruptive activity at Mount St. Helens, USA, 

1975-1984: Journal of Geodynamics, v. 3, p. 397-423. 

Tazieff, Haroun, and Sabroux, J.C., eds., 1983, Forecasting 

volcanic events: Amsterdam, Elsevier, 635 p. 

(Developments in Volcanology 1 ). 

Tilling, R.I., ed., 1989, Volcanic hazards: Washington, D.C., 

American Geophysical Union, Short Course in Geology, v. 

1, 123 p. 

UNESCO, 1972, The surveillance and prediction of volcanic 

activity; a review of methods and techniques: Paris, United 

Nations Educational, Scientific, and Cultural Organization, 

166 p. 

Introduction 3 

1. A Real-Time Seismic-Amplitude 

Measurement System (RSAM) 

By Thomas L. Murray and Elliot T. Endo 

Abstract 

Although several real-time earthquake detection and 

recorder systems exist, few address the problem of 

continuously measuring the amplitude of seismic signals 

during volcano-crisis conditions, when individual events 

are difficult to recognize. We developed a real-time 

seismic-amplitude measurement system (RSAM) that 

uses an inexpensive eight-bit analog-to-digital converter 

controlled by a laptop computer to provide 

1-minute-averaged, absolute-amplitude information for 

eight seismic stations near Mount St. Helens. The 

absolute voltage level for each station is digitized at 60 

samples/second, averaged, and immediately transmitted 

to a host computer for analysis. The RSAM provides a 

convenient-to-access, continuous time history of seismic 

activity at the volcano. RSAM systems calculating 

1 0-minute amplitude averages have been installed at the 

Cascades, Alaska, and Hawaiian Volcano Observatories. 

The RSAM has been a useful tool in predicting eruptive 

activity at Mount St. Helens and Redoubt Volcano, 

Alaska. 

INTRODUCTION 

During a volcanic crisis, seismicity commonly 

reaches a level at which distinguishing individual seismic 

events is difficult Analog seismic records provide some 

information, but rapid analysis is not always possible 

without substantially disturbing the c.ontinuity of 

recording. Although several real-time earthquake 

detection and recorder systems exist, most fail to provide 

quantitative information during periods of high 

seismicity that are commonly seen prior to a volcanic 

eruption. Yet it is during such conditions that 

quantitative seismic information needs to be processed 

most rapidly. To fill this need, we developed a simple 

and inexpensive real-time seismic-amplitude 

measurement system (RSAM). 

The RSAM measures the absolute amplitude of as 

many as eight seismic signals. The sampling rate is 

roughly 60 samples/second per signal. From these data, 

two measurements are computed for each incoming 

signal. 

1. The 1-minute average amplitude (absolute 

voltage) for each incoming signal is computed by 

summing the measurements made in the minute and 

dividing by the number of measurements. We have found 

that storing and analyzing 1-minute data for periods 

greater than a few days is cumbersome, owing to the 

sheer volume of data (1,440 measurements/day per 

signal). If a 10-minute average is computed using the the 

1-minute averages, data are stored and analyzed more 

easily. At the Hawaiian Volcano Observatory (HVO) and 

Alaskan Volcano Observatory (AVO), 10-minute 

averages are used for analysis. There appears to be no 

loss of pertinent information using the 10-minute 

averages compared to the 1-minute average. 

2. The number of events for each incoming signal 

occurring in a 10-minute period are counted by 

comparing successive 2-second average amplitudes. 

When the following conditions are met, the event counter 

for that input is incremented. 

IF A(n) is greater than THRESHOLD 

and 

IF A(n) is greater than RATIO times A(n-2) 

where A(n), n=1,2,3 ... are the successive 2-second 

averages. THRESHOLD defines the minimum amplitude 

for an event and RATIO defines how much greater than 

background the amplitude must be to be considered an 

event. The default value for THRESHOLD is 5.0 RSAM 

units and for RATIO is 2.0. 

Since no effort is made by the RSAM to 

discriminate seismic events from landslides, rockfalls, 

and other surface noise, the measurement is referred to 

as an "RSAM event," not a seismic event or earthquake. 

The data are transferred at 1 0-minute intervals to a 

host computer, where they are immediately available for 

analysis (Murray, chapter 4). In this manner, seismicity 

can be quantitatively monitored in near-real time (within 

10 minutes of acquisition), even during periods of 

intense tremor. 

1. A Real-Time Seismic-Amplitude Measurement System (RSAM) 5 
Energy release can also be monitored in 

near-real time. By squaring the average amplitude, a 

number proportional to the electrical energy 

generated by the movement of the geophone is 

computed. More study is required to determine the 

exact relationship between this electrical energy and 

seismic energy. 

Although the method seems almost too simple to 

be effective, the RSAM proved useful predicting the 

May 1985, May 1986 (fig. 1.1), and October 1986 

dome-building episodes at Mount St. Helens and the 

major January 2, 1990, eruption of Redoubt Volcano, 

Alaska (fig. 1.2). The RSAM data from stations close 

to the lava dome (Yellow Rock and St. Helens West) 

began to rise above background noise some 48 hours 

before the time of extrusion of a new lobe (fig. 1.1). 

The amplitudes continued increasing and peaked near 

the probable time of extrusion (exact time of extrusion 

is not known). The spikes and high amplitudes 

following extrusion were due to surface activity 

resulting from the emplacement of a new lobe. 

The energy release curve prior to the January 2, 

1990, eruption of Redoubt Volcano, Alaska, is shown 

in figure 1.2. The steepening of the curve beginning on 

January 1 was instrumental in the prediction of the 

eruption {I. Power, oral commun., 1990). 

640 

480 (/) 320 Iz 160 ::J 

:::::2: 0 <( 

(/) 

a: 

~ 120 

w· 90 0 

::J 

I- 60 

::::i 30 a.. 0 :::::2: 

<( 

_J 

<( 

z 

CJ 20 

Ci5 15 

10 

5 

0 

2 3 4 5 6 7 8 9 10 11 12 

START DAY IS MAY 1, 1986 G.M.T. - JULIAN DAY 121 

Figure 1.1. Fifteen-minute average amplitudes for three 

seismic stations during the May 1986 Mount St. Helens 

dome-growth episode. Stations Yellow Rock and St. 

Helens West began showing increased activity 2 or 3 

days before the extrusion of the new lobe on or around 

May 9 and showed dramatic increases the day prior to 

the extrusion. The signal amplitude at Elk Rock during 

the episode did not increase much above background 

and is smaller in amplitude than wind noise recorded on 

May 1, 2, 5, and 12. This shows the importance of using 

the RSAM in conjunction with analog records. 

The simplicity of the RSAM can aid in 

communicating levels of seismicity to public officials, 

who may be unfamiliar with the standard jargon used 

by seismologists. An RSAM time-series plot can, in 

many cases, show relative seismic activity more 

effectively than can graphs showing earthquake depths 

and magnitudes, or the number of long-period versus 

short-period earthquakes (Fairbanks Daily 

News-Miner, 1989). 

The RSAM is not meant to replace a conventional 

seismic system. Instead, it complements a conventional 

system, giving real-time information on tremor-amplitude 

levels while earthquake locations and magnitudes are 

being computed by other systems. During times of little 

or moderate activity, the RSAM may be only marginally 

useful and especially susceptible to contamination by 

wind or other surface noise. During these times the 

RSAM should be used in conjunction with analog 

seismic records. But during times of tremor or intense 

earthquake activity, when conventional seismic systems 

fail to keep pace with activity and analog records 

become a blur, the RSAM can become the primary 

monitor of seismicity. 

The RSAM can be used as a standalone unit, but 

we highly recommend that it be configured to transfer 

data to a more powerful host computer at 10-minute 

intervals. The host computer archives the data and 

enables them to be integrated with other 

volcano-monitoring information for near-real-time 

analysis (Murray, chapter 4 ). 

15 20 DEC 25 31 1 JAN 

START DAY IS DEC 12, 1989 A.S.T. - JULIAN DAY 346 

Flgure1.2. Cumulative RSAM energy release for 

stations RON and RED for the January 2. 1990, eruption 

of Redoubt Volcano. Alaska. RSAM energy is calculated 

by squaring the average amplitude. Because the relation 

between RSAM energy and seismic energy is not yet 

known. the shape of the curve, not the actual amount of 

energy, provided the information used in predicting the 

eruption. 

6 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
GENERAL DESCRIPTION 

The real-time seismic-amplitude measurement 

system consists of a Tandy (Radio Shack) Model 100 

laptop computer (or Model 102, which is essentially the 

same except for the system bus socket) and an 

in-house-designed data-acquisition board. The entire unit 

fits easily in a space 25.4 em by 33 em by 5.0 em. Low 

power consumption (90 rnA at 12 V) allows the unit to 

be powered by a car battery and solar panel if necessary. 

The data acquisition board (fig. 1.3) buffers the 

eight seismic input signals and puts them through 0.1-Hz 

high-pass filters to eliminate any DC offsets. The 

multiplexer selects the desired signal for sampling. The 

signal is then full-wave rectified to convert any negative 

component to a positive voltage. The signals are 

digitized with an eight-bit analog-to-digital converter. 

Communication between the acquisition board and the 

computer is through the computer's system bus, freeing 

the computer's other ports for connection to other 

peripherals. 

Generally, data from the RSAM are shown in 

RSAM units. RSAM units are the average value of the 

output of the eight-bit analog-to-digital converter, 

multiplied by 10 so as to be an integer. In a system set 

up for discriminators with a ±2.5-volt output, one RSAM 

unit is roughly 1 millivolt. For more precise 

measurements, each unit should be individually 

calibrated. 

SEISMIC 

DISCRIMINATOR 

OUTPUTS 

BUFFER 

FILTER 

MULTIPLEXER 

FULL WAVE 

~ RECTIFIER r----

DATAACQUISITION BOARD 

MODEL 100/102 LAP COMPUTER 

Figure 1.3. RSAM hardware block diagram. 

8 BIT 

AID 

Data Acquisition and Processing 

The computer calculates the average signal 

amplitude once a minute for each input by simply 

dividing the sum of each input's digitized samples by the 

number of samples. Taking the average over a 1-minute 

period allows the cessation of data acquisition for short 

periods in order to process the data. This greatly 

simplifies programming, because data acquisition and 

processing do not have to be performed concurrently. 

The data processing requires roughly 0.5 second for 

every 2 seconds of data acquisition at 60 samples/second. 

Most of the processing time is due to the event detection 

routine. 

At the beginning of each minute, a call to the 

data-acquisition subprogram causes the computer to 

digitize 125 samples for each seismic input at a rate of 

approximat~ly 60 samples/second/input and return the 

sums of the digitized values. The returned sums are 

added to running totals for the entire minute. At this 

point the data are checked to determine if an RSAM 

event has occurred. Another call is then made to the 

data-acquisition subprogram, and the cycle continues 

throughout the minute. At the end of the minute, the 

average amplitudes are computed by dividing the running 

totals by the number of samples. The process then starts 

again for the next minute's data (fig. 1.4). Depending on 

the specific site setup, the averages can be sent to a more 

powerful computer via an RS-232C link for analysis, 

stored in memory for later access, or merely sent out to a 

printer. The Model 100/102, though only a 32 kilobyte, 

8085-based computer, still allows for numerous options. 

A full listing of the standard program used by the 

RSAM is in Murray and Endo (1989). 

DATA ACQUISITION HARDWARE 

The RSAM data-acquisition board can be divided 

into three sections: (1) power supply, (2) system-bus 

interface, and (3) signal conditioner/converter. The 

circuit schematic is shown in figures 1.5 and 1.6, and the 

parts are listed in table 1.1. 

Power Supply 

The power supply (US through U8) converts the 

12-volt input (12) to ±12 volts to power the analog 

section of the circuit, ±5 volts for the multiplexer, and 

+6 volts to power the computer via J3. This allows the 

entire unit to be powered by a 12-volt battery. Current 

draw is under 100 milliamps. 

1. A Real-Time Seismic-Amplitude Measurement System (RSAM) 7 
System-Bus Interface 

The system-bus interface (U1 thru U4) performs 

the address decoding for the programmable interface 

adapter (PIA), U1, and the analog-to-digital converter 

{A/D), U9. A 40-conductor ribbon cable between J1 and 

the Model 100/102' s system bus socket connects the 

board to the computer. Input-output {1/0) addresses 

0-127 of the Model 100/102 are available for external 

uses such as this acquisition board. Switch S 1 selects the 

address block (32-63, 64-95, or 96-127) in which the 

PIA will reside. The three blocks allow as many as three 

of the boards to be hooked to a single Model 1001102 

(although at present adding the second or third board has 

not been implemented). S2 sets the 1/0 address for the 

A/D. Although S2 can be set to any address under 128, 

SAMPLE FOR 2 

SECONDS AT 60 

SAMPLES I SECOND 

UPDATE 

RUNNING TOTALS 

CHECK FOR EVENTS 

YES 

COMPUTE 

1-MINUTE AVERAGES 

RESET COUNTERS 

BEGIN SAMPLING 

TRANSFER 

1 0-MINUTE DATA 

TO HOST AND 

RAM 

YES 

DISPLAY AVERAGES 

UPDATE 1 0-MINUTE 

RUNNING TOTALS 

Figure 1.4. RSAM software flow chart. 

Table 1.1. Parts list for data acquisition board. 

Part Value Oesaiption 

Cl-C2 lOmF Electrolytic, 16 V 

C3-C4 lOOmF Electrolytic, 16 V 

CS-C6 lOmF Electrolytic, 16 V 

C7 lSOpF Ceramic disk 

C8-C12 O.lmF Mallory CKOSBX104K 

Cl3 33mF Kemet T352-F336K-010AS 

C22-C35 33mF Kemet T352-F336K-010AS 

C37-C38 O.lmF Mallory CKOSBX104K 

RNl lOOK Bourns 4610x-101 lOOK 

R2 lOk Pot 

R3 2.7k 5%,1/4 w 

R4 lOK 5%,1/4 w 

R22-R43 lOOK 5%, 1/4 W (8 resistors total) 

R82-R86 lOK 5%, 1/4 W (5 resistors total) 

01 LM 385-1.2 for ±2.5 V inputs 

LM 336-25 for ±5.0 V inputs 

02-03 1N914 

04 1N5818 

05-06 1N914 

010-011 1N914 

Ul National Semiconductor NSC810 

U2 74HC138 

U3 CD4011B 

U4 74HC688 

us ICL7662 

U6 78LOSCP 

U7 LM7806CK 

us ICL7662 

U9 ADC0803LCN 

U10-Ul7 TL022 

U18 CD4051B 

U19 LM358 

we recommend that it be set to one in the 0-31 block, 

leaving the higher addresses for the PIA's. 

Note that only three of the PIA's 22 digital 1/0 

lines are used to control the multiplexer. The remaining 

lines are available for circuit enhancements. 

Signal Conditioner/Converter 

The analog seismic signals enter the board via J5. 

All the signal lows are shorted to ground. These signals 

8 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Note 1) • D5, D6 and C13 are located on the 

solder side of the board. 

2) Pins 5 and 6 of U2 were switched 

during board production. 

3) D1 is an LM385-12 for 2.5 volt 

output discriminators. 

REF VOLTAGE 

BIT6 

BITO 

ADCOB03 D1 VREF DBO is an LM336-25 for 5.0 volt 

output discriminators. DSC 

11'191'1 

DB1 f-'-'----'-'-''-'--1 

--.--,.,...,---~='---"-1 +VIN DB21----'-"---""'-=--I 

-VIN DB3 1--'-"---'-'-'=--t 

AGND DB'! 1---'-'--='-'---f 

~~~ ~=illldf--t----t:all:L=~ vee DB? 1-'-'----"-'-''-'---' 

CLK ES f-'-----' 

4) The pin configuration for the Model 

102 system bus as listed in the 102 

manual is for the 40-pin DIP socket 

that was used on the Model 100, 

not the mass termination connector 

used on the 102. J1 shows the 

proper pin-outs for the model 102. 

t---'l/lr---'-'-1 CLKR RD 

5) Default address/switch settings 

S1 32-63 (all positions off except 

for 2) for the PIA address. 

S2 5 (all positions off except for 1 

and 3) for the AID address. 

MUXA 

MUXB 

MUXC 

DGND WR 

iNfR U9 

INTERFACE TO THE RADIO SHACK MODEL 102 SYSTEM BUS 

WITH 

8 BIT ANALOG-TO-DIGITAL CONVERTER 

U S GEOLOGICAL SURVEY 

TOM MURRAY 

1988 

32-63 

6'1-95 

96-127 

Flgure1.5. Circuit diagram for power supplies, analog-to-digital converter, and Model100/102 system-bus interface of 

data-acquisition board. 

should be tapped from either the outputs of the 

discriminators or the inputs to the analog drum recorders. 

U10-U17 buffe:r each of the eight signals and send 

them through 0.1 Hz high-pass filters to remove any DC 

offsets. Bits 0-2 of port A of the PIA control which input 

the multiplexer (Ul8) switches to the AID (U9) for 

digitizing. Since the A/D will accept only positive 

voltages, the signal must be full-wave rectified (U19) 

before it is digitized. 

Dl supplies the reference voltage for the A/D. 

The full-scale inpult voltage for the AID is twice the 

reference voltage. For signals with a range of ±2.5 volts, 

a 1.22-volt reference~ (LM385-1.22) is used. For signals 

with a range of ±5.0 volts, a 2.5-volt reference 

(LM336-2.5) is used. Note that R2 can be used to set the 

LM336-2.5 to precisely +2.500 volts, but no such 

adjustment is possiblle with the LM385-1.22. 

CONCLUSION 

The RSAM is not meant to replace analog seismic 

recorders or conventional seismic data-acquisition systems. 

Its inability to distinguish cultural noise from volcanic 

seismicity precludes using it without some method to 

visually inspect the analog seismic records. It is also of 

only marginal value during times of low seismicity. 

However, as seismicity increases during a volcanic crisis, 

the RSAM's value increases. The higher the level of 

seismicity, the more potentially important the RSAM 

becomes. Depending on the nature and level of seismicity, 

the RSAM may provide the only near-real-time quantitative 

measure of seismic activity. The RSAM has repeatedly 

demonstrated its utility in monitoring and predicting 

eruptions and has become an essential monitoring tool at 

the Cascades and Alaska Volcano Observatories. 

1. A Real-Time Seismic-Amplitude Measurement System (RSAM) 9 
INPUT BUFFER 0 I Hz HIGH-PASS FILTER MUL TIPLE><ER 

JNPUT6 

R85 

!OK 

R86 

MU><A 

FULL-WAUE 

RECTIFIER 

REAL-TIME SEISMIC AMPLITUDE 

METER 

SIGNAL CONDITIONER/MULTIPLEXER 

CIRCUIT 

U10-U17 TL022 

U19 LM358 

All capacitors 1 0% 

All resistors 5% 

C21-C36 must be tantalum 

US GEOLOGICAL SURVEY 

TOM MURRAY 

1988 

Figure 1.6. Circuit diagram for signal-conditioner/ multiplexer of data-acquisitiion board. 

REFERENCES CITED 

Fairbanks Daily News-Miner, 1989, Travelers wait as Redoubt 

rumbles anew: December 19, p. 5. 

Murray, T.L., and Endo, E.T., 1989, A real-time seismic 

amplitude measureme.nt system (RSAM): U.S. Geological 

Survey Open-File Report 89-684, 26 p. 

10 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
2. A Low-·Data-Rate Digital Telemetry System 

By Thomas L. Murray 

ABSTRACT 

Telemetering datcl from electronic devices that sense 

ground tilt, temperature, strain, gas concentration, and other 

phenomena can provide valuable information regarding 

current or impending volcanic activity. To facilitate such 

monitoring, I developE~d a low-data-rate digital telemetry 

system that transmits; data at 1 0-minute intervals over 

VHF/UHF radio links to a central receiving site. Analog 

voltages are digitized with a 12-bit-plus-sign analog-to-digital 

converter in the field. The data are transmitted in Bell 103 

format so that most cc,mputers with a Bell 103 compatible 

modem attached to their serial port can receive, process, 

and store the data. A n-gallon steel drum houses the entire 

transmitter including ratdio, antenna, and sufficient batteries 

to power the unit for a year. Design considerations imposed 

by Mount St. Helens resulted in a robust, low-cost, 

low-power system with wide applicability at other volcanoes. 

INTRODUCTION 

Measuring ground tilt, gas concentrations, 

temperature, and othe.r phenomena at 10-minute intervals 

can be useful in assessing volcanic activity (Dzurisin and 

others, 1983; McGe·e and others, 1987). Telemetering 

such measurements to a central receiving site provides 

near-real-time infonnation on the status of a volcano 

when personal observation or on-site data collection is 

impractical due to high hazards or inclement weather. 

The system decribed here is a low-data-rate system in 

which measuremelllts are made and transmitted at 

intervals greater than or equal to 1 minute. Its operation 

resembles that described by Roger and others (1977). 

Individual field transmitters accept up to 16 analog 

inputs. These inputs are digitized, and the data are 

transmitted by radio to a central receiving site at 

preselected intervals, usually every 10 minutes. 

Transmissions typically take less than 10 seconds, 

allowing the transmitter to be in a low-power sleep state 

most of the time. This procedure not only conserves 

power but also allows numerous transmitters to share the 

same radio and audio frequency. Each transmitter in turn 

activates itself, transmits its data, and goes back to sleep. 

Other transmitters can then transmit their data. The 

transmissions are received by a common radio receiver 

and relayed through the modem port of a 

battery-powered laptop computer for decoding and data 

storage. 

The system was frrst used at the Cascades Volcano 

Observatory (CVO) in 1984. Since then, systems have 

been installed in Colombia, Ecuador, Guatemala, and 

Alaska. Data from tiltmeters, temperature sensors, 

magnetometers, strainmeters, gas sensors, and 

water-quality sensors have been transmitted through the 

system. 

The data are usually transmitted back to a receiver 

located in an established volcano observatory, but this 

does not have to be the case. A four-transmitter network 

established in 1987 at Yellowstone Lake, Wyoming, 

transmits data to a Radio Shack Model 100 computer 

housed in a shed behind the nearby Lake Ranger Station. 

The data are transferred weekly to CVO through the 

phone system. Though a faulty fuse at the receiver 

caused some loss of data in the first year, the system has 

been operating since without a problem. 

The following is a general description of the 

transmitter. I describe the operation of the transmitter in 

more detail in Murray (1988). 

DESIGN CONSIDERATIONS 

Telemetering data from the crater of Mount St. 

Helens presents several problems that constrain the 

design of the transmitter. The sites are located on and 

around the lava dome, which is situated in a 

horseshoe-shaped crater opening to the north. Some of 

the monitoring sites are situated on small ledges on the 

flanks of the dome. The sites can be buried by as much 

as 3 m of snow during the winter, making them 

inaccesible for up to 6 months of the year. Radio power, 

physical size, and power consumption were the primary 

considerations in designing the system, and each is 

discussed below. 

2. A Low-Data-Rate Digital Telemetry System 11 
Radio Power 

Because of the horseshoe shape of the crater, a 

transmitter on the south flank of the dome does not have 

line-of-sight radio contact with the repeaters located to 

the north. This geometry requires the radio signal to be 

sufficiently powerful to reflect off the crater wall and out 

to the north. High-gain antennas are impractical owing to 

site considerations, so high-power ( 4 watt) radios are 

used instead of the 100-250 m W radios used at many 

telemetered seismic stations. 

Physical Size 

The stations on the flanks of the lava dome are 

commonly located on narrow ledges situated at the to~ 

of talus slopes. These ledges can be as small as 0.3 m2• 

Although the talus is sufficiently stable to stand upon 

when installing or servicing a site, it does not provide a 

stable platform for the transmitter. Hence, the entire 

transmitter package (electronics, battery, and antenna) 

must be small enough to fit on the ledges. 

Power Consumption 

Many of the sites on the dome are completely 

inaccesible from December to June owing to winter 

snowfall. Primary batteries must be used at these sites, 

since ashfall, wind-blown silt, and topography make 

solar panels impractical. Access to these sites is by 

helicopter (expensive) or by foot (time-consuming), so it 

is desirable that the transmitters operate a full year 

without replacing the batteries. The limited space on the 

ledges limits the size and number of batteries that can be 

used, in turn constraining the power consumption. 

SYSTEM FEATURES 

Using the above considerations as guidelines, I 

designed a digital transmitter with the following features: 

1. Low power cons urn ption. The average power 

consumption for the digital transmitter, when it is 

transmitting four measurements at 300 baud every 10 

minutes, is under 300 J1A at 12 volts. The power 

consumption of the radio is much larger. A 4-watt radio 

draws as much as 900 rnA at 12 volts while transmitting; 

its average power consumption for the above example is 

6.2 rnA at 12 volts. The total power requirement for the 

site (excluding that required for the sensors) for a year is 

just under 60 amp-hours. Two 20-amp-hour 6-volt 

alkaline batteries connected in series can power the 

digital transmitter for 1 year. The radio requires four 

40-amp-hour 7.5-volt alkaline batteries connected in 

series-parallel to power it for a year. 

2. Transmitting data at 300 baud in standard Bell 

103 format. The transmitter utilizes a single-chip modem 

(Motorola MC14412FP) to transmit data over radio links 

in Bell 103 format. This allows nearly any computer with 

a 300-baud Bell 103 compatible modem to receive the 

data. No custom circuitry other than the cable is required 

between the radio and the modem. The Bell 103 format 

also allows the transmissions to be summed with seismic 

signals and share radio links, though at the expense of 

two seismic audio bands (either 1,020 and 1,360 Hz or 

2,040 and 2,380 Hz). When summed with the seismic 

signals, a filter may be required at the receiver if the 

modem filter does not discriminate the signal well 

enough. 

3. Sharing a single radio and audio frequency. The 

short amount of time required to transmit the data also 

allows multiple transmitters to share ·a single audio and 

radio frequency. The situation is analogous to having 

numerous field crews communicating with each other 

using the same voice radio frequency. Each crew's radio 

is in transmit mode for only the duration of the message, 

or 2 seconds. At the end of the message, the radio 

transmitter is turned off, allowing another crew to 

transmit a message. Just as when two people try to 

transmit a message at the same time, transmissions may 

interfere with each other. Like the system described by 

Roger and others (1977), the system described here does 

not try to prevent these "collisions" but instead transmits 

data at semi-random intervals often enough that 

occasional collisions can be tolerated. The transmit 

interval is randomized by using inexpensive, relatively 

inaccurate oscillators. The transmission interval, though 

nominally set for 10 minutes, actually varies from 8 to 

12 minutes. Transmission collisions between stations do 

occur, but owing to the variability in the transmitinterval, the next transmissions from two "colliding" 

stations are unlikely to overlap, and both transmissions 

will be successfully received. 

4. Utilizing 4-watt radio transmitters. The time 

required to transmit the data (2 seconds) is short relative 

to the time between transmissions (10 min), hence radios 

whose output power is 4 watts or more can be used 

without excessive power consumption. These radios can 

generally eliminate the need for the yagi antennas 

required by radios typically used at seismic stations, 

whose output powers are low (100-250 m W) owing to 

their 100 percent duty cycle. Eliminating the yagis 

reduces cost, bulk, weather-related problems, and 

visibility. 

5. Transmitting data twice to determine its validity. 

Each measurement is transmitted in a six-byte message. 

12 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
The frrst two bytes contain the data, and the third 

contains the channel (measurement) identifier (channel 

ID) indicating which sensor's datum is being transmitted. 

A 

8 

Figure 2.1. A,. Transmitter, radio, and batteries housed 

in a metal drum. Antenna is mounted on lid and is 

protected from snow and ice by a plastic pipe. 8, Radio 

(top) and transmitter (bottom) mounted on the inside of 

the lid. 

The fourth and fifth bytes are bytes one and two 

repeated. The sixth byte is the channel ID repeated. A 

message is considered valid only when the frrst three 

bytes are the same as the last three. 

6. Putting the transmitter, radio, antenna, and 

batteries in a single package. At Mount St. Helens, the 

transmitters are housed in 9-gallon metal drums (fig. 

2.1). The transmitter board and radio are mounted on the 

inside of the lid with alkaline batteries placed in the 

bottom of the drum. The drum has sufficient space to 

house enough batteries to power the system for 1 year at 

a 10-minute transmit interval. A quarter-wave whip 

antenna is attached to the top of the lid. The metal lid 

provides an excellent ground plane for the antenna. 

Plastic pipe 10 em in diameter is attached to the lid to 

protect the antenna from snow and ice. 

7. All logic in hardware, not software. The 

transmitter's logic is hardware based, not microprocessor 

based. Switches are used to set the transmit interval, 

channel IDs, and so on. Delays are determined by 

resistor-capacitor (RC) time constants. The field 

transmitters take samples at set intervals and transmit the 

data. Any changes to this operation, such as averaging 

data or transmitting data more often if certain values 

reach a threshold, cannot be done without modifying the 

board. Such modification is not easy, whereas with 

microprocessor-based systems, the changes could be 

easily implemented in software. Logic based in hardware 

has some advantages, however. Most of the integrated 

circuits are manufactured by several companies and are 

readily available in most countries. Also, troubleshooting 

does not require a high-speed oscilloscope or logic 

analyzer. A relatively slow (200 kHz) oscilloscope, a 

volt-ohm meter, and a frequency counter ( MHz) are all 

that are usually required. 

8. Field troubleshooting. A Radio Shack Model 

100/102 battery-powered laptop computer with an 

integral Bell 103 modem serves as a portable field 

receiver. The Model 100/102 allows viewing of the data 

being transmitted in the field. The data's validity can 

checked before leaving the site. 

9. Low cost. The cost of parts for a complete 

transmitter <~xcluding radio is currently under $550. The 

cost of a radio transmitter ranges between $200 and 

$500. 

THEORY OF OPERATION 

The transmitter consists of 10 blocks (fig. 2.2): 

1. Clock 

2. System power supply 

3. Transmission-control logic 

4. Analog multiplexer 

2. A Low-Data-Rate Digital Telemetry System 13 
5. Analog-to-digital converter (AID) 

6. Channel ID counter 

7. Number-of-channels-transmitted counter 

8. Universal asychronous receiver transmitter 

(UART) 

9. Modem 

10. Radio 

To conserve power, the transmitter is normally 

in a low-power sleep state; power is applied only to 

the clock and, if used, IC U14 (one of two choices 

for the analog multiplexer; see below). At 

switch-selectable intervals, the clock enables the 

system power supply, and the transmitter-control 

logic begins the data-transmission sequence. 

Counters indicating the initial analog input to be 

selected for digitizing and the channel ID are set to 

the proper values. The selected analog input is then 

digitized. 

The radio transmitter, which was off to prevent 

radio frequency interference during the A/D 

conversion, is now enabled. The digitized data and 

its channel ID are converted in turn by the UART from 

8-bit parallel data to 8-bit serial data. The serial data 

stream is converted by the modem to audio tones, which 

are transmitted by the radio. The radio transmitter is 

disabled at the completion of the transmission. The 

number-of-channels-transmitted counter, the 

analog-input-select counter, and the channel ID counter 

are incremented by one. If all of the scheduled channels 

have been transmitted, the system power supply is 

disabled and the digital transmitter returns to its sleep 

state. If, however, another channel is to be transmitted, 

the next analog input is digitized and the data transmitted 

as before. The data-transmission sequence continues 

until the designated number of channels has been 

transmitted. 

CIRCUIT DESCRIPTION 

The circuit schematic for the transmitter is shown 

in figure 2.3 and the parts list is given in Table 2.1. 

+12 VOLT LOGIC I +6 VOLT LOGIC 

I 

l • 

I CHANNELS CHANNEL ID MULTIPLEXER 

CLOCK/ WAKE-UP 

TO 

POWER 

TRANSMIT 

OFF 

~ 

~ 

LOGIC ~ I 

DATA BUS 

i t__ ~ 

~ 

I COUNTER PRESET - ~ 

TRANSMISSION ~ 

~ 

I CONTROL 

POWER ON - LOGIC ~ 

I NEXT CHANNEL _ SIGNAL OUT 

BEGIN 

CHANNEL ENABLE -

_jl I AID 

1 LO BITS ENABLE I--

HI BITS ENABLE VOLTAGE 

REGULATORS I CONVERT 

+6 

I 

TRANSMIT BYTE 1--- DATA BUS 

-6 KEY RADIO~ 

+8 I 

-7.6 

+12 I I 

RADIO POWER UART MODEM 

I BELL 103 OUT SERIAL DATA OUT 

I lr 

I 

Figure 2.2 Block diagram of transmitter. 

14 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 

r--

r--

r--

~ 

~ 

r--

r--

~ 

~ 

r--

,__ 

f--

-

-

-

-

-+ 

0 

0 

~ 
Table 2.1. Parts list for digital transmitter circuit board. 

[Pin-for-pin equivalent components from manufactun:n other than those liated may be substituted] 

Part No. Part Value Comments Part No. Part Value Comments 

Cl ceramic diak .1 JLF R17 l'ellator 100 l/4 watt 5CJ, 

C2 canmic diak .01 JLF cuban 

C3 not used RIB shorted 

C4 electrolytic 10 JLF R19 not used 

cs not used R20 re~istor 1M 1/4 watt 5CJ, 

C6 canmic diak .1 JLF cuban 

C7 canmic diak .01 JLF R21-R33 re~iator lOOK l/4 watt 5CJ, 

CB--Cll electrolytic 10 JLF cuban 

C12 electrolytic 15J.Lf R34 l'eliator lSOK 1/4 watt 5CJ, 

C13 ceramic diak 47p.F cuban 

C14 electrolytic 10 JLF R3S l'eliator lOOK 1/4 watt 5CJ, 

CIS ceramic diak .Olp.F cuban 

C16 polypropylene .lp.F mcctrocube R36 l'eliator 1M 1/4 watt lCJ, 

93SB1BlOSK RN60C 

C17 ceramic diak .01 JLF R37 re~istor 2SOK l/4 watt 5CJ, 

C18 polypropylene .22 JLF mcctrocube cuban 

93SB1B224K R38 re~istor 2.4K 1/4 watt 5CJ, 

C19 polypropylene .47 JLF mcctrocube carbon 

93SB1b474K R39 re~istor lOOK 1/4 watt scr, 

C20 ceramic diak .01 JLF carbon 

C21 not used R40 resistor 1M l/4 watt 5CJ, 

C22 ceramic diak .47 JLF cuban 

C23-24 ceramic disk .001 JLF R41 re~istor lSOK 1/4 watt scr, 

C2S tantalum lp.F carbon 

C26-Tl ceramic diak S6pF Needed if U29 is R42 resistor lOOK 1/4 watt 5CJ, 

a IM47021PE carbon 

C28 ceramic diak .01 JLF R43 shorted 

C29 ceramic diak .01 JLF R44 not used 

C30 electrolytic 10 JLF R4S-RS3 resistor lOOK 1/4 watt scr, 

C31-C32 electrolytic 22JLF cuban 

C33 ceramic disk .01 JLF RS4 not used 1/4 watt 5CJ, 

Rl re~istor 7SOK 1/4 wattS% carbon 

carbon RSS resistor lOOK 1/4 wattS% 

R2 re~istor lOOK 1/4 wattS% carbon 

carbon RS6 resistor 10M l/4 watt 5% 

R3 resistor 200K 1/4 wattS% cuban 

carbon RS7 resistor ISM 1/4 watt scr, 

R4 not used cuban 

RS-R7 re~istor lOOK 1/4 wattS% RS8-RS9 resistor l/4 wattS% 

carbon cuban 

RB not used R60-R61 re~istor Selcctl VCO 

R9 re~istor lOOK 1/4 wattS% output ranse 

carbon R62-R64 potentiometer Spcctrol64W103 

RIO re~iator 3M 1/4 watt 5% R65 potentiometer Spcctrol64Wl04 

carbon R66 potentiometer Spcctrol 64W103 

Rll l'elistor 1M 114 wattS% R67 resistor lOOK 1/4 wattS% 

carbon carbon 

Rl2 re~istor 20 1/4 wattS% R68 re~istor 100 l/4 wattS% 

carbon cuban 

Rl3 resistor 360K 1/4 watt 5% R69 resistor lOOK l/4 wattS% 

carbon cuban 

Rl4 resistor lOOK 1/4 wattS% R70 resistor 20K 1/4 wattS% 

carbon carbon 

RlS re~istor 20 1/4 wattS% R71 l'elistor lOOK 1/4 wattS% 

carbon carbon 

R16 re~istor SlO 114 wattS% Dl-4 diode IN914 

carbon 

2. A Low-Data-Rate Digital Telemetry System 15 
Table 2.1. Parts list for digital transmitter circuit board.-Continusd 

[Pin-for-pin equivalent components from manufacturers other than those listed may be substituted] 

Part No. Part Value Comments Part No. Part Value Comments 

D5 tranzo:rb General U15 integrated cin:uit OP07CZ PMI 

Semiconductor U16 integrated circuit MM74C193N National 

tl.SKE1SCA U17 integrated circuit MM74C193N National 

D6 diode Motorola INS400 U18 integrated circuit MM74C373N National 

(3 amp) U19 integrated circuit CD4043BE National 

D7-D14 IN914 U20 integrated circuit CD40106BE National 

ZO-Z15 tranzo:rb General U21 integrated circuit MC14081BCP Motorola 

Semiconductor U22 integrated circuit ICL71091JL Intersil (exL 

11.5K.E1 .5CA tanp.) 

(optional) U23 integrated circuit LM236H-2.5 National (exL 

Ql MOSFET Siliconix tanp.) 

VP0300L U24 integrated circuit MC4081BCP Motorola 

Q2-Q3 MOSFET Motorola IRF511 U2S integrated circuit CD4075BCN National 

Sl switch 1 0 position U26 integrated circuit MC14017BCP Motorola 

S2-S3 switch DPDY Sprague QSP U27 integrated circuit CD4001BCN National 

1410 U28 integrated circuit IM6402IPL lntcrsil 

S4-S5 switch 8 pole DIP C&KBD08 U29 integrated circuit IM47121PE or lntenil 

S6 switch 4 pole DIP C&KBD04 IM47021PE 

S7 switch 8 pole DIP C&KBD08 U30 integrated circuit MC14412FP Motorola 

s8-s9 switch 4 pole DIP C&KBD04 U31 integrated circuit 1L022CP Texas Instruments 

Yl crystal 3.5195 MHz US Crystal U32 integrated circuit CD4051BF National 

Y2 crystal 2.4576 MHz US Crystal (optioned w/(]14) 

Y3 crystal 1.0 MHz US Cryats; 11 1/0 connector A P 925225-26-R 

Tl transfonner 600 ohm J2 44 pin edge card 

primary/secondary connector 

impedance J3 44 pin edge card Spare PCB 

U1 integrated circuit CD4047BE National connector connector 

U2 integrated circuit CD4020BE National 8 pin DIP socket Au gat (9 total) 

U3 integrated circuit CD4043BE National 508-AG37D 

U4 integrated circuit CD40106BE National 14 pin DIP socket Au gat (7 total) 

us integrated circuit CD40SOBCN National 514-AG37D 

U6 integrated circuit MC14504BCP Motorola 16 pin DIP socket Au gat (18 total) 

U7 integrated circuit ICL7663CPA lntersil 516-AG37D 

U8 integrated circuit ICL7660CPA lntersil 20 pin DIP socket Au gat (1 total) 

U9 integrated circuit ICL7663CPA lntersil 520-AG37D 

U10 integrated circuit ICL7660CPA lntersil 28 pin DIP socket Au gat (1 total) 

Ull integrated circuit MC14017BCP Motorola 528-AG37D 

U12 integrated circuit MC14017BCP Motorola 40 pin DIP socket Au gat (2 total) 

Ul3 integrated circuit MM74C193N National S40-AG37D 

Ul4 integrated circuit lll-506-A Harris (optioned 

w/U32) 

Clock System Power Supply 

The clock (U1, CD4047) is a low-power oscillator 

A positive transition at the output of the clock 

circuit awakens the transmitter from its low-power sleep 

whose output frequency (approximately 2 Hz) is divided state by enabling the power supply. Low-quiescentinto nine switch-selectable transmit intervals. The current linear regulators, U7 and U9 (ICL 7663), and 

intervals range from nominally 36 seconds to 2.5 hours. voltage inverters, U8 and U10 (ICL7660), provide ± 6 

A resistor and capacitor determine the exact output and ± 8 volts. A P-channel metal oxide semiconductor 

frequency of the oscillator. Using standard-grade field effect transistor (MOSFEn, VP0300L, provides a 

components with accuracies of± 5% or poorer ensures switched +12 volts for use in powering sensors. Power 

that the transmission interval will be variable between ground is switched to the radio through an N-channel 

transmitters. A nominal 10-minute transmit interval may MOSFET. However, the radio is not put into transmit 

vary between 8 and 12 minutes. mode at this time. 

16 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
~ 

,.. 

; 

~ ;: 

:a 

ID 

I' 

52 

fiQ 

! 

~ 

i' 

3 

s 

-< 

U) 

'< • ; -.... 

Figure 2.3. Circuit schematic for transmitter. 

NOTES~ 

1 • .A..Ll- DIODe:.<:; A.ii::.E 

AS ~TeD 
Transmission-control Logic 

ICs Ul9-U20 and U24-U27 control the sequence of 

events after the transmitter is awakened from its sleep 

state (fig. 2.4). Using resistor-capacitor circuits for 

timing delays, the control logic increments the counters, 

initiates the AID conversions, controls the radio 

transmitter, and sequentially gates data and the channel 

ID into the UART (discussed below) for the 

parallel-to-serial conversion. 

Analog Multiplexer 

Either of two multiplexers, the lfl-506-A (Ul4) or 

the CD4051 (U32), is used to switch the analog input 

signals to the A/D converter for digitizing. The 

HI-506-A allows up to 16 inputs and is overvoltageprotected; it can withstand ± 25 volts applied to its 

inputs even when it is not powered. The HI-506-A is 

manufactured by Harris Semiconductor and currently 

costs about $20. The CD4051 is an 8-channel 

multiplexer with no overvoltage protection. It is 

readily available from numerous suppliers for less than 

$1. 

I originally thought that the HI-506-A, with its 

overvoltage protection, would be used in situations 

where analog input signals are present while the 

transmitter is in its sleep state. This would occur if the 

sensors were always on. If the sensors are active only 

when the transmitter is in its active state, the CD4051 

could be used. 

I later found that although the HI-506-A is not 

damaged by applying signals to it when the power supplies 

are off, the applied voltage leaks through the multiplexer to 

its power pin. Owing to the low power requirements of the 

-SLEEP MODE- -WAKE UPTURN ON RADIO DIGITIZE ANALOG ONLY THE CLOCK 

ENABLE POWER SUPPLIES CHANNEL IS ACTIVE 

PRE-SET COUNTERS 

" 

-POWER DOWN- CHANNELS 

POWER SUPPLIES TO TRANSMIT KEY RADIO 

AND RADIO EQUAL CHANNELS TRANSMITTER 

TURNED OFF TRANSMITTED? 

rUN-KEY RADIO 

TRANSMITTER ~ TRANSMIT DATA ~ 

INCREMENT 

COUNTERS 

Figure 2.4. Flow chart for transmission control logic. 

18 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
transmitter, this leakage can be sufficient to power the 

board. When this occurs, the transmitter never returns to 

its sleep state, but instead continues to transmit. This 

problem was solved by powering the HI-506-A 

continuously from the + 12 volt supply. 

Conner (1990) presented a more elegant solution to 

the problem. He inserted diodes between the multiplexer 

and its power supplies (fig. 2.5). This blocks the leakage 

current from powering the transmitter. He also stated that 

this scheme will protect "most CMOS [complementary 

metal oxide semiconductor] switches from damage caused 

by analog signals that are present when the switches' power 

supplies are off." This suggests that a CD4051 with series 

diodes on the power supply pins may be used in situations 

where the sensors are always operating. The HI-506-A 

needs to be used only if more than eight input channels are 

to be transmitted or if the possible output swing of the 

sensors' voltages is greater than the maximum voltage 

rating for the CD4051 {± 9 volts). 

A pre-settable binary counter, U13, controls which 

analog input (0--15) is selected by the multiplexer. The 

counter is incremented after each channel is transmitted, 

and the multiplexer consequently switches the next input to 

the AID converter. The initial analog input to be sampled is 

designated by switch S6. When using the 8-channel 

CD4051 multiplexer, the fourth bit (MSB) of the counter is 

ignored. 

Before going to the analog-to-digital converter, the 

output of the multiplexer is buffered by a voltage follower, 

U15 (OP07). 

Analog-to-digital Converter 

U22 (ICL7109) is a 12-bit-plus-sign dual-slope 

integrating AID converter. The output of buffer U15 is 

connected to the INPUT HI of the AID converter through 

R36. INPUT LO is connected directly to the analog input 

ground. 

Full-scale input is twice the reference voltage. R62 

is adjusted to give the voltage reference U23 

Lf051 

+6 volts ...._ _ __;;,_;16""-i u -" + 

lNLf llf8 

-6 volts 

8 .------i G ND 

lN't 1 'tB 

Figure 2.5. Circuit for preventing current from analog 

inputs leaking onto power rails. Since input range of the 

AID is ± 5 volts and multiplexer is powered by ± 6 volts, 

voltage drop owing to diodes does not affect analog-input 

range of signals (after Conner, 1990). 

(LM236-2.5) an output of +2.500 volts so that the 

transmitter has a full-scale input range of ± 5 volts. The 

maximum conversion time is 33.3 ms. The resolution of 

the converter is 1.22 m V. 

For proper operation, C16, CIS, and C19 must be 

capacitors with low self-adsorption such as 

polypropylene. 

Channei-ID Counter 

Binary counters U16 and U17 (both CD40193) 

determine the channel ID that is transmitted with the data 

for each analog input. Each analog input for each 

transmitter has a specific ID (0-255) assigned to it. The 

receiver uses the channel ID to associate the correct 

sensor with the data in the message. The initial channel 

ID for the transmitter is set by switch S7. After each 

input is transmitted, the channel ID is incremented. Thus 

the channel ID's for each individual transmitter are 

incremented sequentially from the initial channel ID of 

the transmitter set with S7. For example, if three 

transmitters, each transmitting four channels, were 

deployed, transmitter 1 should have an initial channel ID 

of 1 and transmit channels 1-4. Transmitter 2 should 

have an initial channel ID of 5 and transmit channels 

5-8. If it was expected that two more channels would be 

added to transmitter 2 later, channels 9 and 10 should be 

set aside for this expansion. Thus transmitter 3 should 

have an initial channel ID of 11 and transmit channels 

11-14. 

Number-of-channel-transmitted Counter 

Johnson decade counters U11 and U12 count the 

number of analog signals that have been transmitted and 

return the transmitter to its sleep state after the 

programmed limit has been reached. The counters are 

configured so that after each channel is transmitted, one 

of 16 outputs, in turn, goes high. Setting one of the 16 

switches of S4 and S5 to "on" selects which counter 

output is connected to the circuit that returns the 

transmitter to its sleep state. 

If no switch on S4 or S5 is on, the transmitter will 

continue to transmit until power is disconnected . 

Universal Asynchronous Receiver 

Transmitter (UART) 

The UART, U28 (IM6402), converts the 8-bit 

parallel data to 8-bit, even-parity, 2-stop-bit serial data. 

Switch S8 sets the baud rate. Either 50 or 300 baud is 

2. A Low-Data-Rate Digital Telemetry System 19 
used at CVO; 300 baud is preferred because its quicker 

transmissions decrease the time the radio is transmitting, 

thereby reducing power consumption. The 50 baud rate is 

used on the telemetry link which utilizes the 455-505 Hz 

audio band instead of the normal Bell 103 1,070-1,270 

Hz audio band. The frequency of the 455-505 Hz band is 

too low to allow 300-baud transmissions. 

If the data are to be received by an IBM PC or 

compatible computer executing a GWBASIC program, 

the circuit board must be modified to disable parity 

generation (fig. 2.6). GWBASIC does not allow parity 

bits with 8-bit data. 

Modem 

A frequency-shift keying (FSK) modem, U30 

(MC14412), encodes the serial output of the UART in 

audio tones. The transmitter is normally configured to 

transmit in Bell 103 Originate mode. In this mode, the 

modem produces a 1,270-Hz tone when it detects a logic 

high on the serial output of the UART. A 1,070-Hz tone 

is produced when it detects a logic low. The Bell 103 

Answer mode can be selected using switch S8. In 

Answer mode, the modem produces tones of 2,225 Hz 

and 2,025 Hz for logic high and low. 

The transmitters can share radio links with analog 

seismic data if the audio frequency band used by the 

transmitter is available. For Originate frequencies, this 

means that standard seismic channels with center 

frequencies of 1 ,020 and 1,360 Hz cannot be present. For 

~ ~ 

ooo 

0 CUT 

J,~ ~ ., ocuT o~ o 

::!J,~ 0 

~~0 /::: ;: 

::: ~~ ~= 

-~--=-

::~ 0 

oo 0 

,. : : 

~ __.._ 

~ ··~; O e JUMPER 

O 0 oJUMPER O 

0 -OJ~ 0 

~= ~ ;: 

::: ~~ ~= 

-......:..__---- ::~ ....!__ 

oo 0 

,. : : 

Figure 2.6. Modification to circuit board to disable parity 

generation. 

Answer frequencies, the seismic channels with center 

frequencies of 2,040 and 2,380 Hz cannot be present. 

Precautions must be taken to ensure that the 

low-data-rate telemetry does not interfere with the 

seismic telemetry. Since the low-data-rate transmitters 

send data in bursts, the radio receiver is likely to receive 

bursts of noise as the radio transmitters turn on and off. 

To prevent this, the low-data-rate signals need to be 

buffered. This is done with a single-chip modem (fig. 

2. 7). In this circuit, the output of the radio receiver is 

demodulated by the receive section of the modem. The 

demodulated output is sent to the transmitter section of 

the modem, where it is remodulated and put on the 

seismic line. Noise from the radio receiver will cause the 

output of the modem to modulate between its two output 

tones but will not cause the output to deviate outside that 

band. 

Note that the output of the "modem buffer" will be 

the Originate tones if the input is the Answer tones, and 

vice versa. A second modem buffer would be needed to 

reverse the tones again if this were a problem. 

Radio 

Power and controls for the radio are available from 

the board via J5. Power ground is switched on and off to 

the radio through Q2. With the transmitter in its sleep 

state, Q2 is off and the radio ground is essentially 

disconnected from the battery ground. When the 

transmitter is active, Q2 is switched on, providing a 

low-impedence path between radio ground and battery 

ground. By switching the ground in this manner, the 

radio voltage does not necessarily have to be the same 

voltage as that powering the transmitter. The drawback is 

that the radio ground must be isolated from the power 

ground. 

To prevent radio-frequency interference while 

sampling the analog signals, the radio transmitter is 

activated only when data are to be transmitted. Radios 

are normally put into transmit mode by shorting the 

2.2K 

U3 

78L05 

RIO 

!.OK 

Figure 2.7. Circuit that buffers transmissions, enabling 

them to share a radio link with analog seismic data. 

20 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
push-to-talk control line (PIT) to ground with a switch. 

Q3 provides this function. When on, Q3 provides a 

low-impedence path between P1T and power ground, 

activating the radio transmitter. 

The output of the digital transmitter is 

transformer-coupled through T1 to the radio. The output 

level is normally 0.8 volt peak-to-peak. If the radio uses 

signal inputs in the millivolt range, I recommend that a 

resistor divider network be placed as near the radio as 

possible, to bring the signal down to the proper level. 

This configuration minimizes interference problems. 

POWER CONSUMPTION 

The power consumption of the digital transmitter is 

not a major problem. Even transmitting all 16 channels 

every 10 minutes at 300 baud, two 6-volt 20-amp-hour 

alkaline lantern batteries (Duracell ID9260) provide 

enough power for a year. The formula for determining 

average current consumption is 

AVERAGE CURRENT = QUIESCENT + 

((CHNLS · 2/ INTERVAL)· ACTIVE) 

where 

QUIESCENT= 

CHNLS = 

2= 

INTERVAL= 

ACTIVE = 

the quiescent current 

consumption (0.06 mA if U32 

is used, 0.4 mA if U14 

is used), 

the number of channels to 

transmit, 

the number of seconds to 

transmit one channel at 300 

baud (substitute 5 for 50 

baud), 

the time between transmissions 

(in seconds), and 

the active current consumption 

(13 rnA+ current drawn by 

external sensors). 

Powering the radio is a greater problem. A typical 

4-watt radio may require 900 rnA while transmitting and 

20 rnA while in receive mode. The formula for 

determining average current consumption for the radio is 

RADIO CURRENT CONSUMPTION = ((RCV · 1) 

where 

RCV = 

1 = 

XMIT = 

1 = 

+ (XMIT · 1)) * CHNLS I INTERVAL 

radio current consumption in 

receive mode, 

seconds/channel that the radio 

is in receive mode, 

radio current consumption in 

transmit mode, 

seconds/channel that the radio 

CHNLS = 

and 

INTERVAL= 

is in transmit mode at 300 

baud (substitute 2 for 50 baud), 

number of channels to be 

transmitted, 

time between transmissions (in 

seconds). 

Batteries such as alkalines and carbon-zinc do not 

have flat-discharge voltages. Instead, the voltage slowly 

drops as the battery is discharged. Though a 12-volt 

battery may be rated at 10 amp-hours, 5 of those 

amp-hours may be at voltages below 10 volts. Below 10 

volts the power output of the radio may fall to an 

unacceptable level. If the radio can operate safely at 16 

volts, it is preferable to power it with 7 .5-volt batteries in 

series/parallel to provide a nominal 15-volt supply 

(Lockhart and others, chapter 3). However, the nominal 

15-volt supply would exceed the maximum operating 

voltage for the digital transmitter. For this reason, the 

digital transmitter allows the use of a separate set of 

batteries to power the radio. 

CONCLUSION 

The system described here has been in use at eva 

since 1984. The low cost, small size, and robustness of 

the transmitters have allowed sensors to be installed in 

areas at Mount St. Helens and other volcanoes that might 

otherwise have been considered impractical. Data 

transmitted using the system continue to provide 

essential monitoring information at Mount St. Helens and 

elsewhere. 

The digital transmitters are not as flexible as 

microprocessor-based transmitters. However, more 

complex data processing in the field is usually not 

needed when telemetering such data as tilt, strain, or 

temperature. Until it is demonstrated that more complex 

processing is necessary, this system will continue to be 

useful. 

REFERENCES CITED 

Conner, D., 1990, Analog switches and multiplexers: EDN, vol. 

35, no. 6, 8 p. 

Dzurisin, D., Westphal, J.A., and Johnson, D.J., 1983, Eruption 

prediction aided by electronic tiltmeter data at Mount St. 

Helens: Science, vol. 221, no. 4618, p. 1381-1382. 

McGee, K.A., Sutton, A.J., and Sato, M., 1987, Use of satellite 

telemetry for monitoring active volcanoes, with a case 

study of a gas-emission event for Kilauea Volcano, 

December 1982, in Decker,R.W., Wright, T.L., and 

2. A Low-Data-Rate Digital Telemetry System 21 
Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological 

Survey Professional Paper 1350, p. 821-825. 

Murray, T.L., 1988, A system for telemetering low-frequency 

data from active volcanoes: U.S. Geological Survey 

Open-File Report 88-0201, 28 p. 

Roger, J.A., Johnston, M.J.S., Mortenson, C.E., and Myren, 

G.D., 1977, A multi-channel digital telemetry system for 

low-frequency geophysical data: U.S. Geological Survey 

Open-File Report 77-490, 111 p. 

APPENDIX 

Transmitter Specifications 

Power Requirements: 

Supply voltage 

Supply current 

(U32 installed) 

(U 14 installed) 

(without radio) 

8.7- 15 volts DC 

60 J.I.A quiescent 

400 JlA quiescent 

15 rnA transmitting 

Average current (four channels transmitting every 

10 min, 300 baud, and U32 installed): 

Analog Inputs: 

Range 

AID converter 

AID converter 

output range 

Resolution 

Number of inputs 

Power Output: 

Switched +6.0 volts 

Switched -6.0 volts 

Switched +8.0 volts 

Switched -7.4 volts 

Switched supply voltage 

Data Out: 

<300 J.I.A 

± 5 volts 

integrating 

12-bit-plus-sign 

0-8192 

1.22 mV 

16 single ended (U14) 

8 single ended (U32) 

10 rnA at 5.7 volts 

5 rnA at 5. 7 volts 

25 rnA at 7.8 volts 

5 rnA at 7.0 volts 

>100 rnA 

8 bit binary, even parity, 2 stop bits 

(incorrectly stated as odd parity in Murray, 1988). 

Baud switch selectable 50, 75, 110, 150, 300 

Bell 103 standard FSK frequencies 

Originate HI. 1,270 Hz 

LO 1,070 Hz 

Answer HI 2,225 Hz 

LO 2,025 Hz 

Each channel's data are transmitted in a 6-byte 

message. The frrst 2 bytes contain the data. Because the 

output of the AID converter is 13 bits (12 bits plus one 

sign bit) and data are transmitted in 8-bit bytes, 2 bytes 

are necessary to transmit the channel's data. The 8 

least-significant bits are transmitted in the frrst byte and 

the 5 most-significant bits are transmitted in the second 

byte. The channel ID is transmitted in the third byte. 

Then the 3 bytes are transmitted again to complete the 

message. The data and ID are transmitted twice to 

provide a means to check the validity of a message. 

Packaging: 

Watertight steel drum 35.1 em diameter x 36 em 

high. 

Antenna sheath constructed with 10 em diameter 

plastic pipe. 

Receiver Requirements 

Most computers with a Bell 103-compatible 

modem can be programmed to receive data sent by the 

transmitter. BASIC programs have been written for the 

Radio Shack Model 100/102 and an IBM compatible 

computer (Toshiba T1000) to receive the data. 

Although one radio receiver and computer can 

receive data from numerous transmitters, the total 

number of channels should be kept to less than 30. 

Transmissions from separate transmitters will interfere 

with each other too often if more than 30 channels are 

used. 

FIELD RECEIVER PROGRAM FOR THE 

RADIO SHACK MODEL 100/102 

A Radio Shack Model 100/102 computer is used 

for the portable field receiver. The following BASIC 

program for the Model 100/102 monitors the modem. 

When a valid message is received, the channel ID and 

data are printed on the screen. Owing to the slow speed 

of the Model 100/102's BASIC, a portion of the program 

is written in machine language specific to the Model 

100/102. 

22 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
The transmitted signal is connected to the Model 

100/102 through the modem port. Signal high goes to pin 

4 and signal low to pin 2. The modem selecter switch 

should be in the ACP position and, if the transmitter is 

transmitting on the normal originate frequencies, the 

ANSwer/ORiGinate switch should be in the ANS 

position. If the transmitters are using the answer 

frequencies, the switch should be in the ORG position. 

Lines 21-31 contain the machine code for the 

routine that receives the data from the modem. The 

routine first checks the data for parity and framing 

errors. If no errors are detected after six bytes are 

received, it will determine if the first three bytes of 

the message are the same as the last three bytes. If 

they match, the message is considered valid and the 

routine is exited with the value at location 62700 

equal to 6. The routine is also exited every 10 

seconds to check for keyboard input. In those cases, 

the value at location 62700 does not equal 6. 

20 'FLDRCVR 

21 DATA OO,OO,OO,E3,DS,CS,ES,C9,00,E1,C1 ,01 ,E3,C9,00,48,0C,62,6B 

22 DATA C3,24,FS,78,91,3C,32,EC,F4,AF,32,EB,F4,C9,CD,ED,F4,EB,48 

23 DATA OC,23,0D,CA,1D,FS,7E,2B,77 ,23,C3,11 ,FS,CD,F3,F4,C9,CD,6B 

24 DATA FS,CD,ED,F4,CD,87 ,FS,C6,00,C2,A 1 ,FS,OO,OO,OO,CD,6D,6D,CA 

25 DATA 27,FS,CD,7E,6D,CD,F3,F4,C2,F9,F4,77,23,0D,C2,24,FS,ES,2B 

26 DATA CD,AF ,FS,C2,61 ,FS,2B,CD,AF,FS,C2,61 ,FS,2B,CD,AF ,FS,C2,61 

27 DATA FS,E1,CA,OO,FS,E1,CD,OB,FS ,OC,2B,C3,24,FS,C9,47 ,23,7E,SF 

28 DATA 23,7E,S7,6B,62,3A,EB,F4,4F,CS,AF,47,09,C1,78,91,3C,4F,C9 

29 DATA 00,00,00,00,00,00,21 ,E2,F4,CD,OF,19,3A,E8,F4,21 ,EA,F4,BE 

30 DATA 77 ,3E,OO,CA,9D,FS,3E,01 ,OO,OO,OO,C9,CD,F3,F4,78,91,3C,32 

31 DATA EB,F4,AF,32,EC,F4,C9,ES,7E,2B,2B,2B,96,E1,C9 

32 ' clear the area for the machine code 

33 CLEAR 512,62650 

34 PRINf "LOADING MACHINE CODE" 

35 ' the followina POKEs the above machine code into memory 

36 FOR 1=0 TO 204 

38 READ A$ 

40 H%•ASC(LEFT$(A$,1)) 

42 L%•ASC(MID$(A$,2,1)) 

44 IF H%60 TIIEN H%•16*(H%-SS):GOTO 48 

46 H%•16*(H%-48) 

48 IF L%60 TIIEN L%•L%-SS ELSE L%=L%-48 

49 ' the code is inserted into memory startin& 

SO ' at location 62698 

51 H%•H%+L%:POKE(62698+1),H% 

52 NEXT I 

60'------------------------------------------

85 PRINf "PROGRAM STARTED" 

135 'DA% is used to decode the 6 byte messaae 

140 DIM DA%(6) 

600 ' line 604 opens the modem port. if the transmitters' 

601 ' parity generation bu been disabled, il should 

602 ' read u is. Otherwise it should be opened u 

603 ' "MDM:8E2D" 

604 OPEN "MDM:8N2D" FOR INPUr AS 1 

60S ' the default baud rate is 300. for SO baud execute the 

606 • followin1 

610 ·our 188,0: our 189,76: our 184,195 

612 'for 75 baud execute the followina 

615 • our 188,0 :our 189,72: our 184,195 

616'---------------------------------------------

617 'Q$ is the buffer to receive the 6 byte mesaaae. 

618 'il must be one byte lonacr than the messaac 

620 Q$•"1234567" 

621 ' if the followinaline is not executed, data received 

622 ' into Q$ will appear in line 620 the next time the 

623 'propm is executed. 

624 MID$(Q$,1,6)•MID$(Q$,1,6) 

625'-----------------------------------------

626 ' go look for a valid meaaaae 

630 CAlL 627S3,S,VARPTR(Q$) 

635 ' if the peek-6 then a valid messaae bas been received. 

636 ' otherwise it wu just a 10 second time-out. 

640 IF PEEK(62700)-6 THEN GOSUB 900 

645 ' go back for more data 

650 Garo 630 

731 ·-----------------------------------------

900 'decode the message 

902 'fin:t load the 6 bytes into DA% 

1000 FOR 1%•1 TO 6 

1020 DA%(1%)•ASC(MID$(Q$,1%,1)) 

1021 NEXT 1% 

1090 'convert the binary data to an inteaer value 

1100 DA %(2)=DA %(4)+(DA %(S)AND1 5)*256 

1120 IF DA%(S)AND32 THEN GOTO 1122 

1121 DA%(2)-4096-DA%(2):GOTO 1140 

1122' DA%(2) will hold the data, DA%(6) already 

1123 ' baa the channel ID 

1124 DA%(2)=DA%(2)+4097 

1130 ' go print the message on the screen 

1140 GOSUB 1300 

1162 RETIJRN 

1300'----------------------------------------

1301 'print the decoded measaae 

1500 PRINT USING "'D-##1 ";DA%(6); 

1501 PRINT USING "DATA_, ";DA%(2); 

1502 ' calculate the voltage from the data 

1502 VI•((DA%(2)-4097)*.00122) 

1503 PRINT USING "+#.#lUI VOLTS ";VI; 

1504 PRINT TIME$ 

1540 BEEP 

1541 ' go back for more 

1550 RETIJRN 

1600 ' end of program 

2. A Low-Data-Rate Digital Telemetry System 23 

3. Operating Low-Power Telemetry Networks 

in Severe Environments 

By Andrew B. Lockhart, Thomas L. Murray, and Bruce T. Furukawa 

ABSTRACT 

The major problems affecting the low-power 

telemetry networks at Mount St. Helens have been 

weather and corrosive gases, with power supply problems 

secondary. Lightning and vandalism have also been 

concerns. We have found that these problems can be 

minimized with proper site preparation and instrument 

packaging. Airtight instrument cases protect sensitive 

electronic equipment from corrosive gases and moisture. 

Nonmetal instrument cases are used in highly corrosive 

environments where metal cases quickly deteriorate. 

Instrument cases are protected from direct exposure to 

the environment by enclosures ranging from metal drums 

to small buildings. Problems due to rime ice formation on 

the antennas are minimized by using antennas protected 

by radomes or by using log-periodic antennas. Circuit 

damage from lightning and from improper power 

connections during battery changes may be minimized by 

grounding and use of diodes and transient-suppressant 

devices. To power remote sites, we have used solar 

panels with rechargeable batteries, primary batteries, and 

propane-fueled thermoelectric generators. We find solar 

panels with rechargeable batteries to be the cheapest 

long-term power source although primary batteries are 

the most reliable power source. On-site repair and 

maintenance is made more difficult by poor conditions, 

requiring prior planning and realistic expectations. 

Attention to detail is the key to operating low-power 

telemetry networks. 

INTRODUCTION 

Telemetering data from sensors located on active 

volcanoes provides valuable information with which to 

assess the volcanic hazards in real time. The sensors and 

telemetry equipment in the field are subject to a number 

of problems, both environmental and cultural, which may 

damage equipment and interrupt the flow of data to the 

observatory. Problems unique to active volcanoes include 

corrosive gases and tephra. 

The major problems encountered in maintaining 

electronic equipment on active volcanoes are 

1. Weather and corrosive volcanic gases. 

2. Power surges due to lightning or incorrect 

hookup to batteries. 

3. Maintaining sufficient power for remote 

operation. 

4. Vandalism. 

Weather and corrosive gases have been the primary 

cause of problems in field equipment at the Cascades 

Volcano Observatory (CVO); power-supply concerns are 

secondary. Although storm-caused lightning and 

vandalism have not been a major problem at Mount St. 

Helens, they have been serious problems at other 

volcanoes we have visited. 

Proper site preparation and instrument packaging 

can minimize, though not eliminate, most problems. We 

discuss methods for protecting and powering field 

instrumentation based on our experience at Mount St. 

Helens and other volcanoes. We also outline a 

troubleshooting guide and list tools we have found useful 

during the installation and maintenance of field 

instruments in severe conditions. Our report is directed 

toward people who must prepare and install the 

instruments. 

INSTRUMENT PACKAGING 

Improper packaging of electronic equipment used 

in severe environments can allow moisture or corrosive 

gases to damage the sensitive circuitry. Moisture is an 

especially insidious problem, and its presence can cause 

total failure of electronic circuits. Perhaps worse, it can 

cause incorrect or noisy data by its presence on 

high-input-impedance circuits. Such a problem may go 

undetected if the circuit is otherwise unaffected. 

Corrosive gases can magnify the problem. Sulfur dioxide 

combines with moisture to form sulfuric acid, which 

greatly accelerates corrosion of improperly protected 

circuits, wires, and metal enclosures. 

The primary method to protect electronic circuitry 

from moisture and .corrosive gases is to package the 

3. Operating Low-Power Telemetry Networks In Severe Envlronmnets 25 
circuit to prevent the moisture and gases from contacting 

it. We divide packaging into three categories, (1) 

instrument cases that contain the electronics, (2) 

protective housings that surround the instrument cases, 

batteries, and incidental wiring, and (3) interconnection 

hardware and printed-circuit-board coatings. 

Instrument cases 

Instrument cases are the primary barrier between 

the electronics and the environment. A good instrument 

case is airtight to allow neither moisture nor corrosive 

gases to contact the circuitry. The case should have a 

gasket or 0-ring seal on the lid and around each hole 

that penetrates the case, such as the screws or rivets 

attaching a handle or hinge. If no gasket or 0-ring is 

present, a sealant such as Scotchkote or roomtemperature-vulcanizing (RTV) silicone should be 

liberally applied to the area in question to prevent 

moisture and air from entering the case. 

Some RTV's contain small amounts of acetic acid. 

Though the acid is not sufficiently corrosive to penetrate 

metal cases, it can damage copper wires, resistor leads, 

and sockets. A nonacidic RTV such as Dow Corning 

3145-RTV should be used with metal or electronic 

components and cabling. 

Nonmetal cases, such as those manufactured by 

Pelican Products Inc., are preferred for use in areas 

where corrosive gases are concentrated enough to rapidly 

destroy metal cases. Most nonmetal cases use some metal 

parts for bolts or pins in hinges. A protective coating 

should be applied to these parts if they are not stainless 

steel. 

As the nonmetal cases do not provide any shielding 

for the electronic circuits, the circuits should be enclosed 

in a grounded metal box inside the instrument case. 

In less corrosive environments, metal cases such as 

those manufactured by Zero Corp. can be used. The 

metal cases, if grounded, give better electrical shielding 

to the circuitry than the nonmetal cases, decreasing the 

possibility of interference by the radio transmitter. They 

also provide a suitable ground plane on which a whip 

antenna may be mounted. An alternative to metal cases 

designed specifically to house electronic circuitry are 

"ammunition cans," commonly found in army surplus 

stores, or small (4 or 9 gallon, approximately 16 or 36 

liter) metal drums. They provide inexpensive airtight 

containers when their gaskets are in good shape. 

Whenever possible, connectors should be used to 

pass signals and power through the case. Do not just pass 

a wire through a hole drilled in the case, and then apply 

a sealant: One tug of the wire can destroy the integrity of 

the seal by breaking its bond to either the wire or the 

case. Jacketed cables that pass through the case provide 

a conduit for moisture and also should be avoided. 

We use Amphenol MS-series environmentalresistant standard circular connectors to pass signals and 

power through the instrument enclosure. Moisture cannot 

pass through these connectors, although the mounting 

holes must still be sealed. Non-environmental connectors 

are less expensive than the environmental ones and may 

be substituted if they are carefully sealed on the inside of 

the case with Scotchkote or other sealant. One drawback 

to using the metal MS-series connectors is that corrosion 

of mating connectors causes their threaded parts to bond 

together, making it difficult if not impossible to separate 

them. A light brushing with paint or Scotchkote will 

usually prevent this. 

We use "N" connectors, preferably with gold 

contacts, for radio-frequency connections external to the 

instrument case. Unlike BNC or PL-259/S0-239 

connectors, the "N" connectors are inherently 

waterproof, though we still wrap them with electrical 

tape for added protection if they are located outside a 

protective housing. 

Desiccant should be placed in the instrument case 

to remove any moisture that may leak into the case or 

condense after the case has been closed. The desiccant 

should be replaced annually or whenever the case has 

been opened. 

Protective Housings 

Protective housings prevent direct exposure of the 

instrument cases, miscellaneous interconnecting wires, 

and batteries to weather, dust and ash, and corrosive 

gases. They can also provide security against vandals. 

The housings can range from generic 55-gallon steel 

drums, which provide enough room to house a seismic 

station and batteries, to small buildings, which offer 

room to work on equipment out of the weather (figs. 

3.1-3.3). Plastic, fiberglass, and wood survive well in 

corrosive conditions that would quickly destroy steel. 

If the bottom of the protective housing is below 

ground level such as is typically the case for partially 

buried 55-gallon drums, large (5-10 em diameter) holes 

should be cut in the bottom of the housing to allow 

accumulated water to drain. Unless the housing sits in 

soil of low permeability, such as is found in the crater of 

Mount St. Helens, the chance of ground water flooding 

the enclosure is less than that of flooding by water 

entering from the top. 

The protective housings also serve to prevent 

vandals and souvenir hunters from easily tampering with 

the equipment. Given that a persistent vandal will 

eventually get into any site, the focus of site security 

should be to deter the merely curious or the only mildly 

26 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
malicious passer-by from entry. Housings constructed of 

concrete or steel are more difficult to vandalize than 

those made of plastic or sheet metal (fig. 3.4). Hardening 

a field site against vandals may entail as much work as 

installing the equipment itself. 

In many cases, simply securing the equipment with 

a lock will prevent the curious, law-abiding passer-by 

from unintentionally damaging equipment. The locks at 

Figure 3.1. Radio repeater installation at Guacamole, 

north flank of Mount St. Helens. Batteries and repeater 

are housed in 55-gallon steel drum {left of antenna mast). 

Antenna mast is an unguyed wooden post cemented into 

ground. Cylindrical enclosure mounted at top of mast 

protects receiver's whip antenna inside from snow and 

ice. Vagi antenna is used by transmitter. Solar panel 

attached to mast provides power for site. Bands of tape 

secure antenna and solar panel cables to mast. 

the different field sites should open with the same key. 

A variety of locks, each using a different key, ensures 

that one day the key to a site will be forgotten or lost, 

and the servicing technician will have to damage the 

housing or the lock to reach the equipment. For the same 

reason, if combination locks are used they should all 

have the same combination. A lock housing open at the 

bottom limits access to the lock from underneath and 

greatly increases the difficulty of removing a lock by 

sawing or cutting it in two. 

Camouflaging or hiding the enclosure will limit its 

visibility from nearby paths. Painting the enclosure to 

Figure 3.2. Radio repeater installation at Harrys Ridge, 

8 km north of Mount St. Helens. Signals from numerous 

seismometers are received, summed, and retransmitted 

at this site. White enclosures {radomes) protect UHF 

antennas inside from snow and ice. Building provides 

protective housing for batteries and radios. At sites such 

as this that require numerous antennas, a tri-legged tower 

cemented into ground is used as a mast. 

3. Operating Low-Power Telemetry Networks In Severe Envlronmnets 27 
blend in with the background, burying it, or hiding it 

among shrubs or behind rocks will limit the number of 

people who notice the site and hence the chances of 

someone tampering with it (fig. 3.5). 

Interconnection Hardware and 

Printed-Circuit-Board Coatings 

Spraying printed-circuit boards with a urethane or 

acrylic coating can prevent accumulated moisture in the 

instrument case from affecting the circuit. Though not 

effective if the board is submerged in water, the spray 

can protect the circuit from high humidity or 

condensation. 

All connections should be soldered. Even crimp 

connections should be soldered after crimping for added 

protection against corrosion. Soldering, in effect, 

transforms a stranded conductor to a solid conductor, and 

we have found that a solid conductor is less susceptible 

to corrosion than its equivalent gauge of stranded wire. 

The connections should be insulated with either electrical 

tape or heat-shrinkable tubing and, whenever possible, 

sealed with ScotchKote. Use of wires insulated with 

Teflon instead of polyvinyl chloride (PVC) will reduce 

the effect of corrosive gases permeating the insulation 

and damaging the conductor. 

ANTENNAS AND ANTENNA MASTS 

Snow and ice pose the major problems for 

antennas. Rime on the elements of a yagi antenna will 

detune it and may bend the elements under the extra 

weight. Yagis buried in snow may suffer bent or broken 

elements from the weight of the snow. An antenna with a 

build-up of rime presents more surface area to the wind 

and is thus more liable to rotate on its mast from its 

correct orientation. 

All of these problems are compounded with 

increasing size of the antenna. Larger antennas present 

more surface area to the wind and collect greater weights 

Figura 3.3. Yellow Rock vault tiltmeter installation, on crater floor 1 km north of dome at Mount St. Helens. 

Custom-built fiberglass housing placed over a cement pad was buried to form a vault to house a tiltmeter, batteries, and 

telemetry equipment. Data are transmitted through whip antenna enclosed in cylinder at top of wooden post. This site is 

buried under 3-5 m of snow in winter. Pipe located to left of vault is used to locate vault when snowpack buries 

antenna. 

28 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
of rime than do smaller ones. Larger antennas are more 

difficult to protect from the weather by enclosures or 

sheaths and are more easily seen by souvenir hunters. In 

extreme conditions, the best antenna is the smallest one 

that will suffice. Whenever possible, we use a whip 

antenna instead of a yagi. 

One method of preventing rime from detuning an 

antenna is to enclose the antenna in a plastic housing, 

commonly referred to as a radome. UHF yagis protected 

by a fiberglass radome are commercially available (fig. 

3.2). VHF yagis are too large to conveniently enclose in 

a radome; we have either used a broad-band, log-periodic 

yagi or mounted the antenna inside a small wooden 

building. The log periodics are heavier, bulkier, and 

more expensive than the standard yagis and are used 

only when necessary. For short radio ·paths ( 30 km) we 

use standard yagis, because the signals are strong enough 

to be received even while the antenna is iced. Most of 

our yagis and log-periodic antennas are painted black to 

improve solar warming and hasten deicing. 

Whip antennas are also susceptible to icing. The 

wind can cause the ice-covered antenna to oscillate like 

an inverted pendulum, eventually snapping it off near its 

base. To avoid this, we construct a sheath from 4-inch 

(10 em) plastic sewer pipe to act as a radome for the 

antenna. The pipe is attached to the top of the instrument 

case or ground plane with a plastic-pipe floor flange. A 

pipe cap on the top completes the sheath. With the 

sheath protecting the antenna, the radio can transmit 

through deep snow (fig. 3.3). 

Solidly tightening the antenna-mounting bracket to 

the mast is nonnally sufficient to prevent the antenna 

Figure 3.4. Concrete vault with steel door prevents 

vandals from easily gaining access to telemetry 

instrumentation. 

from rotating on its mast. At particularly windy sites we 

drill a 3-mm-diameter hole through the mounting bracket 

into the mast and insert a steel pin or bolt to ensure that 

the antenna will not rotate on its mast. 

Ice and snow also present problems for the antenna 

cable. If the cable is not securely attached to the antenna 

mast, wind, snow, or ice can literally pull the cable out 

of its connector. The cable should be taped, stapled, or 

secured in some manner to the mast to prevent strain at 

the connector-cable junction. Enough slack should be left 

near the antenna to prevent any rotation of the antenna 

from straining the cable-connector junction. 

We have found that a length of 1 1/2-inch to 

2-inch (3.75 em to 5 em) steel pipe rigidly attached to an 

unguyed 4 by 4 (8 em by 8 em) wood post cemented into 

the ground makes an excellent antenna mast. The pipe 

mount pennits a more careful orientation of the antenna 

or solar panel than a square post. We have had no 

problems with this setup, even where the mast has been 

buried by more than 3m of snow. We avoid guyed masts 

because of the possibility of problems with snow-loading 

on the guys. A further problem with guy wires is that 

wind-induced vibration can increase high-frequency 

noise at seismic stations. 

LIGHTNING AND POWER-SUPPLY 

PROTECTION 

Field equipment is liable to damage from lightning 

and from improper power connections during battery 

changes. Protection diodes, transient-suppressor diodes, 

and plasma-discharge tubes can be easily installed in 

equipment to counter these effects. 

Protecting From Improper Power Connection 

Batteries usually are the power source at the field 

sites. The biggest danger in using batteries is reversing 

the polarity of the power leads after changing batteries. 

The instruments can be protected from reversing the 

battery leads by using either a blocking diode placed 

in-line with one of the power leads or a shorting diode 

placed across the leads. 

The blocking diode prevents any current from 

flowing through the circuit if the battery leads are 

reversed, but at a cost of reducing the voltage applied to 

the equipment by an amount equal to the voltage drop 

across the diode (fig. 3.6A). A standard silicon-junction 

diode has a diode drop of about 0.6 volt. A 

Schottky-barrier diode with a drop of 0.3 volt is 

preferred. The diode's current-carrying capacity must be 

greater than the peak current needed by the equipment it 

is protecting, or the diode will be damaged. 

3. Operating Low-Power Telemetry Networks In Severe Envlronmnets 29 
Placed across the voltage input, a shorting diode 

provides a short to ground for the reversed voltage (fig. 

3.68). The shorting diode should be used in situations 

where the voltage drop of the blocking diode is 

unacceptable. However, if the current-carrying capacity 

of the shorting diode is exceeded, damaging the diode 

and removing the short, the instrument circuitry may in 

turn be damaged. To prevent this, either the diode must 

be able to conduct the maximum output current of the 

battery or a fuse must be placed in the circuit between 

the battery and the diode. The current rating of the fuse 

must be less than that of the diode to ensure that the fuse 

will open before the diode fails. For instance, a diode 

rated at 3 amperes should be protected with a fuse rated 

at 1 ampere. 

Establishing conventions for the pin connections 

for all in-house connectors also helps maintain proper 

battery polarity. At CVO, we have agreed that for all 

in-house connectors, whether they have 2 pins, 5 pins or 

A 

10 pins, pin A will be +12 volts and pin B will be 

ground. If a connector is not supplying +12 volts, its pin 

A is either unconnected or capable of withstanding + 12 

volts applied to the pin. This prevents damage to 

equipment if connectors are inserted in the wrong place. 

We always assign +12 volts to the red wire and the 

ground to the black. 

Lightning Protection 

Nothing can completely protect low-power 

electronic equipment from damage by a direct lightning 

strike. However, damage from voltage ttansients induced 

by nearby lightning strikes can be minimized through the 

use of transient-suppressor diodes, plasma-discharge 

tubes (gas pots), and proper grounding techniques. 

We have found proper grounding to be the primary 

defense against lightning damage. Even without 

8 

Figure 3.5. Telemetered tiltmeter installation on Galeras Volcano, Colombia. A, Partially buried plastic garbage 

container houses batteries, electronics, radio and antenna. B, Installation as seen from road (arrow). Vegetation later 

placed around container made site virtually invisible from road. 

30 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
protection devices such as transient-suppressor diodes or 

gas pots, good grounding will minimize lightning 

damage to a system. The CVO radio repeater located on 

Coldwater Peak shares a building with repeaters installed 

and operated by other agencies. The other agencies' 

repeaters have twice been damaged by lightning, whereas 

the CVO repeater has suffered no damage. The only 

difference in the installations is that the CVO repeater is 

well grounded and the others are not. 

In lightning-prone areas we recommend that the 

negative terminal of the power source (typically the 

battery) be connected to the ground rod with heavy-duty 

cable (12 gauge or greater). The antenna and instrument 

case, if metal, should also be shorted to the ground rod 

with a heavy-duty cable. Exposed connections to the 

ground rod should be coated with a heavy, conductive 

grease to retard corrosion. 

One exception to this grounding configuration is 

the digital telemetry ttansmitters used by CVO (Murray, 

Chap. 2). The circuitry switches power to the radio by 

connecting and disconnecting the radio ground to power 

ground. If a radio with a switched ground is used, the 

switched ground must be insulated from the power 

ground. Otherwise, the switched ground and power 

ground will be shorted together at the ground rod, 

A 

BLOCKING 

~------~~·-------- .!..L DIODE ~ POWER TO 

-li~---+----------~~INSTRUMENT 

:::F 

8 

+ 

FUSE 

SHORTING 

DIODE 

POWER TO 

INSTRUMENT 

Figure 3.6. Protection of circuits from damage due to 

reversed battery leads. A, Blocking diode. Using a p-n 

silicon junction diode drops input voltage by 0.6 volt; a 

Schottky diode drops voltage only 0.3 volt. 8, Shorting 

diode. Fuse must be included to prevent diode from 

exceeding its current-carrying capacity. 

negating the switch. In such instances, only the power 

source ground should be connected to the ground rod. 

Transient-suppressor diodes such as Tranzorbs act 

similarly to zener diodes but clamp the voltage much 

more quickly (1 picosecond). Placed across the input 

power and signal lines, the Tranzorb will prevent 

lightning-induced voltage spikes from rising above 

acceptable levels. We use the 1.5KE series of transient 

suppressors. They have a peak pulse-power-dissipation 

rating of 1,500 watts and are available in a range of 

voltages from 6.8 volts to 400 volts. Bipolar types are 

available for use with audio signals or with sensors such 

as tiltmeters that may have a ±5 volt output range. 

Plasma-discharge tubes installed between the 

antenna cable and the antenna protect the radio from 

voltage ttansients induced in the antenna. Although we 

have no direct evidence of the gas pot's effectiveness, 

we have been strongly advised to use them in areas of 

frequent lightning activity (H.P. Boller, oral common., 

1987). 

POWERING REMOTE INSTRUMENT SITES 

We have used three methods to supply power at 

our remote instrument sites: (1) solar panels with 

rechargeable batteries, (2) primary batteries, and (3) 

propane-fueled thermoelectric generators. 

Solar Panels and Rechargeable Batteries 

Of the three methods, solar panels and 

rechargeable batteries are the cheapest long-term 

solution. In this method, a solar panel charges one or 

more 12-volt batteries, which in turn power the 

instruments. The batteries act as an energy reservoir, 

storing the excess energy produced by the solar panel 

during the day to provide power during the night and 

periods of little sunlight. 

A blocking diode and voltage regulator connect the 

panel and the batteries. A blocking diode allows charging 

current to flow into the batteries but prevents current 

from leaking back through the solar panel, thus 

discharging the batteries and possibly damaging the 

panel. If possible, a Schottky barrier diode should be 

used instead of a standard p-n junction silicon diode, 

because the lower voltage drop of the Schottky barrier 

diode decreases the power dissipated by the diode. The 

voltage regulator prevents overcharging of the batteries. 

For panels of 40 watts or smaller, a 50-watt, 14-volt 

zener diode (for example, Motorola # 1N3313B) 

connected across the solar-panel output prevents 

overcharging. Zeners of lower wattage can be used with 

smaller panels. The zener should be placed as close as 

3. Operating Low-Power Telemetry Networks In Severe Envlronmnets 31 
possible to the battery to minimize any voltage drop due 

to line loss between the zener and the battery. 

In areas such as the Cascades, where a typical 

installation has a large battery capacity compared to the 

size of the panel, the regulator may not be necessary. In 

these cases, the panel is incapable of overcharging the 

batteries. We have developed a formula at CVO to 

determine the need for a regulator. If 15 times the 

maximum output of the solar panel (in watts) is less than 

the total reserve battery capacity (in amp-hours), no 

regulator is needed. For example, a system using a 

15-watt panel and 300 amp-hours of reserve capacity 

would not need a regulator, as multiplying the panel 

wattage (15) by 15 gives a result of 225, well below the 

300 amp-hours of capacity. When operating without a 

regulator, the electrolyte level must be checked 

periodically, preferably in late summer (Solar Power 

Corp.). 

Nonregulated systems require caution. The open 

circuit voltage of a solar panel can be as much as 23 

volts. When connected to the battery, this will drop down 

to the normal 11-14 volts and not be a problem. If the 

battery is taken out of the circuit (to be changed, for 

instance), leaving the panel powering the instrument 

directly, the instrument may not present enough load to 

drop the voltage more than 1-2 volts. If this happens, the 

maximum voltage rating for the instrument may be 

exceeded and its circuitry damaged. To minimize this 

possibility, at least two batteries wired in parallel should 

be used to power the station. The solar panel should be 

attached to one battery and the instrument to another. As 

long as batteries are changed or added one at a time, this 

configuration will prevent the solar panel from powering 

the instrument directly without at least one battery to 

buffer the voltage. 

A balance must be reached in determining the 

necessary output of the solar panel and the necessary 

reserve capacity of the storage batteries. A very large 

panel may generate enough charging current on even the 

cloudiest day to allow the use of a small storage battery, 

one with just enough capacity to provide power through 

the night. At the other extreme, a panel just large enough 

to supply the energy needs on a yearly basis may operate 

well with a nominal battery reserve capacity in the 

summer but require a huge battery reserve to keep the 

instrument powered through the winter months when 

sunshine is scarce. 

Finding the optimum combination of battery 

capacity and panel size is largely a matter of experience. 

In many areas, mountaintops are typically in the clouds 

while valley floors are sunny. In such areas, sites placed 

high on the mountain will need larger panels and (or) 

more batteries than those in the valley. Local topography 

or vegetation may also limit the amount of sunshine 

striking the panel. Our experience in the Cascades has 

been that for every 130 milliamperes (rnA) of average 

current draw, a minimum battery capacity of 300 

amp-hours coupled with a minimum 15-watt solar panel 

should be used. Even in the sunniest climates, we feel 

that the minimum setup to be used for an average current 

draw of 125 rnA is a 10-watt solar panel and 125 

amp-hours of reserve battery capacity. 

Another easily overlooked factor in deciding the 

necessary amount of reserve battery capacity is the effect 

of volcanic ash on the panel. Ash is especially a problem 

in the tropics, where the panels are mounted almost 

horizontally for maximum solar input. Deposits of ash on 

the panel can completely block the sun and render the 

panel useless until it is cleaned by hand or rain. Extra 

reserve battery capacity should be included to carry the 

site through periods of ash fall. 

Problems with ash fallout are compounded if a 

solar panel has a silicone protective surface instead of 

glass. Ash will become embedded in the soft surface, 

permanently blocking sunlight from the solar cells. For 

this reason, solar panels with soft surfaces should be 

avoided. 

We have had good luck with 100-amp-hour 

batteries designed specifically for use with solar panels 

("photovoltaic" batteries) or deep-discharge marine/RV 

batteries. They are a good compromise between cost, rate 

of self-discharge, volume, and weight. Batteries 

advertised to last 10-20 years in stationary operation are 

available but due to their higher cost (two to three times 

that of a marine/R V battery) and the uncertain longevity 

of the field sites, we have only used them at our voice 

repeater sites. We have not had good luck with gelled 

electrolyte batteries. Apparently they are not as tolerant 

of under- or over-charging as the liquid-cell batteries. 

The batteries must be placed in a ventilated 

enclosure, otherwise hydrogen gas produced while 

charging the batteries can collect and explode. Such an 

explosion destroyed a concrete vault in California 

containing seismic equipment (F. Fischer, oral commun., 

1990). Other explosions have occurred where the 

batteries were vented via tubing running from the battery 

vents to the outside. The hydrogen concentration in the 

tubing was sufficient to ignite and destroy the batteries 

when lightning struck near the outlet of the tube (L. 

Craven, oral commun., 1980). 

Primary Batteries 

Primary batteries are the most reliable method for 

providing power. Their only drawback is long-term cost. 

The cost of the primary cells required to power a typical 

seismometer station for one year is $250-$300. In 

contrast, a solar-powered site requires a $500 initial 

investment but should run for at least 5 years with no 

32 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
additional investment. The savings over 5 years is at 

least $750, not counting procurement and 

battery-disposal (hazardous waste) costs. 

We use primary batteries at sites that get little 

sunlight, at sites prone to vandalism, or at sites that are 

likely to be subject to ballistics from the volcano that 

could damage the panel. Whenever possible, we place 

enough batteries at a site to last at least 13 months. Then 

battery changes need to be made only once per year, 

with an extra month for leeway. All batteries at a site 

should be changed at the same time. Fresh batteries will 

be drawn down by any partially discharged batteries that 

are left in the circuit. 

For sites whose average current draw is greater 

than 20 rnA, we use 2.5-volt, 1,000-amp-hour primary 

batteries such as those manufactured by SAB/Nife Inc. 

Five such batteries connected in series provide the 

necessary 12.5 volts to power the field sites. Though the 

1,000 amp-hours will supply a site drawing 105 rnA for 

13 months (396 days), we conservatively round off this 

number to 100 rnA, which is sufficient for a typical 

seismic site. For sites drawing more than 100 rnA, we 

connect additional sets of five batteries in parallel to 

provide the necessary capacity to last a year. 

A limitation to these primary cells is that they 

cannot provide a current exceeding 1 ampere. At sites 

requiring periodic high current output (1 ampere), but 

low average current draw, we add one more cell in 

series for 15 volts total. The primary cells charge a 

secondary battery wired in parallel through a blocking 

diode. Unlike solar panels, which use Schottky-barrier 

blocking diodes to minimize the voltage drop, here a 

p-n junction silicon diode is desired in order to drop 

the voltage to an acceptable 14.4 volts, eliminating the 

need for a voltage regulator. With this configuration, 

the primary batteries provide the current to keep the 

secondary battery charged and to satisfy the long-term 

energy needs. The secondary battery provides the 

short-term high-current needs. 

For sites whose average current draw is less than 

10 rnA, alkaline primary cells are used. They are 

available with smaller amp-hour ratings than the 

SAB/Nife batteries mentioned above, with 

corresponding reductions in size and weight. They also 

do not need to be vented and can be enclosed in an 

airtight instrument case. This allows the batteries and 

instruments to be packaged in a single airtight case, 

with no need for a protective housing (fig. 3.7). 

The main drawback of alkaline batteries is that 

their output voltage declines gradually throughout the 

life of the cell (Union Carbide Corp., 1976, p. 288). A 

standard 6-volt 40-amp-hour alkaline is rated using a 

cutoff voltage of 3.2 volts (Duracell Inc., 1986). To 

take full advantage of the energy stored in the battery, 

the equipment it is powering must be able to work 

when the supply voltage drops to 3.2 volts. By contrast, 

the output voltage of the SAB/Nife batteries remains 

virtually at the same level until the battery is almost 

completely discharged. When using alkaline batteries, 

always choose the battery which has the highest voltage 

the equipment can safely accept so that as much of the 

Figure 3.7. Oops telemetered-tiltmeter site, east flank of 

lava dome at Mount St. Helens. Tiltmeter is located on a 

shelf in rock wall behind steel plate at top of photograph. 

Telemetry equipment and batteries are housed in 9-gallon 

drum at lower right. Whip antenna is inside plastic pipe 

attached to lid of drum. Strain relief for cable connecting 

tiltmeter to telemetry is provided by attaching cable to 

P-K nails hammered into rock. 

3. Operating Low-Power Telemetry Networks In Severe Envlronmnets 33 
total energy of the cell as possible can be utilized (Union 

Carbide Corp., 1976, p. 288). The radios in our 

low-data-rate telemetry transmitters (Murray, Chap. 2) 

operate at a nominal 12 volts, but we have found that 

they can operate safely from supplies of up to 17 volts 

(if for no other reason than that they are in transmit 

mode for only a few seconds every 10 minutes; this is 

not recommended for radios in continuous operation). 

To power these radios, sets of two 7 .5-volt 40-amp-hour 

batteries are connected in series to give a nominal 

15-volt supply. This configuration provides 28 useful 

amp-hours to a cutoff of 10.0 volts. A comparable setup 

using 12-volt 40-amp-hour batteries would yield only 15 

amp-hours until the 10.0-voltcutoffis reached. 

Carbon-zinc primary cells should be avoided. 

They have a steeper voltage dropoff rate than do 

alkaline batteries (Duracell Inc., 1986), lower energy 

density, shorter shelf life, and poor cold-temperature 

performance (Union Carbide Corp., 1976, p. 20). 

Thermoelectric Generators 

We no longer use propane-fueled thermoelectric 

generators. Their advantage was the ability to provide 

relatively high continuous-output currents (3 amperes). 

Although the amount of propane required was not large 

(380 liters per month), the high cost of transporting the 

propane to the remote field sites by helicopter forced us 

to find alternative solutions. In place of the 

thermoelectric generators, we now combine large banks 

of solar panels with reserve battery capacity. Also, we 

cycle the equipment on and off to lower the average 

current consumption. Still, at sites where propane can be 

delivered by truck, thermoelectric generators should be 

considered. 

FIELD REPAIR IN POOR CONDITIONS 

Even in good weather, instrument repair is never as 

easy in the field as in the laboratory. In conditions of 

wind, rain, and noxious gases, only the most rudimentary 

repairs are likely to succeed. To aid in making field 

repairs, we carry a few items not necessary in the lab: 

1. A 12-volt soldering iron and battery. The 

soldering iron may also be powered by the 12-volt 

batteries at solar-powered sites. We have found 

butane-powered irons to be unreliable in cold weather or 

at high altitudes. 

2. A light tarp. This can keep us and the 

instruments dry in driving rain or sleet. 

3. Extra desiccant. This should be carried in an 

airtight container, such as a sealable plastic freezer bag, 

so that the desiccant remains dry until needed. Desiccant 

should be replaced annually or after an instrument case 

has been opened and exposed to high humidity. 

4. "WD-40" penetrating lubricant in an aerosol can. 

Use this in wet conditions to displace water from 

connectors before reconnecting them. Also, it may ease 

threaded connections that have corroded together. Do not 

use WD-40 in dry, dusty conditions; dust will collect on 

oiled connectors and 0-rings, preventing a good seal. 

5. A sealant such as RTV silicone sealant or 

Scotchkote to protect exposed connections from 

corrosion and to seal gaps. With enough sealant and 

electrical tape, virtually anything can be made 

moistureproof. The Scotchkote may be used on wet 

surfaces, as it displaces water. 

6. Low expectations. If possible, avoid working on 

equipment in inclement weather. Because of impatience 

and added moisture, the site may be left in worse 

condition than before the visit. 

TIPS FOR TROUBLESHOOTING IN THE 

FIELD 

Volcano-monitoring equipment is of such 

diversity, with many one-of-a-kind sensors, that the 

need to troubleshoot unfamiliar, poorly documented 

equipment is not uncommon. The following basic 

steps can aid in troubleshooting both familiar and 

unfamiliar equipment. 

Before departing for the field: 

1. Try to hypothesize the cause of the problem 

by studying the symptoms. Did the station stop 

transmitting completely or is it only transmitting 

intermittently? Is it transmitting correctly, but the 

data are apparently incorrect? Did the problem start 

during a rain or lightning storm or after a site visit? 

Did hunting season just begin? Can it be connected 

with a problem at another site? A station that quit 

suddenly during a lightning storm most likely 

suffered lightning damage and, at the least, will need 

its radio replaced. A site that appears to be operating 

but whose data are getting progressively less 

plausible most likely has a moisture problem. A 

solar-powered site that operates only during the day 

most likely needs a new battery. 

2. Check the receiver to ensure that the problem 

is in the field and not at the observatory. 

3. Check to ensure that the equipment taken in 

the field to replace damaged equipment is in proper 

operating condition. 

At the field site: 

1. First, make a visual inspection. Look for 

corroded or broken connections, signs of damage, and so 

forth. 

34 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
2. Second, confmn that proper power is going to 

the equipment. This includes checking that power is 

reaching the equipment and that any voltage regulators 

or DC/DC converters are operating correctly. 

3. Attempt to isolate the problem by following 

signals through the circuit to see where they deviate from 

what is proper. Always be particularly suspicious of 

connectors. 

4. Make sure that all integrated circuits are pressed 

snugly into their sockets. Chips sometimes work their 

way out of the sockets. Look for chips that have one side 

a little higher than the other. 

5. A wet circuit board should be replaced. Though 

it is possible that a board only needs to be dried in the 

sun to return to a functioning condition, this should be 

attempted only in an emergency. The circuit may appear 

to be operating properly after drying, but intermittent 

problems or noisy data may appear later. Trimmer 

potentiometers and electrolytic capacitors are 

components that may have to be replaced after being 

exposed to water. Remember to find the source of the 

leak and seal the hole. 

6. if there is sufficent time, try to determine 

precisely the cause of the problem before replacing a 

board or instrument. The problem may not be 

reproducible in the electronics laboratory. 

7. Simply because equipment starts operating 

properly, do not assume that it has been repaired. A 

marginal contact or component may have been 

temporarily prodded into a functional state. Try to isolate 

the problem in order to ensure that the equipment will 

continue to operate. 

CONCLUDING REMARKS 

The procedures and techniques outlined here may 

not always be applicable. In some situations, we deviate 

from them. But they do provide guidelines to follow in 

site preparation and maintenance, especially in snow and 

ice environments such as those at Mount St. Helens. 

One point has not been stated explicitly but is the 

key to operating low-power telemetry networks: attention 

to detail. No matter how much effort has been expended 

in installing equipment, one unsealed hole in an 

instrument case may cause the equipment to fail. At sites 

accessible only by helicopter, one cold-solder joint can 

cost hundreds if not thousands of dollars to repair. 

Paying attention to the details of site preparation and 

instrument packaging is the most important factor in 

achieving long-term reliability. 

PRODUCTS/MANUFACTURERS 

MENTIONED IN THIS TEXT 

RTV sealant/adhesive 

Scotchkote 

Dow Corning 

Box 0994 

Midland, Michigan 48686-0994 

3M/Electronic-Products Division 

P.O. Box 2963 

Austin, Texas 78769-2963 

Instrument cases 

Connectors 

Antennas 

Pelican Products Inc. 

2255 Jefferson Street 

Torrance, California 90501 

Zero Corp. 

777 Front St. 

Burbank, California 90503 

Amphenol Corp. 

358 Hall Ave. 

Wallingford, Connecticut 06492 

Scala Electronic Corp. 

P.O. Box 4580 

Medford, Oregon 97501 

Larsen Electronics, Inc. 

P.O. Box 1799 

Vancouver, Washington 98668 

Transient suppressor diodes 

General Semiconductor Industries Inc. 

2001 W. 10 Place 

Tempe, Arizona 85281 

Plasma-discharge tubes 

Polyphaser Corp. 

P.O. Box 1237 

Zener diodes 

Batteries 

WD-40 

Gardnerville, Nevada 89410-1237 

Motorola Semiconductor Products, Inc. 

P.O. Box 20912 

Phoenix, Arizona 85036 

SAB/Nife 

P.O. Box 7366 

Greenville, North Carolina 27834 

Duracell Inc. 

Berkshire Industrial Park 

Bethel, Connecticut 06801 

WD-40 Co. 

San Diego, California 92110 

3. Operating Low-Power Telemetry Networks In Severe Envlronmneta 35 
REFERENCES CITED 

Duracell Inc., 1986, Duracell primary battery systems: Bethel, 

Conn., publication PBSB-7/86, 8 p. 

Duracell Inc.,1986, Duracell specification summary: selected 

products: Bethel, Conn., publication PSS-8/86, 4 p. 

Solar Power Corporation, no date, Solar electric generator 

systems-principles of operation and design concepts: 

Woburn, Mass., 11 p. 

Union Carbide Corp., 1976, Eveready battery engineering data: 

Danbury, Conn., 830 p. 

36 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
4. A System for Acquiring, Storing, and Analyzing 

Low-Frequency Time-Series Data in Near-Real 

Time 

By Thomas L. Murray 

ABSTRACT 

Timely interpretation of data is essential in volcano 

monitoring. I developed a system at the Cascades Volcano 

Observatory (CVO) to provide near-real time acquisition and 

display in graphic form of low-frequency data (sample rates 

less than or equal to 1 minute). Data from the low-data-rate 

telemetry network and real-time seismic-amplitude system 

are transferred to a central computer at 1 0-minute and 

1-minute intervals, respectively. Data collected manually are 

entered into the central computer by hand, while data from 

remote telemetry networks are collected weekly through a 

dial-up procedure. In the central computer, the program 

BOB, which I developed at CVO, provides for rapid 

interpretation of data by displaying them in graphic form. 

, Although at CVO a VAX 11fl50 is used as the central 

computer, the system can also use an IBM PC XT/AT or 

compatible for the central computer, increasing the system's 

portability. 

INTRODUCTION 

To provide timely warnings of impending volcanic 

eruptions, the scientific staff of the Cascades Volcano 

Observatory (CVO) must collect large amounts of data 

and analyze new data within minutes of collection. 

Comprehensive analysis of data is not required in real 

time, but precursors to eruptive activity may be missed if 

the data are collected and stored without being inspected. 

At CVO, I developed a system (fig. 4.1) to acquire and 

process low-frequency data (data sampled at intervals 

greater than or equal to 1 minute) for detecting 

anomalous activity and to archive data for further 

analysis. 

Ground tilt, fumarole temperature, gas 

concentration, and earthquakes per hour are typical 

low-frequency measurements. A single telemetered 

tiltmeter transmitting three measurements every 10 

minutes (radial and tangential tilt components plus 

temperature), produces 432 measurements per day, or 

over 150,000 measurements per year. Seismic-amplitude 

measurements from eight stations sampled at 1-minute 

intervals produce more than 10,000 measurements per 

day. Acquisition of these real-time data from the 

low-data-rate telemetry networks (Murray, 1988) and the 

real-time seismic-amplitude measurement system 

(RSAM) (Murray and Endo, chapter 1) is done by 

inexpensive laptop computers. Each laptop stores data in 

its internal memory and, at either 1- or 10-minute 

intervals, transfers the most recent data to a central 

analysis computer. Data collected manually, such as 

single-setup tilt measurements, sulfur dioxide flux 

measured with a Correlation Spectrometer (COSPEC), 

electronic distance meter (EDM) measurements, and the 

number of earthquakes per hour are entered into the 

central computer manually. Once in the central computer, 

data are immediately available for viewing and 

correlation with other measurements through the program 

BOB (Murray and Endo, 1986). 

CENTRAL COMPUTER 

At CVO, the central analysis computer is a VAX 

11/750 minicomputer. Full multitasking, multiuser 

capability allows numerous users to access data 

simultaneously. Several dial-in ports allow scientists to 

keep abreast of the latest data while absent from the 

observatory. The resources of the VAX (such as dial-in 

ports and hundreds of megabytes of disk space) enable 

data collection not only from Mount St. Helens but also 

from local telemetry networks in Yellowstone National 

Park, Wyoming, and Kileaua Volcano, Hawaii, and from 

various other sites via the GOES satellite network. All 

the data go into a data base, accessible by a single 

program, BOB. 

4 A System for Acquiring, Storing, and Analyzing Low-Frequency Time Series Data In Near-Real Time 37 
The VAX functions superbly in an established 

observatory, but is not easily transported. To monitor a 

volcano far from the observatory, data could be 

transmitted to the CVO VAX from the volcano (as is 

done with the Yellowstone telemetry network), and 

scientists at the volcano could use the VAX's dial-in 

ports to view the data. However, dedicated phone lines 

are expensive, and an eruption could disrupt the phone 

system. It is preferable to have the computer on site for 

the most efficient flow of information to the scientists 

monitoring a volcano. To this end, I modified the CVO 

system's programs to run on an IBM PC XT/AT or 

compatible computer. Installation and users' guides for 

the PC version are available (Murray 1990a, 1990b). 

GAS SENSOR 

DISPLACEMENT 

METER 

TILTMETER r~ TEMPERATURE 

SENSOR , LOWDATA-RATE 

NETWORK 

TILTMETER lr~ 

TILT METER lr~ 

RECEIVER 

REAL-TIME 

SEISMICAMPLITUDE 

METER 

RADIO LINKS 

't ' ' 

SEISMOMETER r-c_ , 

SEISMOMETER r-=- , 

SEISMICNETWORK 

RECEIVERS 

SEISMOMETER r-z_ , 

Differences between using a VAX and an mM PC as the 

central analysis computer are minor and usually 

transparent to the user. The major drawback to the PC is 

that it is not inherently a multitasking, multiuser 

computer. Commercially available software (Desqview), 

however, provides the computer with multitasking 

capability to enable automatic data acquisition in 

"background" mode while the computer is performing 

other tasks, such as plotting data, in the foreground. 

Furthermore, the computer need not be dedicated only to 

collecting and displaying low-frequency data; it can also 

be used as a standard PC. But the computer is still 

limited to one keyboard and monitor, and only one 

person can use it at a time. Experience has shown this 

CENTRAL - ANALYSIS 

COMPUTER 

1 

PHONE 

LINE 

rl SERIAL EDM 

PORT I 

SINGLE 

SETUP 

KEYBOARD LEVELING 

INPUT 

EARTHQUAKE 

COUNTS 

I SD2 FLUX I 

Figure 4.1. CVO low-frequency data-acquisition system. 

38 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
not to be a problem during crisis situations, because at 

such times most of the analysis is performed by looking 

at trends and correlating data from different precursors. 

The 10 minutes typically available between telemetered 

updates is usually sufficient time to plot and inspect the 

data. 

The lack of a dial-in port is no longer a major 

problem for the PC, since the proliferation of facsimile 

(fax) machines has provided a means for quick 

distribution of plots to scientists outside the observatory. 

Current developments in networking PCs should provide 

the means in the future for multiuser sharing of the data 

and dial-in ports to the system. 

With their decreasing cost, IBM PCs and 

compatibles are the current central-analysis computer of 

choice. The minimum recommended system for a PC to 

serve as a central-analysis computer is the following: 

Hardware: 

IBM AT or compatible 

EGA, VGA, or Hercules monitor 

Math coprocessor 

2 megabyte EMS 4.0 memory with memory 

manager 

30 megabyte hard disk 

5 1/4 inch floppy drive, 360 kilobyte minimum 

3 1/2 inch floppy drive, 720 kilobyte minimum 

Required software: 

DOS 3.2 or 3.3 by Microsoft, Redmond, 

Washington 

Interpretive BASIC CGWBASIC) by Microsoft, 

Redmond, Washington 

Geograf Graphics Utilities by Geocomp, Concord, 

Massachusetts 

Desgview by Quarterdeck Office Systems, Santa 

Monica, California 

ODOS II by Gazelle Systems, Provo, Utah, or 

similar utilityprogram with an ASCII word processor 

Substituting more powerful hardware for the 

equipment listed above, such as an 80386-based machine 

or a larger hard disk, is encouraged. Though an IBM XT 

or compatible may be used in place of the AT, the XT's 

slow clock speed and 8086 processor may slow the 

computer's response time to unacceptable levels while 

running Desqview. 

DATA ACQUISITION 

The central computer acquires data by one of three 

methods: (1) automatically by direct connection of 

data-acquisition computers to the central computer, as 

with the Mount St. Helens low-data-rate telemetry 

networks, (2) manually by keyboard entry, as with data 

not recorded electronically such as EDM or 

magnetometer-survey measurements, and (3) 

semiautomatically by periodic transfer of data over 

phone lines, as with remote low-data-rate telemetry 

networks at Yellowstone and Hawaii. 

Data collected automatically can be viewed in 

near-real time as the data files on the central computer 

are automatically updated within 10 minutes of the time 

of measurement. Phone-line and labor costs limit how 

close to real-time data collected semiautomatically or 

manually can be viewed. For example, a dedicated phone 

line could be established between the telemetry network 

at Yellowstone and the CVO VAX. Data could be 

transferred automatically at tO-minute intervals, as is 

now done with the Mount St. Helens networks. The 

phone line and modem would be transparent to the VAX, 

which would see no difference between the Yellowstone 

network and the Mount St. Helens networks. 

Automatic Data Acquisition 

Data from the low-data-rate telemetry network and 

the real-time seismic-amplitude meter are automatically 

acquired by several Radio Shack Model 100 or Toshiba 

1000 laptop computers connected to a serial port of the 

central computer. The laptops collect and periodically 

transfer the data over a serial line to the central 

computer. There are several advantages to using laptops 

for data collection: 

1. They relieve the central computer from 

data-acquisition tasks so that its greater processing power 

can be put to better use in analysis and other tasks. 

2. They provide redundancy, as the laptops can 

store a few days of data in the event the central computer 

is inoperative. 

3. They increase immunity to power failures and 

brownouts because the laptops are battery powered. 

Battery-powered computers consume less power than 

normal desktop computers and can operate longer during 

power failures when powered by an uninterruptible 

power supply. 

4. They increase the robustness of the system as it 

is not dependent on a single machine. 

The low-data-rate receiver's operation will not be 

affected by a malfunction of the RSAM, and vice versa. 

If the central computer fails, the ability to quickly plot 

and correlate data is lost, but the laptops would still 

acquire and display data that could be monitored and 

plotted manually, if necessary. 

The laptops share a single serial port of the central 

computer. This sharing, done by a circuit I developed at 

4 A System for Acquiring, Storing, and Analyzing Low-Frequency Time Series Data In Near-Real Time 39 
SIGNAL 1 

XMITTING 

SIGNAL 2 

XMITTING 

SIGNAL 3 

XMITTING 

SIGNAL '+ 

XMITTING 

C3 

~ ~3 

FOUR RS-232 TO 

ONE SERIAL PORT 

TOM MURRAY 

CASCADES UOLCANO OBSERUATORY 

1989 

Figure 4.2. Circuit to allow up to four devices to transfer data through a single serial port. 

CVO (fig. 4.2), has enabled as many as four laptops 

(three low-data-rate telemetry receivers and one RSAM) 

to use a single serial port. 

As currently configured, data are transferred only 

from the laptops to the central computer. No data or 

commands are sent from the central computer back to the 

laptops. This eliminates the need to establish any 

protocol for communication between the computers, 

other than that the data must be sent to the central 

computer in a specified format. Thus, no possibilty exists 

for the central computer to cause a malfunction in the 

laptops by sending them a strange control character, such 

as might occur if the central computer were to be 

accidentally reset. The laptops will continue to transmit 

data to the central computer whether or not it is 

operational. 

A major problem with this system is the possibility 

of two of the laptops attempting to transfer data to the 

central computer at the same time. Simultaneous 

messages will interfere with each other, the result being 

that no data will be correctly received. To prevent this, 

each laptop has an assigned time window (minimum of 

10 seconds) in which to transmit data. Weekly 

synchronization of the laptop's clocks keeps them in 

their respective windows. 

Manual Entry of Data 

Another method for the central computer to acquire 

data is by direct keyboard entry. Data collected 

infrequently are best entered in this manner. For 

example, EDM or single-setup leveling data collected in 

the field are entered manually into the computer and a 

program is run to update the data files. Other data files 

are updated simply by editing the files using a word 

processor. 

Semiautomatic Data Acquisition 

The third method for data acquisition is a 

combination of the previous two. Laptop computers 

operating in Yellowstone National Park and in Hawaii 

receive data from local low-data-rate telemetry networks. 

These laptops, with 256-kilobyte memory modules for 

greater data-storage capability and 1200-baud modems 

for phone-line communications, run a modified version 

of the programs run by the laptops at CVO. Once a 

week, these "dial-up" networks are called and their data 

transferred to the CVO VAX. A program is then run to 

update the data files. A similar procedure is used to 

40 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
retrieve data transmitted via the GOES satellite to a 

ground station at Wallops Island, Virginia. 

DATA STORAGE 

The system divides the data into two groups. One 

group consists of data collected at irregular intervals, 

such as EDM measurements. These data may consist of 

as few as 30 readings per year. In contrast, readings from 

a single axis of a telemetered electronic tiltmeter taken 

every 10 minutes throughout the year result in more than 

52,000 data points per year. I decided not to attempt to 

use a single file format for both types of data sets but 

instead to provide two file formats, one for large 

amounts of data collected at regular intervals and one for 

small data sets collected at irregular intervals. 

Data collected at frequent, regular intervals are 

stored in binary format in direct-access files (fig. 4.3). 

Each file is allocated memory sufficient for one year's 

worth of data for the specific measurement. For data 

collected at 10-minute intervals, this amounts to just 

under 215 kilobytes. These files are subdivided into 366 

records (367 records for a leap year), one record for each 

day of the year and an initial record indicating the 

number of data points per day (which is equivalent to the 

data points per record). Each record contains the data for 

its day of the year. The time interval between data points 

is calculated by dividing 1,440 (number of minutes in a 

day) by the number of data points per day. Intervals that 

are not integral divisors of 1 ,440, such as 17 minutes, are 

not allowed. 

Serial access, ASCII-format files are used for the 

data sampled at irregular intervals (fig. 4.4). Data in 

these files are in columns, with the date and time of the 

measurements in a standard format at the beginning of 

00:00 00:10 23:50 

01 JAN 89 DATA DATA I I DATA 

02 JAN 89 DATA DATA I-I DATA 

03 JAN 89 DATA DATA I-I DATA 

ACTUAL FILE 

30 DEC 89 DATA DATA DATA 

31 DEC 89 DATA DATA DATA 

Figure 4.3. Direct-access binary data file format. 

Example uses 1 0-minute sample rate. 

each line. New data are appended to these files through 

programs or with a word processor. The entire file must 

be read to access the latest data. For long files, this may 

require an unacceptably long time. If necessary, large 

files can be subdivided into smaller, separate files for 

each day, month, or year to speed access. 

The data-plotting and analysis program BOB, 

which I developed at CVO, provides easy access to data 

from both types of files. BOB also has the ability to 

access data from an archive device as well as the current 

on-line drive. If data are not found on the on-line drive, 

BOB searches for data on an archive device such as an 

optical disk. BOB treats data retrieved from separate 

devices as one continuous data set. Programs also exist 

that convert files from one type to the other. 

DATA ANALYSIS 

The interactive, command-driven program BOB is 

the core of the data analysis. It provides quick retrieval 

of data from either file format and easy conelation 

between the data sets. BOB was developed especially 

for quick analysis of time-series data in crisis situations. 

The time axes in BOB plots are in day-month-year 

format instead of julian days for quick determinations of 

when things happened, especially for nonscientists. Time 

periods as short as one day or as long as 50 years can be 

plotted to see the relation between recent trends and 

long-term records. Missing data points are handled 

easily. Making multiple plots in which each data set has 

a different number of data points per day requires no 

special commands. The maximum number of data points 

allowed by BOB is just over 400,000 data points on the 

VAX, while on the PC it is limited only by the available 

disk space. BOB also converts between GMT and local 

time if necessary. The following lists the BOB 

commands that produced the plot of earth tides and 

DATE TIME DATA 1 DATA2 

01 JAN 89 08:10 + 1.233 -456.7 

07 JAN 89 13:30 + 1.238 -998.0 

12 FEB 89 10:42 + 1.238 -998.0 

12 FEB 89 14:13 + 1.235 -455.9 

21 FEB 89 12:05 + 1.235 -460.0 

Figure 4.4. Sample ASCII data file. Note that -998.0 is 

used to indicate no measurement made. 

4 A System for Acquiring, Storing, and Analyzing Low-Frequency Time Series Data In Near-Real Time 41 
RSAM for the first two weeks of the 1989 eruption of 

Redoubt volcano, Alaska, shown in figure 4.5. 

EXEREDOUBT 

B_DATE 12 DEC 89 

E_DATE 2S DEC 89 

STIDRSAM 

MEAS RDNZ 

FILLA 

AV06A 

Select Redoubt volcano 

Select beginning date 

Select ending date 

Choose the station to be RSAM 

Choose the measurement to be 

for seismometer RDNZ 

Put the data for the time period 

and measurement indicated 

into column A 

Average every six measurements 

to produce an hourly average 

TITLE A RON SEISMIC AMPUTUDE Assign a title 

LABEL A HOURLY A VERAOE 

STIDTIDE 

MEAS TIDE 

FILLB 

TITLE B EARTH TIDES 

TZONEAST 

TOPTITLE REDOUBT VOLCANO 

PLOT2A B 

to column A 

Assign a label for 

the units of A 

Choose station for tides 

Choose earth tide measurements 

Put the data in column B 

Assign a title to column B 

Set time zone for Alaska 

Standard Time 

Assign the overall title 

for the plot 

Plot the data in columns A 

and B 

A sample plot produced by BOB for the October 

1986 Mount St. Helens dome-building episode at Mount 

St. Helens is shown in figure 4.6. 

Simple macros, batch files, and menu-driven 

programs provide easy access to the data for people with 

little or no knowledge of the system. For the PC as the 

central computer, these programs are written in BASIC 

so that modifications and enhancements can be made 

easily by staff members with programming experience in 

almost any language. For the VAX, these programs are 

written in FORTRAN. With these programs, observatory 

staff can print plots for all the data for the last few days 

or months with a single command. In interactive mode, 

BOB provides the means to look closely at short-term 

trends and then see how they relate to long-term trends. 

w 

~ a: 

w 

~ 

~ 

800 

600 

400 

a: 200 ::J 

0 

I O~l=~~--r~--~----~--~ ____ L_i_J__L_L~ 

(/) 0.195 

;;}_ 0.115 

g 0.035 _J 

~ -0.045 

-0.125 c__:_ __ ~--'--~~-----'--~-.L ____ _; _ __j_~--'---_j___ _ __j__~--~---= 

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 

START DAY IS DEC 11, 1989 A.S.T. - JULIAN DAY 345 

Figure 4.5. Example plot produced by the program 

BOB for Redoubt volcano. See text for explanation. 

Various types of data can be plotted on a common time 

scale to search for crosscorrelations. 

BOB also provides the user with convenient 

locations for files containing pertinent information about 

the field sites and instruments. These files can contain 

such information as instrument serial numbers, 

calibration factors, and site locations. Such information 

can be critical during a crisis. 

If the user requires more statistical analysis or 

greater graphics capability, BOB can write the data in 

ASCII formats usable by commercial data-analysis 

programs. The greater statistical or graphics capabilities 

of these programs can then be utilized. 

CONCLUSION 

The system in use at eva can provide the core for 

volcano monitoring systems for the future. The VAX 

version, in operation since 1985, has evolved from a 

method to merely plot tiltmeter data to its present state. 

The PC version performed well in crises at Galeras 

Volcano, Colombia, in 1989, and Redoubt volcano, 

Alaska, in 1989-90. 

As currently configured, the system provides a 

flexible means for gathering, analyzing, and archiving 

low-frequency measurements, whether telemetered 

directly to the system, stored in computers at remote 

locations, or collected manually. As more powerful and 

less expensive hardware and software become available, 

improvements will be made. 

(/) 

z I 

<( 0.20 c- Sweet Lips radial tilt 0 0.15 <( :c.. 

a: 0.10 =-

~ 0.05 ::-

~ 0 

i= 

z>-

-:. C§ 600 

~ a: 450 

~ ~ 300 ---6 

I 

-

-= 

-

-= /I 

~ ~ 150 COSPEC sulfur dioxide 

w~ OC-'---'----'---'----'---..L__--'----'---'---'---'---'---=---'---_J___J___J__-'J 

20 04 08 12 16 20 21 04 08 12 16 20 22 04 08 12 16 20 23 

START DAY IS OCT 20, 1986 G.M.T.- JULIAN DAY 293 

Figure 4.6. Example plot produced by the program 

BOB for the October 1986 dome-building eruption of 

Mount St. Helens. 

42 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
REFERENCES CITED 

Murray, T.L., 1988, A system for telemetering low-frequency 

data from active volcanoes: U.S. Geological Survey 

Open-File Report 88-201, 28 p. 

__ 1990a, A user's guide to the PC-based time-series 

data-management and plotting program BOB: U.S. 

Geological Survey Open-File Report 90-56, 53 p. 

__ 1990b, An installation guide to the PC-based time-series 

data-management and plotting program BOB: U.S. 

Geological Survey Open-File Report 90-634A, 25p. 

Murray, T.L., and Endo, E.T., 1986, BOB, a computer graphics 

tool for real-time integrated volcanic monitoring (abs.): 

Eos,v.67,no. 16,p.397. 

4 A System for Acquiring, Storing, and Analyzing Low-Frequency Time Series Data In Near-Real Time 43 

5. Seismic Data-Acquisition Systems at the 

Cascades Volcano Observatory 

By Elliot T. Endo and Gloria Smith 

Abstract 

From late 1987 to early 1990 the Cascades Volcano 

Observatory (CVO) installed and operated two different 

seismic-data-acquisition systems for a small seismic 

network. The systems were set up to detect and record 

small earthquakes beneath Mount St. Helens from a 14-

to 16-station seismic network. The first data-acquisition 

system is a PC/AT system, which utilizes software 

distributed by the International Association of Seismology 

and Physics of the Earth's Interior (IASPEI). The second 

system uses a Sun workstation, an analog-to-digital 

subsystem with an SCSI interface, and data-acquisition 

software written for computers that use the UNIX 

operating system. Recording with both systems in 1989 

gave CVO the opportunity to evaluate two relatively 

inexpensive digital seismic data acquisition systems. Both 

systems were easy to install and performed as expected. 

INTRODUCTION 

Since December 1987 the Cascades Volcano 

Observatory has been a Beta test site for testing and 

evaluating seismic-data-acquisition and analysis software 

written by Tottingham and others (1989), Lee and V aides 

(1989), and Valdes (1989) for IBM PC/AT and 

compatible computers. We began testing early versions 

of the current release of the IASPEI software in 

November 1988 (Toolbox for seismic data acquisition, 

processing, and analysis). An AST Premium 286 

computer (an 80286-based PC/AT clone) used for 

seismic-data acquisition has been in continuous operation 

for over 2 years. The system currently records 15 

channels of seismic data, 8 for seismic stations 

maintained by eva and 7 for seismic stations 

maintained by the University of Washington's 

Geophysics Program (UWGP) in the Mount St. Helens 

area (fig. 5.1). During normal activity, seismic data 

recorded by the PC/AT system are routinely processed 

on a daily basis. In 1989 more than 600 earthquakes 

were picked for routine locations, about 400 of which 

qualified as "A" or "B" quality (Lee and Valdes, 1985) 

earthquakes (figs. 5.2-5.4). 

In late January 1989 we began operation of a Sun 

workstation (WS) seismic-data-acquisition system using 

one of the first production Data Samplers (analogto-digital subsystem) built by Cutler Digital Design 

(Ward and Cutler, 1987). The seismic-data-acquisition 

system utilizes the same software that Peter Ward of the 

U.S. Geological Survey modified for a similar 

workstation that records data from the Katmai seismic 

network. The CVO system currently records Mount St. 

Helens seismic signals from 13 single-component seismic 

stations and 1 three-component site. For a number of 

reasons, routine processing of earthquakes recorded by 

the Sun workstation system has not been implemented, 

thus a rigorous comparison of the two systems was not 

possible. 

Implementation History 

After the two dome-building events in 1986, it was 

clear that eva required more than portable digital 

seismic-event recorders for analyzing Mount St. Helens 

earthquakes. Our preliminary research results pointed out 

the need for relatively complete digital recording of 

precursory eruption seismicity at a higher sampling rate 

than previously to detect evidence for hypocenter 

migration. We also required data from additional stations 

within the crater to improve locations of shallow 

earthquakes associated with magma migration. In fall 

1986, when a decision was made to install a real-time 

seismic-data-acquisition system, the only appropriate 

software available was Caltech-USGS Seismic 

Processing system (CUSP, Tanigawa and others, 1987) 

written by Carl Johnson. CUSP required a VAX 

processor with the Digital Equipment VAX VMS 

operating system. We could have installed CUSP on the 

existing VAX 11n50 at CVO, but the cost for the 

required analog-to-digital converter (A/D), graphics 

terminal, and high-speed interface board was estimated 

5. Seismic Data-Acquisition Systems at the Cascades Volcano Observatory 45 
to be about $30,000 in 1987. One potential problem with 

such an installation was the impact it would have on 

existing VAX users and on high-priority processes. A 

reasonable solution was to procure a MicroVAX WS, 

which we assumed would complement the existing VAX 

11nso and provide us with the required high-speed 

graphics capability. CVO was also faced with the 

problem of implementing CUSP, for which no 

documentation existed. In spring 1987 a Micro VAX RC 

graphics WS was ordered through a national Micro VAX 

procurement effort. The GSA schedule price for a fully 

configured MicroVAX RC was $17,600. That price did 

not include the roughly $12,000 cost of the AID 

subsystem. 

While we were waiting news of the national 

procurement effort, P.L. Ward was involved in 

identifying a low-cost recording system that could be 

used for a project at Katmai National Monument, Alaska. 

~PAK 

~ SEISMOMETER STATION 

• TILT, 

\ 

EDM STATION 

~0 () 

0 

122° 20' 

Analog recording was out of the question owing to the 

labor-intensive process of converting the data to a digital 

format. Real-time digital data acquisition was the only 

reasonable solution. Portable recorders were considered 

and rejected because of the time-consuming process of 

playback into a computer system for analysis and severe 

limitations in network expansion. Commercially 

available seismic-data-acquisition systems as well as 

CUSP were rejected owing to their high hardware and 

software costs as well as the high cost of site 

_requirements. CUSP would also have been unnecessarily 

large and complex for such a small network as Katmai. 

A PC-based system was rejected because of the 

limitations of the DOS operating system. Ward also 

wanted to use his expertise with UNIX and the UNIX 

seismic-data-analysis software that was available from 

Lamont Doherty Geological Observatory (LDGO) and 

Lawrence Livermore National Laboratories. Because no 

HOA 

~ 

~ 

JUN 

5 KILOMETERS 

~VEL 

ITNKR 

~,...-SAUN 

GDN 

Je.- FAMS 

010~ ~~ ) el NSP ~REM SWLP ~ • •• ! 

EMU ~ ~ 

\ -LONE ~OOPS 

./"'~ I "'0 RAZA 

~ 

~KM 

Figure 5.1. Distribution in 1989 of seismic stations at Mount St. Helens. Seismic stations ERT (15 km west), CDF 

(14.5 km southeast), and MTM (20 km south) are not plotted. Locations of a subset of telemetering tiltmeter sites are 

plotted in insert. Contours are in feet. 

48 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
AID subsystem was available to operate under generally 

available UNIX operating systems, Ward together with 

Reese Cutler (Cutler Digital Design) designed and built 

the Data Sampler (Ward and Cutler, 1987), which could 

interface with a UNIX type of computer system via a 

standard small computer standard (SCSI) interface. 

Although any computer using the UNIX operating 

system could have been used, the choice of the Sun WS 

was probably due to the fact that LDGO was well along 

in its software development under the Sun window 

environment. The Sun WS was not designed for 

expansion via a computer bus as were VAX computers. 

In September 1987, while working with Ward at 

King Salmon, Alaska, Endo had the first opportunity to 

observe the Sun WS and prototype Data Sampler in 

operation. He was impressed by the fact that the Data 

Sampler was designed, built, and tested with the Sun WS 

and operational in a period of about 3 months. The Sun 

WS system could be monitored from Menlo Park, 

California, via a modem port. With help from the 

National Park Service, data were written to a magnetic 

tape, which was then mailed to P.L. Ward in Menlo 

Park. Attractive features of the data sampler were its 

estimated $5,000 price for a 16-channel version and the 

low cost for expansion. 

Shortly after returning to CVO near the end of 

September, we were informed that the Micro VAX RC 

was no longer available on GSA schedule and that an 

equivalent replacement system would require another 

$12,000 or so from the project. The additional funding 

required at such a late date in the fiscal year was out of 

the question. Even if the funding were available, the 

cost of the Micro VAX system was approaching an 

unreasonable level. We were given less than a day to 

(/) 

1-

z 

400 

~ 300 UJ 

LL 

0 

ffi 200 

CD 

~ ~ z 100 MAR.1 f'-~_r 

/~'FEB. 18 

J 

\ 

DEC. 8 

'-.OCT. 10 

'AUG. 24 

1989 FEB APR JUN AUG OCT DEC 

START DAY IS NOV 30, 1988 G.M.T.- JULIAN DAY 335 

Figure 5.2. Cumulative number of "A" and ·e· quality 

Mount St. Helens earthquakes recorded by CVO PC/AT 

seismic-data-acquisition system from November 1988 to 

January 1990. 

make a decision regarding the disposition of the 

previously committed funding. After having observed the 

Sun WS in action, there was no hesitation to select the 

Sun WS as replacement for the MicroVAX RC WS. For 

the $17,600 originally committed to the Micro VAX RC, 

we were able to order a 141-megabyte Sun WS with 

1/4-inch tape drive and a Sun WS with a 71-megabyte 

disk drive. The Sun WS arrived at CVO in early 

December 1987. An unknown at that time was when 

would a production-model Data Sampler be available for 

the Sun WS. We still had an immediate need for a 

real-time system capable of recording up to 16 channels. 

In late December 1987 a copy of the earliest 

version of the IASPEI data-acquisition software for the 

PC/AT system was obtained. Due to its relatively low 

cost and commitment to PC-type hardware, which 

could be used for other purposes if we decided to 

abandon the PC/ AT, a decision was made to use the 

PC/AT system until we could fully implement the Sun 

WS system. The PC/ AT system was a low-risk system, 

because of the relatively inexpensive AID board. 

Owing to the simplicity of DOS, the PC/AT system 

was installed in a few hours. Much of that time was 

used for wiring discriminator outputs to the AID and 

verifying inputs. 

The original version of the program was a 

disappointment. While the program did pick and locate 

earthquakes in real time, a programming error resulted 

in numerous files being generated for one earthquake. 

It was in essence a one-earthquake system. In spring 

and summer 1988 no analysis software was available 

for the PC/ AT to confirm computer picks. In October 

1988 we received an improved version of the seismicdata-acquisition software and, a few weeks later, 

data-analysis software written by Carlos V aides. The 

122° 10' 

Figure 5.3. Epicenters of "A" and ·e· quality 

earthquakes located during the period from November 

1988 to January 1990. Lines A-A' and 8-8' are locations 

of profiles in figure 5.4. Contours are in feet. 

5. Seismic Data-Acquisition Systems at the Cascades Volcano Observatory 47 
latest version has been operating and detecting 

earthquakes since October 15, 1988. To this date we 

have detected no problems with the software. The 

analysis software is currently being used for manual 

picks to routinely locate Mount St. Helens earthquakes. 

The Sun WS Data Sampler, which was ordered at 

the end of June 1988, arrived on September 27, 1988. 

By late October, wiring to the Sun WS room was 

completed, the GOES clock installed, and a 

discriminator rack rewired for a ribbon-cable connector. 

After about 2 weeks of studying UNIX manuals and 

P.L. Ward's help, Endo upgraded the UNIX operating 

system to version 3.5, a requirement for operation of the 

data sampler. On November 9 the Data Sampler was 

successfully operated in develocorder mode. On 

December 5, we received additional software from Ward 

to put the Sun WS in detection mode. Further 

installation was temporarily delayed until we received 

additional discriminators for the rack located in the Sun 

WS room. On January 26, 1989, the Sun WS 

seismic-data-acquisition system was put into full-time 

operation. 

SOUTH DOME NORTH 

A I A' 0 ~~--.......--~~--~--~~ 

1 

2 

en 3 

ffi 4 ~ 

~ 5 

0 

~ 6 

z 7 

I 

~ 8 a.. 

gs 9 

10 

11 

12 

0 

+ 

4'- + .+ :Mf.-t + 

++ +++ + t 

+ + + ++ 

2 4 6 

DISTANCE, IN KILOMETERS 

PC/ AT Seismic-Data-Acquisition System 

and Analysis 

The PC/AT (AST Premium 286) system utilizes 

the seismic-data-acquisition software published by 

IASPEI (Lee, 1989). The current version is capable of 

monitoring 16 channels (single ended) and detecting and 

recording earthquakes, optionally locating earthquakes in 

real time. The CVO system currently monitors 15 

seismic stations. The sixteenth channel is used for 

recording IRIG-B (Inter Range Instrumentation Group) 

time code from a satellite clock. Although not essential 

for locating earthquakes with the AT clone, the time 

code (preferably IRIG-E) is required to tie the 

computer-system time to that of other seismic networks 

and to determine the correct origin time. 

With help from Lee the input-parameter file was 

modified to trigger and record successfully most of the 

small Mount St. Helens earthquakes observed on analog 

seismic records. Any event recorded at 20 to 25 mm 

peak-to-peak amplHude and with coda lengths greater 

than 5 seconds appears to be recorded by the system. As 

WEST DOME EAST 

o a.---~........,....----.__,c;;,_.,.'_r---__ ......,.........,a, 

1 

2 

en 3 

a: 

~ 4 

w 

~ 5 

0 

.....J 

~ 6 

z 

7 

I 

~ 8 

w 

0 9 

10 

11 

12 

+ 

0 2 4 6 

DISTANCE, IN KILOMETERS 

Figure 5.4. South-north and west-east cross-section plots of "A" and "B" quality hypocenters. Locations on figure 5.3. 

48 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
with any online system, telemetry noise occasionally 

produced numerous false triggers that filled the hard 

disk system. About 27 megabytes (approximately 

100-150 events) of disk storage is available for 

recording earthquake data and false triggers. A typical 

file with 16 traces digitized at 200 samples per second 

for 35 seconds, requires 230,400 bytes of storage. A 

sampling rate of 100 samples per second would be more 

than adequate for most small networks and would 

double the number of events that could be recorded. Our 

practice has been to check the file system daily. Analog 

records are checked and earthquake files identified. 

Files for false earthquake triggers (telemetry noise, 

avalanches, or rock-fall signals) were deleted and 

earthquake files written to an optical disk (200 

megabytes capacity) or floppy disk (1.2 megabytes 

capacity). Because we do not have a PC Local Area 

Network (LAN) system for file inspection and transfer, 

the data-acquisition system is offline for the 15 to 60 

minutes required for the file transfer. We are currently 

using IASPEI software written by Carlos Valdes 

(Valdes, 1989). With EGA graphics and a mouse, it is 

easy to pick earthquake phases and locate earthquakes 

using HYP071PC (Lee and Valdes, 1985). A version 

of the picking program allows the user to select a time 

window for spectral analysis. If the system's absolute 

response is known, seismic moment for individual 

earthquakes can also be computed. 

Following is our current hardware configuration 

for the AST 286 system: 

40 megabyte hard disk, 28 ms access time 

1.2 megabyte 5 1/4-inch floppy drive 

2 megabyte RAM, 100 ns chips 

80287 math coprocessor 

One serial, one parallel port 

EGA video card 

NEC multisync color monitor 

Keyboard 

MS-DOS 

IBM 3363 200 megabyte WORM drive 

Data translation DT2821 AID+ license to 

use A TLIB software 

Hercules compatible monochrome graphics card 

Monochrome monitor 

Cost for above system: 

AST 286 Premium system 

ffiM 3363 WORM drive 

DT2821 AID and license 

Total 

$3,595 

$1,858 

$1,524 

$6,977 

For display of seismic traces a monochrome 

monitor and card added about $200 to the AST system. 

For data analysis an Everex 1700 12-MHz AT clone is 

used. It has the following configuration: 

40 megabyte hard disk 

1.2 megabyte 5 1/4" floppy 

720 kilobyte 3 1/l" floppy 

1 megabyte RAM 

80287 8 MHz math coprocessor 

EGA graphics card and NEC monitor 

Microsoft mouse 

mM 3363 WORM drive 

The approximate cost for the Everex system, 

without WORM drive, was about $3,000. The actual 

Everex system procured was a $2,100, 1-megabyte, 

monochrome system without EGA graphics, mouse, 

math coprocessor, or WORM drive. Components (EGA 

video card, mouse, EGA monitor) from an existing XT 

system normally used for word processing, graphics, 

and other tasks were put into or used with the Everex 

system. An AID board was not installed on the 

Everex, which was not configured to be identical to 

the AST system. Total cost for the two AT clone 

systems was about $12,000. Continuous data 

acquisition and routine analysis cannot be 

accomplished with one AT clone; two identical 

systems are required to ensure continuous recording. 

While analysis takes place on one system, the user can 

be recording on the other system. The second system 

also serves as a backup in case of failure of the first 

system. Should we have a major earthquake swarm, 

our current plan is to transfer all data to the optical 

disk to minimize recording down time. If the 

data-acquisition and analysis systems are in close 

proximity, the use of a LAN system would eliminate 

the need for a second AID board and the need for a 

similarly configured system. Furthermore, trigger files 

could be transferred to the analysis system without 

interrupting data acquisition. Only one WORM drive 

would be required for the archiving of data. Systems 

procured for use elsewhere should consider the 

problem of continuous recording (backup system) and 

routine data analysis. Analysis software written by 

Carlos Valdes could operate (somewhat slower) on a 

PC/XT with EGA graphics, math coprocessor, hard 

disk, and mouse. 

The implementation process for the PC/ AT 

software was a relatively simple task. The user needs 

only to set up appropriate directories, copy files from 

floppy to hard disk, change config.sys and autoexec.bat 

files, and edit input files for station names, channel 

numbers, triggering parameters, and so forth. A 

monochrome graphics board and the AID board had to 

be installed with jumpers appropriately set. The 

monochrome monitor was used for continuous display 

of seismic traces. 

Like all other PC equipment at CVO, none of the 

above equipment is on a maintenance contract, since 

5. Seismic Data-Acquisition Systems at the Cascades Volcano Observatory 49 
eva has in-house capability to handle much of the 

maintenance. The relatively low cost of PC hardware 

also discouraged a maintenance contract. 

Sun Workstation System 

The Sun WS system implemented at CVO consists 

of two Sun 3/50 workstations. One WS is configured to 

handle data acquisition via a Data Sampler (A/D 

subsystem, Ward and Cutler, 1987) built by Cutler 

Digital Design. The Data Sampler used has one input 

module for 16 channels. Additional modules can be 

installed in the data sampler to provide inputs for up to 

256 channels. The primary functions of the Data Sampler 

are to accept analog seismic inputs from a number of 

channels, digitize signals at a specified rate, and provide 

temporary storage until transfer to a host computer. The 

use of parallel binary coded decimal (BCD) time from a 

GOES satellite clock for the four least significant data 

bits is unique to the Data Sampler. If parallel BCD time 

is not available, the user is provided the option of using a 

less accurate internal clock. The Data Sam pier appears to 

the UNIX operating system as a SCSI nine-track tape 

drive. Software to test the Data Sampler and to detect 

and record earthquakes was written or modified by P .L. 

Ward. 

The eva system is fully operational, with the 

sampling rate set at 100 per second. During the latter 

part of 1989 we started to process earthquake data 

recorded by the Sun WS system to evaluate its 

performance along with that of the PC/AT system. The 

second WS was set up primarily for data analysis, but a 

directory structure similar to that of the data-acquisition 

system has been installed to facilitate the use of the 

system as backup hardware for data acquisition. 

SUNPICK from LDGO is the primary software used for 

routine analysis. At CVO, the Sun WS's are connected 

by ethemet. The ethemet system provides a convenient 

way to share files and resources. 

is: 

The current configuration for the Sun WS system 

One Sun 3/50 WS with: 

320 megabyte hard disk 

60 megabyte 1/4-inch tape drive 

4 megabyte RAM, floating point accelerator 

19-inch monochrome monitor 

One Sun 3/50 WS with: 

141 megabyte hard disk 

60 megabyte 1/4-inch tape drive 

4 megabyte RAM 

floating point accelerator 

19-inch monochrome monitor 

16 ch~nnel Data Sampler 

GOES satellite clock 

292 megabyte Delta Microsystems SCSI disk 

drive 

Cost for above system: 

Two Sun 3/50 systems 

One Delta Microsystems drive 

One Data Sampler 

One GOES satellite clock 

One 320 megabyte disk upgrade 

Total 

$17,421 

4,702 

4,975 

4,275 

6,500 

$37,873 

The 292-megabyte Delta Microsystems disk 

provides space for seismic-data-acquisition software and 

224 megabytes of data. Approximately 500 to 700 

earthquakes 75-100 seconds long can be recorded on the 

available data space. This configuration provides backup 

hardware for the seismic-data-acquisition system. 

Hardware maintenance for the above workstations will 

be $800 for this fiscal year, and we expect software 

maintenance to be about $1,600 for both workstations. In 

this final hardware configuration, the Sun workstations 

cost $37,873. The data sampler can be expanded to a 

256-channel input device for $3,500. 

Preliminary Evaluation of the PC/AT and 

Sun WS Systems 

In 2 years (December 1987 to December 1989) of 

continuous operation, the AST Premium 286 PC/AT 

experienced no hardware failures. Approximately 75 to 

85 percent of the earthquakes detected at three or more 

seismic stations in the crater of Mount St. Helens were 

successfully recorded by the PC/AT system. Recording 

failures were largely due to late triggers that resulted in 

missed P-wave arrivals at the seismic stations. Selection 

of a lower sampling rate and a longer pre-event time 

partially solved that problem. Some events were missed 

during a sequence of closely spaced earthquakes; perhaps 

a longer post-event time could solve this problem. A 

problem in proper triggering in late 1989 was related to 

eva network configuration, the trigger net which 

utilizes mostly CVO seismic stations, and the relatively 

lower gains of crater seismic stations. A significant 

number of small earthquakes at depths greater than 5 km 

were not recorded by the PC/AT system. We are 

experimenting with different trigger parameters to correct 

the problem. 

Telemetry failure creates the most serious problem 

with the PC/AT system. Numerous event files have been 

generated by the detection program whenever there was a 

telemetry disturbance on either one of two main 

telemetry links from Mount St. Helens. Discriminators 

such as the USGS 1120, which clamp on the loss of 

50 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
carrier signals, would eliminate the problem of numerous 

false triggers. Without a LAN, routine data analysis is 

awkward compared to that of the Sun WS, because data 

cannot be acquired during the time needed to determine 

which files were appropriate for routine analysis. If a 

swarm had occurred, all files would have been copied to 

an optical disk, the only mass-storage device installed 

on the CVO PC/AT computers. 

During the 10 months of operation, the Sun WS 

system has gone through one monitor failure and one 

hard-disk failure. The Sun WS systems appear to have 

triggered on almost all Mount St. Helens earthquakes 

detected at three or more seismic stations. There have 

been no failures to completely record all P-wave arrivals 

of earthquakes. The Sun WS system has the advantage 

of using time bits from the parallel BCD output of a 

satellite clock for an absolute time stamp on the digital 

trace data. The Sun WS system also has the advantages 

of a larger disk system and the use of the UNIX 

operating system, which enables flexibility in directory 

and file names, use of script files, use of aliases, use of 

the window environment, and higher-resolution 

graphics. 

Both of the systems detect and record local 

earthquakes well, and each has its advantages and 

disadvantages. The PC/AT system implemented at CVO 

was one-third the cost of the Sun WS system and was 

simple to install, but it suffers from the limitations of 

the PC/AT and DOS. A brief earthquake swarm in 

August 1989 quickly filled the available disk space on a 

standard 32-megabyte partition of the DOS 3.3 

operating system. During a lull in activity, event files 

had to be moved to an optical disk to make room for 

additional events. By comparison, the Sun WS was 

more difficult to install, because of the need to 

understand UNIX and the details required for its 

configuration. The Sun WS recorded more earthquakes 

during the same period and required little attention 

owing to the substantially greater disk space available. 

Files could also be checked without interrupting the data 

acquisition process. In addition, the accurate timing of 

Sun WS files required no time correction of picked 

phases. 

Operational Philosophy 

The implementation of a seismic-data-acquisition 

system at CVO was motivated by the need to better 

locate shallow earthquakes at Mount St. Helens. Part of 

the problem in locating small shallow earthquakes is the 

limited flexibility in reconfiguring telemetry in the 

regional seismic network (about 128 seismic stations) 

operated by the University of Washington. Another 

problem associated with recording small earthquakes by 

a regional seismic network is that of inefficient use of 

data storage. A small earthquake that might be detected 

by only 15 seismic stations less than 20 km from Mount 

St. Helens would require recording digital data for 112 

more distant seismic stations in the network. Recording a 

subset of the regional network and a few additional 

stations was more efficient and less costly on 

smaller-capacity seismic-data-acquisition systems such as 

the PC/AT or Sun WS. 

The installation of two different seismic-dataacquisition systems at CVO arose from the need for a 

real-time system that could routinely provide CVO with 

quick earthquake locations and a means of doing the 

best possible job of recording precursory seismicity 

associated with future dome-building activity of Mount 

St. Helens. We were caught in the evolution of two 

systems. The PC/ AT system was very easy to install, but 

its limitations for our requirements were recognized and 

hence the Sun WS system was tested. Some additional 

work on the Sun WS is required for real-time locations, 

plots, and a complete data-analysis system using PL. 

Ward's proposed data format. After complete 

installation of the Sun WS system, our plan is to keep a 

PC/ AT system available as a backup. These systems 

allowed CVO staff involved in seismic studies to 

improve their skills in seismic-data acquisition and 

analysis. During the past year we have been able to 

participate in the routine analysis of Katmai seismic 

data, thanks to the systems. A half -time seismic analyst 

assists with the routine evaluation of Mount St. Helens 

and Katmai digital seismic data. We have processed 37 

DC600 ( 60 meg) cartridge tapes of Katmai seismic data. 

The Sun WS system is an example of the 

decreasing cost in hardware and software development, 

owing to the use of preexisting documented software. 

The availability of Sunpick for X-windows, XPICK 

from the University of Alaska, and the portability of 

UNIX software should result in lower software 

development costs for future real-time seismicdata-acquisition systems. Another advantage of 

documented software, and software used by other 

institutions, is the potentially lower maintenance costs 

of software. Both systems obviate the need for full-time 

computer specialists for programming and software 

maintenance. The Sun WS system is the system we 

would consider selecting for a seismic network with 16 

or more channels of data. 

The widespread availability of PC/ AT computers 

(and clones) in developing countries makes The PC/AT 

the ideal seismic-data-acquisition and analysis tool for 

volcanologists from such countries. With data-analysis 

software written by Carlos Valdes (V aides, 1989) and 

the DOS format of data files, digital seismograms can 

be available to anyone with an IBM compatible 

computer and EGA graphics capability. Recent 

5. Seismic Data-Acquisition Systems at the Cascades Volcano Observatory 51 
hardware and software development for the PC/ AT 

system and faster computers indicate a potential for 

recording and processing data from as many as 128 

channels. 

CONCLUSION 

Our experience with both the PC/AT and Sun WS 

seismic-data-acquisition systems indicates that either 

system is appropriate for use with small seismic 

networks. Both have their advantages and disadvantages. 

The less costly PC/AT system is relatively simple to 

install and maintain and utilizes PC hardware available 

throughout the world. The disadvantage of a 

PC/AT-based system lies primarily with the limitations 

of DOS 3.3 and earlier versions. The Sun WS offers the 

advantages of UNIX and the availability of more than 

one type of data-analysis software, but currently uses a 

more expensive Data Sampler and is somewhat more 

difficult to implement and more costly to maintain. An 

ideal system would be one that utilizes the best features 

of both systems. The PC/ AT system would be ideal for 

the task of acquiring digital data, and a UNIX type 

workstation that supports X-windows would excel in 

routine data analysis. Whatever system is chosen, it 

should be one that minimizes the need for expensive 

hardware, high-cost maintenance of hardware and 

software, burdensome programming costs, and 

potentially expensive changes in computer operating 

systems. 

ACKNOWLEDGMENTS 

We are indebted to Willie H.K. Lee, Carlos 

Valdes, and Peter Ward for their effort in developing 

software for PC/AT and Sun workstations. The results 

from both seismic-data-acquisition systems would not 

have been possible without the work on the CVO seismic 

network by Tom Murray, Bruce Furukawa. and other 

CVO staff. We also appreciate the work of the 

University of Washington staff who maintained the 

UWGP portion of the seismic network at Mount St. 

Helens, and the University's seismic analysis staff for 

seismic information. 

REFERENCES CITED 

Lee, W.H.K., 1989, Getting started, in Lee, W.H.K., ed., 

Toolbox for seismic data acquisition, processing, and 

analysis: International Association of Seismology and 

Physics of the Earth's Interior, p. 1-46. 

Lee, W.H.K., and Valdes, C.M., 1985, HYP071PC: A personal 

computer version of the HYP071 earthquake location 

program: U.S. Geological Survey Open-File Report 

85-749, 30 p. 

Tanigawa, W.R., English, T.T., Johnson, C.E., Koyanagi, R.Y., 

Nakata, I~S., and Tomori, A.H., 1987, Seismic processing 

at the Hawaiian Volcano Observatory: Abstract volume, 

Hawaii Symposium on How Volcanoes Work, p. 249. 

Tottingham, D.M., Lee, W .H.K., and Rogers, J.A., 1989, User 

manual for MDETECT, in Lee, W.H.K., ed., Toolbox for 

seismic data acquisition, processing, and analysis: 

International Association of Seismology and Physics of the 

Earth's Interior, p. 49-122. 

Valdes, C.M., 1989, User manual for PCEQ, in Lee, W.H.K., 

ed., Toolbox for seismic data acquisition, processing, and 

analysis: International Association of Seismology and 

Physics of the Earth's Interior, p. 175-201. 

Ward, P.L., and Cutler, R., 1987, An inexpensive, buffered 

analog-to-digital converter for use on the SCSI bus with 

the UNIX operating system; U.S. Geological Survey 

Open-File Report 87-624, 81 p. 

52 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
6. Methods Used to Monitor Deformation of the Crater 

Floor and Lava Dome at Mount St. Helens, 

Washington 

By Eugene Y. lwatsubo and Donald A. Swanson 

ABSTRACT 

The lava dome at Mount St. Helens grew episodically 

between October 1980 and October 1986, and throughout 

this period, many techniques and procedures were 

established to help monitor, study, and predict dome-building 

episodes. These included measurement of displacements on 

radial cracks and thrust faults on the crater floor and dome, 

leveling the crater floor, and repeated occupation of 

trilateration and distance-measuring networks on the crater 

floor and dome. Displacement meters were also installed to 

continually monitor radial cracks on the crater floor and 

dome. Distance-measuring networks became one of the 

most reliable methods used for predicting dome-building 

episodes. New and old methods have been adopted to 

establish and measure such networks. The use of both 

distances and angles has allowed us to locate stations in a 

coordinate system that offers a three-dimensional view of 

ground deformation in the crater. 

INTRODUCTION 

Many methods have been used at Mount St Helens 

to monitor the deformation of the dome and adjacent 

crater floor. These methods range from simply using a 

steel tape to measure distances across cracks and thrust 

faults (Chadwick and others, 1983; Chadwick and 

Swanson, 1989) to the use of an electronic distance 

meter (EDM) and continuously monitoring displacement 

meters (lwatsubo, Ewert, and Murray, chapter 9). As the 

Figure 6.1. CVO scientists measuring distance across a radial crack on crater floor. Cracks would widen as 

magma intruded beneath lava dome. By measuring accelerating widening of crack, we were able to predict 

dome-building episodes. 

6. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helena, Washington 53 
20 25 OCT 5 10 15 

START DAY IS SEP 17, 1980 G.M.T. - JULIAN DAY 261 

Figure 6.2. Widening of four radial cracks prior to October 

1980 explosive episode. On October 15 (the day before 

the explosive episode), many new cracks had formed and 

were observed by field crews. 

N 

I 

10 

11 

12 

0 10 CENTIMETERS 

VECTOR SCALE 

13 ~ INCLUDES j EXTRUSION 

Figure 6.3. Displacement vectors for "Christina's radial," 

a crack located adjacent to lava dome on crater floor. 

Vectors show both widening and strike-slip 

displacements, owing to combined extension and 

movement of a thrust plate bounded by crack. Just before 

and during extrusion of September 6, 1981 (vector 13), 

sense of movement was mainly extensional (from 

Chadwick and Swanson, 1989). 

dome grew and changed, so did the methods used to 

monitor and predict its activity. Many of the techniques 

described in this paper helped to predict dome-building 

episodes (Swanson and others, 1983). 

Many of the techniques we used were adapted 

from elsewhere and modified to fit our needs. The 

principles of monitoring volcanic activity by measuring 

horizontal deformation had proved useful in Japan and 

Hawaii (Yokoyama and others, 1981; Kinoshita and 

others, 1974) for many years before 1980. Before and 

after the May 1980 eruption, we monitored the outer 

flanks of Mount St. Helens using these adopted 

techniques (Lipman and others, 1981; Swanson and 

others, 1981; Iwatsubo, Topinka, and Swanson, chapter 

8). In 1981, when we recognized that the dome deformed 

in response to magma intrusion, we established 

distance-measuring networks inside the crater to monitor 

the dome. This has become one of our best tools for 

predicting eruptive episodes at Mount St. Helens. 

HISTORY 

In mid-September 1980, cracks started forming in the 

crater floor radially from the vent, which was plugged by a 

small dome that formed in August. Pieces of reinforcing 

rod (rebar) were driven into the ground on both sides of the 

cracks to form "crack stations," and the distances between 

rebar stakes were measured with a steel tape (fig. 6.1). Our 

inten~on was to monitor these cracks for any type of 

movement; prediction of future eruptive activity on the 

basis of such movement was considered unlikely. However, 

prior to the October 1980 explosive episode, the cracks 

widened at an accelerating rate in response to a rising 

magma body (fig. 6.2). On October 15, observers noted 

many new radial cracks that had formed since the previous 

visit on October 8. An explosive episode occurred on 

October 16-18, after which a new dome started to form; 

this dome, with subsequent additions, has survived to the 

present. Many new radial cracks formed on the crater floor 

as the dome continued to grow. Crack stations were 

established to monitor them (Swanson and others, 1981), 

and the crater-deformation program became firmly 

established 

Many useful data were obtained from the crack 

measurements. Distance changes across a crack were the 

most important of these, but displacement vectors could 

also be calculated for each station (fig. 6.3). We found that 

most cracks showed nearly pure dilational opening but that 

some cracks, which formed tear faults bounding thrust 

plates, showed both dilational and strike-slip components of 

displacement (Chadwick and Swanson, 1989). 

Thrust faults in the crater floor were frrst observed 

during the December 1980 dome-building episode. 

54 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Figure 6.4. Measuring distance between rebar stakes at toe of a thrust fault. Near rebar is on lower 

plate of thrust, and far rebar is on upper plate. Upper plate of thrust overrode lower plate as an eruptive 

episode neared, and measured distance would consequently shorten. Rate of shortening would 

accelerate as magma neared surface of lava dome. 

Thrust stations, established across the leading edge of the 

upper plate, were monitored in the same way as were the 

crack stations (fig. 6.4). As the upper plate of the thrust 

overrode the lower plate, measured distances shortened. 

The rate of shortening increased prior to dome-building 

episodes, just as did the rate of widening of cracks. The 

changing rates were used to predict the start of the next 

eruptive episode (Chadwick and others, 1983; Swanson 

and others, 1983). We also leveled between rebar stakes 

and determined that the upper plate was being uplifted as 

it thrust forward. 

In May 1981 a stainless steel Parker-Kalon (PK) 

masonry nail was hammered into the lava at the base of 

the dome, and the distance to a rebar stake on the crater 

floor was measured with a steel tape. The taped distance 

shortened at an accelerating rate as the next eruptive 

episode neared (fig. 6.5). This was the first time we 

recognized that the dome itself was responding to the 

slow influx of new magma before an eruptive episode. 

The significance of this data set eventually led us into a 

new phase of monitoring. 

A leveling network circling the dome was 

established in summer 1981 to determine the amount of 

uplift and subsidence of the crater floor. This type of 

monitoring yielded interesting results but was very timeand manpower-consuming, particularly given the 

helicopter logistics, uncertainties in weather, and 

volcanic hazards. One of the most severe problems was 

poor visibility in many parts of the crater owing to fume 

rising from fumaroles and to steam rising from the hot 

crater floor. The leveling data showed uplift of the crater 

floor centered around the dome prior to a dome-building 

episode. Leveling was completed before and after the 

June 1981 dome-building episode, and subsidence was 

observed along the whole line when compared to the 

pre-eruptive line. Subsidence was greatest nearest the 

dome and possibly resulted from (1) loss of volatile 

pressure acting against the crater floor, (2) loss of 

magma volume beneath the floor after extrusion, and (3) 

loading of the crater floor by the addition of the new 

magma. After 1981, we discontinued the leveling 

because it proved too time-consuming to be completed 

routinely for prediction purposes. Given more favorable 

circumstances, leveling would have been continued 

because of the unique data it provided. 

In 1981, a trilateration network was established to 

monitor the crater floor north of the dome. Trilateration 

is a method that measures the length of triangle sides and 

can be used for establishing horizontal control (Sturgess 

and Carey, 1987); however, our intent was to calculate 

displacement vectors from the data. Trilateration was not 

conducted as a tool for predicting eruptive episodes, but 

6. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helens, Washington 55 
(/) 

ffi 0 't+-----------F!---------R~ tu~ 

~ ~-10 

m~ () <( -20 za: -f-

~ ~-30 Z<( 

~~ (/) 0-40 

off: 

w 

a... -50 0 

_J 

(/) -6o ~~~~~~~~~~~~~~~~~~~ 15 20 25 JUN 5 10 15 

START DAY IS MAY 12, 1981 G.M.T. - JULIAN DAY 132 

Figure 6.5. Movement of southeast lobe of lava dome 

relative to crater floor. May-June 1981. Taped distance 

between dome and crater floor shortened as eruptive 

episode neared. This was the first indication that the 

dome as well as crater floor responded to rising magma. 

Taped distance was approximately 18 m. 

N 

I 

0 100 METERS 

L_____j 

105 

WAHU 

169 

Figure 6.6. Sketch map showing instrument sites and 

targets inside crater and on dome in 1981. This was the 

first complete network using an EDM to monitor the 

dome. The longest measurement was 1,000 m. 

instead was a research experiment. The results added to 

our understanding of the processes that were occurring 

on the dome and crater floor. Comparing the leveling and 

trilateration data, it was clear that horizontal 

displacements dominated vertical displacements during 

the same time period. For example, one station on the 

crater floor moved 50 em horizontally but only a few 

millimeters vertically. The trilateration data also showed 

that horizontal displacement of the crater floor was 

cumulative. Once a station moved, it remained there 

rather than returning to its previous position. 

By the end of summer 1981 we felt relatively at 

ease climbing the dome, and so we began an intensive 

monitoring program of the dome itself, in part spurred by 

the data in figure 6.5. In October 1981, plastic highway 

reflectors screwed on boards were placed at various 

locations on the dome. Instrument stations were 

established on the crater floor and distances to the targets 

were measured with an EDM. The first crater EDM 

network is shown in figure 6.6. 

A theodolite was used to measure zenith angles to 

the same EDM targets. Combined with the EDM data, 

horizontal and vertical components of displacement 

directed toward the EDM site were calculated for each 

target. Horizontal angles were measured only to selected 

targets to observe the changes in horizontal spacing of 

two or more targets as the dome grew. In retrospect, 

horizontal angles should have been measured to every 

station on the dome and referenced to a known point. 

Coordinates for the targets could have been established 

at a later date if this had been done. Some horizontal 

angles referenced to points on the crater rim showed 

relative horizontal movement of the dome, but 

coordinates could not be calculated from these data. 

During the March 19, 1982, eruptive episode 

(Waitt and others, 1983), all cracks and thrust faults on 

the crater floor were buried by pumice or stripped away, 

thereby ending one major phase of monitoring. New 

EDM networks were established around the dome, and 

monitoring continued. With no cracks and thrust faults to 

measure, EDM measurements became the primary 

method of monitoring dome deformation and are still 

used today. 

A triangulation network on the north side of the 

dome was implemented in 1983, during a year-long 

period of slow, continual dome growth. Triangulation is 

the method where one side of a triangle (baseline) and at 

least two interior angles are measured to calculate a 

location for a station (Shafer, 1987). The purpose of this 

network was to monitor the inaccessible parts of the 

dome where it was impossible to place targets for EDM 

measurements. Horizontal and vertical components could 

be calculated for the triangulation data (fig. 6. 7) and 

were a key part in determining where the dome was most 

56 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
active on the north side. Other parts of the dome were 

monitored with an EDM and theodolite. 

A new dimension was added to our monitoring 

program in 1984 when Peter Otway of the New Zealand 

Department of Scientific and Industrial Research (DSIR) 

introduced the use of coordinates based on the state 

plane-coordinate system. Initially, triangulation was 

attempted from two instrument stations at Kid and Sugar 

Bowl (fig. 6.8), respectively. Zenith and horizontal 

angles were measured to dome targets and coordinates 

were calculated. This method was time- consuming and 

not cost-effective. Standard surveying methods were then 

adopted to use both EDM and theodolite measurements 

to locate targets. As EDM measurements and zenith 

angles were already being measured, only horizontal 

angles referenced to a known point had to be measured 

to enable us to make the coordinate calculations. 

Bird Dog 

Garden 

0 20 40 METERS 

'--------'----.. _j 

VECTOR SCALE 

Dome 

N 

r 

0 100 METERS 

L___j 

MAP SCALE 

Figure 6.7. Horizontal displacements determined by 

triangulation for stations on northeast part of dome. 

Contour is schematic. Time period for these 

measurements was mid-July to late September 1983, 

during a period of prolonged dome growth chiefly 

concentrated in northeast part of dome. Two triangulation 

instrument stations were Garden and Bird Dog. Targets 

on nearby vertical faces of dome were marked by paint 

bombs dropped from open door of a helicopter. Other 

parts of the dome were monitored by EDM and 

theodolite; small x's identify instrument and target 

stations. 

As the dome continued to spread, the available 

working space on the crater floor between the dome and 

the crater walls narrowed. On every side except the 

north, it became increasingly dangerous to work in the 

crater owing to rockfalls off the unstable crater walls in 

summer and snow avalanches in winter. Permanent snow 

and ice fields mixed with rock-avalanche debris formed 

on the east, west, and south parts of the crater floor, 

effectively precluding surveying from these areas. In 

1985, because of the growing lack of working space on 

the crater floor, we transferred most of our monitoring 

activities onto the dome itself. Only the north side of the 

dome continued to be monitored from the crater floor. 

EDM networks were established at various locations on 

the dome. Owing to limited helicopter landing areas on 

top of the dome, some instrument stations could be 

occupied only during winter months when snow covered 

the rocky, irregular surface. In order to obtain 

deformation data from the dome in summer, a single 

instrument station, Shoestring Notch, was established on 

the southeast crater rim (fig. 6.8, 6.9B). Shoestring Notch 

could not be occupied in the winter, because conditions 

were often too hazardous in the notch; however, we were 

122.11' 

46.12' 

0 1 KILOMETERS L______j 

Figure 6.8. Critical stations on rim and inside crater of 

Mount St. Helens. Kid and Sugar Bowl we~e two principal 

triangulation instrument stations in 1984. ShJestring Notch, 

on crater rim. was used during summer months of 1985-88 

to monitor parts of dome. Guacamole, Sugar Bowl. and 

Shoestring Notch are all horizontal reference marks used 

from various crater instrument stations in 1990. Guacamole 

is referenced from Tinker and Little Village. Sugar Bowl is 

referenced from Lonesome. and Shoestring Notch is 

referenced from Earl. Contours are in meters. 

6. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helena, Washington 57 
generally able to work on top of the dome during winter. 

Thus we had two monitoring networks, one for summer 

and one for winter (fig. 6.9). 

In 1989, owing to a shrinking budget and lack of 

volcanic activity, the Shoestring Notch measurements 

were discontinued. To compensate, one more instrument 

station on top of the dome was established to monitor the 

south part of the dome, and landing sites for summer 

operation have been found or constructed. 

MONITORING METHODS 

Crack and Thrust Fault Stations 

Monitoring of cracks and thrust faults is relatively 

simple and inexpensive, requiring only rebar and a sturdy 

25-m steel tape measure. We cut rebar into stakes 40-50 

em long and drive them into the ground, leaving enough 

A To Step2 and Yellow Rock I I 

0 100 METERS L_____j 

of the rebar exposed to provide ground clearance when 

the tape is stretched between them. Many rebars, 

particularly if not driven in far enough, can be hit and 

disturbed by rocks cascading off the dome or crater wall. 

Some pieces of rebar have had to be replaced because 

they were literally eaten away by corrosion from the 

bottom up, owing to warm acidic gas and water in the 

ground, until only the portion above the surface 

remained. 

Displacement meters have been installed to 

monitor several cracks on both the crater floor and dome. 

The cost is much greater owing to the meter itself, and 

digital telemetry or an on-site recorder. More time and 

care are needed to establish such a site, but continuous 

measurement data can be obtained. Iwatsubo, Ewert, and 

Murray (chapter 9) describe how displacement meters are 

used to monitor cracks on the crater floor and dome. 

When establishing a crack or thrust station with 

rebar, it is essential that consistency be maintained in 

procedures for numbering stakes and reading the distance 

between the rebars. We have been confused often enough 

B 

0 1 KILOMETER L____j 

Figure 6.9 Schematic maps of Mount St. Helens. A, Digitized, schematic map showing crater EDM network in 1990. 

Distance to Yellow Rock is 374 m and to Step2 is 1,621 m. Prior to 1990, this was the winter monitoring network on 

the dome, and instead of Earl instrument station, we had shots from Lonesome to Earl area. 8, Summer EDM network 

shown with Shoestring Notch as main instrument station to monitor southeastern part of dome. Contour intervals are 

in meters. All station locations were plotted by coordinates calculated from field observations. Instrument stations are 

identified by soW<~ triangles. 

58 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
to sttess this point! Three rebars fanning a triangle spanning 

the crack or thrust fault is typical. A braced quadrilatenll 

would be more precise but is not warranted in our situation. 

We set up a end station by placing one rebar (designated 

#1) on one side of the aack (or on the upper plate of a thrust 

fault) and two on the other side (#2 and #3), numbered in 

clockwise faWon. By measming distances between 1 and 2 

and between 1 and 3, widening or closing of the crnck or 

thrust fault can be monitored. The 2-3 distance measurement 

serves~ a baseline and check on me$wing precision (1-2 

mm). With the combined three measurements, vector 

displacements can be calculated for rebar 1 relative to rebars 

2 and 3. For the index point for both ends of the tape, we 

generally center-punch the top of the rebar and use this mark. 

One person holds the zero made of the tape over the center 

mark of one rebar, and the tape is stretched tight and read 

over the center mark of the other rebar. 

Leveling, Trllateratlon, and Triangulation 

Leveling and trilateration were time- and 

manpower-intensive. Three to four people were needed for 

\ -.28 .-2 

\ -6~ '?. ... li: 0/'1 v v\ ~ 

-396 ... 

0 100 200 300 METERS 

-117 

g 

....... 

~116 

Figure 6.10. Vertical displacements in millimeters, 

determined by leveling before and after June 18-20, 

1981, dome-building episode. Contour lines are drawn at 

-50, -100, and -200 mm. Station 120 was reference 

fencepost and was monitored by vertical angles from 

north crater floor. Anomalous, positive displacements are 

located on thrust faults that uplifted during this episode. 

"SE December,• "February,• and "April• identify lobes of 

dome extruded respectively in December 1980 and 

February and April 1981. 

leveling, and at least four or five for trilateration. All 

leveling and trilateration stations were either steel 

fenceposts 1.5 m long that were marked with metal 

identification tags, or thrust-fault rebar stakes. The 

distance between leveling marks was approximately 

50-100 m. We used two fiberglass, collapsible, 4-m 

leveling rods, and a Lietz B4 automatic level. The 

leveling was referenced to a fencepost on the east crater 

floor; its movement was monitored by measuring vertical 

angles from the crater floor north of the dome. Vertical 

displacements for the June 18-20, 1981, dome-building 

episode are shown in figure 6.1 0. For this episode, the 

reference fencepost was station 120; it rose 75 mm from 

April 14 to June 15 before the episode but had returned 

back to its April 14 elevation by June 24 following the 

extrusion. 

The trilateration instrument stations were located 

approximately 700 m north of the dome and were 

assumed to be stable. A distance was measured between 

the two instrument stations to establish a baseline. The 

movement of the instrument stations was checked by 

measurements from Harrys Ridge north of the dome 

(Iwatsubo, Topinka, and Swanson, chapter 8). The EDM 

was set up over one instrument fencepost while people 

held prisms on the target fenceposts. The instrument was 

then moved . to the second instrument station and the 

distances to all the targets were measured. The large 

displacements observed during the September 1981 

episode (fig. 6.11) offset any minor movement that might 

have occurred at the instrument stations. 

In 1983, a triangulation network was established 

to monitor the dome. Two instrument stations on the 

crater floor were established and targets on the dome 

were marked by fluorescent paint dropped in plastic 

bags from the door of a helicopter hovering above 

inaccessible areas of the dome. The length and 

azimuth of the baseline between the two instrument 

stations were measured. At the first instrument station, 

zenith and horizontal angles (referenced to the second 

instrument station) were measured to all the targets. 

The theodolite was then quickly moved to the second 

instrument site and the procedure repeated. The data 

using this method were not as accurate as those 

acquired with the EDM, partly owing to the difficulty 

of pointing the theodolite at the same part of the 

spattered paint-bomb targets from both instrument 

stations. But over time, reliable trends and vector 

displacements were determined. 

These methods were of limited use during winter 

months, because of snow cover. Trilateration could be 

completed if snow accumulation was less than about 1 

m, the typical height of the fenceposts above the 

ground surface, but leveling could be accomplished 

only when the ground was snow free. Triangulation 

6. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helens, Washington 59 
was also discontinued because snow covered the 

inaccessible targetsandcouldnot be removed. 

EDM Networks 

Since the fmt crater EDM network was set up, 

there have always been at least three instrument stations 

to monitor as much of the dome as possible. From each 

instrument site, the distances and angles to several (2-8) 

targets on the dome and crater floor were measured with 

an EDM and a theodolite. For example, in 1990 there 

were four instrument stations and approximately 19 

reflector targets (fig. 6.9A). The configuration of the 

network and number of targets changed as both 

instrument and target stations were destroyed by 

rockfalls or dome growth. 

In 1985, we began monitoring the dome from 

Shoestring Notch during the summer months. 

Monitoring from Shoestring Notch required that we 

modify our data-collecting technique. Rebar stakes 

visible from Shoestring Notch were established high on 

the southeast half of the dome. The west and north 

sectors of the dome could not be seen from Shoestring 

Notch but could be monitored by the EDM networks 

owing to favorable helicopter landing sites on the dome 

and crater floor (fig. 6.9B). Approximately 15-20 rebar 

stations were established for the Shoestring Notch 

measurements. The rebar stakes were painted fluorescent 

orange to make them visible from Shoestring Notch 

through the telescope of the theodolite. With an 

instrument person and a recorder at Shoestring Notch, 

one person on the dome held a reflector on each rebar 

while an EDM measurement was taken, then quickly 

moved to the next rebar until all the distance 

measurements were completed. Radio communication 

was essential for this type of survey. Zenith and 

horizontal angles were then measured to the top of every 

rebar: no person had to be present at the rebars during 

the angle measurements. Horizontal angles we!_e 

referenced to a known mark on the crater floor. 

Instrument and target stations 

Instrument stations are established by driving rebar 

stakes, 40-50 em long, into the ground as far as possible 

or until 5-10 em is remaining above the ground surface. 

PK nails, 5 em long, are used to install stations on large 

boulders. To center over the same point each time, the 

center of the rebar is marked with a center punch; PK 

nails are commercially purchased with pre-punched 

heads. Most of the instrument rebar we used had a 

diameter of 0.38 in. (1 em), but now a larger diameter 

0.5 in. (1.3 em) rebar is being used to take advantage of 

small aluminum caps (commercially available) that are 

A 

B 

N 

r 

0 100 

MAP SCALE 

Garden ... 

0 100 200 MILLIMETERS 

VECTOR SCALE 

H184 

H184 ~ 

I I i 

I 

Figure 6.11. Trilateration data spanning September 6, 

1981, dome building episode. A, Network shown with 

those stations that we were able to measure from both 

instrument stations. Normally, there are many more 

targets, but adverse conditions limited data. 8, 

Displacement vectors calculated from measurements. 

Changes in slope distance and displacement vectors are 

in millimeters. Slope distance changes were cumulative 

and permanent. "Feb," "April," and "June" identify lobes 

extruded in February, April, and June 1981. 

60 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
pre-marked and fit snugly on the heavier rebar (fig. 

6.12). The caps can also be stamped for identification. 

Initially, target stations consisted of a cluster of 

plastic highway reflectors (each reflector is 8 em in 

diameter) mounted on 30-cm square boards (figure 

6.13A) and placed on the dome and crater floor. The 

boards, painted fluorescent orange for visibility before 

attaching the reflectors, were supported by short pieces 

of rebar driven into the dome, by PK nails and stainless 

steel wire, or by a combination of rebar, nails, and wire. 

"Whatever works" was our guiding principle. The 

crater-floor targets were mounted on fenceposts driven 

into the ground. Highway reflectors are inexpensive and 

, work for short EDM distances ( <200 m). As the dome 

grew, however, the distances increased and better 

reflectors were needed. 

One type of alternate reflector was a single 

corner-cube glass prism (7 em diameter) mounted 

through a hole drilled in the target board. The prisms 

allowed longer distances to be measured, but were 

expensive ($100-$150), particularly given the short life 

expectancy of the targets owing to rockfalls, dome 

growth, and etching of glass (see "Target and Reflector 

Problems" below). Consequently we tested a lower cost 

($45-$60) glass retroreflector that proved sufficient for 

our needs. This type of reflector consists of three mirrors 

glued together orthogonally to emulate the comer of a 

cube. These glass reflectors are fixed to the board in the 

same fashion as that used for the comer-cube prism (fig. 

6.13B). The cost is lower, therefore allowing more 

targets to be placed on the dome. The retroreflectors are 

Figura 6.12. Rebar (0.5 in., 1.3 em diameter) with 

aluminum cap used as instrument stations on dome and 

crater floor. Aluminum caps can be stamped with station 

identifications. The caps were commercially purchased. 

A 

8 

Figura 6.13. A, Target board with five plastic highway 

reflectors mounted on it. We point to the center of middle 

reflector on top row to measure angles. 8, Target board 

with a glass retroreflector mounted in it. A hole is drilled 

through board and reflector is then fitted and glued with 

silicone seal to board. 

8. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helena, Washington 81 
Figure 6.14. Scientists measuring distances and angles on top of dome. Lietz Red 1A EDM is mounted on top 

of T2 theodolite. 

not of the quality or accuracy of the glass comer-cube 

prisms, but the large movements occurring on the dome 

make these disadvantages insignificant 

EDM and Theodolite 

In October 1981, we first used an EDM, a 

Hewlett-Packard (HP) model 3808A infrared 

medium-range instrument (Iwatsubo and Swanson, 

chapter 10), to measure distances from the crater floor to 

the dome. Zenith angles are measured with an old style 

(inverse reading) Wild T2 theodolite. The HP3808A was 

the smallest EDM available to us in 1981, but its size 

and bulk made it difficult to carry and use in the crater. 

The HP3808A could measure long distances but required 

up to I minute per measurement Because of the slow 

measurement rate, changing steam conditions on the 

dome often made shots difficult to complete. In addition, 

to measure angles the HP3808A had to be removed from 

the tripod so that the theodolite could be mounted. When 

measurements were taken repeatedly at the same 

instrument station, it was difficult and time-consuming to 

continually switch instruments. This was a problem in 

particular during times when rates of movement were 

several centimeters or more per hour. 

In 1982, a Lietz Red lA EDM was purchased 

specifically for use in the crater. This is a relatively 

small EDM that could be mounted directly on top of the 

theodolite using a yoke mount (fig. 6.14; table 6.1). The 

size and weight of the Lietz allowed crews to walk from 

instrument site to instrument site; as a result, 

considerable helicopter time was saved, with the 

important bonus that the crater floor and dome could be 

observed in more detail than previously during the course 

of a survey. The Lietz performed well for as long as we 

used it, but in 1988 a Geotronics Geodimeter 210 was 

purchased. The Geodimeter 210 is even smaller and 

lighter than the Lietz Red lA and is more precise (table 

6.1). The Geodimeter 210 mounts on the telescope of the 

theodolite for easy use. 

Single endpoint temperature and pressure are 

recorded at the instrument station for all measurements. 

Temperature, pressure, and humidity are the atmospheric 

parameters involved in slope-distance corrections. For 

the crater networks, we use a mercury-filled, hand-held 

thermometer for air temperature because the lines are so 

62 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Table 8.1. Specifications for the Lietz Red 1 A and 

Geodimeter 210 EDM's. 

SpecifiCation Uetz Geodimeter 

Range: 1 Priam 1.4km 2.3km 

3 Priam 2.0km 4.0km 

Accuracy: i(S mm + S ppm) ±(S mm + S ppm)1 

Weight3: 

±(5 mm + 3 ppm)2 

3.Sq 1.4kg 

Dimenaions: 160x9Cbc180 mm 17Sx8Sx8S mm 

Temperature nne: -2CrC to +SO"C -2o-C to +SO"C 

1

For a standard single measurement. 

2

For arithmetic mean value (every measurement display is an 

updated mean). 

:Weight includes internal battery. 

short that temperature has little effect on the corrected 

distance. A Thommens altimeter/barometer is used to 

measure pressure. Humidity is not recorded, because of 

its negligible effect. 

Tripods and Setups 

There are two common methods for centering 

tripods over a mark, (1) a tribrach with an optical 

plummet and (2) a telescoping centering rod. Each has 

its own advantages and disadvantages. The type of 

monitoring that is required could decide the choice of 

tripod and centering method. Regardless of which type 

is chosen, it is critical to check and keep the tripod 

leveling system true. Both systems are equally easy to 

check and adjust, and a procedure for doing so should 

become part of a routine maintenance schedule. 

Trlbrachs and standard tripods 

A tribrach with an optical plummet (hereafter 

referred to as a tribrach) requires a uipod with a flat 

head and a 5/8x11 threaded screw (standard tripod). 

The advantages of a tribrach and standard tripod are 

that (1) most theodolites and EDM yokes will fit into a 

tribrach, and (2) the standard tripod can be set up 

close to the ground. This is useful when work needs to 

be done in high winds or over a large rock in which 

the station is set. Not only are winds less severe near 

the ground surface, but instruments are easier to shield 

and can be made more stable because the tripod legs 

can be spread wide apart. 

The disadvantages of the tribrach and standard 

tripod are the requirement to measure the height of 

instrument (ID) above the station mark and ·to correct 

for different HI's during routine monitoring. If a 

method is established to measure the m the same way 

each time, and everyone complies, problems will be 

minimized. However, errors can be introduced if 

individuals measure them inconsistently. 

While monitoring the dome, it is important to 

examine raw data in the field to see if there have been 

significant changes since the last measurement. This 

practice also provides a check for gross measurement 

errors. If a tribrach is used and no attempt is made to 

establish a constant HI for each instrument station, 

comparing raw data on site will be complicated. 

Measured slope distances and zenith angles are both 

dependent on the m. If the setup is at a different· HI 

each time, the raw slope distance and zenith angle will 

differ even if the target has not moved. To compare 

successive measurements, the data must be reduced to 

either a horizontal or mark-to-mark distance. This can 

be accomplished on site if a programmable calculator 

or small computer is used in the field. This procedure 

takes time and could be a critical factor when weather 

and eruptive conditions change rapidly. 

The better method is to consistently use the same 

HI each time at a given instrument station. However, 

this also is difficult and time-consuming with a 

tribrach setup. One easy way to accomplish this is to 

always use the same tripod with fully extended legs 

and mark where the legs should be placed. 

Alternatively, cement pads with small holes for the 

legs can be used to position the tripod in the same 

location for each setup. We have successfully used this 

technique, which we borrowed from colleagues at the 

Hawaiian Volcano Observatory, for some stations at 

Mount St. Helens (lwatsubo, Topinka, and Swanson, 

chapter 8). 

Centering-rod tripod 

Centering-rod tripods simplify routine 

monitoring and have become our preferred method. 

The centering-rod tripod system is easy and fast to set 

up using the telescoping centering rod (stem). Every 

instrument station has a fixed stem height, and the 

tripod is set up at this stem height for each occupation. 

Because the stem height is always the same, raw slope 

distances can be compared because atmospheric 

corrections have little effect on the short lines 

measured (generally <400 m). There is no 

tape-measuring error with a centering rod, but errors 

can be introduced by an incorrect reading of the stem 

height. 

There are two major disadvantages of 

centering-rod tripods. Centering rods allow the tripod 

to be lowered only to the minimum length of the stem, 

which is 1 meter. The instrument may therefore sit too 

high for working in windy conditions or over a high 

rock. The second disadvantage is that centering-rod 

tripods have a unique head that connects to the stem 

and requires special adapters (about $125 apiece) for 

mounting standard surveying equipment. This is no problem 

8. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helena, Washington 63 
if all the pieces are together and present, but if an 

adapter is forgotten or lost, little or no work can be 

.:complished. It is wise to caay a spare adapta' plate in 

the field. 

Equipment Maintenance and C&llbratlon 

Equipment maintenance and calibration are important 

and should be part of a routine schedule. Tribrachs, 

centeling-rod tripods, and standard 1ripods can be tested and 

oojusted by the usez. EDM's shoo1d be aili1nted at base lines 

established by agencies Sldl as the Nalional Geodetic Survey; 

oowever, they can also be checked in areas where 1heze are no 

calilnted base Hnes. The accunq' of the EDM will 10 be 

known, but its precision will be, and precision is the key fimr 

for routine monitoing. If the EDM meaues the base line 

with precision, the data in the field are believable. Anotla" 

check on an EDM is to measure its modulation frequency (the 

~Y that is used to measme distances). This frequeocy 

can be found in the rnaJ11Jfa:turel''s manual. It is important to 

use a frequency countrz that is one mb- of magnitude higher 

than the modulation frequency to ensure correct 

measurementS. lbese countezs are expensive and are not 

standard equipment; CVO does oot have ooe. If 1here is a 

discrqmcy between base-line <r frequency meauanents, the 

EDM must be checked and calibrated at the factmy. 

Theodolites are the only instruments that must be tesled 

by the factory or authorized dealer. The amount and 

conditions of use should detamine the schedule for calibration 

and cleaning of both the EDM and theodolite. 

The procedure f<r checking and oojusting Uilnchs or 

centering rods is simple and should be dooe often. fnslructions 

are usuaDy iocluded with the inslrument manual, oc a general 

surveying reference (Brinlcez and Minnick, 1987) can be used. 

Routine fieldwork is hard on equipmenL Bumping and 

shaldng while riding in a vehicle <r helicopter can cause 

surveying equipment to go out of oojustment. If the centering 

method is out of adjusunent, the data wiD be affected. Once at 

Mount St Helens, the crater centering-rod Uipod came out of 

oojustment, and this was 10 immediately noticed. The data 

collected during that time period showed significant scatter 

and caused concern O\U the erratic behavior of the dome. 

After the centenng rod w. adjusted, the data smoothed out 

and showed that no actual defamation had occurred. 

Target and Reflector Problems 

The main reflector problem encountered over the 

years has been etching and sandblasting of both plastic and 

glass reflectoiS. Hydrofluoric acid mixed with steam can 

badly etch glass in as short a time as one week. Other 

volcanic gases deposit sublimates that also take their toll. 

Windblown asia is a sttong abrasive. These factors must 

be taken into IICCOunt when placing targets on the dome. 

If there is much gas present, the target will not last long • 

We have learned to avoid areas that are frequently near 

or downwind from fumaroles, even if the fumes seem 

weak. Even with such precautions, reflectors placed in 

areas apparently free of gases and protected from winds 

and steam commonly became etched within one week to 

one month. For these reasons, it is wise to place targets 

in more different locations than are absolutely necessary. 

Rockfal15: from higher on the dome, and spalling of 

rock faces holding targets, have caused the demise of many 

targets. It is not uncommon to lose 90 percent of the targets 

before and durilrtg a dome-building episode. 

Dming winter months, problems develop when snow 

and ice cover the reflectors. From 1981 to 1984, we went 

out frequently to monitor the dome, even during winter. 

Cold temperatures do not allow melting of snow or ice on 

the targets and require someone to visit and clean every 

obscured reflector. Even if the reflector is ice free, steam 

can often condense on the reflector, and someone has to 

continually wipe it dry until the measurement is completed. 

This is time-consuming and often exciting (due to rockfalls 

off the dome or to icy conditions) for the person doing the 

cleaning. In the late 1980's eruptive activity slowed, and 

crews no longer go out as often, and then only during good 

weather; consequently reflectors are often snow free. 

Steamy conditions also hinder the ability to complete 

a measuremenL No infrared EDM can measure through 

steam. This has been a problem since the fiiSt network was 

established, particularly during winter months. It is 

common during winter to wait an hour or more for the 

steam to clear in order to complete a measuremenL Prior to 

dome-building episodes the data are critical, and such time 

delays, although frustrating, are deemed worthwhile. 

If angles are to be measured, be consistent in where 

the reference point on the target is. If prisms are used, use 

the center of the prism or prisms as the reference point for 

all angles. This will simplify the procedure and is easy to 

remember. 

COORDINATES 

The coordinate system we use is based on the state 

plane-coordinate system converted to meters. The initial 

known points were located by colleagues of the National 

Mapping Division as part of the control for producing 

topographic maps of the dome and crater. All instrument 

stations, reference marks, and targets have x (casting) 

and y (northing) coordinates, plus an elevation. 

Coordinates are calculated each time a target is measured 

and can be compared with the previous measurement to 

create a displacement vector. Prior to dome-building 

64 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
episodes, these vectors can indicate the location of the 

pressure source within or at the base of the dome. 

Network Setup 

We establish a network using coordinates by 

starting with two nominally stable points, one instrument 

and one reference mark, each with known longitudes, 

latitudes, and elevations. The longitude and latitude are 

converted to x and y coordinates respectively, which are 

easier to manipulate for routine data reduction. The 

conversion is made by a commercially purchased 

software package (Department of Commerce, National 

Geodetic Survey Division). Where it is not possible to 

use two known points, an alternative method is to pick 

two prominent points from a map and establish 

longitudes, latitudes, and elevations. All subsequent 

locations will be relative to these starting points. The 

starting points may not have exact locations, but all 

calculated locations will be internally consistent. 

When establishing the network, we try to limit the 

number of reference marks. The ideal situation would be 

a single reference mark for all of the instrument stations. 

The best type of reference marks that we use are steel 

towers that were installed for other EDM measurements 

around Mount St. Helens (Swanson and others, 1981; 

lwatsubo, Topinka, and Swanson, chapter 8). We also 

use steel fenceposts and antenna poles. Anything that is 

uniquely recognizable can be used, taking into account 

summer haze and winter snow (see figure 6.8 for 

horizontal references used in 1990). 

One of the most important requirements for using a 

coordinate system is to locate instrument and reference 

marks on a yearly basis. This is especially critical if 

stations are located on possibly active or unstable sites, 

such as the dome or crater floor. All of the dome's 

instrument stations are located at least annually, even 

when no eruptive activity has taken place. When eruptive 

activity occurs, we try to relocate each instrument station 

on the dome. This always creates an offset in the 

coordinate data, but it is necessary. One way to prevent 

the offset is to locate the instrument station during each 

occupation. This is fine in theory but not so easy in 

practice. Two ways could be used to locate the 

instrument station: (1) measure a distance and zenith 

angle to a known point, and a horizontal angle between 

that point and one other known mark, and (2) locate the 

station from some other instrument station, as a regular 

target would be located. In order to use the fmt method 

without a reflector person, a reflector must be attached to 

a known tower, pole, or fencepost. At Mount St. Helens, 

this type of station is typically located on the crater rim 

for visibility. During summer months it is possible to 

measure the distance, but in winter, rime and snow often 

cover the reflector. It is not cost-effective to fly to and 

clean the reflector for each visit. The cost prohibits the 

second method as well. A special trip would have to be 

made each time. The best that we can achieve is to 

locate each instrument and reference mark at least once a 

year, or whenever it is made necessary by significant 

movement of the dome. 

SOFTWARE AND NOTES 

Reduction of EDM data can be done with 

programs written in-house or purchased commercially. 

Most of our data reduction and plotting programs have 

been written in-house. We have progressed from data 

reduction by hand, to an HP-41C hand-held computer, to 

a VAX minicomputer, and finally to a PC computer. 

The advantage of in-house programming is the ability to 

custom design programs and to allow for growth. As we 

gain experience, new knowledge can be applied to 

expanding and improving existing programs. 

Slope distances corrected for atmospheric 

parameters form the base from which horizontal and 

mark-to-mark distances are calculated. We always enter 

the atmospheric parameters into the formula given by the 

manufacturer of the EDM, rather than using the 

atmospheric or environmental correction charts that come 

with the instrument, as the charts are not accurate 

enough for most applications and the formula is more 

consistent. To calculate mark-to-mark or horizontal 

distances, either zenith or vertical angles must be 

measured or station elevations must be known. 

Programs were initially written to reduce data to a 

slope distance at a fixed stem height, using only 

instrument temperature and pressure for atmospheric 

corrections. This approach worked until the fixed stem 

height had to be changed for several measurements. A 

program was then written to con vert the slope distance 

from a different stem height to the slope distance at the 

correct stem height. This conversion program was critical 

when it became necessary to monitor the dome from 

Shoestring Notch on the crater rim. At Shoestring Notch, 

a centering-rod tripod was fmt used to establish a stem 

height. But because of usually windy conditions, it was 

necessary to use a standard tripod and tribrach to set up 

close to the ground, and consequently a different m was 

used each time. The conversion program was the only 

way to compare slope-distance changes. 

After years of dealing with the problems of EDM 

data, we recommend reducing data to mark-to-mark 

distances. We routinely measure zenith angles, and it is a 

simple matter to calculate mark-to-mark distances. There 

is no need for conversion programs, and any tripod 

8. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helena, Washington 65 
(/) 

a: 

~ 0 

w 

~ -0.25 

z 

w -0.50 

~ -0.75 

~ 

0~~~~~~~~-~~~~~~~~~ 

-1 o1 _-s-_---s-_--a-B--__ 

-20 

(/) -1.00 -30 

~ -1.25 

a.. 

-40 

-50~ 

0 -1.50 

_J 

-60 t -70~·~·~~·~~~·~~~~~~~~~~·' 

~ -1.75 1 0 20 S E P 1 0 20 OCT 1 0 20 

START DAY IS AUG 4, 1986 

(/) -2.00 w 

CJ 

z -2.25 

<( 

I -2.50 

0 JUL AUG SEP OCT NOV 

START DAY IS JUNE 23, 1986 G.M.T.- JULIAN DAY 174 

Figure 6.15. Distance changes for a typical dome station during quiet and eruptive periods. This 

station, located on northwest side of dome, was measured from an instrument station on north crater 

floor. Inset is a closer view of data leading up to October 1986 dome-building episode. By observing 

curve as episode nears, onset of activity can be predicted. A new lobe on dome was first observed on 

the morning of October 22 (vertical line). 

system can be used to obtain the raw data. When it is 

possible to measure only a slope distance but no zenith 

angle, mark-to-mark distances can still be calculated if 

there has been no large-scale deformation. Because of 

the short distances (200-400 m), the zenith-angle 

difference needed to change the mark-to-mark distance 

by 1 mm is approximately 2.5 minutes of arc for an 

84-degree zenith angle (average zenith angle at Mount 

SL Helens). The typical error for zenith angles in the 

crater is less than 15 seconds of arc, so the 2.5 minutes is 

well above our noise level. 

Computer files are created to store the reduced 

data. Saving all pertinent raw data will save time in 

reentering numbers if new calculations need to be made. 

Raw slope distances, zenith angles, temperatures, 

pressures, and instrument and reflector heights will allow 

fast reduction of the data at a later date. We did not store 

these in the computer for several years, but as we learned 

more, reprocessing of some data was required, and all 

the raw data had to be reentered into the computer. Now, 

all pertinent raw data are stored along with the reduced 

data. We have three data files for each target: (1) a file 

containing the raw, reduced distance data and associated 

changes, (2) a file containing vertical and horizontal 

components and daily rates of change calculated from the 

slope distance and zenith angle, and (3) a file containing 

calculated coordinates, elevations, and changes 

associated with these data. It was necessary to create 

three files, because all of the data would not fit into one 

or two large files. 

Plotting the data presented a totally different 

problem. When we converted all data processing to 

computers, there was no commercially available package 

that would plot time-series data with adequate tick marks 

and labeling of days, months, or years. An in-house 

plotting program was written to plot slope-distance 

changes versus time (fig. 6.15). As other types of data 

were produced, plotting programs were adapted to 

66 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
accommodate them. The coordinate system has allowed 

stations to be plotted on digitized maps (fig. 6.9). All the 

stations were located from coordinates calculated from 

field observations. 

We have learned to keep good field notes on any 

changes that are made to instrument and target stations. 

In later years, these notes will become invaluable. If data 

are to be stored in computers, we try to add all or as 

many of the field notes as possible. The better the 

documentation, the easier it will be for the person who 

will have the responsibility to write a report. 

DISCUSSION AND SUMMARY 

The ability to reoccupy stations is a critical factor 

in determining the equipment used and precision 

required. The accuracy of the EDM should meet the 

resolution that the monitoring requires. It is better and 

more versatile to use a more accurate EDM. If targets are 

to be measured frequently, the precision need not be as 

tight as for those stations that are measured only 

periodical!y. Frequent measurements allow trends in the 

data to be established and lessen the effect of a few bad 

points. These guidelines are important to follow, because 

deviations from ongoing trends normally imply changes 

in the volcano. We never trust single measurements but 

instead always look for patterns and remeasure as soon 

as possible. 

Consistency is important for methods used in the 

field and in the office. We have learned, the hard way, 

that inconsistency will ultimately lead to more work. Try 

to make methods simple without compromising the data 

that to be collected. 

Once equipment and field procedures are 

established, data processing and storing become 

important. For historical and research purposes, we 

advise keeping complete field notes on any changes and 

anything different that was done in the field. We also 

advise establishing a method for data reduction that fits 

the needs of the office and that people are comfortable 

with. It is a waste of time and money to purchase or 

design powerful software when all that is needed is to 

reduce a slope distance. Perhaps in the future these 

powerful software packages will be useful, but for 

inexperienced workers they can lead to confusion and 

frustration. 

Establishing and implementing a small deformation 

network has been successful in the crater and on the 

dome of Mount St. Helens. Techniques ranging from 

simple measurements with a steel tape to networks using 

electronic distance-measuring instruments have helped to 

predict all but one of the dome-building episodes. The 

above description of the methods used at Mount St. 

Helens is intended to be used as a guide. Every volcano 

will have certain aspects that are unique to it. We hope 

that our descriptions are general enough to be helpful 

and that they can be changed and adapted to meet the 

different needs of researchers at other volcanoes. 

ACKNOWLEDGMENTS 

We thank all the people who helped in installing 

and measuring the crater networks. We especially thank 

Bill Chadwick, John Ewert, Christina Heliker, and Peter 

Otway (New Zealand DSIR) for spending long hours 

helping to design and install many of the methods that 

have become so important to our project. We also thank 

Robin Holcomb, who was instrumental in the leveling 

effort. 

REFERENCES CITED 

Brinker, R.C., and Minnick. Roy, eds., 1987, The surveying 

handbook: New York. Van Nostrand Reinhold, 1270 p. 

Chadwick. W.W., Jr., and Swanson, D.A., 1989, Thrust faults 

and related structures in the crater floor of Mount St. 

Helens volcano, Washington: Geological Society of 

America Bulletin, v. 101, no. 12, p. 1507-1519. 

Chadwick. W.W., Jr., Swanson, D.A., lwatsubo, E.Y., Heliker, 

C.C., and Leighley, T.A., 1983, Deformation monitoring at 

Mount St. Helens in 1981 and 1982: Science, v. 221, no. 

4618, p. 1378-1380. 

Kinoshita, W.T., Swanson, D.A., and Jackson, D.B., 1974, The 

measurements of crustal deformation related to volcanic 

activity at Kilauea volcano, Hawaii, in Civetta, L., 

Gasparini, P., Luongo, G., and Rapolla, A., eds., Physical 

volcanology: Amsterdam, Elsevier, p. 87-115. 

Lipman, P.W., Moore, J.G., and Swanson, D.A., 1981, Bulging 

of the north flank before May 18 eruption-geodetic data, in 

Lipman, P.W., and Mullineaux, D.R., eds., The 1980 

eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 143-155. 

Shafer, L.M., 1987, Triangulation, in Brinker, R.C., and 

Minnick, Roy, eds., The surveying handbook: New York, 

Van Nostrand Reinhold, p. 296-339. 

Sturgess, B.N., and Carey, F.T., 1987, Trilateration, in Brinker, 

R.C. and Minnick. Roy, eds., The surveying handbook: 

Van Nostrand Reinhold, New York, p. 340-389. 

Swanson, D.A., Casadevall, T.J., Dzurisin, D., Malone, S.D., 

Newhall, C.G., and Weaver, C.S., 1983, Predicting 

eruptions at Mount St. Helens, June 1980 through 

December 1982: Science, v. 221, p. 1369-1376. 

Swanson, D.A., Lipman, P.W., Moore, J.G., Heliker, C.C., and 

Yamashita, K.M., 1981, Geodetic monitoring after the May 

18 eruption, in .Lipman, P.W., and Mullineaux, D.R., eds., 

The 1980 eruptions of Mount St. Helens, Washington: 

U.S. Geological Survey Professional Paper 1250, p. 

157-168. 

6. Methods Used to Monitor Deformation of the Crater Floor and Lava Dome at Mount St. Helens, Washington 67 
Waitt, R.B., Pierson, T.C., MacLeod, N.S., Janda, R.J., Voight, 

Barry, and Holcomb, R.T., 1983, Eruption-triggere4 

avalanche, flood, and lahar at Mount St. Helens-Effects 

of winter snowpack: Science, v. 221, p. 1394-1397. 

Yokoyama, I., Yamashita, H., Watanabe, H., and Okada, Hm., 

1981, Geophysical characteristics of dacite 

volcanism-The 1977-1978 eruption of Usu volcano: 

Journal of Volcanological and Geothermal Research, v. 9, 

p. 335-358. 

68 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
7. Electronic Tiltmeters for Volcano Monitoring: 

Lessons from Mount St. Helens 

By Daniel Dzurisin 

Abstract 

The recent proliferation of electronic tiltmeters 

suitable for volcano monitoring presents both an 

opportunity and a challenge to volcanologists charged 

with providing timely warnings of impending eruptions. On 

one hand, tiltmeters are valuable monitoring tools 

because they are widely available, relatively inexpensive, 

easy to install and operate, and capable of continuously 

monitoring ground deformation in remote and hazardous 

areas. On the other hand, tiltmeters have several 

characteristics that can introduce misinformation and 

confusion into real or perceived volcanic emergencies 

unless considerable caution is used in the installation and 

maintenance of the tiltmeters and in the interpretation of 

tiltmeter data. Most tiltmeters are short-baseline 

instruments that measure tilt over horizontal dimensions 

of a few centimeters to a few meters-several orders of 

magnitude smaller than the scale of ground deformation 

associated with most volcanic activity. Local effects 

caused by such factors as ground slope or soil 

characteristics can overwhelm larger-scale effects of 

volcanic deformation. Therefore, it is imperative to verify 

tiltmeter data and relate it to the larger-scale deformation 

field by making geodetic measurements that span the 

entire area of interest. Other characteristics of tiltmeters 

that affect their applicability to volcano monitoring include 

secular drift, trade-offs between resolution, repeatability, 

and range, and susceptibility to environmental factors 

such as temperature, site preparation, precipitation, 

freeze-thaw cycles, and ground-water changes. Coupled 

with the fact that the pattern of ground tilt associated with 

volcanic unrest can be extremely variable in space and 

time, both at any one volcano and from one volcano to 

another, these factors make the interpretation of tiltmeter 

data extremely difficult in a crisis situation. 

Several steps can be taken to mitigate these 

problems. First, tiltmeters with specifications (resolution, 

repeatability, range, drift characteristics) that are 

appropriate for the specific application should be 

selected. Ideally, the magnitude and dimension of ground 

deformation should be determined by geodetic 

measurements before tiltmeters are installed. Second, all 

reasonable effort should be made to couple tiltmeters to 

the ground in a stable and meaningful way, and to shield 

them from extraneous environmental effects. The best 

approach to each installation depends on the resources 

available, logistical constraints, and field conditions, but in 

general subsurface installations in bedrock or 

well-drained sediments are preferable. In order to 

determine both the spatial and temporal patterns of 

ground tilting, no fewer than three tiltmeter stations 

should be established in a network that spans the area of 

concern. Stations on the upper flanks of a restless 

volcano or elsewhere where large tilts are expected 

should take precedence, because at such sites 

environmental factors are less likely to overwhelm 

long-wavelength crustal tilts caused by the unrest. If 

possible, two tiltmeters should be installed at each station 

for redundancy, and any difference between the two tilt 

records must be weighed heavily in assessing the 

reliability of the measurements. A third important step is 

to acquire enough baseline data from each tiltmeter 

station to fully characterize its response to environmental 

factors, so that extraneous effects can be recognized and 

factored into interpretations of volcanic unrest. A fourth 

essential step is to interpret data from each tiltmeter only 

within the context of all available information that 

addresses the current state of the volcano, the trend of 

recent changes, and the range of likely outcomes of the 

unrest. This context should include (1) knowledge of the 

volcano's recent eruptive history based on historical 

records and stratigraphic studies, (2) the temporal and 

spatial patterns of ground deformation determined from 

tiltmeter data, leveling surveys, EDM measurements, or 

other geodetic techniques, and (3) other monitoring 

information including visual observations, seismic 

information, and volcanic gas measurements. Toward this 

end, collective sites should be chosen for various types of 

measurements in order to develop a common data base 

for comparison. 

Experience shows that the same tilt pattern 

measured by different methods at different places or at 

different times can have vastly different implications for 

the outcome of volcanic unrest. In view of the high stakes 

involved in predicting eruptions of explosive volcanoes in 

populated areas, it is incumbent on volcanologists to 

base their predictions on the most complete and reliable 

data set possible. Electronic tiltmeters can make a unique 

and valuable contribution to such a data set, but only if 

7. Electronic Tlltmeters for Volcano Monitoring: Lessons from Mount St. Helens 69 
they are installed, operated, and interpreted in a careful 

and responsible manner. 

INTRODUCTION 

During the past decade, several manufacturers have 

introduced relatively inexpensive electronic tiltmeters 

that are suitable for monitoring ground deformation 

associated with volcanic unrest. When combined with 

modem radio telemetry and recording systems, these 

instruments are capable of recording ground tilts at 

remote or hazardous sites on a virtually continuous basis, 

thereby complementing various types of geodetic surveys 

and contributing to eruption predictions (for example, 

Dzurisin and others, 1983a). My personal experience 

with electronic tiltmeters at the Hawaiian Volcano 

Observatory (HVO) and the Cascades Volcano 

Observatory (CVO) illustrates the utility of electronic 

tiltmeters for monitoring volcanic unrest, but also 

highlights several pitfalls that deserve careful 

consideration. Recently, I have grown increasingly 

concerned that the easy availability of inexpensive 

tiltmeters plus the widely publicized fact that tiltmeter 

data have contributed to successful predictions of 

dome-building episodes at Mount St. Helens (Swanson 

and others, 1983; Dzurisin and others, 1983a) might set 

the stage for inappropriate applications of tiltmeters at 

other hazardous volcanoes, with potentially tragic 

consequences. 

My goal here is to encourage the prudent use of 

tiltmeters by describing their strengths and weaknesses as 

revealed by experience at HVO and CVO from 1976 to 

1989, with emphasis on applications at Mount St. Helens 

since 1980. This paper is more nearly a scientific essay 

than a research report. Its recommendations are 

supported by a combination of data and experience, but 

most of them are generalizations that must be adapted to 

specific situations. Nonetheless, I believe its message is 

an important one for volcanologists attempting to deal 

with hazardous volcanoes: Tiltmeters are valuable tools 

when used properly, but like any tool they can be 

dangerous and should be handled with care. 

Types of Electronic Tlltmeters 

Tiltmeters are available in assorted shapes, sizes, 

and price ranges that are suited to a wide variety of 

applications. They range from relatively simple designs 

used as security devices in executive briefcases to 

sophisticated instruments capable of recording the effects 

of solid Earth tides or providing inertial guidance for 

aircraft and missiles. For volcano applications, the most 

common types use a fluid to measure tilt changes along a 

baseline that ranges from a few centimeters to a few 

meters in length. 

Water-tube tiltmeters are fluid-filled U-shaped 

tubes, typically a few meters long. As the tube tilts, the 

fluid (usually water) rises relative to one end of the tube 

and drops relative to the other end. The change in water 

level can be measured manually or sensed electronically 

and converted to an equivalent tilt change. A 

state-of-the-art, half-filled, water-tube tiltmeter 

developed by R. Bilham features a 1-km baseline, 

extteme precision (resolution of one part in 10 ~, and 

very low long-term drift rate (Agnew, 1986). It is 

relatively easy to install and has been used successfully 

to monitor uplift of the resurgent dome at Long Valley 

caldera in east-central California. 

Bubble tiltmeters use the motion of a bubble in a 

fluid to accomplish the same task. Typically, a bubble a 

few millimeters in diameter floats in an electrolytic fluid 

sealed in a vial a few centimeters across. Motion of the 

bubble caused by tilting of the vial is measured 

electronically by electrodes that project into the fluid 

(Westphal and others, 1983). Tilt along a single axis can 

be measured using a cylindrical vial, and tilt along two 

axes can be measured using a disk -shaped vial. 

Mercury tiltmeters employ a pool of mercury as a 

capacitance plate in an LC resonant bridge circuit; tilting 

of the opposite plate relative to the surface of the 

mercury causes a c-apacitance change that can be 

measured electronically. 

Because the bubble tiltmeter is the most commonly 

used type at CVO and other volcano observatories, the 

following sections were written with bubble tiltmeters in 

mind. However, most of the ideas apply equally well to 

other types, including more specialized designs not 

mentioned above. 

Choosing an Appropriate Tlltmeter 

Resolution, Repeatability, Range, and Cost 

The selection of a tiltmeter inevitably will involve 

trade-offs among resolution, repeatability, range, and 

cost. The resolution of a tilt sensor is the smallest tilt 

increment that produces a measurable change in sensor 

output. Repeatability measures the range of outputs 

produced by the same input following tilt excursions; it 

differs from resolution because most tilt sensors are 

subject to hysteresis. Thus, a tilt sensor might be capable 

of resolving tilt changes as small as 0.1 microradian, but 

with a repeatability of only 1.0 microradian. The range 

of a tilt sensor is the maximum tilt change that it can 

measure reliably; a related concept is linearity, which 

measures a sensor's non proportional response to tilt 

changes near the limits of its range. Most tiltmeters used 

70 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
for volcano monitoring have ranges of 1 to 60 degrees 

and linearities of 1 to 5 percent. In general, better 

resolution correlates with narrower range, better 

linearity, and increased cost 

The most precise tiltmeter (that is, the one with the 

smallest resolution and repeatability) that is affordable 

may not be the best choice, because better resolution and 

repeatability generally mean narrower range. It 

accomplishes little to resolve a tilt change of 0.1 

microradians through a range of 100 microradians if 

diurnal fluctuations or volcanic unrest cause tilts of 

several hundred microradians or more (1 microradian = 

0.206 arc-seconds= 5.730 x 10-s degrees). Conversely, it 

is futile to monitor microradian-scale tilt changes with a 

tiltmeter designed to resolve changes of a few 

arc-seconds through a range of several degrees. 

Ideally, the magnitude and dimension of ground 

deformation should be determined first by surveying 

techniques such as ttilateration or leveling, as discussed 

elsewhere in this volume. This is not always feasible or 

advisable in a volcanic emergency. In such cases, I 

recommend tiltmeters with at least 10-microradian 

resolution and repeatability over as wide a range as 

possible (usually a few thousand microradians). 

Experience shows that most volcanic eruptions cause tilts 

of at least tens of microradians within a few kilometers 

of the vent, so sub-microradian resolution is not required. 

This is particularly true if the tiltmeter installation is 

subject to diurnal tilt changes larger than 1 microradian, 

which is common. An exception might be a deep 

installation (deeper than 2 m) relatively far from the 

vent, where diurnal effects are negligible and 

volcano-related tilts are expected to be small. Such 

installations are particularly useful for monitoring 

deep-seated magma reservoirs, which can cause 

relatively small tilts over broad areas. For such 

applications, choose a tiltmeter capable of resolving 1 

microradian or less, and bury it at least 2 m deep to 

reduce diurnal and seasonal effects. 

More likely to be encountered near the vent of a 

restless volcano are very large tilts that are detectable 

with virtually any commercially available tiltmeter. In 

this case, range is more important than resolution. 

During several dome-building episodes at Mount St. 

Helens, tilts greater than 10,000 microradians have been 

measured on the flanks of the lava dome, while on the 

crater floor a few hundred meters away, measured tilts 

have been less than 1,000 microradians (figs. 7.1, 7.2). 

On the dome, most tiltmeters with microradian-scale 

resolution would be beyond their useful ranges during 

the most critical part of the precursory period before 

dome-building episodes, while on the crater floor 

tiltmeters with very wide ranges would be incapable of 

resolving small tilt changes to provide a useful record of 

the event 

··················· Figure 7.1. A, Generalized outline of Mount St. Helens 

with locations of seismometers mentioned in text: YELS, 

Yellow Rock; GRDN, Garden; SHWS, St. Helens West; 

EDMS, East Dome. Approximate outline of lava dome is 

indicated by circular contour at center. Contours are in 

feet. 8, Generalized outline of lava dome and crater floor 

at Mount St. Helens, with locations of EDM stations 

(triangles), tiltmeters (squares), a seismometer (circle), and 

a gas sensor (x) mentioned in text: TNKR, Tinker EDM 

station; SAUN, Sauna tiltmeter; GRDN, Garden 

seismometer and tiltmeter (located together and indicated by 

circle only); RUBY, Ruby EDM station and tiltmeter (located 

together and indicated by triangle only); FAMS, Famous 

tiltmeter; AMOS, Amos EDM station; HOSR, Haser EDM 

station; LONE, Lonesome gas sensor; OOPS, Oops 

tiltmeter; LOVE, Lovelorn tiltmeter; RAZA, Raza tiltmeter; 

SWLP, Sweet tiltmeter; HIKE, Hike tiltmeter. The October 

1986 lobe is outlined by several closed contours midway 

between HIKE and LONE; bold dotted line indicates 

approximate base of dome. Contours are in meters. 

7. Electronic Tlltmetera for Volcano Monitoring: Lessons from Mount St. Helena 71 
At CVO, we solve this problem by installing two 

single-axis tiltmeters at each station on the dome: a 

high-gain instrument with a resolution of about 5 

microradians and range of ±1 degree (17 ,450 

microradians; approximate cost $2,000), and a low-gain 

instrument with a resolution of about 1 arc-minute (350 

microradians) and a range of about 1 radian (±60 

degrees) (cost, $100). We use single-axis tiltmeters to 

reduce cost and because the most important component 

of tilt for purposes of eruption prediction and hazard 

assessment at Mount SL Helens is radially outward from 

the center of the dome. The high-gain insttuments are 

sensitive enough to detect precursory swelling of the 

dome several weeks before the culmination of 

dome-building episodes, but they are sometimes beyond 

their useful range during the last few days of most 

intense activity. By that time, the low-gain insttuments 

are capable of tracking the intense deformation that 

occurs just before magma reaches the surface of the 

dome. Combining data from both types of tiltmeters 

provides a relatively complete record of ground tilt from 

the inception of precursory activity through the most 

intense period of dome building and, unless the station is 

destroyed by explosions or rockfalls, during the period of 

structural adjustment to the newly extruded lobe (fig. 

7.3). 

On the crater floor near the base of the dome, 

ground tilts associated with dome-building episodes are 

typically one to three orders of magnitude smaller than 

on the dome itself. Accordingly, we install two-axis 

16,000 Station Ruby, radial tilt J- 12,000 

(/') 8,000 z 

<( 

0 4,000 

<( 

a: 

0 

a: 

0 

~ 

~ 400 

t-=' 

....J 300 i= 

200 

o~~~~~~~~~~~~~~~~~ 

MAY 2 3 4 5 6 7 8 9 1 0 

START DAY IS MAY 1, 1986 G.M.T. -JULIAN DAY 121 

Figure 7.2. Tilt records from stations on north flank of 

dome (top) and crater floor north of dome (bottom) for 

May 1-10, 1986, which included the May 1986 

dome-building episode. Net tilt change on dome was 

more than 40 times as large as that on crater floor, 

although stations were only about 300 m apart. Arrows 

indicate onset of extrusion. 

tilbneters on the crater floor that are more sensitive than 

the instruments used on the dome, and we attempt to 

shield them more thoroughly from environmental effects 

(see below). Close to the dome (within 700 m of the 

vent), we use sensors with ±1.0 microradian resolution 

and 10 degree range; farther from the dome, we use 

models with 0.1 microradian resolution and ±1 degree 

range. The approximate cost for each model is $3,000. 

Two-axis tiltmeters provide directional 

information that helps to locate the deformation source, 

which in this case is consistently beneath the center of 

A f High gain 

1,200 ~ 

9oo r 

~ 600~ 

<( 

0 

<( 

a: 

0 

a: 

0 

~ E Low gam '"'''T"'"f"'"- l ~ 1.200 r l 

~ 900 [ jl_ __ J Jl--l 

300 

600~ ~---l[- -llllli[-l I~ -~ ~ I I I 

Or I . I. "' _l__LL_u_j_,_.c_J~~L.J._;_j__~-'--LL..L.L OCT 2 3 4 5 6 7 8 9 1 0 

START DAY IS OCT 1, 1986 G.M.T.- JULIAN DAY 274 

Figure 7.3. A, Records from a high-gain (top) and a 

low-gain (bottom) tiltmeter at station Sweet on west flank 

of dome for October 1-10, 1986. Difference in gains 

between the two tiltmeters was approximately a factor of 

100 (3.6 microradians/count for the high-gain tiltmeter 

versus 355 microradians/count for the low-gain tiltmeter). 

Diurnal1 pattern in high-gain record is caused primarily by 

temperature fluctuations. Discrete steps in low-gain 

record occur because tiltmeter does not respond to tilt 

changes smaller than 355 microradians. More than two 

weeks before beginning of extrusion on October 21-22, 

precursory tilt was well resolved by high-gain instrument 

but barely detected by low-gain instrument. 8, Records 

from the same tiltmeters for October 11-20, 1986. The 

two records are similar during this period of slowly 

accelerating deformation of dome. Both tiltmeters were 

offset by similar amounts by a magnitude 3 earthquake 

on October 20 (arrows). C, Records from the two 

tiltmeters at Sweet for October 21-22, 1986. Note 

differing vertical scales. High-gain tiltmeter went off-scale 

early on October 21 after recording more than 20,000 

microradians of outward tilt. Low-gain tiltmeter continued 

to operate for an additional 18 hours and recorded about 

1.5 radians (87 degrees!) of outward tilt before going 

off-scale. Station was destroyed by a rockfall about 9 

hours later. Extrusion probably began about 0800 G.m.t. 

on October 22 (arrows). 

72 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
the dome. However, this is not always clear from the 

tiltmeter data alone, because local site effects often 

complicate the pattern of measured tilt changes. In 

theory it should also be possible to determine the depth 

of the pressure source using an array of several tiltmeters 

installed at various distances from the dome, but in 

practice this has not worked well at Mount SL Helens for 

at least two reasons. First, tilts associated with 

dome-building episodes are small enough on the crater 

floor to be partly obscured by spurious environmental 

effects. Second, except possibly for the very early stage 

of precursory activity, deformation of the dome and 

crater floor is strongly inelastic, anisotropic, and 

therefore difficult to model. 

8 

~ High gain 

16,000 ~ 

12,000 

~ 8,000 <t 

0 4,000 E- <t '- § t,---;-~,- I I 

a: 

() 

~ ~:~~~~~~~~,~~~~~~~~ 

~ 16 000 ~. Low gain Earthquake'~ 

r: ' ~ ....J t 

t= 12,000 t 

8,ooo=-

4,QQQ, -~.---cr __ ,., ,~r-.-r :--~"'"'·~ - - - -·-·,C' ~ 

0 ~l' I ~LL,~_L..'._L-: 

11 1 2 1 3 14 15 1 6 17 18 1 9 20 

START DAY IS OCT 11, 1986 G.M.T.- JULIAN DAY 284 

c e---r-----r- . ' 

[ High gain 

~20,000't ~ ~ ~-----~­

~ 19,000 ~ a: ~ 

0 ' a: 18,000t I () ~I 

~ 17,000 f 

en 

z 

<t 

0 

<t 

a: 

16,000 L_~---'------'--'---

Low gain 

1.6 ~ 

1.2 :--

0.8 =-

0.4 ~-

/ 

0 -~ 

t 

Eruption 

210400 1200 2000 22 0400 1200 2000 

START DAY IS OCT 21, 1986 G.M.T.- JULIAN DAY 294 

Figure 7.3. Continued 

Platform Versus Borehole Models 

It is worth noting that platform and borehole 

tiltmeters measure physically different quantities, 

although for most volcano monitoring applications the 

difference is negligible (C.E. Mortensen, written 

commun., 1989). If x andy are the horizontal axes, z is 

the vertical axis, uz and u1 are the horizontal components 

of displacement, and u, is the vertical component of 

displacement, then the horizontal tilts measured by a 

platform tiltmeter are given by 

au, d dllr 

ax an aJ' 

whereas the vertical tilts measured by a borehole 

tiltmeter are given by 

dUz and ~ az az 

The horizontal and vertical tilts will differ if there 

is a significant stress gradient within the rock volume 

sensed by the tiltmeter, owing to deformation of the 

rock volume itself. This is particularly true if 

heterogeneities are present. However, for the case of 

zero stress, as at or near the surface where there can be 

no tractions, these tilts are equivalent 

Another consideration is that the platform 

configuration is first-order sensitive to temperature, 

while in the borehole configuration this is a 

second-order effect (C.E. Mortensen, written commun., 

1989). This is because the platform configuration relies 

on mechanical elements such as screws for coarse 

leveling of the instrument, and the lengths of these 

elements are directly proportional to temperature. 

Therefore, any initial length difference in these elements 

will cause a first-order response to temperature 

fluctuations. In contrast, the borehole configuration is 

radially symmetric and therefore temperature 

fluctuations cause volumetric changes within the 

tiltmeter housing but no first-order tilt response 

(although the bubble itself is still temperature sensitive). 

In addition, borehole tiltmeters typically are installed 

deeper than platform models, where temperature 

fluctuations are smaller. To reduce temperature effects, 

platform tiltmeters can be temperature-compensated, or 

various temperature stabilizing approaches can be 

adopted. Nonetheless, for applications in which 

temperature sensitivity is a critical issue, the borehole 

configuration is preferred. In most applications at 

restless volcanoes, this is not a limiting factor, and the 

choice between borehole and platform configurations 

can be based on other considerations. 

In my experience, the choice depends primarily on 

local conditions and on such factors as relative 

availability, price, and ease of installation. Borehole 

models are used more widely in Hawaii, while platform 

models are used almost exclusively at Mount St. Helens. 

7. Electronic Tlltmeters for Volcano Monitoring: Lessons from Mount St. Helens 73 
This is partly a historical accident and partly a 

consequence of differing local conditions. In Hawaii, the 

deformation field is usually broad enough that tiltmeters 

can be located in relatively safe areas that survive several 

eruptions. At Mount St. Helens, in contrast, measurable 

deformation occurs only on the lava dome and 

surrounding crater floor. Therefore, tiltmeters must be 

placed close to the vent in hazardous areas in order to be 

useful. As a result, most tiltmeter stations survive only a 

few eruptive episodes. Given the short life expectancy of 

tiltmeters at St. Helens, we have opted for platform 

models that are easier to install and adjust, and also 

generally less expensive, than borehole models. 

Another consideration is the nature of surficial 

material at prospective tiltmeter sites. In Hawaii, 

borehole tiltmeters can be installed in young basalt flows 

or soil, whereas at Mount St. Helens the crater floor is 

composed of heterogeneous debris from eruptions, lahars, 

and rockfalls. Digging or angering in such material is 

very difficult. In addition, for several years after the 

1980 eruptions, this material was still too hot for 

subsurface tiltmeter installations. Therefore, we decided 

to use platform models installed at the ground surface. 

More recently, a lens of permanent snow and ice 

intercalated with rockfall talus has accumulated on the 

south crater floor, making that area unsuitable for surface 

installations. Experience at Long Valley caldera shows 

that borehole tiltmeters work very well under snowpack 

(Mortensen and Hopkins, 1987), but such installations 

are not feasible in the crater at Mount St. Helens, where 

the lens of snow and talus is locally more than 100 m 

thick and growing. 

In my experience, the difference between platform 

and borehole tiltmeters is less important than the manner 

in which each type is installed and coupled to the 

ground. A carefully installed platform tiltmeter that is 

shielded from most environmental effects and well 

coupled to the volcano-related deformation field is better 

than a poorly installed borehole tiltmeter, as vice versa. 

Single-Axis Versus Two-Axis Types 

Two-axis tiltmeters provide useful information on 

the location of the deformation source, and therefore are 

preferable to single-axis types in most cases. A two-axis 

tiltmeter determines the vector to the deformation source, 

whereas a single-axis tiltmeter measures only one 

component of the tilt vector. Measurements from several 

two-axis instruments indicating tilt vectors that are 

demonstrably radial to a possible vent might have very 

different implications for volcano hazards than 

measurements from single-axis instruments that are 

sensitive only to the radial tilt component. In the first 

case, the source of the deformation is indicated to be 

near the vent, but in the second case the source is 

unknown and might be completely unrelated to the vent 

(perhaps each station is unstable and tilting in some 

random direction, and the radial tilt components 

fortuitously defme a consistent pattern). Redundancy is 

another advantage of two-axis tiltmeters, which use a 

separate sensor and amplifier for each axis (useful 

information can be obtained even if one axis fails). To 

take advantage of this feature, the sensor axes can be 

rotated approximately 45 degrees from the radial 

direction, so both axes are sensitive to radial tilt changes. 

A possible exception to the general preference for 

two-axis tiltmeters is the case of a known vent such as 

the lava dome at Mount St. Helens, where in order to 

reduce cost we have opted for single-axis tiltmeters 

oriented radial to the center of the dome. The same 

compromise might be considered for monitoring the 

flanks of a restless stratovolcano, where the probable 

location of the vent is known. However, the increased 

cost of two-axis instruments is usually outweighed by the 

additional information they provide. 

When Not to Use Tlltmeters 

It is important to remember that electronic 

tiltmeters generate a nearly continuous stream of data 

that must be recorded, processed, interpreted, and 

archived-and that recording, processing, interpreting, 

and archiving zeroes is a very time-consuming and 

unrewarding task. In addition, routine maintenance and 

repair of tiltmeters (and associated telemetry or data 

logging systems) can be an onerous job, particularly 

during periods of bad weather when access to the 

stations is difficult or impossible. This is especially true 

at remote volcanoes, where access may be difficult even 

under the best conditions. Accordingly, it is seldom a 

wise use of limited resources to install tiltmeters on 

dormant or long-inactive volcanoes in anticipation of the 

time when the volcano might become restless. In such 

cases it is usually adequate to monitor earthquake 

activity with a modest seismic network and to make 

repeated geodetic surveys on the volcano, either 

according to a predetermined schedule or as dictated by 

seismicity or other signs of unrest. 

This is the approach that CVO has taken to 

monitoring the currently dormant volcanoes of the 

Cascade Range (that is, all except for Mount St. 

Helens). Earthquake activity near each of these 

potentially active cones is monitored by regional seismic 

networks that are capable of . detecting and locating 

earthquakes larger than some threshold magnitude that 

varies from volcano to volcano (generally magnitude 

2.5). In addition, CVO has installed baseline geodetic 

networks on most of the potentially active cones 

(Chadwick and others, 1985; Dzurisin and others, 1982, 

1983b; Iwatsubo and others, 1988; Yamashita and 

74 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Doukas, 1987; Iwatsubo and Swanson, chapter 10). 

These networks will be remeasured every few years or as 

conditions warrant We made a conscious decision not 

to install tiltmeters or other continuously recording 

devices, because in view of the long recurrence intervals 

between past eruptions (centuries to millennia), we 

judged the cost of installation and maintenance, both in 

dollars and manpower, to be too high to justify the 

effort 

Even if tiltmeters are affordable, they are usually a 

poor choice for monitoring dormant volcanoes. This is 

because, although tiltmeters are well suited to measuring 

short-term tilts, they also are subject to long-term drift 

from a variety of sources, which can confuse 

assessments of volcanic unrest The causes of long-term 

drift include aging of electronic components, 

adjustments of the ground to the installation process or 

to the presence of the station, and seasonal or secular 

environmental factors (fig. 7 .4). Especially over time 

scales longer than a few months, tilt changes indicated 

by electronic tiltmeters must be verified by geodetic 

surveys. Therefore, continuous seismic monitoring and 

periodic geodetic surveys are adequate at most dormant 

volcanoes; tiltmeters should be considered only after the 

beginning of unrest, and then only to monitor relatively 

short-term changes. 

C/) t" , ' , ' I I----,----,-- I -,--- ' 1 ~ 600 ~-Tangential Tilt . -

~ 450 ~- ~"'"~ -

~ 300 ~ ~-~ ~\ }'\ ?1 

~ 15~L- I' I 'I i ~~~~~~~~~I' j 

Figure 7.4. Tilt and temperature records for 1988 from a 

tiltmeter mounted on a large boulder on crater floor north 

of dome. Note relatively large reversible tilt change 

(several hundred microradians) caused by precipitation 

and snow loading while station was buried under snow 

from January through early July. When station emerged 

from under snow in mid-July, long-period tilt changes 

virtually stopped and diurnal effects became apparent. 

Types of Tlltmeter Installations 

General Considerations 

In order to provide useful information for volcano 

monitoring, tiltmeters must be coupled to the ground in 

such a way as to provide a representative measure of the 

spatially-averaged deformation field in the surrounding 

area. Although this problem may seem straightforward at 

frrst, in practice it is usually the single greatest obstacle 

that must be overcome in order to obtain meaningful 

tiltmeter data. Many sophisticated and expensive 

tiltmeters have been operated at poorly designed 

installations, recording countless bits of precise but 

useless information on site-specific ground movements. 

Even when considerable time, money, and effort are 

invested in more elaborate tiltmeter installations, some of 

them inevitably prove to be inadequate. Experience 

shows that no single recipe can be trusted to consistently 

produce high-quality tiltmeter installations: What works 

well in one place or at one time may fail miserably in 

another. Only a long record of high-quality baseline data 

can demonstrate the reliability of a given installation. 

Theoretical discussions of potential topographic 

and thermal effects on tiltmeters are given by Harrison 

(1976) and Harrison and Herbst (1977). Those authors 

argue that, in order to reduce such effects, tiltmeters 

should be installed as deep as possible at sites that are 

topographically symmetric. 

In my experience, healthy doses of common sense 

and perspiration provide a good start toward effective 

tiltmeter installations. The more effort expended on the 

installation, the better it is likely to be. There is no 

substitute for hard work, whether it is spent digging a 

hole to shield the tiltmeter from environmental effects or 

pouring concrete to produce a massive foundation. In 

general, the deeper the installation, the better. 

High-precision borehole tiltmeters installed 30-100 m 

beneath the surface will resolve the daily solid-earth 

tides (a few parts in 107

) with virtually no other diurnal 

effects and seasonal effects of only a few microradians 

(Meertens, 1987; Meertens and others, 1989). 

Near-surface installations are quicker and easier, 

but they are subject to large environmental effects. 

Diurnal tilts of several hundred microradians are 

common at surface installations exposed to outdoor 

temperature fluctuations (figs. 7 .5, 7 .6). For comparison, 

the magnitude of diurnal tilt caused by solid-earth tides 

is approximately 0.1 microradians at temperate latitudes. 

Indoor installations below ground level are a good choice 

if the area is not subject to heavy vehicular or foot 

traffic. In such cases, the tiltmeter should be 

mechanically isolated from the surrounding building, 

which may deform to a surprising degree in response to 

thermoelastic effects. If drilling or augering is an option, 

7. Electronic Tlltmeters for Volcano Monitoring: Lessons from Mount St. Helens 75 
the tiltmeter should be placed at least 2 m below the 

ground surface and the hole should be backfilled to 

insulate the tiltmeter from diurnal and seasonal 

temperature changes. I prefer installations in bedrock 

wherever possible, but another school of thought 

recommends well-drained sediments such as dry sand or 

gravel. In the latter case, choose an area that is relatively 

level and isolate the tiltmeter from at least the upper few 

decimeters of substrate, to minimize the effects of 

near-surface creep and freeze-thaw cycles. In practice, 

the choice is often dictated by local conditions: On many 

stratovolcanoes, exposed bedrock is scarce because most 

of the surface is covered by loose fragmental material. 

In all cases, be flexible. Choose what seems to be the 

best installation option in the area of interest and then 

make the best installation possible, given the time and 

resources available-then distrust the site until it proves 

itself to be reliable. 

Surface Installations 

The simplest approach is to install a platform 

tiltmeter on a boulder or concrete platform that is partly 

buried below ground level and seemingly well coupled to 

the ground. I prefer boulders that "ring" when struck 

with a hammer, but this test is arbitrary, depends on the 

~ 1 ,200 ~ R~d~i~l ;i,ll I ' ' ' I ' 

~ 900 

a: 

~ 600 

(.) 

~ 300 

C/) 

::::> 

U5 

_J 

w 

(.) 

C/) 

w 

w 

a: 

(9 

w 

Cl 

Figure 7.5. Tilt and temperature records for a 1 0-day 

period in September 1989 from station Oops, a surface 

tiltmeter installation on east flank of dome. Diurnal effects 

such as these can be modeled and removed from 

tiltmeter data, but it is better not to remove such effects 

and to compare tilt and temperature records visually. 

During quiescent periods, diurnal changes are welcome 

evidence that station is operating normally. Before most 

eruptions, diurnal effects are overwhelmed by precursory 

ground deformation as magma forces its way to the 

surface. 

nature of rock available, and is scant assurance of 

stability. This method was used with good results for 

several tiltmeter installations on the crater floor at Mount 

St Helens during the early 1980's, when dome-building 

episodes were frequent and the average lifetime of most 

stations was only a few months. Tiltmeters were placed 

on ceramic tiles cemented to large (up to several meters 

in diameter), relatively flat boulders that were mostly 

buried in ash. The meters were covered to protect them 

from ash and the elements, and the required batteries and 

telemetry system (Murray, chapter 2) were housed 

nearby in a sturdy container. Starting in 1985, similar 

installations were made in the summit area of the dome, 

where the tiltmeters were placed on boulders buried in 

tephra or on recently extruded bedrock. The main 

advantage of this method is ease of installation, but a 

major disadvantage is susceptibility to large 

environmental effects (typical diurnal variations are 

several hundred microradians). Accordingly, tuch 

installations are appropriate only when time is limited, 

other approaches are not feasible, or the expected tilt 

signal is very large. 

Another approach to installing tiltmeters that has 

worked well at Mount St Helens is to attach them to 

steep cliffs on the flanks of the lava dome. The initial 

motivation for such installations was to protect the 

meters from ballistic ejecta during small explosions on 

the dome. This was accomplished by bolting a sturdy 

C/) 

::::> 

U5 

...J 

w 

(.) 

C/) 

w 

w 

a: 

(.9 

w 

0 

Figure 7.6. Tilt and temperature records from station 

Lovelorn, located on a boulder that was partly embedded 

in tephra on surface of dome. Diurnal tilt changes of 

several hundred microradians per day occurred WhC3never 

station was exposed to atmosphere, but they virtually 

stopped while station was buried under snow during 

March 6-18 and March 24-26. 

76 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
aluminum housing to a cliff or by using a portable 

electric jackhammer and generator to make an opening in 

the cliff fa<:e large enough to house one high-gain and 

one low-gain tiltmeter. We continue to use this approach 

even when explosions are infrequent, because on the 

upper flank~ of the dome steep cliffs are more common 

than large, stable boulders suitable for the type of 

installation described above. Diurnal effects at cliff sites 

are comparable to those at boulder sites, typically a few 

hundred mic;roradians per day. 

Subsurface Installations 

Whenever possible, tiltmeters should be buried at 

least 2 m below the ground surface to reduce 

environmental effects. At bedrock sites, a relatively 

small diameter hole can be drilled for a borehole 

tiltmeter, or a larger hole can be drilled for a platform 

tiltmeter. At Long Valley caldera, we drilled five 

20-cm-diameter holes 5-10 m deep in bedrock, and 

cemented a platform tiltmeter to the bottom of each hole. 

Using a detachable metal rod, each tiltmeter was lowered 

to the bottom of a hole where a specially designed 

leveling mechanism at the base of the tiltmeter was 

embedded in wet cement. Coarse leveling was 

accomplishc~ by measuring the output of the tiltmeter at 

this stage and positioning it with the rod. After the 

cement hardened, final adjustments were made using a 

tool attached to the rod, while the bottom of the hole was 

illuminated with reflected sunlight. The procedure was 

tedious but usually required less than an hour. 

If bedrock is unavailable or badly fractured, holes 

should be drilled and cased in dry sandy sediment or 

tephra; cll!.y and water-saturated material should be 

avoided. To allow for removal or coarse releveling of the 

tiltmeter, it can be carefully packed near the bottom of 

the hole with well-sorted sand rather than being 

cemented permanently in place. 

Another approach to burying the tiltmeter in soil or 

fragmental material is to excavate a large hole, drive 

rods and JlOur concrete at the bottom, and install the 

tiltmeter on the concrete pad. One formula that has 

worked for us is to dig a cylindrical hole about 2m deep 

and 1 m in diameter, and case the lower 1 m with 

corrugated metal drain pipe. Drive at least three metal 

rods into soil at the bottom of the hole, at least 2 m deep 

but preferably until they can be driven no farther. Metal 

fence posts or sections of 3/8-inch or 1/2-inch-diameter 

rebar are widely available and suitable for this purpose. 

If necessary, cut the rods off 5-10 em above the bottom 

of the hole and encase them in a concrete pad at least 50 

em across and 10 em thick. Install the tiltmeter on the 

pad, cover it, and fill the drain pipe with insulating 

material that can be removed for easy maintenance of the 

tiltmeter (a sturdy bag full of plastic foam pellets works 

well). Cover the top of the drain pipe with a lid or plate 

and bury it under 1 m of soil. If vandalism is a problem, 

the entire installation except for a telemetry antenna can 

be buried or otherwise hidden from view. 

Installations similar to this have been used by staff 

of the Rabaul Volcano Observatory (RVO) in Papua 

New Guinea and by the USGS Volcano Crisis Assistance 

Team (VCAT; Lockhart, Murray, and Furukawa, chapter 

3) in Central America. Most of these stations have 

performed well after an initial settling period, but a few 

have had to be replaced owing to unacceptably large 

diurnal or ground water effects. 

At Mount St. Helens, CVO uses a similar approach 

to install tiltmeters and other monitoring instrumentation 

on the 1980 crater floor. Instead of drain pipe, we use 

rectangular fiberglass vaults about 3m x 1.5 m x 1.5 m 

high. The vaults are custom made for CVO at a cost of 

about $600 each; similar vaults can be fabricated 

wherever fiberglass technology is available. At Mount St. 

Helens, the vaults were buried by tunneling into a small 

ridge or levee, which has the advantage of providing 

easy access to the front entrance. The vaults are 

floorless, so rods can be driven and a concrete pad 

poured to accommodate tiltmeters, seismometers, and 

other instruments. By burying the tops of the vaults 

under 1-2 m of crater fill, we have reduced diurnal tilt 

changes from hundreds of microradians at surface 

installations to a few microradians in the vaults (fig. 7 .7), 

while typical seasonal effects are reduced from hundreds 

of microradians to tens of microradians. 

(/) 

z 

<( 

0 

<( 

a: 

0 

a: 

g 

~ 

i 

(f) : Temperature 

~ 13.30=--

~ 13.15 ~- r~~~- C/) ~ ~'-~_j \ ,r tH 13.00 ~ i~----,J " j~~~ 

a: ~ ~~ 

S112.85 ~ ''"-•cj 

O 1 2. 7 0 [\ I l-'--'-'--'-LLLJ.__J__LJ_--LLLLLJ_J_..L.l._LJ_j_J 

20 21 22 23 24 25 26 27 28 29 

START DAY IS SEP 20, 1989 G.M.T.- JULIAN DAY 263 

Figure 7.7. Tilt and temperature records from Garden 

Vault, an instrument enclosure buried about 1 m beneath 

crater floor north of dome. Note that diurnal effects are 

greatly reduced relative to those at typical surface 

installations (compare with figs. 7.5, 7.6). 

7. Electronic Tlltmeters for Volcano Monitoring: Lessons from Mount St. Helena 77 
Sources of Extraneous Tilt Signals 

Even if an appropriate tiltmeter has been selected 

and carefully installed, it still may be subject to several 

signal sources that are unrelated to volcanic activity. As 

noted above, diurnal and seasonal temperature changes 

can cause tilt signals as large as several hundred 

microradians in surface installations. Temperaturesensitive components in the tiltmeter or telemetry system 

may be a factor, but I suspect that thermal strains in the 

tiltmeter or its housing are the dominant cause in most 

cases. The best solution is to insulate the tiltmeter from 

thermal effects by burying it as deeply as possible. 

Thermoelastic strains caused by heating and cooling of 

the ground surface can be transmitted to the tiltmeter 

through the surrounding rock or soil mass and therefore 

are difficult to eliminate (C .E. Mortensen, written 

commun., 1989). In most volcano-monitoring 

applications, such effects are believed to be negligibly 

small. 

Another common source of extraneous tilt signals 

is precipitation. At Mount St. Helens, most of the annual 

precipitation falls as snow during the period from 

October to May, but brief periods of heavy precipitation 

rJ) 

~ 400 f Radial tilt, Sauna Rock/~ 

§5 200 ~ ~ ~ 300 r- , ~ 

5100 : ~~ -

~ 0 .r:. , .. '-"-~J 

rJ) 

~ ~~ t T~mp~r~t~r~. ~a~na :k ~~ • pv~;;;, 

~ -5 ~ I • , ' 1 ~ 

~ 87 FEB MAR APR MAY JUN JUL AUG SEP 

START DAY IS JAN 1, 1987 G.M.T.- JULIAN DAY 001 

Figure 7.8. Tilt. precipitation. and temperature records 

from January 1987 through September 1987. Tilt and 

temperature data are from a station on crater floor near 

north base of dome; precipitation record is from a station 

near Spirit Lake. about 1 0 km north of dome. Note 

relatively large tilt changes (1 00-200 microradians) during 

periods of heavy snowfall on February 1 and March 1-4. 

and also during spring snowmelt in May and early June. 

Starting in mid-June when station melted out of snow and 

precipitation became much less frequent. tilt record was 

relatively flat except for minor diurnal effects. 

occur throughout the year. Particularly during heavy 

snowfall or rainfall, tiltmeters in the crater commonly 

record rapid tilt changes (fig. 7 .8) that define no coherent 

pattern of tilting throughout the crater, even though the 

general direction of tilting at each site may be consistent 

from storm to storm. Stations on the dome are usually 

less susceptible than stations on the crater floor, 

presumably because the dome is mechanically stronger 

and better drained than crater-fill material. To minimize 

precipitation effects, install tiltmeters on bedrock or in 

well-drained granular material, and record precipitation 

at one or more tilbneter sites for comparison with the 

tilbneter data. 

Extraneous tilt signals also can be caused by 

changes in the local ground-water table during periods of 

heavy precipitation or rapid snowmelt This is the case at 

Yellowstone Caldera, where tilts of several microradians 

were detected during periods of rapid spring snowmelt 

by high-precision tiltmeters installed in 30-m-deep 

boreholes (Meertens, 1987; Meertens and others, 1989). 

The larger the water-table fluctuation, the larger the 

effect on the tilbneters. Similar effects have been noted 

at Long Valley Caldera, where spring snowmelt in the 

Sierra Nevada and resulting ground-water flow through 

porous materials flooring the caldera cause tilt excursions 

that are clearly recorded by an array of tiltmeters in 

shallow boreholes (Mortensen and Hopkins, 1987). 

Record water-table changes near the tiltmeter array if 

possible, but in any event be aware of such changes and 

their possible effects on tilbneters. 

Many other processes can cause tilt signals that are 

not directly related to volcanic activity, including 

regional strain or large earthquakes, landslides, and 

human or animal activity. These are too numerous and 

varied to discuss here. 

TILTMETERS USED TO AID ERUPTION 

PREDICTION: A CASE HISTORY 

The October 1986 dome-building episode at Mount 

St. Helens illustrates the use of electronic tiltmeters as 

part of CVO's integrated volcano-monitoring effort, 

which has led to successful predictions of numerous 

eruptive episodes since May 1980 (Swanson and others, 

1983, 1987; Swanson and Holcomb, 1990). Following 

the May 1986 dome-building episode, which was 

preceded by a series of small explosions that destroyed 

most of the monitoring stations on the dome and 

surrounding crater floor, the dome entered a typical 

period of post-eruptive quiescence. From June through 

August 1986, all monitored parameters, including seismic 

activity, ground deformation, and gas emission, were at 

background levels. The only change noted in the crater 

was slow gravitational spreading of the dome as 

78 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
indicatc:d by electronic distance meter (EDM) 

measureJDents (fig. 7.9). 

Tite first hint of further unrest was noted in 

mid-September 1986, when a subtle increase in the 

number of small earthquakes occurring beneath the dome 

was acc:ompanied by tilt changes of a few hundred 

microractians (only slightly larger than diurnal effects) at 

two of four tiltmeter stations on the flanks of the dome 

(fig. 7.10). As described above, each tiltmeter station 

contained two single-axis tiltmeters oriented radial to the 

center oJ the dome. The radial tilt components measured 

in mid-September and throughout the rest of the 

precursory period were directed outward from the center 

of the (J orne, consistent with swelling and endogenous 

growth. Additional flurries of small earthquakes that 

culmin~.ted on October 1 and October 5 were 

accompnnied by further small tilt changes at stations 

Famous and Sweet; the latter event was the most 

pronounced to date (fig. 7.11). 

Following the October 5 event, there was a brief 

lull in earthquake activity but Famous and Sweet 

continued to tilt outward at a slowly increasing rate. 

EDM measurements showed that deformation of the 

dome, particularly the west flank where Sweet tiltmeter 

was located, had begun to accelerate (fig. 7 .9). On 

October 16, new cracks were noted on the dome and in 

snow mantling the west crater floor. On that date, 

seismic ;activity beneath the dome and tilt rates at Sweet 

and Famous were steadily increasing (figs. 7.12, 7.13). 

Comparison of the EDM and seismic data with patterns 

from previous dome-building episodes indicated that 

another episode was likely to begin within three weeks, 

and a public advisory to that effect was issued on 

October 16. 

(f)1 

a: 

~ 0 w 

~ -1 

~ 

u.i -2 () 

~ -3 1-

(f) 

0-4 

w 

g, -5 _J 

(f) -6 

~ 

~ -7 

c:l 

~ -8 I 

() -9 

I 

rrI 

JE;up~ior\ I I I I I 

~ 

-~~ 

~ 

~~ Eruption 

~ l RUBY 

~ ~ 

I 

~ L3@8 @XD03g:~ -~ 

I I I I I I 

1986 MAR MAY JUL SEP NOV 1987 MAR MAY JUL SEP NOV 

-

-

-

-

-

-

0 

Figure 7.9. Line-length changes from EDM station 

Tinker to two reflectors on north flank of dome from 

January 1986 to December 1987, which included 

dome-bl.tilding episodes in May 1986 and October 1986. 

For each line, an arbitrary nominal length was subtracted 

from me<asured line-lengths. 

(f) 

z 

<( 

0 

<( 

a: 

0 

a: 

0 

~ 

(f) 

~ 3,200 

~ 2,400 

~ 1,600 

~ 800" 

oUW~~~~~~~~~~~~~u 

SEP 5 10 15 20 25 

START DAY IS SEP 1, 1986 G.M.T.- JULIAN DAY 244 

Figure 7.10. Tilt records from two high gain tiltmeters on 

the dome (top, middle) and an RSAM (B.eal time S.eismlc 

Amplitude Measurement; Endo and Murray, 1991) record 

from Garden seismometer, each for September 1-30, 

1986. RSAM provides a measure of seismic activity 

based on numerical integration of electrical output of a 

seismometer. Vertical scale for RSAM record Is arbitrary; 

values shown are 1 Q-minute averages. A small flurry of 

shallow earthquakes beneath the dome on September 15 

was accompanied by small tilt changes at Famous and 

Sweet, but was too small to be evident on RSAM record. 

(f) 

z 

<( 600 

0 

<( 

a: 

0 

a: 

0 

450 

300 

150 

Radial tilt, station Famous 

~ o~~~~~~~~~~~~~~~~~~~ 

~ 4,000 

~ 3,000 

~ 2,000 

~ 1,000 

a: o~~~wu~~~~~~~~~~~~~~ 

OCT 2 3 4 5 6 7 8 9 1 0 

START DAY IS OCT 1, 1986 G.M.T. -JULIAN DAY 274 

Flgure7.11. Tilt records from two stations, Famous and 

Sweet, and an RSAM record (1 0-minute average) from 

Garden seismometer for October 1-10, 1986. A 

pronounced flurry of small earthquakes beneath dome 

that culminated on October 5 was accompanied by onset 

of slow but steady tilting at Famous and Sweet. Note 

differing vertical scales for tiltmeter data; RSAM scale is 

arbitrary. 

7. Electronic Tlltmeters for Volcano Monitoring: Lessons from Mount St. Helens 79 
By October 18, EDM measurements and the tiluneter 

data from Sweet indicated that the most rapid swelling was 

occurring on the north and west flanks of the dome, while 

tiluneters at stations Raza and Oops indicated that the south 

and east flanks of the dome also had started to tilt outward 

(figs. 7.9, 7.13). An updated advisory issued on October 19 

narrowed the prediction window from 3 weeks to 2-10 

days. New cracks were noted on the dome and crater floor 

on Octobez 20, when another advisory was issued to narrow 

the prediction window to ''within the next 5 days, most 

likely within the next 3 days." The predictions were based 

most directly on the EDM data and seismicity, which 

indicated rapidly accelerating deformation according to a 

pattern that had generally repeated itself during each 

previous dome-building episode. The pattern of tilt changes 

was less useful foc detailed predictions because it was less 

repeatable from episode to episode. Several factors 

contributed to this shortcoming, including (1) extreme 

deformation of the dome that changed pattern from 

episode to episode; (2) destruction of many tilbneters by 

eruptive activity, which precluded long-term records 

from the same sites; and (3) the very short base line of 

tiltmeters relative to EDM lines, which makes tilt 

measurements more sensitive to local deformation. On 

the other hand, tilbneters played a strong supporting role 

in the predictions by providing real-time deformation 

data at night and during periods of bad weather, when 

visual observations and EDM measurements were not 

feasible. 

3,200 ~ ~~c•~m Garden .. · ILi'J 

2,400 f I 

1,600 ~ I • • 

sogr.. .•• L .. ~..~.u.," .... J.I.u.l.l,~~,L,J. ,lJ~u.L.L 

3,200 r~l~~ We;r'TT'T; ,,-TT'""T'''''F'''-""'i"'''l 

2.400. • 11. 

1 ,600 ~ I . J i I ' i ! 

808. I ~~IlL, J1,LI ~~<ll.l,li ... Jiill ~Li/d LJI.t ·~, 11 1 2 1 3 14 15 16 17 18 19 20 

START DAY IS OCT 11, 1986 G.M.T.- JULIAN DAY 284 

Figure 7.12. RSAM records (1 0-minute averages) from 

four seismometers at Mount St. Helens for October 

11-20, 1986. Seismic activity increased steadily during 

this period, accompanied by inflation of the dome at an 

increasing rate. 

By October 21, seismic activity beneath the dome 

was very intense (numerous M 2-3 earthquakes), severe 

disruption of the dome was obvious to observers in the 

crater (many new cracks and frequent rockfalls), the 

high-gain tiltmeter at Sweet was off-scale, and the flux 

of sulfur dioxide measured above the dome had increased 

above background levels (figs. 7.14-7.16). The Sweet 

low-gain tiltmeter recorded almost 90 degrees(!) of 

outward tilt before it was destroyed late on October 21, 

while the other tiltmeter stations recorded lesser amounts 

of tilt that were generally consistent with EDM 

measurements to nearby reflectors. In anticipation of 

outward tilting during dome building, the Sweet low-gain 

tiltmeter had been adjusted during installation to make 

use of most of its range of ±60 degrees (120 degrees 

total). 

Exttusion of a new lobe probably began during the 

night of October 21, as suggested by (1) a change in the 

character of seismic events to predominantly rockfall 

signals, (2) destruction of Sweet tilbneter station, (3) a 

temporary reversal in the tilt trend at Raza, ( 4) the onset 

of more rapid tilting at Famous and Oops (possibly an 

accommodation to extrusion), and (5) a sudden increase 

in the flux of volcanic gases measured by a continuously 

recording sensor on the dome (Sutton and others, chapter 

18; fig. 7 .16). A new and growing lobe was first 

observed on October 22, eventually from a safe vantage 

point on the dome itself. An advisory issued on October 

22 reported the beginning of extrusion and noted that, 

12,000 ~ Stat"io"n "sw,-e"el-rt "'""~"' "I T""T"nCTTTTTTl".,l rrl TTTll :-rTT"I Tl

1 

"""I/~ 

9,000~ ~ ~ 

3

6,000~ 

•

008 ~-L'"""~' I 1, I I 1 , I 

~ 

1 I I 1 I I 1 I ~ 1 I 1 J 

~ 4,000 ~-Station Famous 

<( 3 000 "• 1 1

1

1

1

1

1

1

] 

~ 2:ooo[ ~ 1,oog~-~-.~~~-~-~~~ -~ -:-:-1 I-:-~ a: 

(.) 

~ 1 ,600 r St~tion Rci~a i I I I I I I I I II I I 

- 1,200 =- ~ 800~ ~~~--v-~ 

i= 408 tJ I I I I I I I I 

I I I I I ~ 

j I I I II 

400 rst~~l-06;;~· T TTTTJTT T ~ 

200[ 

300~ - ~.~~ 1og [_ ~~v<~Ck=l~ , ~ 

11 12 13 14 15 16 1 7 18 19 20 

START DAY IS OCT 11, 1986 G.M.T.- JULIAN DAY 284 

Figure 7.13. Records from four tiltmeters on the dome 

for October 11-20, 1986. See figure 1 for locations of 

tiltmeters. Note differing vertical scales. Offset at RAZA 

on October 11 was caused by a local earthquake during 

early phase of precursory activity. 

80 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
although seismic activity remained high, the new lobe 

appeared to be stable and therefore collapse of the lobe 

or other part of the dome was unlikely. On October 27, a 

fmal advisory reported that seismicity, deformation, and 

gas emission had decreased markedly, and therefore 

hazards had returned to their pre-eruption levels. 

Each of the data sets mentioned above provided 

valuable information concerning the October 1986 

eruptive episode, but it was the thorough integration of 

all the information that contributed most to our 

successful predictions and scientific understanding of the 

event By studying in real time plots such as those in 

figure 7.16, which combines five separate data sets 

ranging from EDM surveys every few days to seismic 

amplitude measurements every minute, the CVO staff 

was able to track the situation as it unfolded, assess the 

associated hazards, and communicate our conclusions to 

appropriate public officials in a timely fashion. Clearly, 

this would not have been possible if only one or two 

types of data had been available, or if the various data 

sets had been interpreted independently. Consider, for 

example, what conclusion might have been drawn from 

the tiltmeter data alone (fig. 7.15). Late on October 21, 

one tiltmeter station had literally fallen over (Sweet), 

4,000 ~- Garden · + Eruption l 

2,000 

3,000~-~ I Wu1.~:, i .~~i ,r ~ ' 'II · _j 

1 000 ~ , 1 . I . 

' 0 ,, I .LlL.IL~~ --'______l_---"--------'--~- ···- ' 

~ ' ~--~----~~ 

z 4,000 ~E Yellow Rock , 

::J 3,QQQ , " 1 

~ 2,QQQ I , · J 

(/) 1 ,000 ' 

a: 0 _______l___L _ __,____ 

z 

§ 6,000 r.:- East Dom, e . ' ' . l 

t:i j:~gg ~j j' I l Jlj I' I Jlil,il ~I Lillll.L ~~ikll,ll, I il ~ 50~~ulljiJ1~Ll.~ . '~r~·,~·III!'L1 

j 

4,000 1 

3,000 j 

2,000 j 

1 ,cog W....hl.l.J...WJ....J..illl.LJJ~~~j__---" _ _____L__,__"'!ruiL........Ji 

21 0400 1200 2000 22 0400 1200 2000 

START DAY IS OCT 21, 1986 G.M.T.- JULIAN DAY 294 

Figure 7.14. One-minute RSAM data for period of most 

intense activity on October 21-22, 1986. Extrusion 

probably began at about 0800 G.m.t. on October 22 

(arrows). Offset in Yellow Rock record at about 2030 

G.m.t. on October 20 was caused by a gain change. 

Reduced scatter in RSAM data from all four stations from 

about 0000 to 0800 on October 21 indicates a change in 

character of seismic activity from numerous discrete 

earthquakes that produce RSAM spikes to virtually 

continuous small earthquakes that maintain RSAM value 

at a high level. 

another was temporarily tilting inward toward the center 

of the dome (Raza), and two others had not yet started to 

tilt rapidly outward. Based on this confusing pattern of 

tilt changes alone, no specific prediction would have 

been possible. However, on the basis of a variety of 

seismic, deformation, and volcanic gas data, CVO was 

able to issue three increasingly specific and accurate 

predictions of the onset of extrusive activity. 

RECOMMENDATIONS 

1. Be skepticaL Don't believe tiltmeter data unless 

compelled to do so by reliable information. Resist the 

temptation to interpret tiltmeter data until adequate 

base-line information is available for each station. 

Remember that substantial ground tilt can be caused by 

site responses to installation of the tiltmeter, diurnal 

variations, seasonal variations, precipitation, freeze/thaw 

cycles, ground-water changes, and other factors unrelated 

to volcanic activity. 

2. Never believe a single tiltmeter. Design your 

tiltmeter network with enough redundancy to compensate 

for unreliable stations that are likely to exist, regardless 

of the amount of effort expended during installation. For 

(f) 

z 

<( 

0 

<( 

a: 

2.1 ~Station Sweet, low gain 

1.7 ~ 

1.3 t 

0.9 t t Eruption 0.5 ~~~~~===:::::____L _ ____~, __ __;__ _ __j_ __ .L____---L.____j 

~~--~~~----~--~~--~~~ 

12,000 t- Station Famous, high gain ~-j 

9,000~. // ~ 6,000 / j 3,oog ~---~- --~ t 

(/) 

z 

<t: 4 000 E- Station Raza, high gain 

~ 3:ooo~ ~/ ~ 2,000~ ------ a: 1,000 !Co o~~____L--~__j_--~~~--~~--~~--~ 

~ ~ ---,--------,----,--------,----------

3,200 ~Stations Oops, high gain -- ~ -j 

2,400 ~- 1 1,600~- ,/ 

8og~------~-~~-r-~/~ t 

21 0400 1200 2000 22 0400 1200 2000 

START DAY IS OCT 21, 1986 G.M.T. -JULIAN DAY 294 

Figure 7.15. Tiltmeter records for October 2o-21. 1986. 

Low-gain tiltmeter record at Sweet went off-scale early on 

October 21 after recording more than 20,000 

microradians of outward tilt change. Low-gain instrument 

continued to operate for another 18 hours. h recorded 

nearly 90 degrees of outward tilt before going off-scale 

late on October 21 and then being destroyed on October 

22. Extrusion probably began at about 0800 G.m.t. on 

October 22 (arrows). 

7. Electronic Tlltmeters for Volcano Monitoring: Lessons from Mount St. Helens 81 
the foreseeable future, the cornerstones of volcano 

monitoring and eruption prediction will continue to be 

seismology and geodesy. With limited resources, priority 

should be given to seismic and ground deformation 

techniques that are reasonably assured of yielding 

unambiguous data for interpretation. 

3. Never believe a single data set. Interpret 

tiltmeter data only within the context of other monitoring 

information and in light of the recent eruptive history of 

the volcano. Eruption prediction is a young and largely 

intuitive endeavor. Experience is the best teacher, and 

the more information that can be brought to bear on the 

problem, the better. 

4. Be conscious of the social and economic 

impact of your work. Impacts can be either positive 

(reduction of damage to life or property) or negative 

(hardships caused by unnecessary evacuation or decrease 

in property values), but in either case they are likely to 

be substantial. For your own peace of mind, be very sure 

of your information before reaching a conclusion that 

may have a direct impact on local residents. 

>- _ . . . , . '1"",--;--r-TTTT~TTTT''''T---,-T~"T'TTTTfT""'j 

ffj <C soo t Sulfur dioxide flux ~. -· ' . 1 

~~ 450§ _a·· ·.,~-- Ow 3oo r . . s ____ _ 

I- c.. 1 58 ~~ ' I [lj . I __J d H + b ~ -IF - ' '~ - Tf t- - -Q . ~ I ' lL_!__c_j_ ' ' El ' 

15 16 17 18 19 20 21 22 23 24 

START DAY IS OCT 15, 1986 G.M.T. -JULIAN DAY 288 

Figure 7.16. Comparison of EDM. RSAM. tiltmeter, and 

gas flux data for October 15-24. 1986. See figure 18 for 

locations of measurement stations. RSAM values in 

second plot represent 1 0-minute averages of RSAM 

signal; vertical scale is arbitrary. Fourth plot shows flux of 

volcanic gases measured continuously by a sensor at 

LONE on dome (Sutton and others. chapter 18). Bottom 

plot shows flux of sulfur dioxide measured above dome at 

1- to 2-day intervals using a COSPEC spectrometer 

mounted in a fixed wing aircraft. Extrusion probably 

began at about 0800 G.m.t. on October 22 (arrows). 

ACKNOWLEDGMENTS 

Andy Lockhart and Carl Mortensen shared their 

considerable experience with tiltmeters; Barry Voight 

and the editors, Don Swanson and John Ewert, made 

helpful suggestions. Jim Savage and colleagues in the 

Tectonophysics Branch conbibuted a healthy dose of 

skepticism concerning tiltmeters, for which I am grateful. 

Tom Murray and Gene Iwatsubo developed computer 

software used to generate the figures. 

REFERENCES CITED 

Agnew, D.C., 1986, Strainmeters and tiltmeters: Review of 

Geophysics, v. 24, p. 579--624. 

Chadwick, W.W. Jr., Iwatsubo, E.Y., Swanson, D.A., and 

Ewert, J.W., 1985, Measurements of slope distances and 

vertical angles at Mount Baker and Mount Rainier, 

Washington, Mount Hood and Crater Lake, Oregon, and 

Mount Shasta and Lassen Peak, California, 1980-1984: 

U.S. Geological Survey Open-File Report 85-279, 96 p. 

Dzurisin, D., Johnson, D. J., Murray, T. L., and Myers, B., 

1982, Tilt networks at Mount Shasta and Lassen Peak, 

California: U.S. Geological Survey Open-File Report 

82-670, 42 p. 

Dzurisin, D., Westphal, J.A., and Johnson, D.J., 1983a, 

Eruption prediction aided by electronic tiltmeter data at 

Mount St. Helens: Science, v. 221, no. 4618, p. 

1381-1383. 

Dzurisin, D., Johnson, D.J., and Symonds, R.B., 1983b, Dry tilt 

network at Mount Rainier, Washington: U.S. Geological 

Survey Open-File Report 83-277, 19 p. 

Endo, E.T., and Murray. T., 1991, Real-time seismic amplitude 

measurement (RSAM): A volcano monitoring and 

prediction tool: Bulletin of Volcanology, v. 53, no. 7, p. 

533-545. 

Harrison, J.C., 1976, Cavity and topographic effects in tilt and 

strain measurement: Journal of Geophysical Research, v. 

81, p. 319-336. 

Harrison, J.C., and Herbst. K., 1977, Thermoelastic strains and 

tilts revisited: Geophysical Research Letters, v. 4, p. 

535--537. 

Iwatsubo, E.Y., Topinka, L., and Swanson, D.A., 1988, 

Measurements of slope distances and zenith angles at 

Newberry and South Sister volcanoes, Oregon, 

1985-1986: U.S. Geological Survey Open-File Report 

88-377' 51 p. 

Meertens, C.M., 1987, Tilt tides and tectonics at Yellowstone 

National Park: Boulder, University of Colorado, Ph.D. 

thesis, 236 p. 

Meertens, C., Levine, J., and Busby, R., 1989, Tilt observations 

using borehole tiltmeters II: Analysis of data from 

Yellowstone National Park: Journal of Geophysical 

Research, v. 94, 587-601. 

Mortensen, C.E .• and Hopkins, D.G .• 1987, Tiltmeter 

measurements in Long Valley caldera, California: Journal 

82 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
of Geophysical Research, v. 92, no. B13, p. 

13,767-13,776. 

Swanson, D.A., Casadevall, T.J., Dzurisin, D., Malone, S.D., 

Newhall, C.G., and Weaver, C.S., 1983, Predicting 

eruptions at Mount St. Helens, June 1980 through 

December 1982: Science, v. 221, p. 1369-1376. 

Swanson, D.A., Dzurisin, D., Holcomb, R.T., lwatsubo, E.Y., 

Chadwick, W.W. Jr., Casadevall, T.J., Ewert, J.W., and 

Heliker, C.C., 1987, Growth of the lava dome at Mount St. 

Helens, Washington, (USA), 1981-1983, in Fink, J.H., 

ed., The emplacement of silicic domes and lava flows: 

Geological Society of America Special Paper 212, pp. 

1-16. 

Swanson, D.A., and Holcomb, R.T., 1990, Regularities in 

growth of the Mount St. Helens dacite dome, 1980-1986, 

in Fink, J.H., ed., Lava domes and flows: Emplacement 

mechanisms and hazard implications: New York, Springer 

Verlag (IAVCEI Proceedings in Volcanology, v. 2), p. 

3-24. 

Westphal, J.A., Carr, M.A., Miller, W.F., and Dzurisin, D., 

1983, An expendable bubble tiltmeter for geophysical 

monitoring: Review of ScientifiC ln5truments, v. 54, p. 

415-418. 

Yamashita, K.M., and Doukas, M.P., 1987, Precise level lines 

at Crater Lake, Newberry Crater, and South Sister, 

Oregon: U.S. Geological Survey Open-File Report 87-293, 

32p. 

7. Electronic TUtmetera for Volcano Monitoring: Leaaona from llouftl Sl. Helena 83 

8. Slope-Distance Measurements to the Flanks of 

Mount St. Helens, Late 1980 through 1989 

By Eugene Y. lwatsubo, Lyn Topinka, and Donald A. Swanson 

ABSTRACT 

A slope-distance monitoring network on the flanks of 

Mount St. Helens was established one month before the 

major eruption of May 18, 1980. By 1990, the network has 

expanded to include 6 instrument and 13 reflector stations 

designed in a star-shaped configuration. The instrument and 

reflector stations are steel towers 3-4 m high cemented in 

the ground. The towers are designed to allow measurements 

to be taken during winter months when snow covers ground 

marks. For atmospheric corrections, measurements of 

temperature and pressure at the instrument station only 

(without corresponding measurements at the reflector 

station) have proved adequate for the precision required. 

Prior to dome-building episodes, measurements taken from 

a station as far as 8 km away can be used as a predictive 

tool. Seasonal variations are possibly observed on some 

'lines. Taken as a whole, the data obtained from 

measurements to the flanks after May 18, 1980, show no 

correlation with eruptive activity. 

INTRODUCTION 

Following the eruption on May 18, 1980, 

slope-distance monitoring of the flanks of Mount St. 

Helens was resumed and expanded to a network of 7 

instrument and 12 reflector stations (Swanson and others, 

1981). One pre-1980 instrument station was occupied for 

a short period but was abandoned in late 1980, leaving 

six permanent instrument stations. A thirteenth reflector 

tower was established in November 1983. 

The purpose of this network is to detect a large 

influx of magma into the present conduit system. From 

late 1980 through 1989, 1 explosive episode and 16 

dome-building episodes have occurred. Although the 

amount of deformation has been large, this movement 

has been localized at or very near the dome; at times, 

only sections of the dome were affected by magmatic 

intrusion. Detectable changes in line length to the flanks 

of the volcano were not associated with any of the 

episodes; thus no significant volume (several million 

cubic meters or more) of magma has intruded the conduit 

system at a shallow depth of about 1 km since the 

eruption of May 18, 1980. An even larger influx of 

magma would have to be introduced at the 7-10 km 

depth, the depth of the reservoir inferred from seismic 

and petrologic studies of the May 18 eruption (Scandone 

and Malone, 1985; Carey and Sigurdsson, 1985; 

Rutherford and others, 1985), before measurable 

line-length changes would be detected by the outer-flank 

network. 

122' 15' 

\flARRYS RIDGE 

1000 " ~ 

46' 15' 

1 KILOMETER 

Flgure8.1. Slope-distance monitoring network 

established at Mount St. Helens. Line from Harrys Ridge 

to dome and crater floor actually represents several lines 

to closely spaced targets. Contours in meters. 

8. Slope-Distance Measurements to the Flanks of Mount St. Helens, Late 1980 through 1989 85 
NETWORK 

The slope-distance network on the flanks of Mount 

St Helens, called the outer net, is shown in figure 8.1. A 

star-shaped configuration was used to obtain coverage 

around the volcano (lwatsubo and Swanson, chapter 10). 

Twenty-eight lines make up the outer net, with distances 

ranging from 2 to 8.5 km. Several additional lines from 

Harrys Ridge measure distances to targets mounted on 

the dome and crater floor. 

Instrument stations used soon after May 18, 1980, 

consisted of two lengths of reinforcing rod (rebar) 

cemented into the ground close to one another; one was 

for the electronic distance meter (EDM) to measure 

distances, and the other for the theodolite to measure 

angles. Because of the danger imposed by the major 

A 

explosive episodes that followed the May 18 eruption 

(the last one occurring in October 1980), both 

measurements were taken simultaneously to minimize the 

time spent at each site. Unfortunately, because of 

volcanic activity or weather, some measurements were 

taken in haste and important information, such as height 

of instrument (HI), was not recorded. 

To resolve the HI problem, a method of tripod 

setup developed at the Hawaiian Volcano Observatory 

(HVO) was used. Tl)ree cement pads were positioned 

around each rebar to mark where the tripod legs should 

be placed. Small holes were chipped into the cement and 

painted for easy recognition. The tripod legs, always 

fully extended, were fit into the holes, thereby assuring 

that the tripod was at the same height each time. An 

optical-plummet tribrach was used to center each 

instrument over its rebar. This cement-pad method 

allowed an easy and quick setup. 

The reflector stations from May to September 1980 

consisted of a cluster of plastic highway reflectors 

mounted on boards bolted to steel fenceposts driven into 

the ground. The reflectors were typically 1-1.5 m above 

the ground surface. For several longer instrument-toreflector distances, standard glass corner-cube prisms 

B 

Figura 8.2. A, Instrument tower with EDM mounted on top tower plate. 8, Reflector tower with prisms mounted near 

its top. Tower heights range from 3 to 4 m. Each leg is cemented into ground. Instrument tower has a walkway for easy 

access to all sides of tower. Temperature pole is supported by walkway and one leg of tower. 

86 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
were mounted on the fenceposts, replacing the plastic 

reflectors. Blowing ash often sandblasted the reflectors, but 

the plastic reflectors were inexpensive and easy to replace. 

The reflectors performed well during summer months but 

would have been bwied under snow during winter months. 

In September 1980, steel towers (fig. 8.2) were 

installed by a local power company at all instrument and 

reflector stations (Swanson and others, 1981) to monitor the 

network throughout the year, especially during winter 

months. These towers stand 3-4 m above the ground and 

allow the instrument and reflectors to be almost always 

above the snow (fig. 8.3). Measurement stations for all 

distances and angles were then transferred from the ground 

marks to the towers and used throughout the year. 

Each reflector tower has glass comer-cube prisms 

mounted near its top to raise them above the snow and 

blowing ash (fig. 8.2B). Each tower has one or two prisms, 

depending on how many instrument stations measure to it. 

Although the prisms were relatively high above the ground, 

loose blowing ash etched some reflectors and was a minor 

problem for several years after the towers were installed. 

Most reflector prisms have been replaced once since 1980, 

and a few have been replaced twice. Ice is the major 

problem during winter months. The glass prisms are 

A 

typically ice covered and need cleaning before a 

measurement can be made, and it is not uncommon to 

have up to 1 m of rime on the towers (fig. 8.4) that has 

to be chopped off with an axe before the prism can be 

cleaned. 

Instability can be a problem with the towers. The 

problem is most severe at those towers installed ·on thick 

unconsolidated deposits from the May 18 eruption, where 

settling has occurred over several years (fig. 8.5, lines 

ROADSGB, HARYSSGB, and HARYSS1P). Step tower, 

located at the north entrance to the crater on loose 

debris-avalanche material, had to be moved in 1986 

because the narrow ridge it was on was eroding. Sugar 

Bowl tower now has one free-standing leg, because pumice 

and ash has been eroded by the wind from the base of that 

leg. Towers located on other sections of the volcano, where 

the 1980 deposits are thin, appear to be stable. 

EQUIPMENT 

The EDM used for the outer net measurements is a 

Keufel & Esser Rangemaster III, capable of measuring 

up to 60 km using 30 prisms at an accuracy of ±(5 mm + 

8 

Figure 8.3. A, Instrument station Clearcut in summer, and 8, winter monitoring conditions. Steel towers enable 

occupation of network all year long. Snow accumulations rarely cover instrument towers and never reflector towers. 

8. Slope-Distance Measurements to the Flanks of Mount St. Helens, Late 1980 through 1989 87 
1 ppm). The Rangemaster III draws 5 amperes (amp) of 

current and requires a large 12-volt battery. Car batteries 

can be used but are extremely heavy. We use a 24 

amp-hour, sealed, gelled-electrolyte, rechargeable battery 

(dimensions are 17x16x12.5 em; weight is 8.65 kg) 

packaged inside a fiberglass box with a handle for easy 

use. We always carry two batteries into the field and 

typically use both during the survey. The EDM's light 

source, a helium-neon gas laser, is visible, and the return 

signal from the reflector can be seen. The visible beam is 

very useful in locating targets that are a long distance 

away or in marginal weather conditions. 

The Rangemaster III is yoke mounted using an 

optical-plummet tribrach. The tribrach, yoke, and EDM 

are mounted directly to the top of the steel instrument 

tower (fig. 8.2A). The top plate of each instrument tower 

has a hole drilled in the center so that the tribrach can be 

mounted. A tapered bolt is used to insure that the 

instrument is always centered in the hole. 

Air temperature and barometric pressure are 

atmospheric parameters measured to correct the slope 

distance. Air temperature is measured 7-8 m above the 

ground surface. A calibrated temperature sensor 

(lwatsubo and Swanson, chapter 10) is mounted inside a 

Figure 8.4. Rime collecting and covering reflector tower 

and prism is a problem during winter months. This tower 

had to be cleared of all ice and prisms cleaned before a 

measurement could be made. 

streamlined shield that fits atop sections of steel pipe 

used to raise the shield. The sensor is mounted inside a 

46-cm-long, 3.7-cm-diameter tube that is nested inside a 

larger 8.5-cm-diameter tube. Phenolic tubing is used, 

because of its property of distributing radiant heat over 

the whole surface of the tube. A fitted tail or rudder is at 

the rear of the tube (fig. 8.6). A short piece of solid brass 

stock (7 em long, 1.3 em diameter) attached to the 

middle of the outer tube perpendicular to its long axis 

fits into the top section of the steel pipe (inner diameter 

of 1.5 em) and allows the shield to spin. The whole 

design acts as a weather vane, so as the wind direction 

changes, the shield pivots to allow air to always flow 

through the tubes and past the sensor. The steel pipe 

holding the shield is supported at its bottom by the 

walkway around the tower and is tied to a tower leg for 

stability (fig. 8.2A). 

From 1980 to 1986, barometric pressure was 

measured using a Wallace & Tieman altimeter. In 1987, 

we switched to the AIR-HB-1A hand-held 

barometer/altimeter (lwatsubo and Swanson, chapter 10) 

for a more accurate measurement. Humidity is not 

measured. 

A Wild T2 theodolite is used to measure zenith and 

horizontal angles. The theodolite mounts directly to the 

top plate using the same tapered bolt as that for the 

EDM. Zenith angles are measured to the same prism 

used for the EDM measurement. The distance is always 

measured frrst. Thus, where there is more than one prism 

per tower, the visible laser of the EDM allows us to 

observe which prism is the correct one, thereby avoiding 

confusion when the angles are measured. 

DATA 

Single-endpoint atmospheric data are collected 

only at the instrument station, because it is not 

economically feasible to fly to each reflector station and 

measure atmospheric conditions. Comparison of slope 

distances calculated by using atmospheric data measured 

at one endpoint with those measured at both endpoints 

shows that although the corrected slope distance differs, 

the changes in slope distance remain approximately the 

same, especially when the elevation difference between 

the instrument and reflector is less than about 300 m 

(Hawaiian Volcano Observatory, unpublished data, 

1989). In other words, the accuracy differs but the 

precision is about the same. 

Single-endpoint atmospheric data can be adequate 

for monitoring purposes, but it is imperative that many 

slope-distance measurements be taken to establish the 

precision associated with each individual line. At Mount 

St. Helens, many single-endpoint measurements are used 

88 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
A 

CJ) 

a: I 

W 0 pri\-...-U--t>v""~ +'/1....;\~{=~- 6 b- tli ~ \/\1 \J c:::? ~ ~ -5 

i= CLEARSRI RMS = 2.440 

aJ-10' ,, I 

() 

w· 

~ Ohl\{\ I 

{1 ffv\ 1~ /1Ar" ~ ~~~-~~ §E -5 e ""V-v "i\1 v v~ 'Z;;j y= "\;::= 

~ -10 I CLEARSES . RMS = 1.401 J 

0 

w 

~ 10 I .....J. 

CJ) 

~ 5 

fB 0 

C) . z -5 ~ 

~ -10 I, CLEARSER I I 

RMS = 2.605 I .I 

I -----, 

I 

-~ ~\i~J(--;it~~~---~~~-___j -10~1~~~~~~~==~~~~~~==~~~ 

1981 1982 1983 1984 1985 1986 1987 1988 1989 

START DAY IS NOV 12, 1980 G.M.T.- JULIAN DAY 317 

B 

I I I 

---./\== 

RMS = 1.146 

1 j 

0 ~ --~- ' • ~ We !} - - 6 -- ~1 _;~t'. M:~:s,:D v • ~ = ~~.:=1.166,~ 1981 1982 1983 1984 1985 1986 1987 1988 1989 

START DAY IS NOV 12, 1980 G.M.T.- JULIAN DAY 317 

Figure 8.5. Cumulative slope-distance changes of lines measured from A, Clearcut; 8, Muddy; C, Road 1 00; D, Harrys 

Ridge; E. Studebaker; and F, Butte Camp instrument stations. A best fit line is drawn through data, and 

root-mean-square error (RMS, in centimeters) around line is indicated in each plot. 

to establish trends and errors (fig. 8.5). Errors related to 

setup procedures, atmospheric conditions, and 

measurements on steep slopes (average elevation 

difference between instrument and reflector station is 

617±286 m) can be as much as several centimeters. 

The cumulative slope-distance changes for all 28 

lines (lines to Step are included although the station was 

moved in 1986) are plotted in figure 8.5. The straight 

line is a least-squares regression and the 

root-mean-square (RMS) error is given in centimeters. 

Table 8.1 lists other statistics. The standard deviations 

(S.D.), in meters and parts per million (ppm), are 

associated with the mean slope distance, and n is the 

number of measurements. The linear regression equation 

is 

y =A+ Bx. 

The mean standard deviation for 25 of the lines is 

0.0188±0.0044 m (4.9068±1.2544 ppm). The mean 

standard deviation does not include the lines measured to 

Step and Sugar Bowl towers, which were not stable 

throughout the period of measurement. 

RELATION BETWEEN OBSERVED 

DEFORMATION AND VOLUME OF 

ERUPTED OR STORED MAGMA 

A simplified Mogi (point-source) model (A. 

Okamura, written common., 1990) was used to estimate 

the maximum expected horizontal displacement and its 

horizontal distance from the source for two volumes of 

magma, 2.5 and 5.0 x 106 m3

, added or subtracted at 

depths ranging from 1 to 5 km (table 8.2). The model 

assumes a volume change in an elastic, isotropic 

material. The depth range is that considered as likely for 

the shallow magma reservoir under Mount St. Helens. 

The average horizontal distance from the reflector towers 

8. Slope-Distance Measurements to the Flanks of Mount St. Helens, Late 1980 through 1989 89 
c 

10 

5 

0 

-5 

0 

-5 

RMS = ~.166 I I 

,----------------

_

10 ~OAD~ML I . RMS = 0.880 uJ 

20 

15 

10 

5 

0 

-5 

ROADSGB RMS = 2.579 -10~~~~~~~~~~=ili~~~~~~hd 

1981 1982 1983 1984 1985 1986 1987 1988 1989 

START DAY IS NOV 12, 1980 G.M.T.- JULIAN DAY 317 

Figure 8.5.-Continusd 

D 

5 

0 

-5 

-10 I I 

HARYSWND RMS = 1.537 

-5 

-10 

-15 

-20 

-25 

-30 

-35 

-40 ~ HARYSSTP 

-45 L RMS = 4.534 

ui 

(.) r z 15 I 

~ 

----~------------, 

(j) 

0 10 

w 

0.. 5 

0 

_J 0 (j) 

~ -5 

(j) 

~ -10 

HARYSSGB RMS = 3.447 I .I 

~ -15 

I 

(.) 

I I 

5

of \~~~~~ ·" JJA 6cj -( y I' ' ur V vv : 

-10 I 

_

15 1_, HAR':'SNE~ , , , f!MS = ~-659 i 

1981 1982 1983 1984 1985 1986 1987 1988 1989 

START DAY IS NOV 12, 1980 G.M.T. -JULIAN DAY317 

90 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
E 

~ ~ I ,~ A L\ r<=-- • --- ~~->=-~~ ~ ~ •VI? v ------~-~~ _ _ _ _ 1 

~ -1-~ 2~~EWRI RMS = 2.341 I 

() 

~ ,--------------------------------

w 5~ 1\ ~~ ~ O' 0A f\ ~A ·. ~ r~V~ v--~ 

~ ;~ L ~T~~LUE-LUN-'-L!W-LUD.L.LLLLC.LLLI~u_u_..j_.LLL._.U_LLLLLl_LU_LLLLL'-.UiRLU-MLU-SLU-=UL2LU-. Ou_u5_._u3 LLLI --'-'-"1 

0 

_J 

~ o~r ~ _;~ ft'\7-~"\ 0-15l vy v 

l 

I STUDESGB RMS = 5.009 

-20 "LU_LL.u.L..u_~~Lw_cLLLLLLul_u_u~.~~.tiuD~_ci-LLli-Lli.LU.cl..u..uJ 

1981 1982 1983 1984 1985 1986 1987 1988 1989 

START DAY IS NOV 12, 1980 G.M.T.- JULIAN DAY 317 

Figure 8.5.-Continusd 

to the center of the dome is 2.1 km, and the amount of 

horizontal displacement at this average distance is also 

shown in table 8.2 (TD). 

The results of the calculations suggest that the 

outer net would detect displacement greater than those 

expected at the 2 S.D. level only for a volume change of 

more than 2.5x106 m

3 at a depth of about 1 km. A 

smaller volume or greater depth would not yield 

displacements greater than those expected at the 2 S.D. 

level. However, repeated measurements over a long time 

interval might result in a convincing trend even if the 

volume or depth were not adequate to satisfy formally 

the 2 S.D. confidence guideline. 

The modeling results are marginally consistent 

with the volume of single dome-growth episodes. The 

average do~e-~owth episode produced a volume of 

about 3.6x10 m (Swanson and Holcomb, 1990), and no 

deformation was detected by the outer net during the 

events. According to the model, this would imply 

withdrawal of magma from a reservoir at a depth greater 

than 1 km, which is within the range of 1-2 km 

estimated from seismic observations (Endo and others, 

1990). Most likely the model is too idealized to be used 

in such a detailed manner, but at least it is consistent 

with observations. Another complication is the 

possibility that magma rose from greater depth to take 

5! ______________ __ 

1 ~~~~ 

~- 0 ~J BUTTEWRA 

() -5 LLJ.LLu~..LLu.u~ RMS = 2.605 uJ 

z 

~ (j) 

0 

the place of the magma erupted, thereby resulting in no 

net deformation related to a shallow source and 

deformation related to a deeper source that is too small 

to be detectable by the outer net 

A related finding is that the outer net did not detect 

anl deformation resulting from the eruption of about 77x 

10 m3 between October 1980 and October 1986 

(Swanson and Holcomb, 1990). According to the 

point-source model, eruption of such a volume from a 

depth of 7 km should have resulted in about 6.6 em of 

horizontal displacement at 2.1 km from the vent; from 

depths of 8, 9, and 10 km, the resulting displacement 

would have been 4.6 em, 3.0 em, and 2.4 em, 

respectively. However, considerably less displacement 

than this was measured at most of the stations (fig. 8.5), 

and in fact nearly all of the measured changes are within 

the 2 S.D. confidence limits. Therefore, we can conclude 

that a volume of 77x106 

m

3

, equivalent to that of the 

dome, could have incrementally risen from a depth of 7 

km or more without geodetic detection. 

DISCUSSION 

Several interesting results have been seen from 

monitoring the flanks of Mount SL Helens. There is a 

•• Slope-Distance Measurements to the Flanks of Mount St. Helens, Late 1980 through 1989 91 
Table 8.1. Statistics for outer net lines station symbols listed in figure 8.5. 

[Average standard deviation (S.D.) is .0188±0.0044 m (4.91±1.25 ppm), excluding measurements to 

unstable Step and Sugarbowl towen. A and B, coefficients in linear regression equation (see text); n, 

number of measurements.] 

Une Mean slope S.D. S.D. A B n 

distance {m} {m~ {~~m} 

CLEARLY A 2612.3518 0.0163 6.24 0.052843 -0.000011 55 

CLEARWRA 5142.2053 0.0255 4.38 0.058339 0.000030 44 

CLEARSRI 3787.7435 0.0207 5.46 -0.131858 -0.000013 50 

CLEARSER 4567.9731 0.0245 5.36 0.019525 0.000018 41 

SLEARSES 3229.0305 0.0143 4.43 0.009890 -0.000008 55 

MUDDYSES 3155.5176 0.0167 5.29 0.015682 0.000006 49 

MUDDYSER 3944.8057 0.0194 4.92 0.031095 -0.000010 44 

MUDDYNEL 3133.9928 0.0165 5.26 0.012470 0.000016 52 

MUDDYEDM 2073.8606 0.0119 5.74 -0.019293 0.000005 53 

MUDDYWND 3559.6672 0.0115 3.23 -0.013443 0.000000 50 

ROADWND 2921.7032 0.0158 5.41 0.100623 0.000015 42 

ROADNEL 4558.1470 0.0198 4.34 0.113864 0.000003 43 

ROADSGB 3240.9759 0.0695 21.44 0.049108 0.000089 42 

ROADSTP 3392.5475 0.0545 16.06 0.032978 0.000092 37 

ROADST2 3243.5133 0.0200 6.17 -0.025132 -0.000002 4 

ROADGML 3823.3190 0.0177 4.63 -0.018813 0.000024 13 

ROADNWD 5079.7687 0.0166 3.27 -0.019029 -0.000005 38 

HARYSSGB 6679.5150 0.0648 9.70 -0.125063 0.000060 237 

HARYSSTP 6346.4131 0.0916 14.43 -0.170432 -0.000148 215 

HARRYSST2 6101.6053 0.0209 3.43 0.009628 0.000030 24 

HARYSGML 6391.8993 0.0257 4.02 0.012151 0.000000 95 

HARYSNEL 8529.7190 0.0265 3.11 -0.035903 0.000000 107 

HARYSWND 7033.3793 0.0171 2.43 0.010598 0.000013 88 

STUDESGB 4461.8031 0.0454 10.18 -0.177775 -0.000031 42 

STUDENWD 3502.0822 0.0206 5.98 0.013641 -0.000003 41 

STUDEWRI 4507.9884 0.0263 5.83 0.009424 0.000017 37 

BUTIEWRI 2814.4481 0.0152 5.40 -0.001051 0.000007 44 

BUTIEWRA 2585.6419 0.0216 8.35 0.111692 0.000022 35 

BUTIESRI 3014.5021 0.0151 5.01 0.031401 0.000006 37 

BUTIELVA 2717.0358 0.0138 5.08 0.012025 0.000008 44 

hint of seasonal fluctuations affecting some slope effects such as sliding, settling, or freeze-thaw activity. 

distances but not others (fig. 8.5). For example, from the The lack of a consistent pattern contrasts with the 

same instrument station, line MUDDYSER shows such general trend of inward collapse, possibly along a 

fluctuations but line MUDDYWND does not. Elevation, northwest-trending axis, that characterized the volcano in 

line length, and site location have no apparent effect. We the summer and fall of 1980 (Swanson and others, 1981) 

have no ready explanation, although we suspect that the and may have resulted from a response of the remaining 

fluctuations are caused by atmospheric rather than shell of the volcano to the sudden formation of the deep 

volcanic effects. crater on May 18, 1980. 

Since late 1980, no pattern of extension or Although not part of the network to monitor the 

contraction consistent throughout the network has been flanks of the volcano, the measurements from Harrys 

defined. Hence the few trends seen, such as those on the Ridge to the dome (8 km) and crater floor are used to 

lines CLEARWRA, BUTTEWRA, BUTTESRI, and help predict dome-building episodes and provide an 

STUDEWRI, may be attributable to slight instability of alternative indicator if volcanic conditions in the crater 

the instrument or reflector towers, or to other local are considered too hazardous to work there. In 1984, 

92 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Table 8.2. Computed maximum horizontal displacement 

(HD) and horizontal distance (D) from the source for two 

increases In volume of magma at depth. TO, expected 

change at reflector towers (calculated for the average 

horizontal distance of the towers to the center of the 

dome, 2.1 km). 

[Calculations based on point source embedded in an isotropic, 

homogeneous, elastic half-space.] 

Depth 

(km) 

2 

3 

4 

5 

D 

2.5 x 106 m3 

volume increase 

HD TD 

(km) (em) (em) 

0.7 15.3 6.6 

1.4 3.8 3.4 

2.1 1.7 1.7 

2.8 1.0 0.9 

3.5 0.6 o.s 

D 

5.0 x 106 m3 

volume increase 

HD TD 

(km) (km) (an) 

0.7 30.6 13.3 

1.4 7.7 6.9 

2.1 3.4 3.4 

2.8 1.9 1.8 

3.5 1.2 1.8 

prior to the February and March dome-building episodes, 

a target on the north side of the dome was measured 

from both Harrys Ridge and Sauna instrument stations. 

Sauna was located approximately 280 m north of the 

dome and was part of the dome-deformation network in 

the crater (lwatsubo and Swanson, chapter 6). Both lines 

reflect the. same characteristics (fig. 8. 7) and were used 

to help predict this dome-building episode. Moreover, the 

similarity of trends from both instrument stations 

indicates that Sauna, on the crater floor near the dome, 

was not moving significantly-a vital piece of 

information otherwise unavailable. 

One benefit of the measurements from Harrys 

Ridge to the dome is that conditions outside the crater 

almost always allow landings at Harrys Ridge. In 

Flgure8.6. Side view of shield used to house 

temperature sensor. Shield spins on steel pipe like a 

weather vane, thereby allowing wind to pass through tube 

and temperature sensor. Shield is approximately 60 em 

long. 

contrast, high, unpredictable winds in the crater 

sometimes make flying and landing in a helicopter 

hazardous. During periods before an eruptive episode, 

when work may be limited to Harrys Ridge owing to 

weather or safety considerations, these measurements 

help to define better the deformation cycle. An example 

is the February 1984 episode (fig. 8. 7). The 

dome-building episode started on February 6. The 

Sauna-Funkier line was measured-on February 5 and next 

on February 14, but the Harrys-Funkier line was 

measured at midday February 7, when windy conditions 

prohibited work in the crater, and again on March 3. The 

Harrys-Funkier distance on February 7, when compared 

to that of March 3, shows that deformation had virtually 

stopped by February 7. Therefore the change in the 

Sauna-Funkier distance measured between February 5 

and February 14 must also have occurred before midday 

on February 7. 

CONCLUSIONS 

Since late 1980, little or no detectable deformation 

has been measured on the flanks of Mount SL Helens. 

All eruptive activity and deformation has taken place on 

the lava dome. 

Single-endpoint atmospheric data are adequate to 

detect large-scale deformation that may occur before an 

eruptive episode, as indicated by the measurements from 

(f) 

fffi 0 ::J-t:t--BIEfJ-....-___~ w 

~ 

~ 

w 

0 

~ -5 

~ 

0 

w 

0... 

0 

~-10 ~ 

I r +-Dome-building episodes -+ 

G -1 5 L-iltiill_l LLJ..LU_LU_j_j_l.LU.J..Ll..LU..Ll.LU.J..L.LLL.lJ..l.L.LLLll.LLU_l_!_Lllll.LLU-'-Lllll.LLU.LLULllLLLJ_l_Ll 

10 20 FEB 10 20 MAR 10 20 APR 

START DAY IS JAN 04, 1984 G.M.T.- JULIAN DAY 004 

Figure 8.7. Cumulative slope-distance changes to dome 

target Funkier from two different instrument stations for 

February and March 1984 dome-growth episodes. Upper 

plot shows data measured from Sauna, 280 m north of 

dome on crater floor. Bottom plot shows data measured 

from Harrys Ridge, 8 km north of dome. Both lines were 

used to predict both episodes. 

a. Slope-Distance Measurements to the Flanks of Mount Sl Helens, Late 1980 through 1989 93 
Harrys Ridge to points inside the crater. It is important to 

collect as many single-endpoint measurements as 

possible to help define errors and trends for each line. 

The data show random variations of about 5 ppm, which 

is considered to be within the expected errors defined by 

the long-term trends. Many lines at Mount St. Helens 

exhibit this type of behavior. 

If a significant new influx of magma enters the 

system at a depth of about 1 km, movement of the flanks 

should indicate this inflation. What "significant" means 

in this context is not known well, other than that the new 

volume would probably have to be greater than 2.5 x 106 

m

3

, as judged from the simple point-source modeling. At 

greater depths a larger intrusion could go undetected by 

the outer net. Until intrusion occurs, we expect little or 

no slope-distance changes along the outer-flank lines. 

REFERENCES CITED 

Carey, Steven, and Sigurdsson, Haraldur, 1985, The May 18, 

1980 eruption of Mount St. Helens 2: Modeling of 

dynamics of the plinian phase: Journal of Geophysical 

Research, v. 90, p. 2948-2958. 

Endo, E.T., Dzurisin, Daniel, and Swanson, D.A., 1990, 

Geophysical and observational constraints for ascent rates 

of dacite magma at Mount St. Helens, in Ryan, M.P., ed., 

Magma transport and storage: New York, John Wiley&. 

Sons, p. 317-334. . 

Rutherford, M.J., Sigurdsson, Haraldur, Carey, Steven, and 

Davis, Andrew, 1985, The May 18, 1980 eruption of 

Mount St. Helens 1: Melt composition and experimental 

phase equilibrium: Journal of Geophysical Research, v. 90, 

p. 2929-2947. 

Scandone, Roberto, and Malone, S.D., 1985, Magma supply, 

magma discharge, and readjustment of the feeding system 

of Mount St. Helens during 1980: Journal of Volcanology 

and Geothermal Research, v. 23, p. 239-262. 

Swanson, D.A., and Holcomb, R.T., 1990, Regularities in 

growth of the Mount St. Helens dacite dome, 1980-1986, 

in Fink, J.H., ed., Lava flows and domes: New York, 

Springer-Verlag, p. 3-24. 

Swanson, D.A., Lipman, P.W., Moore, J.G., Heliker, C.C., and 

Yamashita, K.M., 1981, Geodetic monitoring after the May 

18 eruption, in Lipman, P.W. and Mullineaux, D.R., eds., 

The 1980 eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 157-168. 

M lloftJtoring Volcanoea: Techniques and Strategl• Uaed by the Staff of the Cascades Volcano Observatory 
9. Monitoring Radial Crack Deformation 

by Displacement Meters 

By Eugene Y. lwatsubo, John W. Ewert, and Thomas L. Murray 

ABSTRACT 

Measurements of change in the width of cracks on 

the crater floor of Mount St. Helens, Washington, were a 

major tool used in predicting dome-building episodes 

during 1980-82. These measurements were made by 

field crews using a steel tape measure to determine the 

distance between lengths of reinforcing rod driven into 

the ground on opposite sides of cracks. In 1981, a 

telemetered, electronic tape measure was installed on the 

crater floor to monitor crack movement continuously. 

However, electronic and physical problems kept this and 

a second displacement meter from being viable predictive 

tools. In 1985, using new techniques and instrumentation, 

a third displacement meter was installed on the dome, 

and it successfully monitored the widening of a crack 

days before a dome-building episode. By 1989, there 

were two displacement meters of this type monitoring 

cracks on the dome. 

INTRODUCTION 

Beginning in September 1980, cracks that extended 

radially from the Mount St. Helens dome (fig. 9.1) 

formed in the crater floor. As magma ascended into the 

dome the crater floor deformed, creating segments or 

blocks bounded by cracks that either widened or, less 

commonly, narrowed before and during extrusive 

episodes (Chadwick and others, 1983; Chadwick and 

Swanson, 1989; Swanson and others, 1983). The rate of 

widening or narrowing of the cracks accelerated before 

extrusion occurred. Initially, a steel tape was used to 

measure changes in distance between reinforcing rod 

(rebar) stakes driven into the ground on either side of 

selected cracks (crack stations). This method became a 

reliable tool for predicting dome-building episodes at 

Mount St. Helens. 

To monitor crack movement continuously, 

electronic tape measures (displacement meters) were 

installed to monitor displacements between rebar stakes. 

Measurements from the displacement meters are made 

every 10 minutes and telemetered to the Cascades 

Volcano Observatory (CVO). This paper discusses 

methods used to establish permanent displacement 

meters at Mount St. Helens. Although these methods deal 

with specific problems at Mount St. Helens, they could 

be adapted for use at other volcanoes that display similar 

deformation. 

DEVELOPMENT OF THE METHOD 

Data obtained by monitoring crack stations helped 

in predicting the October 1980 explosive episode 

(Swanson and others, 1981, 1983). With the success of 

this method, more crack stations were established on the 

crater floor to monitor deformation and aid in the 

prediction process. 

Because measurements could only be taken when 

crews were in the crater, instrumentation was designed to 

measure and telemeter crack measurements to CVO. In 

June 1981, the first displacement meter was installed on 

the east crater floor. The displacement meter and 

electronics were housed inside a fiberglass box that was 

connected to a plastic pipe 10 em in diameter and 12 m 

long. The displacement-meter wire (stainless steel) was 

strung through the plastic pipe to a fencepost on the 

other side of the crack. At this time, many of the cracks 

were hot and emitted steam and gas; where the wire 

crossed the crack, the plastic pipe was encased in a 

larger steel pipe for added protection. 

A separate fiberglass box contained the telemetry 

unit. Measurements were telemetered to CVO at 

10-minute intervals. Our intent was to observe movement 

of the blocks on either side of the crack before and 

during an eruptive episode, but condensation of water 

vapor and corrosion by gases caused the electronics in 

the meter to fail. 

After modifying the electronics housing to be 

airtight, the displacement meter was once again installed 

across a crack. As a check on the displacement meter, a 

crack station was established and measured by field 

crews. Prior to the September 1981 episode, the 

9. Monitoring Radial Crack Deformation by Displacement Meters 95 
telemetered data from the displacement meter indicated 

narrowing of the crack; field measurements confirmed 

this trend. It had been expected that the crack would 

widen as magma intruded, and the construction of the 

station was accordingly designed to accommodate 

widening, not narrowing. Consequently, as the crack 

continued to narrow, the stiff plastic pipe buckled and 

eventually broke the wire. As the pipe buckled and 

pulled on the wire, the displacement meter indicated 

widening, even though the crack itself was narrowing. 

Following the March 1982 episode, the radial 

cracks on the crater floor were buried by pumice 

deposits (Waitt and others, 1983). Consequently no 

measurements of cracks were made until 1985, when we 

started to establish geodetic stations on the dome itself 

(lwatsubo and Swanson, chapter 6). At this time a 

commercial displacement meter was purchased and 

installed (station Steger) next to an existing crack 

station (South August). This third installation was 

environmentally secure, and both the displacement 

meter and taped distances showed similar trends 

preceding the May 1985 episode (fig. 9.2). Except for 

minor modifications, this is the system configuration 

used in 1990. 

EQUIPMENT 

The primary equipment is the displacement meter 

itself. The displacement meter used in 1981 was 

designed and installed by Malcolm Johnston and Tom 

Murray and was described in the monthly report of the 

Cascades Volcano Observatory in June 1981. This meter 

used a spring, wheel, axle, stainless-steel wire, and a 

360-degree potentiometer. The stainless-steel wire was 

strung from one end pier, across the crack, and into the 

electronics box containing the displacement meter. The 

wire was wound around the wheel and attached to a 

spring for tension. The wheel was welded to an axle that 

had one end connected to the wiper of the potentiometer. 

As the wire was pulled or wound, the wheel would rotate 

the axle and change the position of the wiper of the 

potentiometer. Five volts was applied across the 

potentiometer, and as the wire moved and rotated the 

potentiometer, the voltage would change 

correspondingly. The signal output of the displacement 

meter was digitally transmitted to CVO. 

Beginning in 1985, with the installation of station 

Steger, a displacement meter purchased from Celesco has 

been used. The displacement meter, model PT101, has a 

Figure 9.1. Dome of Mount St. Helens in December 1980, showing radial cracks that formed on crater floor 

adjacent to it. 

98 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
3.8-m-long wire but can be purchased in lengths that 

range from 1 em to 12.7 m. The Pr101 has an operating 

temperature range of -17.7 to 93.3 ·c, the thermal 

coefficient is reported as 0.0011 percent per ·c, and the 

accuracy is 0.10 percent full-scale maximum. The 

dimensions of the meter are 19 x 13.5 x 11 em. With a 

3.8-m-long wire, the sensitivity of the meter is 1.4 

m V /mm. The PT1 01 works on the same basic principle 

as the displacement meter built by Johnston and Murray. 

Materials and Installation 

Installation costs are minor compared to the cost of 

the meter ($500). All totaled, the materials used for 

installation and wire protection cost approximately $50. 

This section describes materials and methods used to 

install displacement meters at Mount St. Helens. 

The displacement meter is mounted inside a 

surplus metal ammunition can (30 x 15.5 x 18 em) with 

six holes drilled in it, one for the displacement-meter 

wire, one for the signal output, and four to bolt the meter 

to the can (fig. 9.3A). Silicone sealant is used around the 

screw holes to prevent water and fumarole gas from 

seeping into the ammunition can. The meter is powered 

by a single 12-volt alkaline lantern battery (or two 6-volt 

C/) 

ffi100 

fw 

~ 

:J 

....J 

~ 75 

~ 

LLi 

CJ 

z 

<{ 50 I 

() 

w 

() 

z 

~ 25 CJ) 

0 

J 

1 0 1 5 20 25 MAY 5 1 0 1 5 20 25 

START DAY IS APR 09, 1985 G.M.T. -JULIAN DAY 099 

Figure 9.2. Taped (A) and telemetered (B) distance 

changes across South August crack, April-May 1985. 

Telemetered data from Steger displacement meter show 

start of deformation to be on May 18 and define a smooth 

curve as deformation increases. Tape measurements 

reflect same displacements, but could only be measured 

up to May 23. Downward inflections of Steger 

displacement data correspond to graben-forming events 

that occurred during extrusive episode, which began late 

on May 24. Vertical scale shows relative displacement 

from an arbitrary zero point. 

alkaline batteries), which also is inside the ammunition 

can. A 5-volt regulated reference supplies a constant 

voltage to the meter. The batteries power the 5-volt 

reference and displacement meter for one year. 

The displacement-meter wire passes through a 

small hole drilled into the side of the ammunition can. 

To protect the wire from rubbing and breaking on the 

metal edge, it is strung through a hard rubber plug (a 

rubber eraser would also suffice) with a hole drilled 

through it. The plug is then placed inside the small hole 

and glued in place with silicone sealant (fig. 9.3B). This 

A 

8 

Figure 9.3. A. Top view of inside of displacement meter 

housing (ammunition can). A 12-volt (two 6-volt batteries 

connected in series) battery is used to power 5-volt 

reference (small plastic container attached to wires). 

Holes are drilled in ammunition can for wire and to mount 

meter. Hose clamps are to connect housing to 

fenceposts. 8, Closeup view of wire coming out of can. 

Rubber stopper (left side) with hole drilled in it for wire is 

glued in the hole with silicone sealant. Plug on right-hand 

side is signal output to telemetry unit. 

9. Monitoring Radial Crack Deformation by Displacement Metera 97 
allows the wire to move freely in and out, protects it 

from the metal edge, and maintains the environmental 

integrity of the ammunition can. 

Three steel fenceposts are driven into the ground 

and cemented (if the fenceposts are not stable when 

driven in), two at the instrument end, and one on the 

other side of the crack for an end pier. The ammunition 

can is attached to the two fenceposts by two hose clamps 

mounted to the can (figs. 9.3, 9.4) and sits directly on the 

ground for added stability. 

A path between the fenceposts is cleared so that 

the displacement-meter wire can stretch between end 

piers without touching the ground. We typically string a 

temporary wire between end piers, at the height of the 

actual wire, to act as a guide while clearing the path. 

This temporary wire shows where material needs to be 

removed or filled in and can be kicked or hit with debris 

without fear of damaging the instrument. After the path 

is cleared, the temporary wire is removed and the 

displacement-meter wire is strung. 

Once strung, the wire must be protected from 

volcanic and environmental processes. Small explosions 

can impact the wire with ballistic ejecta 1 m or more in 

diameter, and snow accumulation (up to 10 m on the 

dome) during the winter months can pin the wire to the 

ground. To protect and give easy access to both the 

instrument and end piers, wooden boxes (61 em square) 

with large slots to allow the wire free movement are 

constructed and placed over each end pier (fig. 9.4). To 

Figura 9.4. Displacement meter housing attached to 

fenceposts by two hose clamps (rocks are spacers 

between hose clamps and fenceposts). Wooden box 

allows easy access to wire and displacement meter. End 

pier has a similar box surrounding it. This box shows 

signs of the one winter it has survived. 

cover the boxes, we place a separate piece of wood on 

top of the box and hold it down with rocks. 

In 1985, to protect the wire at station Steger, 

0.5-in. (1.3 em) plywood sheets were cut into 

30.5-cm-wide strips and connected together to fonn an 

A-frame over the wire (fig. 9.5). The ends overlapped 

one another to allow the frames to slide if narrowing or 

widening of the crack occurred. Tephra and dome rocks 

were piled against the A-frame for additional lateral 

support. This method perfonned well until the meter was 

destroyed in the May 1985 eruptive episode. In August 

1985, a displacement meter was installed on the dome at 

station Rainbow Warrior. An A-frame cover was again 

used to protect the wire, but by February 1986 10 m of 

snow had accumulated over the displacement meter and 

the A-frame was crushed, breaking the wire. 

A second installation method was tested at station 

Rainbow Warrior. A trench was dug from fencepost to 

fencepost and a plywood box approximately 20.5 em 

square and 10 m long (fonning a tunnel) was placed in 

the trench. The wire was then strung through the tunnel 

and attached to the anchor pier. The top of the box, 

which was level with the ground surface, was covered 

with dirt for added protection. The wire was thus safe 

from snow and ballistics. This method would not have 

been used had previous crack information not indicated 

that widening of the crack would be the dominant sense 

of movement. The buried box method will not work for 

measuring large-scale contraction. 

PROBLEMS 

In 1981, the two major problems that affected 

displacement-meter operation resulted from gases and 

construction design. The frrst displacement meter failed 

because the electronics corroded from condensation of 

gases. An environmental housing was subsequently used. 

The second displacement meter failed because the 

protective wire casing was too stiff and could not 

accommodate narrowing of the crack. Since 1985, the 

displacement meters have been housed in ammunition 

cans because they are airtight, large enough to enclose 

both the meter and battery, and inexpensive. The 

problem of wire protection has been solved as discussed 

above. 

A short time after station Steger was installed in 

1985, we noticed that the telemetered data differed 

significantly from the taped data at South August crack 

station. The problem was found to be electronic; the 

5-volt reference was drifting with time. The 

displacement-meter output signal is directly related to its 

input voltage, the 5-volt reference. In order to obtain true 

displacement readings, the 5-volt reference must be 

stable. A 5-volt reference is used because of the 

98 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
configuration of the digital telemetry. We now use a 

Precision Monolithic Incorporated, REF-02 5-volt 

reference. 

The most common problem we have encountered 

has been broken wires. Wire breakage is typically caused 

by the condensation of corrosive gases on the 

wire-even stainless steel wire is not immune. Breakage 

owing to corrosion is not a problem in the summer 

months, when gases can vent from the monitored crack 

and mix freely with the atmosphere. The problem arises 

during winter months, when the stations are buried under 

deep snow. 

A broken displacement meter can be difficult to 

repair. When the wire breaks, it often will snap back into 

the displacement-meter housing and (1) completely 

unwind from the wheels inside, (2) cause the tension to 

release, and (3) can damage the potentiometer. We 

obtained instructions from the manufacturer on how to 

rewind the wire and set the spring tension, and 

potentiometers can be purchased locally. Therefore we 

are able to repair the instruments when they break. 

To protect the thin (0.038 mm), stranded, stainless 

steel wire of the displacement meter from corrosion 

during winter months, we attach a heavier (1 mm), solid 

stainless-steel wire to the end pier and stretch it across 

the hot crack to within 1-2m of the displacement meter. 

The thinner wire of the displacement meter is then 

connected to the heavier wire. If the heavier wire is too 

long, the relatively weak spring in the displacement 

meter will not be able to hold it off the ground. In this 

case, the total length of the .installation must be 

shortened until the spring holds the heavier wire above 

the ground. 

The heavier wire increases the overall life 

expectancy of the installation, but the remaining thinner 

wire is still susceptible to corrosion over time. Prior to 

the use of the heavier wire, installations have lasted 2-3 

months during winter, but with the heavier wire, 

installations lasted up to 6 months before the thinner 

wire broke. To improve the installations, we now replace 

the manufacturer's wire with nylon-coated, stranded 

stainless steel fishing line (purchased at a fishing shop). 

The diameter of the nylon-coated wire is a little larger 

than the wire supplied with the displacement meter but 

not enough to affect the sensitivity of the meter. 

In addition to modifying the wire, we drill holes 

(2-3 em) in the protective wood box and frame to help 

circulate air and to vent corrosive gases and steam away 

Figure 9.5. Scientist measuring South August crack with a steel tape in 1985. Steger displacement meter is in 

background. Note top of A-frame (behind tape) and dirt piled against it for lateral support. 

9. Monitoring Radial Crack Deformation by Displacement Meters 99 
from the wire. During winter, snow typically melts from 

the ground up because of the warm crack, thereby 

forming caverns under the snow that help improve air 

circulation around the wire. More holes are drilled in the 

area where the wire crosses the crack, because that is 

where the gases and steam are most concentrated. We do 

not drill large holes (>3 em) over colder areas, because 

snow can drift into the holes, pin the wire to the ground, 

and cause erroneous data. 

DATA 

Prior to the May 1985 episode, South August 

crack station and station Steger were monitored 

simultaneously (fig. 9.2). From taped distances alone, 

the start of deformation for the South August crack 

would not have been known, but telemetered data show 

the start to be May 18-19. The frrst day a taped distance 

indicated movement was May 21, after the start of 

deformation. On May 23, taped measurements and 

telemetered data indicated 33 mm and 31 mm of 

extension, respectively. The total amounts of 

displacement between taped distances and telemetered 

data are similar, but the curve is better defined by the 

(/) 

ffi 7 Rainbow Displacement tu 5 ~ 3 

:J 1 E---,,.--,, 

~ -2 

~ ~ 45 

w- 40 _,..-----~""' ai~~wTem erature ~~]- ffi ~ 35 ~ 

w ~ 30 0 25 :_____1__ ~~~~~~--L-~J_~~-~--~ 

(/) 

ffi 4 b~andi Disp-lacement tu 3 

~ 2 -

:J 1 

~ 0 L_____l_____ -'-------'---------'--- -d 

Figure 9.6. Plots displaying a 14-day period of quiet in 

1989 at two displacement meters, Rainbow Warrior and 

Sandi. Straight line for Sandi indicates a stable 

instrument and electronics, while Rainbow Warrior 

(although quiet) displays 1 mm of background noise. 

Temperatures are monitored near instrument pier at a 

depth of 0.5 m. Temperature changes do not affect 

displacement-meter signals. 

telemetered 10-minute data. The reversal in trend on 

May 25 could be attributed to movement of a bounding 

block, south of both stations, which may have pushed the 

meter and crack northward. This corresponds to the 

formation of a large graben during this episode 

(Swanson, 1985; CVO monthly report, May 1985). 

South August crack began to widen, as it was possibly 

falling into the graben, just prior to the loss of telemetry. 

Periods of quiet are more often the norm for 

displacement meters. Figure 9.6 is a plot of a 14-day 

period for the two displacement meters on the dome in 

1989. Ground temperatures are monitored approximately 

0.5 m beneath the surface near each instrument pier and 

show how relatively warm the ground remains. Data in 

figure 9.6 show that the meters are not seriously 

affected by large temperature fluctuations. 

Snow may drift through the holes in the protective 

wood box and frame and accumulate on the wire, 

causing erroneous data (fig. 9.7). On March 25, snow 

accumulated on station Sandi (located on the upper west 

part of the dome) and caused apparent extension. Such 

extension is relatively sudden and steplike, and differs 

from the volcanic-deformation signal observed at station 

Steger (fig. 9.2). As the snow slowly melted, the signal 

returned to within about 1 to 2 mm of its previous 

position. 

20 

~15 r ~ 10 I'~ 

~ : L,='71 ' ' ' ' ' ' I ' ' ' I ' ' ' ' I ' ' ' _L_L__J__---'----J_J_----'----'---J'---L-'-----'----J 

gs 14 

en 

_J 

w 12 

0 

(/) 

ttj 10 

a: 

~ 8 0 

6 ~~~~~~~~~~~~~~~~~~~ 

1 0 15 20 25 APR 5 1 0 15 20 25 

START DAY IS MAR 10, 1988 G.M.T.- JULIAN DAY 070 

Figure 9.7. Effect of snow accumulating on wire of 

station Sandi during one heavy snowfall. These offsets 

normally occur quickly and are steplike as snow falls on 

framing, accumulates, then falls through holes and onto 

wire, causing appearance of crack widening. Signal 

returns to its pre-snowfall position (within 1 to 2 mm) after 

snow melts. 

100 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
SUMMARY 

The monitoring of cracks using displacement 

meters lends itself well to deformation studies of 

near-vent processes at Mount St. Helens. Structures 

likely to show displacement must be identified by field 

observation and carefully studied before installation of 

real-time monitoring equipment The problems that arise 

with the installation of displacement meters have not 

been trivial, but the experience gained from earlier 

failures and successes is important. Continuous 

monitoring of crack movement at Mount St Helens is 

especially critical prior to eruptive episodes. Deformation 

of an active crack corresponds to magma intrusion and 

can give information indicating the rate and amount of 

deformation taking place. During bad weather when 

crews cannot get into the field, displacement-meter data 

can be used with other telemetered data, such as tilt, gas, 

and seismic monitoring, to help decipher eruptive 

activity. 

Having the foreknowledge that a particular crack 

widens or narrows during magma intrusion has helped in 

the design of individual stations. Each environment will 

have a special set of problems and answers. At Mount St 

Helens, each failure has led to a better end product. 

Housing the meter and electronics in environmentally 

sealed cases, protecting the wire from snow and 

ballistics, and solving the problems of corroding wires 

are a few improvements that have helped us to design a 

longer lasting displacement meter. 

REFERENCES CITED 

Chadwick. W.W., Jr., and Swanson, D.A., 1989, Thrust faults 

and related structures in the crater floor of Mount SL 

Helens volcano, Washington: Geological Society of 

America Bulletin, v. 101, p. 1507-1519. 

Chadwick. W.W., Jr., Swanson, D.A., lwatsubo, E.Y., Heliker, 

C.C., and Leighley, T. A., 1983, Deformation monitoring 

at Mount SL Helens in 1981 and 1982: Science, v. 221, p. 

1378-1380. 

Swanson, D.A., 1985, Graben formation, thrust faulting, and 

growth of the dacite dome, Mount SL Helens, Washington, 

May-June 1985: Eos (Transactions American Geophysical 

Union), v. 66, p. 852. 

Swanson, D.A., Lipman, P.W., Moore, J.G., Heliker, C.C., and 

Yamashita, K.M., 1981, Geodetic monitoring after the 

May 18 eruption, in Lipman, P.W., and Mullineaux, D.R., 

eds., The 1980 eruptions of Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 

1250, p. 157-168. 

Swanson, D.A., Casadevall, T.J., Dzurisin, D., Malone, S.D., 

Newhall, C.G., and Weaver, C.S., 1983, Predicting 

eruptions at Mount St. Helens, June 1980 through 

December 1982: Science, v. 221, p. 1369-1376. 

Waitt, R.B., Jr., Pierson, T.C., MacLeod, N.S., Janda, R.J., 

Voight, Barry, and Holcomb, R.T., 1983, 

Eruption-triggered avalanche, flood, and lahar at Mount 

St. Helens-Effects of winter snowpack: Science, v. 221, 

p. 1394-1397. 

9. Monitoring Radial Crack Deformadon by Displacement Meters 101 

10. Trilateration and Distance-Measuring Techniques 

Used at Cascades and Other Volcanoes 

By Eugene Y. lwatsubo and Donald A. Swanson 

ABSTRACT 

From 1980 to 1989, scientists of the U.S. Geological 

Survey's Cascades Volcano Observatory established 

trilateration and distance-measuring networks on Mount 

Baker, Mount Rainier, and Mount St. Helens in 

Washington; Mount Hood, South Sister, Newberry, and 

Crater Lake in Oregon; Medicine Lake, Mount Shasta, 

Lassen Peak, and Mammoth Lakes in California; and 

Augustine Island in Alaska. The networks were installed to 

provide baseline information on potentially active and 

dangerous volcanoes. The experience gained in 

monitoring Mount St. Helens has helped in designing 

guidelines for establishing these networks. One improved 

method of measuring air temperatures (for atmospheric 

corrections to the distance) at each bench mark has led to 

considerable savings in helicopter time by not requiring 

continuous temperature measurements along the path of 

the infrared signal. Bench-mark stability is critical and 

must be carefully evaluated both during installation and 

later if movement is measured. The equipment and 

methods currently in use are sufficient to meet the 

precision (±2-3 ppm about any given distance) that is 

required for baseline data. 

INTRODUCTION 

Trilateration and distance-measuring networks 

have been established on 12 potentially active 

volcanoes in the Pacific Coast States beginning in 

1980. These networks, once the baseline information 

has been collected, can detect surface deformation that 

may reflect magma movement up the conduit. The 

rates of deformation increase as magma approaches the 

surface, and these measurements can therefore help 

determine where and when an eruption may occur 

(Lipman and others, 1981). Before 1980, electronic 

distance meters (EDM's) had been used primarily to 

monitor horizontal deformation during inflation and 

deflation of shield volcanoes (Kinoshita and others, 

1974). Little horizontal deformation monitoring had 

been attempted on stratovolcanoes with the exception of 

Usu volcano, Japan, where up to 160 m of movement 

was obser:ved using trilateration techniques to monitor 

cryptodomes forming in the summit area in 1977-78 

(Yokoyama and others, 1981). Several distances at 

Mount St. Helens and Mount Hood were measured in 

1972 but were not remeasured prior to the reawakening 

of Mount St. Helens in 1980. Several distances were 

measured twice at Mount Hood in the summer of 1980, 

owing to an earthquake swarm, but no complete network 

was established and no significant changes were 

observed. 

In mid-April 1980, measurements of distances and 

angles were initiated at Mount St. Helens, primarily to 

monitor the rate of deformation of the bulge on the 

north side of the volcano. Displacements of 1.4-1.6 

m/day were measured on the bulge prior to May 18, but 

there was little or no significant change outside the 

bulge area (Lipman and others, 1981). A complete EDM 

network was established at Mount St. Helens shortly 

after the catastrophic eruption on May 18, 1980 

(Swanson and others, 1981; Iwatsubo, Topinka, and 

Swanson, chapter 8). 

Monitoring networks were established at other 

volcanoes in the Pacific Coast States, including 

Augustine Island, Alaska, between 1981 and 1989 (fig. 

10.1; table 10.1). All networks, except the Medicine 

Lake and partially completed Mammoth Lake networks 

(both installed 1989), have been reoccupied at least 

once. Periodic reoccupation of these networks is 

planned as part of an overall long-term monitoring 

program. Reoccupation serves as a check on previous 

data, solidifies the baseline information, and provides an 

assessment of the state of the volcano. 

Techniques and equipment developed in the 

1980's have helped to reduce costs for monitoring 

volcanoes without compromising the precision of the 

data. We explain in this report the techniques and 

procedures used by Cascades Volcano Observatory 

(CVO) scientists to measure distances at volcanoes. For 

10. Trllateratlon and Distance-Measuring Techniques Used at Cascades and other Volcanoes 103 
Table 1 0.1. List of years when each volcano has been 

measured 

(Not included is Mount St. Helena, which has been monitored 

continuously since 1980. No networks were occupied in 1987] 

Volcano 1981 1982 1983 1984 1985 1986 1988 1989 

Augustine X X 

Mount Baker X X 

Mount Rainier X X X X 

Mount Hood X X 

Crater Lake X X X X X 

South Silt« X X 

Newbetry X X 

Medicine Lake X 

Mount Shalla X X X 

Lassen Peak X X X 

Mammoth X 

Mountain 

further information on general trilateration and 

distance-measuring techniques, see Brinker and 

Minnick (1987) and Davis and others (1981). 

EQUIPMENT 

Most of the equipment used for surveying can be 

purchased commercially (see appendix). The methods we 

use to install bench marks and to measure distances, 

angles, barometric pressure, humidity, and air 

temperature are described below. These methods, in use 

in 1990, have been evolving since 1981 and should 

continue to improve in the future. 

Bench-mark Installation 

All bench marks installed are 10-cm-diameter brass 

bench marks with 7 -em stems that are cemented into 

bedrock or large boulders (Chadwick and others, 1985). 

Holes approximately 2.5 em in diameter are bored into 

the rock, mainly by hand, using a masonry star drill. The 

bench marks are stamped with the station name and year 

of installation. A fast-ewing cement (15 minutes curing 

time) is used to cement bench marks in place, because 

they are commonly installed at high elevations where the 

possibility exists of freezing, which destroys regular 

cement (24-hour curing time). Immediately after initial 

installation, we shade the bench mark and cement from 

direct sunlight to prevent cracking of the hardening 

cement. EDM bench marks can be used immediately 

after installation, but it is better to wait at least 24 hours 

to allow complete hardening of the cement. See Doukas 

and Ewert (chapter 11) for further discussion of 

bench-mark installations. 

Surveying Equipment 

We have used two EDM's that can measure 

distances up to 10 km, which is the longest distance 

measured in our monitoring program. The typical 

measured distance is 3 to 5 km. The majority of 

horizontal deformation work in the Cascades and other; 

volcanoes is completed by helicopter and short . 

excursions by foot, so the EDM must be portable. From 

1981 to 1988, we used a Hewlett-Pac~d (HP) model 

3808A (no longer manufactured); in 1989, we purchased 

a Geotronics model Geodimeter 114, which is much 

122° 117° 

.t. Mt. Baker 

WASHINGTON 

... Mt. Rainier 

, Mt. St.. H_e!ens. . ....... 

,.• 

• Mt. Hood 

OREGON 

.._South Sister IDAHO 

• Newberry 

• Crater Lake 

... Medicine Lake ... . 

Mt. Shasta 

.t. Lassen Peak 

CALIFORNIA : 

NEVADA 

.._Mammoth 

Mountain 

0 200 KILOMETERS 

Flgure10.1. Volcanoes at which personnel of Cascades 

Volcano Observatory have installed horizontaldeformation monitoring networks. Not included is 

Augustine Island, Alaska. 

104 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Table 10.2. Specifications for HP3808A and Geodimeter 

114 

[Weight includes battery. Distances shown are those quoted by 

manufacturer for number of prisms on clear day (light haze or moderate 

sunlight) with light heat shimmer] 

HP3808A Geodimeter 114 

3.bn 6.S .bn 

3 6.bn 8.0 .bn 

Multiple 10 .bn (6) 9.0 .bn (7) 

Weight lOkg 2.7 kg 

Size 34Sx318x283 mm 220x190x87 mm 

Accuracy ±(S mm + 1 ppm) ±(S mm + 1 ppm) 

Mount Yoke Yoke or Theodolite 

smaller and lighter (table 10.2). Although the 

manufacturer's specifications for the Geodimeter 114 are 

better than those for the HP3808A, in practice our 

HP3808A is able to measure longer distances to the same 

number of prisms than the HP specifications state and 

farther than is possible with the Geodimeter 114. 

The dimensions of an EDM are critical when used 

in moderate to high winds. The bulky HP3808A can 

catch wind and vibrate constantly, making it difficult to 

complete measurements. Each HP3808A measurement 

takes as long as 1 minute, depending on the length of the 

shot, even on calm days. When wind causes instrument 

vibration, measurement can take up to several minutes to 

complete and may even fail. The Geodimeter 114 takes 

14-25 seconds for the fll'St measurement, then repeats the 

measurement every 4.5 seconds. Often, the Geodimeter 

114 can read 10 measurements before the HP3808A can 

measure one. This is a big advantage in marginal 

conditions. 

The lightweight Geodimeter 114 is preferable for 

backpacking. A backpack frame is available for the 

HP3808A and is essential for most field applications; the 

carrying case for the Geodimeter 114 comes with straps 

that can be used as a backpack. Both EDM's use a 

12-volt battery as the main power source. The HP3808A 

battery fits into the bottom of the instrument, whereas 

the Geodimeter 114 battery is connected to the EDM by 

an external cable. We advise purchasing an additional 

battery cable that can be used with a separate 12-volt 

sealed or automobile battery. This battery should be 

carried in the field as a backup power supply. If trips are 

planned to remote areas for extended periods and the 

recharging of batteries is impossible, several batteries 

must be carried. 

The HP3808A has an internal telescope for 

sighting and is mounted on the tripod using its built-in 

yoke. The Geodimeter 114 does not have an internal 

telescope but can be used in two ways. The Geodimeter 

114 is designed to fit on the telescope of the theodolite, 

which is used for sighting on the target. A yoke with a 

telescope (4-7x magnification) mounted on it can also be 

used, and we recommend purchasing the most powerful 

telescope (30X magnification minimum) available. 

Because of budget limitations we did not purchase the 

powerful telescope, and the 4--7x magnification telescope 

on the yoke is not powerful enough for our applications. 

Therefore we always mount the Geodimeter 114 on the 

theodolite. 

When the EDM is yoke-mounted, the optical center 

or vertical axis of the EDM always remains the same, 

regardless of the tilt of the EDM. In other words, the 

EDM pivots over the center of the bench mark. 

However, when the EDM is mounted on a theodolite 

telescope, the optical center of the telescope pivots over 

the center of the bench mark, but the optical center of 

the EDM does not. The difference between the optical 

centers of the theodolite and of the EDM when tilted is 

the eccentricity error. To correct for this error, either a 

zenith or vertical angle must be measured and 

incorporated into the slope distance reduction. The 

formula to correct the slope distance (SD) for the 

eccentricity error (corr) is: 

corr = e • cos(Z) 

corrected SD =raw SD- corr, where Z <90• 

corrected SD =raw SD + corr, where Z >90• 

where e is the distance between the optical center of the 

theodolite and the EDM, and Z is the zenith angle (Bevin 

and Dip, 1983). 

To measure zenith angles for the eccentricity error 

and to calculate station elevations, we use a Wild T2 

(new style) theodolite. The Wild T2 is manually read to 

the nearest 1 second; any good-quality 1-second 

theodolite is recommended for measuring angles. The 

advent of microprocessors has revolutionized most 

surveying equipment, including the theodolite. Wild 

manufactures the T2000 theodolite, that is accurate to 0.5 

second and digitally reads out to 0.1 second (Ewert, 

chapter 15). Its drawback for our purposes is its weight, 

9.8 kg versus 6.3 kg for the T2. 

Instruments and reflectors are typically mounted on 

Kern tripods with centering rods, but standard tripods 

with optical tribrachs can also be used. For more detailed 

discussion of tripods, see Iwatsubo and Swanson (chapter 

6). 

For reflectors, we use standard (accuracy within ±2 

arc seconds), 7 .3-cm diameter, glass comer-cube prisms 

in a triple nontilting mount assembly (a triad). When 

using the HP3808A, we carry two triads in each reflector 

backpack. For almost all of the measurements, two triads 

are sufficient. With the Geodimeter 114, four triads are 

carried in each reflector backpack, with 2-3 triads 

normally being set up for the average 3-5 km line. 

Recently, tilting mounts with smaller diameter (5.9 em) 

prisms became available. Tilting mounts are superior to 

10. Trllateratlon and Distance-Measuring Techniques Used at Cascades and other Volcanoes 105 
nontilting mounts when steep lines are measured, 

because they can be aimed directly at the EDM 

increasing the return signal strength. Steep lines can be 

measured with nontilting mounts but may require more 

reflectors. 

Atmospheric System 

Absolute barometric pressure, humidity, and air 

temperature are needed to correct the raw slope distance 

to an actual slope distance. The distance displayed by an 

EDM is based on some assumed standard atmosphere 

established by the manufacturer. Any deviation from the 

standard atmosphere must be corrected for. A correction 

of 1 part per million (ppm) is produced by 

a 1 ·c change in air temperature, 

a 3.4 mbar change in barometric pressure, or 

a 22.66 mbar change in water vapor pressure 

(Bevin and Dip, 1983). 

Humidity has a very small effect on the 

measurements but nonetheless is . included in all data 

reduction. 

A complete atmospheric package, including 

pressure, humidity, and temperature sensors, is carried in 

all reflector and instrument packs. A barometric-pressure 

sensor we initially used to record barometric pressure 

performed well but was bulky, consumed considerable 

power, and needed frequent maintenance. A hand-held 

digital barometer/altimeter, model AIR-HB-1A, was 

'purchased to replace the barometric-pressure sensor. The 

AIR-HB-1A is commercially available and has upgraded 

our barometric pressure measurements. It has a pressure 

range of 364 to 775 mm Hg with a resolution of 0.1 mm 

Hg; other specifications are listed in the Appendix. The 

size, capabilities, and ease of use of this unit make it 

ideal to be part of the atmospheric package. Another 

commercial barometer package is the Ultimeter model 

12, which has a resolution of 1 mm Hg and a range of 

330 to 787 mm Hg. Other pressure indicators, such as 

the Thommens altimeter/barometer, can be used but 

should be checked daily, before and after use, against a 

more accurate instrument (such as the AIR-HB-1A). 

Each of our reflector packs carries a Thommens altimeter 

as a backup for the AIR-HB-1A. 

Humidity measurements above 0 ·c are made using 

a sling psychrometer, which measures a dry- and 

wet-bulb temperature. When temperatures are below 

freezing, some other method must be found to measure 

humidity. 

Air temperature is the most important atmospheric 

parameter measured. This is another area where we have 

improved our data collecting methods. Endpoint 

temperatures can be affected by ground radiation, but 

W.H. Prescott (unpub. USGS report, 1971) showed that 

measurement of temperatures 7 m or more above ground 

level can minimize this problem. We now use a 

telescoping, 7.6-m-long fiberglass rod as a pole for the 

temperature sensor. By using the 7.6-m rod in place of 

the previous 6-m poles (Iwatsubo and others, 1988), we 

have minimized the ground radiation effects. This longer 

rod is sufficiently sturdy to stand up in moderate winds 

and short enough ( 1. 78 m collapsed) to be used in 

helicopter operations. The 6-m fiberglass poles are 

shorter ( 1.06 m collapsed) and lighter, and are still used 

when considerable hiking is required and there is enough 

wind to allow good air mixing, thereby helping to 

minimize ground radiation effects. 

f 

Flgure10.2. Temperature shield mounted on 

telescoping leveling rod. Sensor is mounted between two 

sets of parallel plates with wires connecting it to a plug 

on bottom of shield. Separate wire is then connected to 

box containing temperature-output display (fig. 1 0.3). 

106 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
To avoid direct sunlight on the temperature 

sensor, a naturally aspirated shield is used to house the 

sensor. The shield consists of four thin aluminum 

plates 10 em square, two above the sensor and two 

below (fig. 10.2). The pairs of plates are 5-7 mm 

apart, and the temperature sensor is mounted between 

the two pairs of plates, which are separated 1 em 

apart. The shield is attached to the top of the leveling 

rod by threaded aluminum stock. The sides of each 

plate facing away from the sensor are painted white to 

reflect light, and the sides facing the sensor are 

painted black to absorb light. 

Figure 10.3. Pelican case containing barometer/ 

altimeter (left side), battery and 5-volt regulator (middle), 

and digital multimeter for temperature display (right side). 

Wire from temperature shield plugs into front of this box, 

just below handle. 

This shield is lightweight and allows adequate air 

circulation. 

A wire for power and signal output connects the 

temperature sensor to a separate box that contains the 

power supply, barometer, and a digital display for the 

temperature sensor. A foam resin case ( 4 7x39x17 em; 

manufactured by Pelican Products, Inc.) that is airtight 

(with a pressure-release valve), corrosion free, and durable, 

houses this equipment (fig. 10.3). A centtal foam section 

that is easily cut to tightly fit any instrument protects the 

equipment inside the case. After years of use, no damage 

has occurred to any of our cases. 

We use a National Semiconductor LM335 precision 

temperature sensor powered by a 5-volt voltage reference; 

an LM78L05 reference is currently being used (fig. 10.4), 

but we will update to an REF-02 for better stability. A 

12-volt sealed rechargeable battery (6.5 Ah), which allows 

long periods of use without recharging, powers the 5-volt 

reference. For short periods, a 9-volt transistor battery can 

replace the larger 12-volt battery. This is sometimes 

necessary, when it is not possible to carry both the larger 

battery and the case because of weight or bulk. The signal 

output of the temperature sensor is displayed by a digital 

4th-digit Fluke multimeter. The LM355 sensor-output 

signal is a voltage that corresponds to the air temperature 

and is displayed to the nearest 0.1· K. A flow chart (fig. 

10.5) shows the path taken to record air temperatures. 

The temperature sensors are calibrated at least 

once a year against a more accurate quartz thermometer. 

An environmental chamber, quartz-thermometer unit 

(HP2804A with a fast-responding probe), 

data-acquisition unit (HP3.421 A), and a personal 

computer are used for calibrating sensors. The range of 

+12 VOLT .., LM78L05 

OUTPUT _ .... 

.33 JlF .01 JlF 

GROUND 

Figure 10.4. Circuit diagram of LM78L05 used to 

regulate voltage for temperature sensor. We will be 

swithching to a REF-02 for a more stable reference. 

10. Trllateratlon and Distance-Measuring Techniques Used at Cascades and other Volcanoes 107 
calibration is from -20 to 50 ~C in 1 ·c intervals 

(lwatsubo and others, 1988). 

All of the above equipment, except for the 

temperature rod and tripod, are packed inside a typical 

external-frame backpack. The tripod can be strapped to 

the backpack if desired. The pack is easy to carry and 

fits into a helicopter. These factors are important when 

setting up a system that must be portable. 

NETWORK SETUP AND MEASURING 

PROCEDURE 

Setting up a network starts with the · best 

topographic map available, preferably one with vegetated 

areas marked on it. About six instrument stations are 

selected at the base of the mountain, approximately 60 

radial degrees apart relative to the summit. Ideally each 

instrument station should be visible from the next station 

so that measurements can be made between them, but 

this is not critical and is commonly difficult to achieve. 

Next, we select reflector sites at medium to high 

elevations on the volcano that can be seen from at least 

two instrument stations. Reflector sites visible from three 

instrument stations are optimal. If only one instrument 

station can be seen and the line is considered important, 

then we install it. We try to form triangles when 

establishing stations and to connect the whole network 

together when possible. This ideal can sometimes be 

accomplished on a map but is rarely met when attempted 

in the field. Topography, vegetation, snow and ice cover, 

SHIELD/ 

SENSOR 

t ~ .I ' 

,J; 

12-VOLT 

SUPPLY 

PRESSURE DIGITAL 

METER l METER 

5-VOLT 

REGULATOR~ 

_l 1' -.!. J 

Flgure10.5 Flow chart representing atmospheric 

system. The 12-volt battery powers 5-volt reference, 

which in turn powers temperature sensor with its 

signal-output (temperature in kelvins) displayed by a 

digital voltmeter. 

and bedrock outcrops ultimately dictate what can be 

established at any volcano. Examples of three monitoring 

networks are shown in figure 10.6 to demonstrate their 

diversity. 

During installation of a network, one person 

occupies a potential instrument site, and one is located at 

each respective reflector site. If a solid rock outcrop or 

large boulder firmly set in the ground is present and 

visibility is good, the bench marks are installed. People 

move around the volcano until the network is completed. 

It is imperative that visual confirmation be made before 

the bench marks are set. There have been times when an 

instrument site had to be moved several hundred meters 

to complete a triangle or to be seen from more than one 

reflector station. 

Bench-mark location descriptions should be written 

at the time of installation and should be improved when 

the station . is reoccupied during the measurement of the 

lines. Writing clear bench-mark descriptions in the field 

is important, so that in the future someone who has never 

been to the bench mark can locate it. We try to be as 

complete as possible. Descriptions should include 

anything that can help locate the station, taking into 

account any seasonal effects such as snow covering the 

bench mark, fallen leaves in autumn, and possibly future 

human intervention. We have had problems with 

vandalism at some sites where bench marks have been 

removed or damaged. If we anticipate vandalism, we 

now bury the bench marks several tens of centimeters 

below ground and write a careful description of where 

one needs to dig. Photographs of the bench mark and 

surrounding rocks are very helpful. 

We typically use four people to measure a 

network, two at the instrument end and two separate 

reflector people. All parties should have two-way radio 

communications for a number of reasons; among other 

things, this ensures that the atmospheric data are 

collected simultaneously. Most often, it is not possible to 

locate the reflector from the instrument station without 

the aid of a signal mirror. Both instrument and reflector 

people flash toward each other, enabling the instrument 

person to aim the EDM and the reflector person to aim 

the prisms. Sometimes a flash from the EDM end of the 

line can be reflected by the prisms at the reflector end, 

and there is no need for the reflector person to flash. On 

cloudy days, bright clothing, flags, or flares are useful. 

Once the reflector is sighted, the slope distance and 

zenith angle are measured. Two lines can measured 

before someone moves. Measuring the network follows 

the same procedure as setting it up; people move from 

site · to site in a leap-frog manner until all the lines are 

completed. 

Complete recording at the instrument and reflector 

sites of all information necessary to reduce the data is 

108 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
critical but easily overlooked. A form that must be 

filled out completely before leaving the site is one way 

to minimize problems. We use a data sheet printed on 

"Rite-in-the-Rain" waterproof paper (fig. 10.7). Both 

the instrument and reflector people fill out appropriate 

parts of the same data sheet. Each backpack is 

assigned a number that corresponds to a specific 

calibrated temperature sensor and is referred to as the 

"BOX #" on the data sheet. Hence the same sensors, 

tripod, reflector set, and notebook are always together. 

A 

46" 55' 

thereby minimizing potential calibration problems. It is 

imperative that the height to the optical center of both 

the EDM and the reflector prisms be recorded in order 

to reduce the data (we reduce to mark-to-mark 

distances). The bottom lines on the data sheet are for 

recording angles. Miscellaneous notes can be recorded 

on the back of each sheet. Such notes might include the 

geometry of an unusual prism setup, sketches of 

landmarks, comments on weather conditions, or 

condition of the bench mark. 

121" 45 

Base from U.S. Geological Survey, Mt. Rainier National 

Park, Wash., 1971, 1 :50,000 

Figure 10.6. Monitoring networks established at three Cascade volcanoes. Contours are in feet. A, Trilateration 

network at Mount Rainier, Washington. Numbers and names identify bench marks. Rainier network is a "typical" 

trilateration network. 8, Distance-measuring network at Newberry, Oregon. Paulina Peak is on highest part of caldera 

rim, and all shots radiate down from it. This is an "atypical" network dictated by topography. Numbers identify each 

line. C, Another "atypical" network to monitor Crater lake, Oregon. line 1-3 was established as an independent 

measurement to check stability of 1-"Lodge" line. 

10. Trllateratlon and Distance-Measuring Techniques Used at Cascades and other Volcanoes 109 
8 

43° 40' 

Flgure10.6.-Continued 

121 ° 15' 

Base from U.S. Geological Survey, Paulina Peak, Oreg., 

1963, 1 :24,000; and East Lake, Oreg., 1982, 1 :24,000 

110 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
DISCUSSION 

The type of equipment and methods used should be 

detennined by the required precision of the data. H the 

project requires the highest quality data, expect large costs 

for specialized equipment and helicopter or airplane hours 

to fly temperature and humidity profiles along the lines. 

Precision of 3 mm and a proportional error of 0.2 ppm of 

line length can be attained using such methods with the 

greatest relative precision at longer distances (Savage and 

Prescott, 1973). However, we have found that owing to 

random errors, which include incorrect centering and 

inaccurate atmospheric conditions, systematic errors such as 

improper calibration and aiming errors (McDonnell, 1987), 

c 122° 10' 

Figure 10.6.-Continued 

and bench-mark instability, our precision is ±(2.46 ±2.26 

ppm) about any given distance. Such changes should be 

random and not defme a pattern, but if a pattern is defined, 

the possibility of real deformation should be considered and 

an attempt should be made to reoccupy the network as soon 

as possible. 

From 1981 through 1986, we used a helicopter to fly 

along each measured line and collect continuous 

temperature and humidity data. Barometric pressures weze 

collected at each endpoint An average index of refraction 

for the line path was calculated from this infonnation and 

used in the data reduction. This is the standard procedure 

used for collecting high-precision strain ~ when line 

lengths are typically 20--30 km and relatively flat, such as 

Base from U.S. Geological Survey, Crater Lake National 

Park and vicinity, Oreg., 1956, 1 :62,500 

10. Trllateratlon and Distance-Measuring Techniques Used at Cascades and other Volcanoes 111 
those used in earthquake studies (Savage and Prescott, 

1973). Lines measured on volcanoes are shorter and 

steeper and are very difficult to fly because they are in 

mountainous terrain where winds are typically strong and 

gusty. For these reasons, we compared distances 

calculated by using two-endpoint air temperature data 

obtained from the new 7 .6-m rods with those utilizing 

the continuously recorded temperature and humidity data 

(table 10.3). There is little difference between the two 

sets of data; all of the differences are within expected 

error. Similar comparisons have been made for a number 

of networks with similar results. Consequently, we no 

longer fly the lines and have saved up to 50% on 

helicopter hours and time spent to complete a survey. 

EDM RECORDING SHEET 

INSTRUMENT? RM/HP/LEITZ/GEOD 114/GEO 210 

NETWORK LINE NUMBER. ____ _ 

INST STA REFL STA. ______ _ 

DATE TIME (LOCAL) ____ _ 

INST TEMP V/F/C 

INST PRESS inHG/FT/MB 

INST HEIGHT ___ SH/HI INST BOX # __ _ 

GUN PERSON ------ W =__ D = __ 

REFL TEMP V/F/C 

REFL PRESS ______ ---inHG/FT/MB 

REFL HEIGHT ____ SH/HI REFL BOX # 

REFL PERSON ______ W =-- D = __ _ 

NUMBER OF TRIADS __ _ 

DISTANCE: M/FT 

Figure 10.7. Data sheet used to record information 

during each measurement, printed on waterproof paper. 

The "W" and "D" spaces are the sling psychrometer wetand dry-temperature readings. 

Therefore, we have ultimately reduced the cost of each 

survey. 

We have established networks on the following 

Cascade volcanoes: Mount Baker, Mount Rainier, 

Mount St. Helens, Mount Hood, Newberry, South Sister, 

Crater Lake, Medicine Lake, Mount Shasta, and Lassen 

Peak. Similar networks were installed at Augustine, 

Alaska, in 1988 and in late 1989 to help monitor 

Mammoth Mountain in eastern California. Repeat 

measurements of each network, except for those at 

Medicine Lake and Mammoth Mountain, both of which 

were first installed in 1989, have established baseline 

data (table 10.1). 

Occasional reoccupation helps assess bench-mark 

instability, the major source of error in such networks. 

All the Cascade volcanoes have high snowfall, and 

bench marks are subject to severe freeze-thaw 

conditions. When establishing the networks, the best 

bench-mark location available at the time was chosen: in 

some cases this choice was dependent on the snowpack 

and led to incorrect assessment of bench-mark stability. 

Mount Rainier provides a good example of such a 

problem. 

The network at Mount Rainier (fig. 10.6A) was 

established in 1982 and partially reoccupied in 1983. No 

major changes were seen during this interval. In 1988, a 

portion of the network was once again measured and 

larger than expected changes were seen on distances to 

several bench marks in an area where glacial-outburst 

flooding had periodically occurred since 1987 (T. 

Pierson, CVO, written commun., 1987). The distance to 

one reflector station high on the cone (station 13 in fig. 

10.6A) had contracted 20 em. Stations 14 and 15, both 

measured from Iron Mountain, had moved -0.036 and 

0.057 m, respectively. We were uncertain if actual 

deformation was taking place, so the entire network was 

again measured in 1989. When the network was initially 

installed in 1982, the rock that contains bench mark 13 

was mostly buried in snow and appeared to be solid. In 

1989, after a winter with low snowfall, the person who 

installed the bench mark noted that the snow had melted 

away from the rock containing the bench mark, revealing 

that the rock was simply a loose slab lying on a rather 

steep slope and was clearly not stable. Data from the rest 

of the 1989 survey, stations 14 and 15 included, are 

within the expected error of the 1982 and 1983 surveys. 

Potential bench-mark instability must be carefully 

watched. Instability can lead to large changes that may 

be interpreted as volcanic deformation. If there is no 

seismic activity and apparent deformation is seen, we 

look at the data very critically and do not accept them 

until the network has been reoccupied again and 

examined for bench-mark instability or atmospheric 

problems. 

112 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Table 1 0.3. Mark-to-mark slope distances (in meters) 

between lines where temperature and humidity were 

flown and lines where a temperature (elevated up to 7.6 

m) and humidity were measured at each endpoint 

[In both cases, barometric pressure was recorded at each endpoinL 

~· eans ~r million] 

Une Two Difference 

flown endpoints m ~~m 

2612.3428 2612.3425 -.0003 -0.11 

5142.2388 5142.2304 -.0084 -1.63 

3787.7485 3787.7478 -.0007 -0.18 

4567.9478 4567.9472 -.0006 -0.13 

3229.0068 3229.0069 .0001 0.03 

3155.4868 3155.4855 -.0013 -0.41 

3944.7571 3944.7550 -.0021 -0.53 

3134.0022 3134.0064 .0042 1.34 

2073.8518 2073.8512 -.0006 -0.29 

3559.6423 3559.6407 -.0016 -0.45 

4558.1172 4558.1149 -.0023 -0.50 

3241.0857 3241.0888 .0031 0.96 

3243.4739 3243.4814 .00'15 2.31 

3823.2954 3823.2971 .0017 0.44 

5079.7373 5079.7372 -.0001 -0.02 

6681.5571 6681.5542 -.0029 -0.43 

6101.5864 6101.5870 .0006 0.10 

6391.9248 6391.9245 -.0003 -0.05 

7573.0171 7573.0166 -.0005 -0.07 

4461.7808 4461.7804 -.0004 -0.09 

3502.0552 3502.0570 .0018 0.52 

4507.9756 4507.9696 -.0006 -0.13 

2814.4336 2814.4325 -.0011 -0.39 

3014.4924 3014.4950 .0026 0.86 

2717.0093 2717.0099 .0006 0.22 

SUMMARY 

Baseline horizontal-distance data have been 

collected on many potentially active volcanoes. We have 

saved considerable time and money by continually 

improving monitoring methods without compromising 

the data. We have seen that, despite care during 

installation, bench-mark instability remains a problem 

and could potentially be misinterpreted as real 

deformation. 

The equipment and methods described in this paper 

are more than adequate to meet the precision for baseline 

data. Equipment and methods will no doubt continue to 

improve over the years, and as networks are reoccupied 

the baseline data on volcanoes in the Pacific Coast states 

will also continue to improve, statistically. 

The intent of this paper was to describe the 

equipment and methods used at CVO to monitor 

volcanoes. We hope that this may serve as a guide for 

establishing equipment needs, methods, and procedures 

for similar projects elsewhere. 

ACKNOWLEDGMENTS 

We thank Teresa Atwill, Steve Brantley, Ed 

Brown, Ken Cameron, Tom Casadevall, Bill Chadwick, 

Dan Dzurisin, John Ewert, Christina Heliker, Dan 

Johnson, Jack Kleinman, Jeff Marso, Bobbie Myers, 

Peter Otway (New Zealand Geological Survey), John 

Power, Dave Schneider, Ben Talai (Rabaul Volcano 

Observatory, Papua New Guinea), Lyn Topinka,.Richard 

Waitt, Dave Wieprecht, Ed Wolfe, and Jon and Ken 

Yamashita for their help at one or more volcanoes. 

Special thanks go to Lyn Topinka for the majority of the 

photography, Tom Murray for electronic guidance, and 

Elliot Endo for computer programming. We also thank 

the many helicopter pilots who made it possible to 

complete these measurements. 

REFERENCES CITED 

Bevin, A.J., and Dip, L.S., 1983, Electronic distance measuring 

and the practical surveyor (second edition): Auckland, 

Continuing Educating Committee, New Zealand Institute 

of Surveyors, 115 p. 

Brinker, R.C., and Minnick. Roy, eds., 1987, The surveying 

handbook: New York. Van Nostrand Reinhold, 1270 p. 

Chadwick, W.W., Jr., lwatsubo, E.Y., Swanson, D.A., and 

Ewert, J.W., 1985, Measurements of slope distances and 

vertical angles at Mount Baker and Mount Rainier, 

Washington, Mount Hood and Crater Lake, Oregon, and 

Mount Shasta and Lassen Peak. California, 1980-1984: 

U.S. Geological Open-File Report 85-205, 96 p. 

Davis, R.E., Foote, F.S., Anderson, J.M., and Mikhail, E.M., 

1981, Surveying theory and practice, sixth edition: New 

York. McGraw-Hill, 992 p. 

Iwatsubo, E.Y., Topinka, L., and Swanson, D.A., 1988, 

Measurements of slope distance and zenith angles at 

Newberry and South Sister volcanoes, Oregon, 1985-1986: 

U.S. Geological Survey Open-File Report 88-377, 51 p. 

Kinoshita, W.T., Swanson, D.A., and Jackson, D.B., 1974, The 

measurement of crustal deformation related to volcanic 

activity at Kilauea volcano, Hawaii, in Civetta, L., 

Gasparini, P ., Luongo, G., and Rapolla, A., eds., Physical 

Volcanology: Amsterdam, Elsevier, p. 87-115. 

Lipman, P.W., Moore, J.G., and Swanson, D.A., 1981, Bulging 

of the north flank before the May 18 eruption-Geodetic 

data, in Lipman, P.W., and Mullineaux, D.R., eds., The 

1980 eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 143-155. 

McDonnell, P.W., 1987, Linear measurements-EDM 

instruments, in Brinker, R.C., and Minnick. Roy, eds., The 

surveying handbook: New York, Van Nostrand Reinhold, 

p. 109-128. 

Savage, J.C., and Prescott, W.H., 1973, Precision of Geodolite 

distance measurements for determining fault movements: 

Journal of Geophysical Research, v. 78, no. 26, p. 

6001-6008. 

10. Trilateratlon and Distance-Measuring Techniques Used at Cascades and other Volcanoes 113 
Swanson, D.A., Lipman, P.W., Moore, J.G., Heliker, C.C., and 

Yamashita, K.M., 1981, Geodetic monitoring after the May 

18 eruption, in Lipman, P.W., and Mullineaux, D.R., eds., 

The 1980 eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 157-168. 

Yokoyama, 1., Yamashita, H., Watanabe, H., and Okada, Hm., 

1981, Geophysical characteristics of dacite 

volcanism-The 1977-1978 eruption of Usu volcano: 

Journal of Volcanological and Geothermal Research, v. 9, 

p. 335-358. 

APPENDIX 

Equipment list used at the Cascades Volcano 

Observatory to measure distances at volcanoes other than 

Mount SL Helens. 

Site Installation 

Bench marks -10-cm-diameter brass bench marks. 

Star drill -3/4"-7/8" diameter. 

Hammer-3-lb sledge. 

Cement -Jet Set, fast-curing cement. 

Surveying Equipment 

EDM-HP3808A and Geodimeter 114. 

Theodolite-Wild T2 (new style) . 

. Tripods-Kern centering-rod style with adaptor plate. 

Any standard tripod with optical tribrach. 

Prisms-One-piece comer-cube glass prism, accurate 

to within 2 arc seconds. 

Atmospheric Equipment 

Temperature 

Unmarked leveling rod, 7.6 m length, Mound City. 

Sensor, National Semiconductor LM355. 

Aspirated shield, made in-house. 

5-volt regulator, National Semiconductor LM78105 

or Precision Monolith Corporation REF-02 

12-volt sealed battery, 6.5 Ah. 

Circuitry for 5-volt reference, in-house. 

Digital multimeter, 4112 digits, Fluke 8060A. 

Pressure 

Model AIR-HB-1A Barometer, Atmospheric 

Instrumentation Research Inc. 

Specifications: 

Temperature range: -10 to 40 ·c 

Size: 3.0x9.11x14.5 em 

Weight: 280 g 

Power: 9-volt transistor battery 

Ultimeter model 12, Peet Bros. Company. 

Thommen altimeter/barometer. 

Humidity 

Sling psychrometer. 

Box 

Pelican case, Pelican Products Inc. 

Accessories 

Spare 12-volt sealed battery and battery cable for EDM. 

Spare 9-volt batteries for digital multimeter and AIR 

barometer. 

Water for sling psychrometer. 

Backpack for HP3808A and backpack straps for 

Geodimeter 114 case. 

Signal mirror. 

Notebook and pencil. 

114 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
11. Installation of Bench Marks and Permanent 

Reflectors for Geodetic Deformation Networks 

By Michael P. Doukas and John W. Ewert 

ABSTRACT 

To measure deformation over a large area at an 

active volcano, Instruments and techniques are borrowed 

from surveying and geodesy. We discuss several 

methods employed at the Cascades Volcano Observatory 

for Installing bench marks and reflector prisms used to 

measure deformation. We emphasize a versatile system 

designed for ease of portability, compatibility of 

components, and quick and easy Installation on restless 

volcanoes. 

Instrument bench marks and reflector prisms are 

Installed Into bedrock by drilling holes or Into 

unconsolidated substrate by constructing concrete 

foundations. · 

Backsights, lines measured away from the volcano 

to areas presumed more stable, are used to test the 

stability of EDM bench marks and are Included in 

deformation networks to provide a means of checking the 

relative stability of the bench marks. Careful site 

selection, building as much redundancy as is practical 

Into· deformation networks, and frequent measurements 

provide a check on reflector site stability. 

INTRODUCTION 

Monitoring surface deformation of a volcano helps 

scientists determine the possible course of an episode of 

volcanic unrest. Deformation may be monitored remotely 

at selected points with electronic sensors (tiltmeters, 

displacement meters, or strainmeters) that send the data 

via radio telemetry to a central recording site. To 

measure deformation over a larger area, however, 

instruments and techniques borrowed from surveying and 

geodesy must be employed. In this paper we discuss 

methodologies employed at the David A. Johnston 

Cascades Volcano Observatory (CVO) for installing 

bench marks necessary to measure deformation with 

surveying instruments such as electronic distance meters 

(EDMs), theodolites, and leveling instruments. 

Floyd (1978) described procedures to install 

geodetic bench marks used in the United States 

vertical-control network in considerable detail. These 

procedures are generally suited only to truck-supported, 

roadside installation of monuments, but the information 

about site selection with respect to substrate type and 

climate is complete and pertinent to the installation of 

bench marks on and around active volcanoes. We 

describe here a versatile system for installing bench 

marks and reflector prisms on volcanoes. This system 

was developed by personnel of the U.S. Geological 

Survey's Volcano Crisis Assistance Team (VCAT). The 

systems and methods are designed for ease of portability, 

compatibility of the components, and quick and easy 

installation of reference points. The methods described 

here are those we have found effective, but availability 

of materials and local field conditions may require 

modification. All permanent bench marks should be 

corrosion resistant. 

BENCH-MARK PLACEMENT 

Most geodetic methods used to monitor volcanoes 

are capable of detecting crustal movement of a few parts 

per million, although on some active volcanoes 

displacements can be several orders of magnitude 

greater. It is important to assure that any changes 

measured truly reflect volcanic behavior, not local 

ground dislocations caused by site instability or other 

nonvolcanic effects, such as ground creep and slumps or 

thermal, diurnal, or seasonal changes (Banks and others, 

1989). Thus the two most important factors in 

bench-mark installation are site selection and proper 

installation. 

Site Selection 

General guidelines during site selection include the 

following: (1) establish EDM lines that are as short as 

permitted by logistical constraints and safety 

considerations in order to maximize precision; (2) take 

11. Installation of Bench Marks and Permanent Reflectors for Geodetic Deformation Networks 115 
account of geologic structure, such as dikes and faults, 

when locating monitoring networks (for example, lines 

normal to dike trends are particularly useful); (3) locate 

level lines on competent ground and radial to the 

deformation source; (4) monitor as much of the 

circumference of the volcano as is possible; (5) locate 

instrument stations with clear lines of sight; and (6) 

choose site locations that have good access, to reduce the 

time required for reoccupation, and that are protected 

from inundation by volcanic products. Geodetic tilt and 

leveling measurements require arrays of bench marks set 

out in accordance with instrumental needs and 

topographic limitations (Dzurisin, chapter 12; Yamashita, 

chapter 14; Ewert, chapter 15). The location of a bench 

mark to be used as an instrument station in EDM 

networks depends on the range and precision of the 

instrument used, length of lines from reference point to 

prisms, number of prisms needed (fig. 11.1), and the 

degree to which the instrument stations are 

interconnected (lwatsubo, Topinka, and Swanson, chapter 

8; lwatsubo and Swanson, chapter 10). 

Caution should be exercised in choosing the 

location for a bench mark so that volcanic products, such 

as mudflows or ash falls, are not likely to bury the mark. 

Bench marks should be placed on the crests of ridges 

rather than on side slopes, for example. This also 

minimizes mark instability owing to surficial ground 

creep, frost action, and movements related to seasonal 

changes in ground-water hydrology. 

I I I 

3 + • • . ... C/) 

~ C/) 

a: 

0.. 

~2 

a: 

w 

co 

~ 

:::::> 

z 

+ • • • 

0~-------~~------~-l _________ ~L--------~ 

0 5 10 15 20 

KILOMETERS 

Flgure11.1. Average range and number of prisms 

required for five models of EOM. cross, Lietz model 

RE02A; dot, Wild model 015; square, Wild model 013000; 

diamond, Cubic Precision model Ranger Va; triangle, 

Geodimeter model 6000. 

A bench mark set in bedrock is less susceptible to 

these effects than is a mark set in an unconsolidated 

substrate. An unconsolidated substrate requires more 

work, hardware, and concrete to support the bench mark 

and create a stable site. Bench marks used in single-setup 

leveling arrays should all be in similar substrate. 

Bedrock installations should not be mixed with 

unconsolidated substrate ones at any given single-setup 

leveling tilt site, because the thermal response of unlike 

substrates can be significantly different, resulting in 

spurious data (Sylvester, 1985). 

Bench marks should not be located in areas that 

receive significant human traffic, to minimize the 

possibility of vandalism. If a mark must be placed at 

such a site, it is wise to bury the mark so it is not visible 

to the casual observer. Burial requires careful site 

description, so that the mark can be uncovered later. In 

general, a good description of the bench-mark location 

should be written with reference to landmarks not likely 

to be destroyed by an eruption. 

Bench-mark Installation 

Two general types of substrate characterize 

volcanic areas, competent rock (typically lava flow) 

outcrops and unconsolidated sediment (typically 

volcaniclastic material). Each type of substrate presents 

its own set of problems related to stable bench-mark 

installations. 

Installation In Bedrock 

A stainless steel anchor bolt is the quickest and 

easiest type of mark to install at a bedrock site. These 

bolts, key elements in our system, are used both as bench 

marks and as anchor points for reflector attachment. The 

bolt consists of a 100-mm shaft, threaded at the top (0.5 

inch 13 National Course thread [1/2-13NC]), with a 

locking ring assembly at the base (fig. 11.2). The top of 

the bolt is center-punched to provide a unique reference 

point. Bolt-type bench marks can be used immediately 

after installation. 

The tools required for installation of the bolt are 

listed in table 11.1. To install a stainless steel bolt, a hole 

is drilled the same size as the bolt but 88 mm deep using 

a star drill and hammer, or preferably a lightweight, 

hand-held, battery-operated hammer/drill with a carbide 

bit. The power tool can shorten the drilling time by a 

factor of 5-10, thereby decreasing exposure at potentially 

hazardous locations and increasing the number of 

stations installed per day. Rock fragments are blown 

from the hole using a plastic tube, and any plant or soil 

fragments are removed from around the hole. The anchor 

bolt is hammered into the hole until firmly seated (fig. 

116 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Table11.1. Tools required for Installation of anchor bolt 

or bench mark In rock 

Field tools for Installation of anchor bolta and bench marks 

Hammer 

Star Drill, 1/2 and 3/4 inch 

or 

Portable hammer/drill, battery operated 

Batteries, with spares 

Bit, 1/2 and 3/4 inch 

Wrench, 3/4 inch 

Plastic tubing, trowel, water, plastic sheet, mortor (1 part 

cement, 3 parts clean sand, 2 parts water) 

Field tools and equipment for prism-mount assembly 

Stainless steel anchor bolts, with nut and washer 4 inches 

long, 1/2-13NC thread 

Bars, 1 1/2 inch x 8 inches long, four boles, 1/2 inch 

diameter equally spaced 

Screws, hex drive, 1/4 inch x 2inch long, 1/2-13NC thread 

Wrench, allen, 1/4 inch drive 

Nuts, 1/2-13NC, washers 

11.3). If the bolt is to be used as a leveling mark, its base 

should rest on the bottom of the hole. The bolt top is 

then peened with a hammer to create a rounded head on 

which a level rod can rest. If the bolt is to be used as an 

EDM mark, a cross is cut into the top of the bolt with a 

hacksaw. A nut is placed on the bolt and tightened, 

drawing the bolt slightly upward. This slides the locking 

ring on the bolt onto the conical wedge at the base of the 

bolt, expanding the ring to press against the hole walls 

and locking the bolt into place (fig. 11.3). In either case, 

the bolt cannot be removed without breaking the rock 

that holds it. A mortar patch is then placed around the 

nut and head of the bolt to protect the installation from 

infiltration of water and to strengthen the site. The last 

step when working with mortar is to cover the patch with 

a piece of plastic or other material to help the patch cure 

properly (fig. 11.4). 

Figure 11.2. Stainless steel anchor bolt. Length is about 

9 em. 

Formal bench marks in use at CVO are made of 

brass with a disk approximately 100 mm in diameter and 

a stem approximately 88 mm long (fig. 11.5). Identifying 

labels and numbers are generally stamped on the upper 

surface of the bench mark, a procedure not possible on a 

bolt-type installation. Bench marks used for EDM work 

have a flat disk with a cross in the center, whereas those 

used for precise leveling and single-site leveling have a 

nipple in the center that serves as a turning point. Two 

general types of stems have been used: a solid or hollow 

stem, round in cross section, and a solid, fluted and 

twisted stem. The fluted twisted shape gives the bench 

mark better grip in concrete than the round cross section, 

once the concrete has properly cured. The tools required 

for installation of a brass bench mark into bedrock are 

listed in table 11.1. 

To install a brass bench mark in a bedrock site, a 

fairly level area on an outcrop that is not highly fractured 

or jointed is selected. A 100-mm-deep hole slightly 

larger than the stem is drilled. A plastic tube is used to 

blow the rock dust from the hole and immediate 

surroundings. Any soil or plant material is also removed 

from the area around the hole. The bench mark is placed 

Figure 11.3. Anchor bolt in bedrock. For vertical stability 

during single-setup leveling, bolt Is firmly seated (left); for 

horizontal stability during EDM monitoring, bolt Is drawn 

up slightly (right). 

11. Installation of Bench Marks and Permanent Reflector• for Geodetic Deformation Network• 117 
in the hole to see that it fits properly and that the disk 

rests flush on the rock surface. The surface in and around 

the hole is wet slightly to improve the holding power of 

the mortar. Mortar is then mixed and placed in and 

around the hole. If a hollow-stem bench mark is being 

used, the stem and concave underside of the disk are 

filled with mortar. The bench mark is placed in the hole 

and seated finnly in the mortar. Mortar may be added 

around the rim of the mark to slightly cover the top 

edges. The last step, when working with mortar, is to 

cover it with a piece of plastic or cloth to help it cure 

slowly and properly. The bench mark can be used 

immediately as an EDM reference point, but the concrete 

must be allowed to cure for 12 to 24 hours before using 

the bench mark for leveling (fig. 11.6). 

Installation In Unconsolidated Substrate 

If a bedrock outcrop is unavailable, a foundation 

for the bench mark must be constructed. In areas where 

the ground freezes, the foundation must reach below the 

local frost line to prevent frost heave. 

The preferred method of installing a vertically 

stable bench mark installation in unconsolidated substrate 

is to attach the bench mark to a single length of rod that 

Bolt 

Lock ring 

Hard rock 

Figure 11.4 Anchor bolt in place with mortar patch and 

plastic sheet. 

Figure 11.5. Types of bench marks. A, Bench mark for 

EDM monitoring with labeled, flat upper surface; 8, 

hollow-stem bench mark; C, nlppled bench mark with 

fluted stem for use In precise leveling networks. 

has been driven to as far as possible (to refusal) through 

the bottom of a shallow hole (fig. 11. 7). This installation 

is not good for horizontal control work unless the mark 

can be prevented from sideways sway by backfilling 

around the mark. Two-meter-long stainless steel or 

copper-clad steel rods are driven with a 

gasoline-powered hammer/drill or sledge hammer. Total 

refusal is 25 to 30 seconds of operation of the hammer 

without movement. If total refusal is not met in the first 

2 meters, a second rod is coupled to the first with a 

threaded and crimped coupling, and driving is resumed. 

This procedure is repeated until total refusal. If the 

supply of rods is exhausted or total refusal not achieved, 

the bench mark should be identified as potentially 

Plastic sheet 

Bedrock 

Figure 11.6. Bench mark installed in hard rock. 

118 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Table11.2. Tools req ul red for bench mark or rod 

assembly Installation and cairn construction In 

unconsolidated substrate 

Field tools for Installation of bench mark with gasoline powered 

hammer 

Gasoline powered hammer (24 kg) 

Hammer bit (3 kg) 

Threaded head 

Gasoline 

Hydraulic crimper 

Crimper jaws 

Adjustable wrench 

1/2 to 1 inch diameter copper-clad, threaded, weld rod, 2 m 

long with threaded couplings 

Field tools for Installation of anchor bolts, bench marks, and 

construction of calms 

Three 2-m reinforcement rods, 9-13 mm diameter 

Shovel, trowel, water, mortar, gravel, and stones 

Plastic sheet, mortar (1 part cement, 3 parts clean sand, 2 

parts water) 

Field tools and equipment for prism-mount assembly 

Sleeve, (cut, drilled, tapped) 3 inches long, 1" I. D. 

1/2-13NC thread 

2-m reinforcement rods, 15 mm diameter 

Bars, 1-1/2 inches wide, 8 inches long four holes drilled 

1/2 inch diameter 

Screws, hex drive, lock, 1/4 inch 2 inches long, 1/2-13NC 

thread 

Wrench, allen, 1/4 inch drive 

Nuts, 1/2-13NC, washers 

unstable. In areas with deep tephra, the total rod length 

may approach 15-20 m. Once at refusal, the exposed 

portion of the rod is trimmed to the desired length and a 

hollow-stemmed bench mark is attached with a hydraulic 

crimper. The weight and bulk of the tools needed make 

this sort of installation better suited to truck- or 

helicopter-supported operations. Table 11.2 lists the tools 

and materials necessary for this type of installation, and 

Aoyd (1978) gives more detailed descriptions of the 

method. 

A foundation must be constructed when it is 

impractical to use the method described above. This 

method is best suited to areas where deep freezing is not 

a problem. A hole is dug about 0.75 m in diameter and 

about 1 m deep. Three lengths of 9- to 13-mm diameter 

(3/8 to 1/2 inch) steel reinforcement rod (rebar), 

approximately 2 m long, are driven through the bottom 

of the hole. If refusal is met before all the rod is in the 

hole, the end of the rod is cut and then bent over back 

into the hole. The hole is then filled to ground level with 

layers of rocks, mortared together around the rebar. 

Wetting the rocks will improve the holding power of the 

mortar. A pad of mortar is created on top of the 

stonework and the bench mark is then set into the wet 

mortar. More mortar may be added around the rim of the 

bench mark to slightly cover its top edge (fig. 11.8). The 

last step is to cover the pad with a piece of plastic to 

help the mortar cure properly. The tools required are 

listed in table 11.2. 

If no brass bench marks are available, a stainless 

steel anchor bolt can be set in the mortar on top of the 

stone work. In this case, the nut and washer are threaded 

onto the bolt and the bolt is placed in the mortar upside 

down to maximize its holding strength (fig. 11.9). 

PRISM ATTACHMENT SYSTEM 

Dependable EDM monitoring of active volcanoes 

requires a horizontally stable reflector installation. 

IPffiiiiJ~'';JllJ;H;Iil:::r.IIIIlillll]] Gasoline-powered 

hammer/drill 

Couplings 

Unconsolidated substrate 

Figure 11.7. Use of gasoline-powered hammer/drill for 

driving steel rod into soft ground for installation of a 

bench mark. A hollow-stem bench mark is crimped onto 

rod after refusal has been reached. 

11. Installation of Bench Marks and Permanent Reflectors for Geodetic Deformation Networks 119 
Depending upon considerations of time, funding, safety, 

and accessibility, it may be more advantageous to place a 

permanent reflector on the volcano than to send a 

reflector crew onto the volcano's flank for each 

measurement. 

At Mount St. Helens, permanent EDM reflectors 

are mounted on 3-m-high steel towers on the outer flanks 

of the volcano, so that they stand above winter snow. 

Similar towers, located off the flanks of the volcano, 

serve as instrument stations on which the EDM is 

mounted (Swanson and others, 1981 ). 

These towers weigh more than 200 kg each and 

were placed by helicopter. This system works well for 

long-term studies (lwatsubo, Topinka, and Swanson, 

chapter 8), but the amount of funding, fabrication time, 

and logistical support necessary to create such a network 

of stations precludes its use in rapid or short-term 

responses. On other Cascade volcanoes a mobile reflector 

system has been used (Chadwick and others, 1985; 

lwatsubo and others, 1988, lwatsubo and Swanson, 

chapter 10). The mobile reflector system is set up over 

an established bench mark at the reflector site and 

consists of a tripod, tribrach with optical plummet or a 

centering stem, prism holder, and prism(s) (fig. 11.10). 

This system works well on volcanoes that are monitored 

Unconsolidated substrate 

Flgure-11.8. Bench mark installed In unconsolidated 

substrate, showing bent rebar and mortared stonework. 

on an infrequent basis with helicopter support (lwatsubo 

and Swanson, chapter 1 0). A distinct advantage in using 

a mobile system is that temperature and pressure data 

can be obtained at the reflector site by the reflector crew 

and the EDM data can then be better corrected for 

environmental conditions. However, use of the mobile 

system is time-consuming and requires more personnel to 

set up and measure. 

During periods of volcanic unrest, the use of a 

mobile reflector system can place personnel at higher 

risk than may be appropriate. For such cases a 

lightweight, easy-to-install reflector kit was developed by 

VCAT members at CVO (see also lwatsubo and 

Swanson, chapter 6). This kit consists of equipment for 

attaching the reflector directly to the ground, mounting 

hardware for attaching the reflector to the anchor, and a 

suitable reflector (tables 11.1 and 11.2). The kit's 

hardware is standardized to minimize the number of tools 

required during installation and to increase the versatility 

of any one component. The anchor bolts (1/2-13NC) are 

the same as those described in the bench-mark section of 

this paper. The permanent reflector generally requires 

one visit for installation and can be set up by one person. 

Some stations require extra time because of the need to 

Unconsolidated substrate 

Figure 11.9. Installation of anchor bolt in mortared 

foundation. Anchor bolt is installed upside down for 

added stability. 

120 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
protect against vandalism or volcanic activity. Shop tools 

are needed to thread the prism stud and prepare hardware 

before going into the field (table 11.3), and field tools 

are needed to attach the prism at reflector sites (tables 

11.1 and 11.2). 

As with bench marks, the anchor system for the 

reflector must be adaptable to either bedrock or 

unconsolidated substrate. Cliff faces with rock outcrops 

offer the best locations for prism installation because 

installation is quick and the prisms tend to be protected 

from the accumulation of volcanic products. To install a 

prism on a cliff face, a hole is drilled parallel to the line 

of sight from the EDM location to the prism, the 

stainless steel anchor bolt is driven in and secured (fig. 

11.11 ), and the prism or a multiple-prism mounting 

D 

Figure 11.1 0. Typical mobile reflector system, including 

(A) tripod, (B) tribrach, (C) prism holder, and (D) prisms. 

Table 11.3. Shop tools for preparing reflector prism and 

prism-mount assembly (sleeves and bars) 

Tap set, handle 

Tap, taper, 1/2-13NC, for sleeve modification 

Tap, bottom, 1/2-13NC, for prism modification 

27/64-inch bit, drill 

Vise 

Metal hacksaw 

Electric drill, drill press 

bracket is attached. The tools required for installation of 

an anchor bolt to bedrock are listed in table 11.1. 

The general idea when working in unconsolidated 

substrate is to secure to the ground a length of rod to 

which a mounting sleeve and prism will be attached (fig. 

11.12). For such an installation it is important to get the 

prism at least 50 em above the ground level (preferably 

twice that or more) to minimize strong near-surface 

refraction effects. A stonework foundation, similar to 

that described for bench marks in unconsolidated 

substrate, may be necessary to support the rod. How 

much work goes into unconsolidated substrate 

installations will depend, to some extent, on how great 

the hazards are at a particular site and how long the 

monitoring activity is expected to continue. For example, 

at Mount St. Helens before the May 18, 1980, eruption, 

reflectors were merely affixed to steel fenceposts driven 

into tephra (Swanson and others, 1981 ). The tools 

required for installation of a reflector rod into 

unconsolidated substrate are listed in table 11.2. 

The mounting system was designed to use the 

same basic, interchangeable hardware for any substrate 

encountered. The mounting hardware for EDM prisms 

consists of readily available, corrosion-resistant parts. 

The prisms we use are solid glass comer cubes in a 

plastic case. These have a hollow stud at the back that 

Prism/ 

Figure 11.11. Anchor bolt Installed In vertical cliff. 

11. Installation of Bench Marks and Permanent Reflectors for Geodetic Deformation Networks 121 
can be threaded to accept thread common to our system 

(1/2-13Nq. Thus, a single prism can be attached directly 

to the threaded portion of the anchor bolt in a bedrock 

installation (fig. 11.13A). To install a multi-prism station 

a rectangular metal bar, long enough to hold two or three 

prisms is attached to the bolt, and prisms are attached to 

the bar (fig. 11.13B) using hex drive setscrews and nuts 

(table 11.1). The metal bar is 25 mm x 6 mm galvanized 

or painted steel with evenly spaced holes drilled to 

accommodate screws by which the prisms are attached. 

Another bar can be attached crosswise to accommodate 

more prisms. 

A sleeve is placed over and attached to the anchor 

rod with a threaded setscrew (fig. 11.13C, table 11.2). 

The setscrew is secured with a nut. The sleeves are 

sections of steel pipe 35 mm outside dimension, 25 mm 

inside dimension (1.4 in. OD, 1 in. ID) cut into 

approximately 75 mm lengths. Holes are drilled and 

threaded to accept the screw size (112-13Nq common to 

the system. One prism or a rectangular metal bar of 

multiple prisms can then be attached to the setscrew (fig. 

,, 11.13C, 11.13D). 

-Mounting sleeve 

Figure 11.12. Mounting sleeve attached to anchor rod 

installed in unconsolidated ground. 

Consideration should be given to protecting the 

monitoring station from volcanic activity, the weather, 

and vandalism no matter what type of anchor-mounting 

system is used. A projecting overhang might be found to 

protect the site at bedrock sites (fig. 11.11, 11.14). 

Artificial protection may be constructed, by drilling holes 

into the rock, placing rods into them, and using mortar or 

rock to construct a roof over the prism, or at locations of 

flat or sloping ground a protective cairn might be built 

out of concrete and rocks (fig. 11.14, table 11.2). This 

A 

B 

Figure 11.13. Single and multiprism assemblies. A, 

Single prism in rock site. 8, Multiprism assembly in rock 

site. C, Single prism in unconsolidated ground site. 0, 

Multiprism assembly in unconsolidated ground site. 

122 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
cairn should be constructed around the rod or anchor 

surrounding the prism. Sometimes vandalism is difficult 

to avoid. One could remove the prisms during quiet 

periods or camouflage the station. 

ASSESSING BENCH-MARK STABIUTY 

The stability of a bench mark should be 

continually evaluated after installation. Seasonal 

movement of the ground around the mark can give the 

false impression of deformation related to volcanism. 

Some method of checking bench-mark stability should be 

built into deformation networks. Backsights, lines 

c 

D 

Figure 11.13. Continued 

'Natural 

overhang 

Figure 11.14. Three types of prism-protection schemes. 

2 4 6 8 10 12 14 

KILOMETERS 

Figure 11.15. Relative position of EDM station with 

reflector at backsight station (left). 

measured away from the volcano to presumably more 

stable areas, can be used to test the stability of EDM 

bench marks (fig. 11.15). Single-setup-leveling and 

traverse-leveling arrays can have redundant bench marks 

to provide a means of checking the relative stability of 

the bench marks. 

Reflector installations may also be subject to 

instability caused by environmental effects. We have 

found that careful site selection, building as much 

redundancy as is practical into deformation networks, 

and measuring the networks as often as possible are the 

best ways to eliminate uncertainty in what is being 

measured. 

REFERENCES CITED 

Banks, N.G., Tilling, R.I., Harlow, D.H., and Ewert, J.W., 

1989, Volcano monitoring and short-term forecasts, in 

Tilling, R.I., ed., Volcano hazards: Washington D.C., 

American Geophysical Union, Short Course in Geology, 

volume 1, p. 26-80. 

Chadwick, W.W., Jr., Iwatsubo, E.Y., Swanson, D.A., and 

Ewert, J.W., 1985, Measurement of slope distances and 

vertical angles at Mount Baker and Mount Rainier, 

Washington, Mount Hood and Crater Lake, Oregon, and 

11. Installation of Bench Marks and Permanent Reflectors for Geodetic Deformation Networks 123 
Lassen Peak, California, 1980-1984, U.S. Geological 

Survey Open-File Report 85-205, 96 p. 

Floyd, R.P., 1978, Geodetic bench marks: National Oceanic and 

Atmospheric Administration Manual NOS NOS 1, 52 p. 

lwatsubo, E.Y., Topinka, L., and Swanson, D.A., 1988, 

Measurements of slope distances and zenith angles at 

Newberry and South Sister Volcanoes, Oregon, 

1985-1986: U.S. Geological Survey Open-File Report 

88-377, 51 p. 

Swanson, D.A., Upman, P.W., Moore, J.G., Heliker, C.C., and 

Yamashita, K.M., 1981, Geodetic monitoring after the 

May 18 eruption, in Upman, P.W., and Mullineaux, D.R., 

eds., The 1980 eruptions of Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 

1250, p. 157-168. 

Sylvester, A.G., 1985, Shortcomings of monitoring volcano 

deformation by dry tilt surveying [abs): Eos, v. 66, no. 46, 

p. 854. 

124 Monitoring Volcanoes: Technique• and Strategle• Used by the Staff of the Cascade• Volcano Observatory 
12. Geodetic Leveling as a Tool for Studying Restless 

Volcanoes 

By Daniel Dzurisin 

Abstract 

Although steep terrain, difficult access, hazardous 

conditions, and limited staff often discourage 

volcanologists from making geodetic leveling surveys at 

restless volcanoes, leveling has several advantages, 

including (1) greater precision for vertical displacements 

than virtually any other method over horizontal distances 

up to at least 100 km; (2) applications that range from 

single-setup leveling or short traverses designed to 

measure local ground tilt to long traverses that measure 

vertical displacements over broad areas; (3) sufficient 

redundancy based on field procedures and the large 

number of bench marks along a typical traverse to virtually 

eliminate measurement error and bench-mark instability 

as sources of uncertainty; (4) well-established 

procedures, relatively inexpensive equipment, and 

straightforward data processing; and (5) the utility of 

vertical displacement data as input for numerical models 

of the deformation source. Disadvantages include (1) 

substantial manpower requirements for long traverses; (2) 

logistical problems imposed by equipment that is difficult 

to transport to remote volcanoes and impractical to use in 

dense vegetation; and (3) the relatively slow pace of 

traverse leveling compared to other volcano monitoring 

techniques. An experienced first-order leveling crew of 

four to six people will traverse 3-6 km per day on mean 

slopes of 10 degrees or less, or 1-2 km per day on slopes 

of 10-30 degrees. Alternatively, a leveling crew can 

measure several short traverses or more than a dozen 

single-setup arrays in a day. The standard deviation of 

vertical displacements determined from successive 

first-order class II leveling surveys is 1 mmlkm~ x L V-2, 

where L is distance along the leveling line (that is, ±3.2 

mm/1 0 km or ±1 0.0 mm/1 00 km). At mafic volcanoes, on 

the lower flanks of stratovolcanoes, at large calderas, and 

in terrain between volcanoes along volcanic arcs, long 

leveling traverses can provide important information on 

large-scale ground deformation. In virtually any terrain 

and particularly during volcanic emergencies, short 

traverses are a useful tool to quickly determine short-term 

deformation rates. 

INTRODUCTION 

Scope 

Geodetic leveling is a long -established technique 

used to measure elevation differences between successive 

bench marks and, by repeated surveys, to measure 

elevation changes (vertical displacements) as a function 

of time. Leveling has been used as a geodetic 

measurement system for more than a century (Vanfcek 

and others, 1980), and has proved its utility at several 

active volcanoes (for example, Wilson, 1935; Mogi, 

1958). However, recent innovations in precise geodesy 

sometimes overshadow the considerable and enduring 

strengths of leveling as a volcano monitoring tool, to the 

extent that leveling is overlooked in situations where it 

could make a unique contribution. 

The goals of this paper are to ( 1) discuss the 

advantages and disadvantages of leveling at restless 

volcanoes, (2) dispel some misconceptions concerning 

the feasibility of leveling in volcanic terrain, (3) illustrate 

the utility of leveling surveys through two examples, and 

( 4) urge volcanologists to consider leveling a primary 

element in any modern volcano monitoring program. A 

companion paper by Yamashita and Kaiser (chapter 13) 

describes the procedures and equipment used for 

ftrst-order class II leveling surveys. Detailed standards 

and specifications for leveling surveys of various orders 

and classes are described in manuals published by the 

National Geodetic Survey (Schomaker and Berry, 1981) 

and Federal Geodetic Control Committee (1974, 1975, 

1984). 

Terminology 

During conventional geodetic leveling, a spirit 

level or other type of leveling instrument (most modern 

levels are self-leveling and do not include a spirit level) 

12. Geodltlc Leveling as a Tool for Studying Restless Volcanoes 125 
and a pair of graduated leveling rods (typically 3m long, 

graduated every 0.5 or 1.0 em) are used to measure the 

elevation difference between permanent bench marks by 

accumulating the elevation differences between a series 

of temporary turning points. The forward turning point 

(relative to the direction of the traverse) is called the 

foresight and the backward turning point is called the 

backsight. Typically, bench marks are spaced 1-3 km 

apart and turning points are 20-100 m apart (depending 

on steepness of the terrain). Adjacent turning points 

bound a setup and adjacent bench marks bound a section; 

measurement of a series of sections constitutes a survey. 

Leveling surveys can be used to serve two different 

but often complementary purposes. Long traverses 

(typically 10 km and longer) measure near-absolute 

vertical displacements with respect to a distant bench 

mark or group of bench marks that is assumed to be 

stable. Short traverses (typically 1 km and less) measure 

relative vertical displacements among a small group of 

bench marks, usually for the purpose of determining 

local ground tilt. Single-setup leveling is a special case 

in which relative vertical displacements within a small 

array of bench marks are measured from a single 

instrument setup to determine local ground tilt 

(particularly useful when deformation rates are high or 

surveying time is very limited) (Yamashita, 1981; 

chapter 14). Each of these approaches has unique 

advantages and disadvantages that determine which one 

is most appropriate for a specific application, as 

discussed below. One effective strategy is to measure 

short segments of a long traverse periodically, then 

measure the entire traverse when significant changes are 

indicated. 

ADVANTAGES OF GEODETIC LEVELING 

Precision 

Recent advances in satellite geodesy and other 

modern methods notwithstanding, conventional 

leveling is still the most accurate technique available 

for measuring vertical displacements over horizontal 

distances up to at least 100 km. Such dimensions are 

of particular interest in volcanology, because they 

apply to most of the processes that affect active 

volcanoes (except for plate tectonics and other 

regional tectonic processes). Therefore, unless there is 

a compelling reason to choose some other method 

(such as safety or logistics), leveling is the preferred 

technique for measuring vertical displacements at restless 

volcanoes. 

Unlike emerging techniques for which 

measurement uncertainties are still being evaluated, the 

empirical accuracy of leveling surveys has been well 

established through decades of repeated surveys and 

comparisons with other methods. Leveling errors usually 

are categorized as blunders, systematic errors, and 

random errors. Blunders (gross reading errors, rod 

movement during a setup, recording errors) are easily 

detected, particularly by surveys that are double-run with 

dual-scale rods (V anfcek and others, 1980). 

Systematic error accumulates in proportion to 

distance, elevation, or elevation gradient. Certain types 

of systematic error, such as those associated with 

imperfect collimation of the instrument, can be virtually 

eliminated through rigid adherence to prescribed field 

procedures. Other types, such as errors in rod length, can 

be tightly controlled through a combination of field and 

laboratory procedures, including frequent calibration 

against an invariant standard (calibrations available at the 

U.S. National Bureau of Standards). Two types of 

systematic error that have received considerable attention 

recently are refraction error (Strange, 1981; Stein, 1981; 

Holdahl, 1982) and magnetic error (Rumpf and 

Meurisch, 1981; Packard and MacNeil, 1983). 

Refraction error can be corrected using temperaturegradient data collected during the leveling survey or 

estimated from climate models. Magnetic error affects 

only self-leveling instruments, and it may vary from 

instrument to instrument or as a function of time with a 

single instrument. It can be eliminated by installing a 

magnetic shield (available as an option from the 

manufacturer). 

Random error is introduced by inaccurate 

observations, nonsystematic imperfections in the 

equipment, and ever-present vagaries in atmospheric 

conditions. Random error generally can be diminished 

through redundancy, such as that achieved through 

double running and the use of double-scale rods 

(Vanlcek and others, 1980). 

The standard deviation of an observed elevation 

difference (OED) due to random error is given by 

cr (h)= cr0 L'h 

where cr(h) is one standard deviation in the OED 

between any two marks, cr0 is a constant for each type 

(order and class) of leveling survey, and Lis the distance 

along the leveling traverse. The contemporary value of 

cr0 is 0.7 mm/kmlA for first-order class II surveys, 1.3 

mm/kmlll for second-order class II surveys, and 2.0 

mm/kmlll for third-order surveys (Vanlcek and others, 

1980). 

126 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
For double-run segments, an estimate of a0 can be 

obtained from the discrepancies between forward and 

backward OED measurements using 

m 

02 = _1_ "8h~ 0 2m .L..J /. 

I 

i=O 

where m is the number of double-run segments, h; 

is the discrepancy between forward and backward OED 

measurements for section i, and I; is the length of section 

i (Pelton and Smith, 1982). The empirical precision of 

any leveling survey can be estimated by double-running 

a representative number of sections and computing a0 

from the equation above. If the computed value exceeds 

the desired standard, field procedures should be checked 

and the survey repeated. Experience of the U.S. 

Geological Survey and National Geodetic Survey shows 

that random errors associated with geodetic leveling 

(hence, the value of a0) are functions of the order, 

vintage, and character of the survey. For example, 

first-order leveling procedures in the United States 

currently are such that where L is given in kilometers, 

the estimated standard deviation in the OED between any 

two marks for class 1 and class 2 surveys is about 0.5 

mm ·L¥1. and 0.7 mm ·L¥1. respectively. However, 

experience shows that the standard deviation for 

first-order leveling in general (class 1 and class 2) was 

about 2.5 mm · L lh prior to 1901, about 2.0 mm · L lh 

during the period 1901-1916, about 1.5 mm · L¥1. during 

the period 1917-1955, and about 1.0 mm · LV2 during the 

period 1956-1974 (Vanicek and others, 1980). In 

practice, the standard deviation also depends on the 

character of the survey; that is, surveys in mountainous 

terrain generally have larger standard deviations than 

surveys in flatter terrain, partly because more setups are 

required to traverse the same distance in steep terrain 

and therefore random error accumulates faster. The best 

approach is to measure a0 empirically by double-running 

a representative number of sections and using the 

equation for a0 given above. For surveys that are entirely 

single-run, values of a0 such as those quoted above 

should be regarded as lower limits on the likely standard 

deviations in most cases. 

The standard deviation of a vertical displacement 

(change in the OED between marks) measured between 

any two surveys, a(Sh), is equal to the square root of 

the sum of the squares of the deviations for each 

survey. The standard deviation of the corresponding 

vertical velocity, a(V), is given by a(Sh) divided by 

the time interval between the surveys (Pelton and 

Smith, 1982). 

Figure 12.1 shows the random error in vertical 

displacements determined from contemporary leveling 

surveys of various orders and classes as a function of 

distance along the leveling route. The standard deviation 

of a vertical displacement between two bench marks 

separated by 1 km is 1.0 mm if determined from 

successive ftrst-order class II surveys, 1.8 mm from 

second-order class II surveys, and 2.8 mm from 

third-order surveys. The corresponding values are 3.2 

mm, 5.8 mm, and 8.9 mm for a separation of 10 km; and 

10.0 mm, 18.4 mm, and 28.3 mm for a separation of 100 

km. For successive first-order class II surveys1 this 

corresponds to a precision of 1.0 x 10-6, 3.2 x 10· , and 

1.0 x 10·7 over distances of 1 km, 10 km, and 100 km, 

respectively. For comparison, the standard deviation of 

repeated GPS observations over distances up to 11 km is 

4-6 x 10·7 (horizontal) and 1-2 x 10-6 (vertical). The 

standard deviation of GPS measurements increases only 

slowly with distance, such that at 225 km the precision 

increases to 3-5 x 10·8 (horizontal) and 2 x 10·7 

(vertical) (Prescott and others, 1989). 

Redundancy 

Another advantage of leveling is built-in 

redundancy that serves to minimize two sources of 

uncertainty that plague other techniques. The fust source 

is measurement blunders, which, as noted above, are 

easily detected and virtually eliminated by strict 

adherence to established procedures. When dual-scale 

rods are used, the elevation difference between foresight 

CJ) ! a: 

w 

f- 20 L 

w I 

~ 

::J 

...J 

~ 10 ·-

~ 

0 20 40 60 80 100 

DISTANCE, IN KILOMETERS 

Figure 12.1. Standard deviation of vertical 

displacements measured by pairs of geodetic leveling 

surveys of various orders and classes as a function of 

distance. Explanation indicates types of surveys for each 

pair: 1.11-1.11 refers to successive first-order class II 

surveys; 1.11-2.11 refers to a first-order class II survey and 

a second-order class II survey, and so forth. Standards 

for various types of surveys are from table 1 of Vanfcek 

and others (1980). 

12. Geodltlc Leveling as a Tool for Studying Restless Volcanoes 127 
and backsight is measured twice at each setup. The 

procedure for making the readings depends on the type 

of instrument used, but the concept is the same. The 

sequence of readings (backsight low-scale, foresight 

low-scale, foresight high-scale, backsight high-scale) 

includes a closure check on the stability of the turning 

points and the instrument (that is, the difference between 

low-scale and high-scale OED measurements at each 

setup must be less than a specified value: 0.30 mm for 

firSt-order class II surveys). 

In our experience using this system over the past 

seven years, blunders rarely escape detection. The most 

common blunder is disturbance of the foresight rod, 

usually by wind, after the backsight rod and leveling 

instrument have been moved ahead to the next setup but 

before the disturbed rod has been measured as a 

backsight. When this occurs, the traverse must be 

repeated from the previous bench mark. A less obvious 

but equally serious error occurs if a setup is measured 

without ftrSt leveling the instrument. It is possible that 

such a setup would pass the low-scale/high-scale closure 

test (both OED measurements can be equally wrong). 

However, in most cases such an oversight would be 

obvious when viewing the rods through the eyepiece, or, 

if the instrument were significantly out of level in the 

direction perpendicular to the traverse, the setup would 

fail the low-scale/high-scale closure test. The only other 

error that has gone undetected is a bookkeeping error in 

which a failure of the low-scale/high-scale closure test 

goes unnoticed, and the survey proceeds to the next 

setup. This possibility has been practically eliminated by 

programming the recording device to output an audible 

tone and text message whenever the low-scale/high-scale 

closure criterion is not met (Yamashita, 1989). 

A second source of uncertainty that is virtually 

eliminated by redundancy in geodetic leveling is 

bench-mark instability. Except for single-setup arrays, 

most leveling networks include a large number of bench 

marks with relatively close spacing, so anomalous 

movement of unstable marks generally can be recognized 

·and discounted. This point is discussed below. In rare 

cases, movement of a single mark can indicate real 

ground movement (such as localized faulting), so it is 

important to consider the geologic context of each mark 

before discounting anomalous movements. 

Even though unstable marks can usually be 

recognized, it is nonetheless important to install only the 

highest quality marks possible to ensure reliable results. 

This is particularly true for relatively short traverses with 

a small number of marks, and for any type of survey in 

steep or unstable terrain. The National Geodetic Survey 

classifies and provides specifications for various types of 

bench mark installations (Floyd, 1978; Federal Geodetic 

Control Committee, 1974, 1975, 1984). A thorough 

reading of these publications is absolutely required 

before marks are set by an inexperienced crew. The 

usual practice is to establish, at 1- to 3-km intervals, 

bench marks set in bedrock, stable manmade structures 

(preferably grounded on bedrock), and concrete posts, 

or secured to rods driven as deeply as possible (or to a 

stable stratum) with a jackhammer or sledgehammer. 

Large buried boulders and foundations not grounded 

on bedrock are sometimes used, but each traverse also 

should include an adequate number of high-quality 

bedrock or rod marks (preferably at least one every 5 

km for long traverses, and one at each corner of short 

ttaverses and single-setup arrays). 

Timeliness 

A third advantage of leveling over more 

sophisticated techniques such as GPS is that leveling 

provides timely results that can be interpreted 

immediately in the field and factored into assessments 

of volcanic hazards. To maximize the accuracy of 

leveling observations, it is necessary to make a series 

of corrections that require access to a computer 

(Balazs and Young, 1982; Yamashita and Kaiser, 

chapter 13), but in most volcano-monitoring 

applications these corrections are small compared to 

the deformation signal. For example, the largest total 

correction along a 43-km leveling traverse across 

Yellowstone caldera that has been measured annually 

since 1983 is typically 2-3 mm, whereas the annual 

signal is 20-30 mm. In a more severe case along a 

193-km leveling loop at Medicine Lake volcano, 

which included more than 600 m of elevation 

difference and atmospheric conditions that produced 

large refraction corrections, the largest total correction 

was 43 mm. This compared to a corrected closure error 

of 19 mm and a maximum subsidence of 389 ±43 mm 

from 1954 to 1989. Thus, except in very steep terrain 

or extreme climatic conditions, corrections to observed 

vertical displacements over distances of 1-100 km can 

be ignored in preliminary assessments of a volcanic 

hazard. 

DISADVANTAGES OF LEVELING (REAL 

AND PERCEIVED} 

Geodetic leveling, especially of long traverses, is 

not commonly used to study volcanic unrest worldwide 

(Newhall and Dzurisin, 1988). This may be due in part 

to a common misconception about the feasibility of 

leveling in volcanic terrain. Admittedly, long traverses 

require the efforts of several people for an extended 

128 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
period. A typical leveling crew of four to six persons 

(crew chief, two rod persons, a recorder, and up to two 

people to direct traffic) will average 1-6 Ian of firSt-order 

leveling per day, depending on the terrain. Only the crew 

chief need be experienced; other crew members require 

only a few hours of training to be effective. This is a 

substantial commitment, but it should be weighed against 

the precision and coverage afforded by long traverses. H 

staff support is limited or at times of rapid defonnation or 

increased hazard, short ttaverses or single-setup arrays are a 

reasonable alternative to long traverses. This is especially 

lrue when deformation is so rapid that closure cannot be 

obtained on long ttaverses. 

Another disadvantage of leveling is limited access to 

most of the world's active volcanoes: A road to the summit 

of a restless volcano is a rare luxury. However, leveling can 

be done along trails or across open country if the 

equipment can be ttansported to the vicinity of the volcano. 

In the absence of roads or trails, dense vegetation can be a 

serious obstacle, particularly at tropical or subtropical 

volcanoes. Helicopter support is a major advantage if the 

leveling rods can be safely ttansported. Even at remote sites 

with dense vegetation and no road access, short traverses or 

single-setup arrays usually offer viable alternatives to long 

ttaverses. 

Time is another consideration that often weighs 

against long leveling traverses at restless volcanoes. Long 

traverses are marathons that require endurance, whereas 

short traverses and single-setup arrays are sprints that 

feature speed. It is true that a leveling crew can measure 

several short ttaverses or even 15-20 single-setup arrays in 

a day, but it is also true that the same crew can ttaverse 

completely around or over most volcanoes in one to two 

weeks! 

As an example, two crews from the Cascades 

Volcano Observatory (CVO), each consisting of an 

experienced crew chief and four others (some with no prior 

experience), measured a 193-km leveling traverse at 

Medicine Lake volcano in northern California to first-order 

class II standards in nine crew-weeks during July-August 

1989, at an average rate of 3.5 km/working-day/crew. Most 

of the route is over steep unpaved roads, with more than 

600 m of elevation gain and loss. At Yellowstone caldera, 

where the topography is relatively flat, CVO crews average 

about 5 km/day/crew, so that a 43-km line across the 

caldera can be measured by one crew in fewer than 10 

days. Even on the steep slopes of stratovolcanoes such as 

Pacaya and Fuego in Guatemala, a leveling crew can cover 

about 1 km/day (J. Marso and J.W. Ewert, written 

commun., 1989). When precision is a less stringent 

requirement than speed, second order surveys may be a 

wise choice. At Kilauea volcano in Hawaii, where large 

movements are associated with frequent eruptive activity, 

leveling crews from the Hawaiian Volcano Observatory 

average about 7 km/day for second-order class n surveys 

(K.M. Yamashita, written commun., 1989). 

Other Considerations 

Like any marathon, long leveling traverses can 

cause human fatigue that must be properly managed 

for the survey to be successful. Fatigue not only 

lowers morale but also increases the likelihood of 

blunders going undetected. Our experience at Medicine 

Lake, camping and working in a remote area, is that 

2-3 weeks of uninterrupted leveling is an upper limit 

to ensure good morale and top performance. For 

longer surveys, crew members or entire crews should 

be rotated occasionally, or the work schedule should 

include adequate time for rest and relaxation. 

The cost and availability of equipment is another 

issue to be considered. A first-order leveling 

instrument costs $4,000 to $6,000, a pair of Invar rods 

and stays (to hold the rods erect) costs about $5,000, 

and miscellaneous equipment including a recording 

system and temperature sensors costs an additional 

$1,000 to $2,000. The total cost of $10,000 to $13,000 

is substantial but generally not prohibitive for 

volcano-monitoring projects with governmental 

support. Owing to its utility in engineering and 

general surveying applications, leveling equipment is 

available in most countries of the world. 

Data processing requirements also must be 

considered, but they are no more severe for leveling 

than for most other surveying techniques (considerably 

less severe than for GPS surveys). Data recording in 

the field can be automated using a hand-held computer 

that is programmed t.o record rod and temperature 

readings, perform a low-scale/high-scale comparison at 

each setup, and calculate the backsight/foresight stadia 

imbalance, stadia distance, and observed elevation 

difference at each setup (Yamashita, 1989). 

Corrections for rod scale, temperature, refraction, and 

other effects (Balazs and Young, 1982; Yamashita and 

Kaiser, chapter 13) can be made at the conclusion of 

the survey, or even daily in the field using modern 

data recorders or computers. 

RELATIVE ADVANTAGES OF LONG 

TRAVERSES, SHORT TRAVERSES, AND 

SINGLE-SETUP ARRAYS 

Long traverses offer several distinct advantages 

over short traverses and single-setup arrays, but the 

latter type of measurement sometimes is feasible under 

conditions that preclude longer traverses. Perhaps the 

12. Geodltlc Leveling as a Tool for Studying Restless Volcanoes 129 
most important advantage of long traverses is the ability 

to determine near-absolute vertical displacements with 

respect to a reference outside the deforming area (also 

possible with vertical-angle or GPS measurements). The 

choice of a reference usually is arbitrary, but the 

resulting uncertainty can be minimized by extending the 

survey beyond the area of unrest as indicated by other 

observations (usually seismicity) and by considering the 

spatial pattern of vertical displacements. For most 

purposes, an adequate datum can be selected from among 

bench marks that (1) are located outside the identified 

area of unrest and (2) did not move with respect to each 

other during the period between leveling surveys. 

A second advantage of long traverses is the 

redundancy inherent in a large number of bench marks. 

Short traverses are almost as good in this regard, but 

single-setup arrays are susceptible to large errors if 

redundant marks are not measured at each site. Consider 

the case of a single-setup array of two bench marks 

spaced 40 m apart. If one of the marks were to move 

vertically by 0.5 mm relative to the other owing to some 

extraneous process (thermal expansion or contraction, 

ground-water changes, cultural disturbance), the resulting 

tilt change would be given by 0.5 mm/(40 x 103 mm) = 12~5 microradians. On the other hand, if the same 

0.5-mm displacement were to occur at one of the marks 

bounding a 1-km section of a leveling traverse, the 

indicated tilt change would be only 0.5 microradians. In 

addition, anomalous movements of several millimeters or 

more usually can be recognized and discounted if the 

unstable mark is one of several closely spaced marks. 

Three lessons from this example are that (1) long 

traverses are less affected by unstable marks than short 

traverses, (2) the more bench marks along a traverse, the 

greater the opportunity to recognize unstable marks, and 

(3) redundant marks always should be included in 

single-setup arrays. 

The third advantage of long traverses over short 

traverses or single-setup arrays is the utility of spatially 

coherent data as input to numerical models of the 

deformation source. In theory, such models work equally 

well with measurements of vertical displacements or tilt 

changes as inpul In practice, however, tilt measurements 

are more susceptible to extraneous small-scale effects 

than are vertical displacement measurements that span a 

much larger area. 

The primary advantage of short traverses and 

single-setup arrays is speed. This is particularly 

important when surveying time is limited or deformation 

rates are high, as during volcanic emergencies. At such 

times, information from long traverses may not be timely 

enough, closure may not be possible, and it may be 

prudent to limit the exposure of field crews to volcanic 

hazards. Short traverses or single-setup arrays can be 

measured quickly with adequate precision for hazard 

assessment during the crisis; longer traverses can be 

measured later as appropriate. 

EXAMPLES OF LEVELING SURVEYS IN 

VOLCANIC TERRAIN 

The following two examples were chosen to 

illustrate the utility of leveling surveys in volcanic terrain 

as a tool for studying active magmatic and tectonic 

processes. Individual traverses range in length from 196 

m at South Sister volcano to 193 km at Medicine Lake 

volcano. Measurement of the South Sister network of 

four short traverses requires one or two crew-days, while 

the Medicine Lake traverse requires about nine 

crew-weeks. Both networks were established as part of a 

CVO effort to obtain baseline geodetic measurements at 

each of the potentially active volcanoes of the Cascade 

Range. 

South Sister Volcano: A Network of 

Short Traverses to Determine Tilt 

Changes 

South Sister volcano is located along the Cascade 

volcanic chain in the Three Sisters Wilderness Area of 

west-central Oregon. The area has been the site of 

recurrent silicic volcanism during Pliocene and 

Quaternary time. Rhyodacite tephra, lava domes, and 

lava flows erupted during late Holocene time from more 

than 20 vents on the flanks of the volcano (Scott, 1987). 

In 1985, CVO established a network of four linear 

leveling traverses ranging in length from 196 m to 317 m 

at South Sister (Yamashita and Doukas, 1987). The 

purpose of the network is to provide baseline geodetic 

information for comparison with future surveys, 

especially at times of volcanic unrest. Triangular or 

L-shaped arrays would have been preferable, but those 

configurations were precluded by the steep terrain. The 

flanks of the cone are sparsely vegetated, so the biggest 

obstacle to longer traverses was the steepness of the 

flanks. The traverses are located at elevations of 2,200 m 

to 2,500 m along radial ridges on the north, east, south, 

and west flanks of the volcano (fig. 12.2). In spite of 

numerous short (10 m and less) setups required by the 

terrain, the network can be double-run in one or two 

days with helicopter support. 

The South Sister network was remeasured in 1986 

to verify the initial observations and determine if 

measurable deform~tion had occurred since the network 

was established. Measured tilt changes ranged from 0.8 

to 6.5 microradians. No pattern was apparent in the 

130 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
changes, which are attributed to measurement error. 

Better precision (1-2 microradians) could be obtained 

from longer traverses (about 1 km), but the time required 

for the measurements would increase accordingly. The 

existing network is thought to be a good compromise, 

adequate to detect precursory deformation in the event of 

renewed volcanic activity. 

Medicine Lake Volcano: A Single Long 

Traverse to Determine Regional 

Deformation 

Medicine Lake volcano is a Pleistocene and 

Holocene shield volcano located in northeastern 

California about 50 km east of Mount Shasta, near the 

western margin of the Basin and Range tectonic 

province. Lava Beds National Monument is located on 

the northern flank of Medicine Lake volcano and 

encompasses mostly basaltic and some andesitic lavas. 

Higher on the volcano, basaltic lava is mostly absent, 

andesite dominates, and rhyolite and small volumes of 

dacite are present, the latter mainly near the 7 x 12 km 

Medicine Lake caldera (Donnelly-Nolan, 1988). 

A 20-km traverse across the summit area of 

Medicine Lake volcano was measured during August 

1988. This first-order class II survey repeated a 

121° 45' 

44° 07' 

Flgure12.2. Locations of four short leveling traverses 

on flanks of South Sister Volcano in west-central Oregon, 

from Yamashita and Doukas (1987). Contours are in feet. 

second-order survey made in 1954 by the National 

Geodetic Survey. After it was recognized that substantial 

subsidence had occurred between the two surveys and 

that the deformation field extended well beyond the 1988 

traverse, a more ambitious leveling effort was conducted 

in summer of 1989. The 1989 survey crossed Medicine 

Lake volcano and the Stephens Pass area, where intense 

swarms of shallow earthquakes occurred in 1978 and 

1981. Closure for the 1989 survey was 19 mm around 

the 193-km circuit, well within NGS requirements for 

first-order class II surveys1 (5 mm · Lv1 = 69 mm; 

Vanfcek and others, 1980). Approximately 80% of the 

1954 bench marks along the route were recovered, and 

missing marks were replaced to maintain an average 

spacing of 1.5 km. 

The most obvious feature of the vertical 

displacement data (figs. 12.3-12.5) is an area of 

A 

(/) 

a: Medicine Lake~ 

::: 2000 

LJ.J ~ ~ 1800 ,&'_Caldera':,= Stephens Pass 

;- 1600 = ~ :'~ 

21400 ~ ~ ~ """ 

~ 1200 ,; ~ ":, Bartle~ 

LJ.J 

~1000~~

'-Bartle 

--~--~--~--~~~~--~--~~ 

~ 

LJ.J 0 ~ ~ ~ ~ 100 1~ 1~ 1~ 180 200 

STADIA DISTANCE, IN KILOMETERS 

8 

1H197 i H~97 

~ -~j-§-~~~~- ===============~~~=- :- J·~~~~~ !l ~ -100 

:J 

_J -150 

~ 

~ -200 

~ -250 

w 

~ -300 w 

~ -350 _J 

fu -400 

Ci 

-

M500 

46M 

~ ·~

-- T502 

~::: 

~~-~ 

V501 

§'-J502 

s 

C500 

EXPLANATION 

Benchmark _ Standard deviation 

-450 ~._____l __ ___!..._ __ _.L.. __ _J.._ __ _.__ __ _.__ __ _,__ __ ,_____. __ __, 

0 20 40 ~ M 100 1~ 1~ 1~ 180 200 

STADIA DISTANCE, IN KILOMETERS 

Flgure12.3. Topography (A) and vertical displacements 

(B) along a 193-km leveling loop across Medicine Lake 

volcano and Stephens Pass, California. Displacements 

were calculated by comparing results from a 

second-order survey in 1954 and a first-order class II 

survey in 1989. Bench mark H197 near Bartle was held 

fixed. Shaded area in 8 indicates ±1 standard deviation. 

1 A common source of confusion is the distinction between the 

standard deviation of an OED, o(h), the standard deviation of an OED 

change, o(8h), and the maximum allowable circuit closure, C, for 

surveys of a ~ven order and class. For first-order class IT surveys, O'(h) 

= 0.7 mm · L ; o(l;h) = 1.0 mm · L ~; and C = S.O mm · L ~ (Vamcek 

and othen, 1980) 

12. Geodltlc Leveling as a Tool for Studying Restless Volcanoes 131 
pronounced subsidence (maximum 389 :t: 43 mm at T502, 

corresponding to an average rate of 11 mm/yr) in the 

summit area of Medicine Lake volcano, near the center 

of Medicine Lake caldera. Subsidence extends beyond 

the caldera rim and across the entire volcanic edifice. 

In addition, there is evidence in several places along 

the leveling route for localized faulting, presumably 

associated with historical crustal extension in the 

western Basin and Range tectonic province (for 

example, near M500, Y500, V501, and C500). 

The apparent inverse correlation between 

topography and vertical displacements raises the 

possibility that the calculated displacements are 

contaminated by systematic error in one or both of the 

leveling surveys. For example, if the leveling rods 

used for the 1954 survey were poorly calibrated or the 

rod corrections were incorrectly applied, error would 

have accumulated as a function of elevation. As a 

result, the difference between the 1954 and 1989 

surveys would indicate elevation-dependent 

displacements. This possibility is currently under 

investigation, but at present the calculated 

displace~ents are thought to be correct because (1) the 

magnitude of the rod-correction error that would be 

required to explain the displacements is implausibly 

large (389 :t: 43 mm in 600 m elevation change is 

approximately 6 x 10-4

); (2) the correlation between 

topography and displacements breaks down in detail, 

particularly in the Stephens Pass area; and (3) a 

0 5 10 15 20 25 30 35 

50 

45 

40 

35 

30 

25 

20 

15 

5 

0 

-5 -5 

-20 -15 -10 -5 0 5 10 15 20 25 30 35 

DISTANCE EAST OF BARTLE, IN KILOMETERS 

general correlation between topography and vertical 

displacements makes sense if the displacements are 

caused by isostatic loading, magmatic deflation, or some 

other volcanogenic process. 

The area of localized subsidence near C500 

(about 20 em with respect to nearby marks) is within the 

epicentral area of the 1978 Stephens Pass earthquake 

swarm, which was accompanied by ground breakage 

(Cramer, 1978; Bennett and others, 1979). Although only 

one bench mark is involved, both its undisturbed 

appearance and proximity to the 1978 epicenters suggest 

that the displacement was caused by faulting rather than 

bench-mark instability or cultural disturbance. 

Unfortunately, several nearby marks were destroyed by 

road work during the period between the leveling 

surveys. The grabenlike feature near V501 probably is 

associated with a swarm of shallow earthquakes that 

occurred just north of Tennant during 1981. No historical 

seismicity has been recognized that might correlate with 

the area of localized faulting near MSOO and Y500, but 

that area is cut by many young faults that bound the 

Whitehorse Mountains and horst-and-graben terrain to 

the west. 

The cause of the 1954-1989 subsidence at 

Medicine Lake volcano is still under investigation, but 

almost surely there are both tectonic and volcanic aspects 

to the problem. Crustal thinning owing to Basin and 

Range extension, isostatic loading by the weight of the 

volcanic pile, and processes within a possible magma 

B 

CJ) 45 a: 

w tu 40 

:::;E 

g 35 

~ 

~ 30 

~ 

~ 25 

I 

~ 20 

I 

t: 15 

0 

z 10 w 

0 

z 5 

~ CJ) 

0 0 

0 5 10 15 20 25 30 35 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

- 0 

-~20 -15 -10 -5 0 5 10 15 20 25 30 35- 5 

DISTANCE EAST OF H197, IN KILOMETERS 

Figure 12.4. Smoothed topography (A) and vertical displacements for 1954-89 (B) In Medicine Lake region of northern 

California. Center of Medicine Lake caldera Is near bench mark T502 (8, upper right). Contour Intervals are 100 m In 

A and 2.5 em in 8. Displacements are relative to bench mark H197 near Bartle. Large negative displacements near 

bench marks M500, Y500, V501, and C500 may indicate local faulting, although spatial distribution of these movements 

is poorly determined. 

132 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
chamber beneath the summit area (Evans and Zucca, 

1988) may each play a role. 

RECOMMENDATIONS 

The following general recommendations are based 

primarily on personal experience from leveling surveys 

at Kilauea volcano in Hawaii, at several volcanoes in the 

Cascade Range, and at Yellowstone and Long Valley 

Figure 12.5. Three-dimensional representations of 

topography (A) and 1954-89 vertical displacements (8) 

in Medicine Lake region. Displacements are relative to 

bench mark H197 near Bartle. View is to southeast. 

calderas. Specific applications at other volcanoes might 

require modifications to accommodate local conditions 

and constraints. 

1. Consider leveling a "technique of ftrSt resort." 

Are there specific reasons why a long leveling traverse is 

inappropriate or not feasible? If so, is a short traverse or 

a network of single-setup arrays a viable alternative? 

What types of information might be gained by a long 

traverse that are otherwise unavailable? Weigh the 

increased effort and surveying time against the 

advantages of greater precision and areal coverage. 

2. Be meticulous in following established 

procedures. Standard deviations calculated for a survey 

of a given order and class of leveling strictly apply only 

if the appropriate procedures and specifications are 

followed exactly. Cutting corners may save a small 

amount of time and seem harmless in the field, but keep 

in mind that the results eventually may be compared to 

surveys conducted many decades later by other 

researchers with other goals or requirements. 

3. Err on the side of precision. Field procedures 

should be matched to the requirements of the individual 

survey to avoid unnecessary complication, but the 

additional time and effort to conform to first-order 

standards is an investment that might pay dividends in 

the future. 

4. Double-run segments to check procedures. It is 

tempting to single-run twice as far, but double-run at 

least 10 percent of each traverse to verify that the desired 

precision is being obtained. 

5. Never skimp on bench marks. No amount of 

measurement precision can overcome the effects of 

inadequate bench marks. Follow established procedures 

for installing all marks (Floyd, 1978). Design short 

traverses and single-setup arrays with enough 

redundancy to eliminate mark instability as a potential 

source of error. 

6. Combine leveling surveys with other types of 

geodetic measurements. No single data set is likely to be 

definitive. The combination of vertical displacements 

from long leveling traverses, tilt changes from short 

traverses or single-setup arrays, and horizontal 

displacements from EDM or GPS measurements provides 

a particularly powerful constraint for models of the 

deformation source and assessments of related hazards. 

ACKNOWLEDGMENTS 

The crew chief is the heart of every leveling crew, 

and CVO is particularly favored in this regard with the 

services of Ken Yamashita. On many occasions in the 

past 10 years, Ken has turned an inexperienced collection 

of students, teachers, bosses, and friends into a hardened 

12. Geoditlc Leveling as a Tool for Studying Restless Volcanoes 133 
leveling crew in a remarkably short time, sometimes 

under extraordinary conditions. Others have served 

~~bly as temporary crew chiefs for special projects, 

mcluding John Estrem, Gene Iwatsubo, Jack Kleinman, 

and Dave Schneider; their considerable efforts are deeply 

appreciated. So, too, are the labors of those who ran 

rods, recorded data, and flagged traffic from Yellowstone 

to Newberry Crater and from Mount Baker to Lassen 

Peak. 

REFERENCES CITED 

Balazs, E.I., and Young, G.M., 1982, Corrections applied by the 

National Geodetic Survey to precise leveling observations: 

National Oceanic and Atmospheric Administration 

Technical Memorandum NOS NGS 34, 12 p. 

Bennett, J.H., Sherburne, R.W., Cramer, C.H., Chesterman, 

C.W., and Chapman, R.H., 1979, Stephens Pass 

earthquakes, Mount Shasta-August 1978: California 

Geology, February 1979, p. 27-34. 

Cramer, C.H., 1978, The Stephen's Pass earthquake swarm of 

August 1978, east ofMt. Shasta, California: Eos, v. 59, no. 

12, p. 1130. 

Donnelly-Nolan, J.M., 1988, A magmatic model of Medicine 

Lake volcano, California: Journal of Geophysical 

Research, v. 93, no. B5, p. 4412-4420. 

Evans, J.R., and Zucca, J.J., 1988, Active high-resolution 

seismic tomography of compressional wave velocity and 

attenuation structure at Medicine Lake volcano, north.em 

California Cascade Range: Journal of Geophysical 

, Research, v. 93, no. Bl2, p. 15,016-15,036. 

Federal Geodetic Control Committee, 197 4, Classification, 

standards of accuracy, and general specifications of 

geodetic control surveys: Rockville, Maryland, National 

Oceanic and Atmospheric Administration. 

__ 1975, Specifications to support classification, standards of 

accuracy, and general specifications of geodetic control 

surveys: Rockville, Maryland, National Oceanic and 

Atmospheric Administration. 

__ 1984, Standards and specifications for geodetic control 

networks: Rockville, Maryland, National Oceanic and 

Atmospheric Administration. 

Floyd, R.P ., 1978, Geodetic bench marks: National Oceanic and 

Atmospheric Administration Manual NOS NGS 1, 52 p. 

Holdahl, S.R., 1982, Recomputation of vertical crustal motions 

near Palmdale, California, 1959-1975: Journal of 

Geophysical Research, v. 87, no. Bll, p. 9374-9388. 

Magi, K., 1958, Relations between the eruptions of various 

volcanoes and the deformations of the ground surface 

around them: Bulletin of the Earthquake Research Institute, 

v. 36, p. 94-134. 

Newhall, C.G., and Dzurisin, D., 1988, Historical unrest at large 

calderas of the world: U.S. Geological Survey Bulletin 

1855, 1,108 p. 

Packard, R.F., and MacNeil, J.H., 1983, A direct comparison of 

spirit and compensator leveling: Geophysical Research 

Letters, v. 10, no. 9, p. 849-851. 

Pelton, J.R., and Smith, R.B., 1982, Contemporary vertical 

surface displacements in Yellowstone National Park: 

Journal of Geophysical Research, v. 87, p. 2745-2761. 

Prescott, W.H., Davis, J.L., and Svarc, J.L., 1989, Global 

positioning system measurements for crustal deformation: 

Precision and accuracy: Science, v. 244, p. 1337-1340. 

Rumpf, W.E., and Meurisch, H., 1981, Systematische 

nderungen der Ziellinie eines prztstons 

Comensator-nivelliers-Insbesondere des Zeiss Ni 

1-Durch Magnetische Gleich- und Wechselfelder, paper 

presented at 16th International FIG Congress, Montreux, 

Switzerland, 26 p. 

Scott, W.E., 1987, Holocene rhyodacite eruptions on the flanks 

of South Sister volcano, Oregon, in J.H. Fink, ed., The 

emplacement of silicic domes and lava flows: Geological 

Society of America Special Paper 212, p. 35-53. 

Schomaker, M.C., and Berry, R.M., 1981, Geodetic leveling: 

Rockville, Maryland, National Oceanic and Atmospheric 

Administration Manual NOS NGS 3. 

Stein, R.S., 1981, Discrimination of tectonic displacement from 

slope-dependent errors in geodetic leveling from southern 

California, 1953-1979, in Simpson, D.W., and Richards, 

P.O., eds., Earthquake prediction, an international review: 

Washington, D.C., American Geophysical Union (Maurice 

Ewing Series v. 4), p. 441-456. 

Strange, W.E., 1981, The impact of refraction correction on 

leveling interpretations in southern California: Journal of 

Geophysical Research, v. 86, no. B4, p. 2809-2824. 

Varucek, P., Castle, R.O., and Balazs, E.l., 1980, Geodetic 

leveling and its applications: Reviews of Geophysics and 

Space Physics, v. 18, no. 2, p. 505-524. 

Wilson, R.M., 1935, Ground surface movement at Kilauea 

Volcano, Hawaii: University of Hawaii Research 

Publication 10, 56 p. 

Yamashita, K.M., 1981, Dry tilt: A ground deformation monitor 

as applied to the active volcanoes of Hawaii: U.S. 

Geological Survey Open-File Report 81-523, 21 p. 

Yamashita, K.M., 1989, Using the "HP-71B" hand-held 

computer for data entry while running ftrst-order class n 

level surveys: U.S. Geological Survey Open-File Report 

89-179, 21 p. 

Yamashita, K.M., and Doukas, M.P., 1987, Precise level lines 

at Crater Lake, Newberry Crater, and South Sister, Oregon: 

U.S. Geological Survey Open-File Report 87-293, 32 p. 

134 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
13. Using First-Order Class II Geodetic Leveling 

Procedures to Monitor Vertical Displacement 

By Kenneth M. Yamashita and William P. Kaiser 

ABSTRACT 

Levels have been used to monitor volcanoes since 

the early 1900's. Members of the Cascades Volcano 

Observatory use first-order class II geodetic leveling 

procedures as defined by the Federal Geodetic Control 

Committee to monitor vertical displacement at active or 

potentially active volcanoes. Equipment needs, 

bench-mark spacing and installation, data acquisition, and 

data reduction procedures are defined and described. 

Field procedures and technical specifications for 

equipment are summarized and consolidated to provide a 

guide for those Interested in using geodetic levels as a 

method of monitoring vertical displacement. 

BACKGROUND 

The use of levels and other surveying instruments 

to monitor vertical displacement in volcanically active 

areas is not a new science. It has been in use in Hawaii 

since the early 1900's, when R.M. Wilson, comparing 

data from sea level to Kilauea volcano in 1926, 

determined that "this large circuit closure error may be 

in part due to ground movement" and stated that "The 

divergence of the 1926 line probably represents an actual 

subsidence of the ground surface" (Wilson, 1935). Since 

that survey, major improvements in both surveying 

instruments and methods have occurred. 

In October 1983 members of the Cascades Volcano 

Observatory (CVO) in Vancouver, Washington, executed 

a first-order class II geodetic level survey to monitor 

vertical displacement at Yellowstone National Park, 

Wyoming (Dzurisin and Yamashita, 1986). Since that 

initial survey, geodetic leveling has been the principal 

method used at eva to monitor vertical displacement for 

volcanic studies in the Western United States. 

The objectives of this report are to (1) consolidate 

the many published reports on geodetic leveling, (2) 

provide a guide for those interested in utilizing leveling 

as a means of monitoring for vertical displacements, and 

(3) outline the procedures and techniques used by 

members of eva. 

FEDERAL GUIDELINES FOR GEODETIC 

LEVELING 

Guidelines to coordinate national mapping, 

charting, and surveying activities were established in 

1919 by the Federal Surveying and Mapping Agencies 

(FSMA). The FSMA was abolished in 1942 and its 

functions transferred to the Bureau of the Budget, which 

in tum transferred its authority to the Federal Geodetic 

Control Committee (FGCC) when it was chartered in 

1968. 

In 1975 and 1984 the FGCC updated its guidelines 

prescribing specific standards and accuracies for surveys 

included in the National Network of Geodetic Control, of 

which geodetic leveling is one technique used for 

vertical control. Under these guidelines, specific 

constraints are imposed as to procedure, accuracies, 

precautions, and equipment that can be used for a 

particular order and class of leveling (Federal Geodetic 

Control Committee, 1975). 

FIRST-ORDER CLASS II LEVELS 

First-order class II levels are used primarily for 

regional engineering and for measuring vertical 

displacement. One of the primary tasks of CVO is to 

monitor active or potentially active volcanoes, on which 

appreciable surface displacements may occur. Therefore, 

CVO crews typically run first-order class II surveys. 

This report outlines procedures and standards for such 

surveys. 

Besides closure error, the criteria imposed to 

determine order and class of the level survey are 

governed by these guidelines: (1) equipment, (2) 

bench-mark spacing and installation, (3) data-acquisition 

procedures, and ( 4) data-reduction procedures. 

13. Using First-Order Class II Geodetic Leveling Procedures to Monitor Vertical Displacement 135 
Equipment 

Level Instrument 

For first-order leveling, an instrument with an 

optical micrometer capable of resolving 0.01 em is the 

first criterion. 

If a spirit level is used, the level vial of the 

instrument must be sufficiently sensitive and consistent 

to set the line of sight within 0.25 second of arc, and for 

compensator levels, the compensator should also be 

capable of setting the line of sight to within 0.25 second 

of arc. The instrument should have an objective lens 

diameter of approximately 50--70 mm, have a telescope 

magnification of about 40 power, and be equipped with a 

horizontal, V -shaped reticule. 

Aside from these requirements, any instrument 

capable of producing results classified as first-order class 

II levels can be used. 

CVO Instrumentation 

The 1983 and 1984 Yellowstone surveys 

(Dzurisin and Yamashita, 1986; Dzurisin and others, 

1986) used the Wild N3 precision level. All other 

surveys by eva used the Wild NA2 compensator 

level. 

The technical information supplied with the Wild 

NA2 indicates that the NA2 has an optional telescope 

magnification of 25, 32, or 40 power. The objective 

aperture is 45 mm, and it has a compensator setting 

accuracy of z0.3 second of arc. The parallel 

micrometer plate is read to the nearest 0.1 mm and 

estimated to 0.01 mm. 

Although the specification for the NA2 is just 

outside limits set by the FGee, our data show that by 

following proper field procedures the NA2 is capable 

of producing first-order class II accuracy (Dzurisin and 

Yamashita, 1987). 

The N3 spirit level has the option of using a 34, 

45, or 46 power magnification and has an objective 

lens of 56 mm. The tubular level has a sensitivity of 

10 seconds and a setting accuracy of the split bubble 

of z0.25 second. The parallel micrometer is read to the 

nearest 0.1 mm and estimated to 0.01 mm. The N3 

therefore meets all the requirements as set by the 

FGee. 

eva purchased a Jena NI 002 A reversible 

compensator level in January 1989. It has a 40 or 50 

power telescope magnification, an objective lens of 52 

mm, and a pendulum setting accuracy of z0.05 second. 

The micrometer scale is read to the nearest 0.1 mm 

and interpolated to 0.01 mm. The Jena NI 002 A 

therefore also meets all the requirements as set by the 

FGee. 

Level Rods 

The level rods used should have rod scales 

composed of the metal Invar and have 1.0 em or 0.5 em 

graduations. Each rod should have two parallel scales, 

but the scales should differ in value by some constant. 

The level rod should have a circular level bubble 

accurate to 10 minutes of arc and be self-supporting, that 

is, capable of accepting stays or braces. 

eva uses Wild GPLE-3 lnvar level rods and Kern 

lnvar rods. Both have an Invar strip with 1-cm 

graduations, and both are capable of accepting braces to 

become free standing. 

The rods are calibrated yearly by the National 

Bureau of Standards in Gaithersburg, Maryland, to an 

accuracy of 0.00005 m. 

Temperature Probes 

The temperature probes should be of the aspirated 

type, where the fans pull rather than push air past the 

temperature sensors. This prevents heat from the fan 

motor from biasing the true ambient temperature. The 

temperature sensors should be shielded from direct 

exposure to the sun and be accurate to 0.1 oe or 0.2 °F. 

There should be two probes, one each located at 

0.5 m and 2.5 m (Balazs and Young, 1982), or 0.3 m and 

1.3 m (Schomaker and Berry, 1981), above the ground 

surface. The probes can be either hand-held or mounted 

on the tripod leg of the level instrument. 

Bench-Mark Spacing and Installation 

Bench marks along a first-order class II level route 

must not be spaced farther than 3.0 km apart or have an 

average spacing greater than 1.6 km apart. Bench marks 

are classified as class A, class B, or others (Floyd, 1978). 

Most bench marks set by eva are either on 

bedrock, and therefore classified as A, or set on rods 

without sleeves, and therefore classified as B marks, as 

per the classifications set by the National Oceanic and 

Atmospheric Administration (NOAA). See Hoyd (1978) 

for complete details as to the classification and 

procedures for bench-mark installation. 

Procedures to Follow During Data 

Collection 

The accuracy of the field data depends on a 

number of factors, the most important of which are (1) 

adjustment of the level instrument, (2) adjustment of the 

level vials, and (3) field procedures. 

136 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Adjustment of Level Instrument 

The instrument should be checked for collimation 

error daily, except for reversible compensator levels 

which are checked weekly (Federal Geodetic Control 

Committee, 1984). 

Parallax Adjustment 

Before the instrument is read, it should be checked 

for parallax by looking through the eye piece, focusing 

on a fixed object, then bobbing the head up and down. 

The horizontal wire should not appear to move with 

respect to the fixed object. If it does, then parallax exists, 

and must be removed. 

To remove the parallax, focus the telescope on a 

plain background, then adjust the focus of the reticule, 

then adjust the focus of the telescope again. Continue to 

focus and refocus the reticule and the telescope. There 

should be minimal movement of the reticule when the 

parallax is removed. 

Peg Test 

Instrument collimation error can be detected and 

corrected by a procedure called a peg test. Tests 

conducted by the National Geodetic Survey (NGS) have 

found that performing a peg test during times of positive 

air-when the ground temperature is colder then the air 

temperature-can cause refraction errors that indicate 

that the instrument is out of adjustment when in reality it 

is not. It is now required that a peg test be conducted 

only during times of negative air temperature. The 

prescribed procedure is to perform a peg test no earlier 

than 2 1/2 hours after sunrise, and no later then 1/2 hour 

before sunset (Poetzschke, 1983). 

Proper procedure for performing a peg test can be 

found in Schomaker and Berry (1981, p. 3-31). 

Adjustment of Level VIals 

The bulls-eye level on compensator levels can be 

checked by first leveling the bubble and rotating the 

level instrument 180°. If the level bubble is within 0.2 

mm of center, the level is considered to be in adjustment, 

otherwise it needs to be adjusted (Schomaker and Berry, 

1981, p. 3-26). 

To adjust the level vial, bring the level bubble 

back half-way into the level circle using the level foot 

screws. Finish leveling the level vial with the adjustment 

screws located under the level vial. Rotate the instrument 

180° and repeat the procedure until the level bubble stays 

within 0.2 mm of center when rotated. 

The level vial on the rods can be easily checked by 

holding a carpenter's level along the edge and front of 

the rod. The level on the rod cannot be checked, or 

adjusted, by rotating the rod, as it was done with the 

level instrument. The rod is leveled using the carpenter's 

level, not the bulls-eye level. If the level is out of 

adjustment the bubble is brought back into the circle the 

full amount using the adjustment screws, and the rod is 

not rotated at all. 

The level vial should stay within 2 mm of center or 

10 minutes of vertical when rotated (Schomaker and 

Berry, 1981, p. 3-42). 

Field Procedures 

Explicit instructions outlining field procedures are 

given by the Federal Geodetic Control Committee (1984) 

for the class of leveling that is being performed. For 

first-order class II level surveys, the following field 

procedures should be followed. 

The level survey should generally start from an 

established permanent survey mark, and at least two 

consecutive "old" bench marks should be incorporated in 

the new survey (Federal Geodetic Control Committee, 

1984). 

Before the level survey actually begins, the 

note-taker should record the date, the type of level 

instrument that is being used, and the instrument serial 

number. The brand type of level rod, the serial number, 

the units that the tern peratures will be recorded in 

(Celsius or Fahrenheit), the geographical area in which 

the survey is made, and the serial number of the rod that 

is on the bench mark should also be recorded. 

A rod is leveled with. brace poles over the highest 

part of the monument. The level instrument is then set at 

some convenient distance from the backsight rod, but at 

a point no farther then 60 m away, and in such a manner 

that the lowest rod scale reading on the rod is no lower 

then 0.5 m because refractions close to ground level can 

cause appreciable errors (Whalen, 1981 ). The distance 

can be approximated by pacing, and the elevation 

difference can be accurately estimated with a hand level. 

The foresight rod is established at a distance that is 

equal to that of the backsight rod from the instrument. 

This can be done by pacing the distance from the 

backsight rod to the instrument, and pacing an equal 

distance beyond the level instrument for the foresight 

position. The imbalance between the two distances 

cannot exceed 5.0 m, nor can the lowest rod scale 

reading on the foresight rod be lower than 0.5 m (Federal 

Geodetic Control Committee, 1984). 

The foresight turning point is established by 

driving either a wooden stake or a metal pin into the 

ground; on a hard surface, a trivet or "turtle" should be 

used. Again, pace to establish the distance and use a 

hand level for the elevation difference. 

Generally the first reading taken is on the 

backsight rod, low scale, then the top stadia wire, then 

13. Using First-Order Class II Geodetic Leveling Procedures to Monitor Vertical Displacement 137 
the bottom stadia wire. The level is rotated to the 

foresight rod, a reading is taken on the low scale, and 

then the stadia are read and computed. If the stadia 

imbalance is greater then 5.0 m, the level instrument or 

the foresight rod should be moved so the imbalance is 

less then 5.0 m. 

If a reversible compensator level is used, the 

compensator position should now be changed. For 

nonreversible compensator levels, the level should be 

disleveled and releveled (this has not been done by eva 

crews or the National Mapping group), then a reading 

taken on the foresight rod, high scale, then on the 

backsight rod, high scale. If the field crew is using a 

level instrument with a reversible compensator, such as 

the Jena NI 002, consult Poetzschke (1983), which 

contains valuable information unique to reversible 

compensator levels. 

The elevation difference computed for the low 

scales and the high scales cannot exceed 0.10 em for 

reversible compensator levels, or 0.03 em for all other 

levels. If the difference is greater, the entire setup should 

be rejected and read again. If the stadia readings were 

acceptable, the stadia need not be repeated. 

Informing the rod people of the true distance as 

determined by the stadia readings will help them to more 

accurately adjust their pacing. 

If the readings are acceptable, the temperatures are 

read top probe first, then the bottom probe. The 

thermometers should be aspirated for at least one minute 

before taking a reading (Schomaker and Berry, 1981). 

When all the readings and checks for the setup are 

acceptable, the backsight rod and the level are moved 

leapfrog fashion past the foresight rod; again balance the 

stadia distance, and do not use the lower 0.5 m of the 

level rod. While the move is taking place, the previous 

foresight rod person should rotate the rod 180°, taking 

extreme care not to disturb the turning point in any way. 

The previous foresight rod now becomes the backsight 

rod, and the previous backsight rod is now the new 

foresight rod. 

The new foresight is established by pacing the 

distance from the backsight rod to the level instrument, 

and pacing an equal distance beyond the level instrument 

for the new foresight position. 

The recorder should be keeping track of the 

running elevation difference between the foresights and 

backsights and also the cumulative stadia imbalance, and 

informing the rod people of these values so that they can 

adjust for any imbalance. An imbalance of 5.0 m per 

setup and 10.0 m between sections (typically 1 to 2 km 

between bench marks) is allowed (Federal Geodetic 

Control Committee, 1984). 

This leapfrog sequence is followed until another 

bench mark is reached. Standards require that two rods 

be used and that the rods be alternately read as the 

foresight rod, then as the backsight rod. 

After the reading on the foresight bench mark is 

made, the bench-mark ID is recorded together with a 

wind and cloud code. 

The cloud codes are "0" if less than 25 percent of 

the readings are taken in sunny condition, "1" if 25 to 75 

percent of the readings are taken in sunny conditions, 

and "2" if more than 75 percent of the readings are taken 

in sunny conditions. The wind codes are "1" if the wind 

speeds average less than 5 mph (10 kph), "2" if the wind 

speeds average 5-15 mph (10-25 kph), and "3" if the 

average is greater than 15 mph (25 kph) (Holdahl, 1985). 

The wind and sun codes are used to predict tern peratures 

if these are not taken at the time of the survey. 

Before continuing to another section, the data 

collected should be checked against previous data to 

ensure that there are no obvious errors. Closure error 

should be computed and verified for double-run or closed 

loops. 

At eva, all data are recorded on an HP-71B 

hand-held computer (Yamashita, 1989), where all the 

checks and balances are made. If a similar data 

collection system is utilized, the field data should be 

printed, or recorded on a digital recorder at this time. 

The HP-71B has enough memory to hold a day's worth 

of level data, but we have had a "memory lost" problem 

with this computer and feel safer by dumping the data at 

each mark. 

eva crews have been reversing the direction of 

the run after each bench mark. In other words, if the 

direction of the first run from bench mark to bench mark 

was from north to south, the next run would be in the 

opposite direction, from south to north. It is 

recommended that the direction of leveling be reversed 

from morning to afternoon or every other day (Federal 

Geodetic Control Committee, 1984). The reasoning is 

that by reversing the direction between runs (for 

single-run lines), cumulative errors due to pin settling 

(accumulation of small systematic errors from turning 

point movement) can be minimized by cancellation. 

Sources of Error 

Errors are normally characterized as being random 

or systematic. Random errors are normally attributed to 

unpredictable variations encountered during the survey. 

These errors can be caused by movement of the tripod or 

the turning points, or by the effects caused by 

imbalanced shot lengths. Systematic errors are those that 

are inaccurate but consistent, such as the effects of the 

Earth's magnetic field on certain kinds of compensator 

levels (Rumpf and Meurisch, 1981), or the errors that 

can be introduced by using instruments or level rods that 

are not in adjustment or poorly calibrated. 

138 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Some errors can be directly attributed to departure 

from accepted field practices, but these can be minimized 

or eliminated by adhering to procedures as set forth by 

the Federal Geodetic Control Committee (1984). 

However, there are other sources of error that are 

inherent and present no matter what precautions are 

taken. These errors can be accounted for, and corrections 

applied, only during data reduction. 

Errors incorporated during the field survey can be 

detrimental to the integrity of the data set and can 

appreciably add time and expense to the survey, not to 

mention the frustration of rerunning sections. The 

following procedures can minimize these errors: 

1. Check for parallax frequently by moving the 

head up and down while watching the reticule for 

bounce. 

2. Perform a peg test daily (but only during times 

of negative temperatures), or whenever there is reason to 

suspect that the instrument has drifted, for example if the 

instrument was jarred during transportation, or if there 

are an inordinate number of "no checks" between 

low-scale and high-scale readings. 

3. Check the level vial on the instrument 

frequently during the day. Glancing at the level bubble 

after the instrument is rotated between foresight and 

backsight is sufficient to see if the level bubble is out of 

adjustment. The level vial on the rod can be quickly and 

easily checked while the instrument is being set up for 

the peg test. Adjustments on the level vials should be 

done before doing the peg test. 

4. The rod person should ensure that nothing is on 

the bottom of the level rod before setting it on either the 

bench mark or the turning point. This can be 

accomplished by wiping off the bottom of the rod before 

setting it on the mark, or by rotating the level rod 

slightly from left to right and listening for any grinding 

noise which would indicate dirt. The rod person should 

also use extreme care to ensure that the level rod is not 

only on the bench mark but on the highest part of the 

mark, and that no part of the rod is touching anything but 

the bench mark. It is critical that the turning points do 

not move during the transition from being a foresight rod 

to becoming a backsight rod. This is one of the few areas 

where there is no way to check for a blunder. 

5. The instrument person can ensure that the tripod 

is stable and not subject to movement between reading, 

and that the reading sequence is as described earlier. 

This gives two independent elevation differences for the 

setup. If the instrument or tripod has not moved between 

the high-scale and low-scale readings the elevation 

difference should be the same to 0.03 em. If the level 

line runs along an asphalt paved road, the instrument 

person should ensure that at least two of the three tripod 

legs are off the pavement because of the possibility of 

the tripod legs sinking into the asphalt. 

6. The recorder is responsible for noting that all 

required information such as equipment serial number, 

wind and cloud codes, date, time, and bench-mark ID's 

are recorded correctly. The recorder needs to ensure that 

the checks between the low-scale and high-scale readings 

are within the tolerance prescribed for that particular 

order and class of survey, and that the elevation 

difference and stadia distance are current. It is important 

that the stadia distance be balanced and any imbalance 

corrected. Balancing the stadia distance will eliminate, or 

at least minimize, errors introduced by a maladjusted 

instrument and by the effects of curvature (where the line 

of sight is not parallel to the equipotential surface 

(Schomaker and Berry, 1981). If the data are recorded 

electronically, the recorder needs to ensure that they are 

properly stored on another electronic medium for transfer 

to a PC or other computer device. 

Both the field chief of party and the recorder 

should calculate double-run, or loop-closure, errors. 

Single-run data should be compared to the previous 

survey, and any discrepancies should be resolved before 

leaving the site. 

The field chief of party must be sure that the 

direction of runs is reversed twice daily, or every other 

day at a minimum. 

Data Reduction 

Data collected in the field give only a close 

approximation of what the true elevation differences 

between bench marks are; they must be corrected for 

several factors in order to maximize precision. 

Corrections for rod-scale error, refraction of the 

light path, changes in length of the ivar strip of the level 

rod, effects of the Earth's magnetic field, closure errors 

for double-run or loop sections, and orthometric heights 

are some of the gremlins that can affect the data. These 

corrections must be applied. The following are equations 

to correct for these factors. 

Computation of Collimation Error 

C = [(Ml-Ah2)-0.27]1-As2 

where c = collimation error in mm/m, 

M1 = elevation difference in mm for first 

setup from middle, 

M2 = elevation difference in mm for second 

setup, and 

6s2 = difference in stadia distance between 

rod and instrument. 

The equation is derived from Schomaker and Berry 

(1981, p. 3-32). The values listed in the refraction and 

13. Using First-Order Class II Geodetic Leveling Procedures to Monitor Vertical Displacement 139 
curvature table (Schomaker and Berry, 1981, p. 3-31) are 

not correct. The term (-0.27) in the above formula was 

taken from Zilkoski and Ward (1989, vol.1). 

If the height difference is greater than %10.0 

seconds or %0.05 mm/m (Federal Geodetic Control 

Committe, 1984), the instrument should be adjusted. 

CF = stadia2 · (C/100) · 4 

where CF = correction factor to be added to the far 

rod reading, 

stadia 2 = exact stadia distance from the 

instrument to the far rod, 

C = collimation error derived previously, 

and 

4 = constant factor for full-centimeter rods 

(8 for half-centimeter rods). 

This equation is modified from Schomaker and Berry 

(1981, p. 3-33). 

Refraction Correction 

where R 

s 

R = -10-5

• 70(S/50)2 • 6(D/2) 

= refraction correction, 

= distance from rod to instrument, in 

meters, 

6 = temperature difference in degrees 

Celsius between the two temperature 

probes, and 

D difference in elevation for the setup. 

The correction is added to the observed elevation 

difference. This equation is modified from Balazs and 

Young (1982, p. 6). 

Orthometrlc Correction 

Co= -2ha sin 2p [1+ (a- 2f3/a) (cos 2p)] sin dp 

where Co 

h 

a 

f3 

p 

dp 

= orthometric correction, 

= average height of the section, 

= 0.02644, 

= 0.000007, 

= average latitude of section, and 

= latitude difference between the 

beginning and end points of the 

section. dp is positive when the ending 

point is north of the beginning point. 

This equation is from Balazs and Young (1982, p. 4). 

The equation as originally published is incorrect in that 

the sine of dp, not dp alone, should appear at the end 

(Emery Balazs, written comm., 1989). 

Rod Temperature Correction 

Ct = (tm - ts) DCE 

where Ct = rod temperature correction (same units 

as D), 

tm = mean observed temperature of the Invar 

strip, 

ts = 25 •c, 

D = observed difference of elevation 

between bench marks, and 

CE = 1 x 10 -6 per degree Celsius. 

This equation is from Balazs and Young (1982, p. 4). 

Level Collimation Correction 

Cc = -(e ·SDS) 

where Cc = level collimation correction, in 

millimeters, 

e = collimation error, in radians· 1000 or 

in mm/m, and 

SDS = accumulated difference in sight lengths 

for the section, in meters. 

This equation is from Balazs and Young (1982, p. 4). 

Equation to Calculate Standard Error for Double-Run 

Segments 

where 0 = 

d = 

k = 

1_~ a- 2n v l:ct> 

error, in millimeters, 

difference between the backward and 

forward measurements of a section, 

distance in kilometers, and 

n = total number of double-run sections. 

This equation is from Whalen and Balazs (1977, p. 22). 

Equation to Calculate Closure Error 

When the level line is closed by running over the 

same route twice (double-run), the equation to calculate 

the closure error is -

4mmVk 

where k = total distance leveled in kilometers 

(twice the one-way distance). 

When the level line is closed by a circuit or loop 

closure, the equation to calculate the closure error is 

SmmVk 

140 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
where k = distance in kilometers for the closure 

loop. 

These equations are from NOAA (1980). 

Magnetic Error 

A linear correction is applied when using the Wild 

NA2level: 

0.1 ·Lmn mm 

where Lmn is the magnetic north component of the 

distance, in kilometers, along the leveling line (Rumpf 

and Meurisch, 1981). CVO has two NA2 compensator 

levels, and both instruments have been magnetically 

shielded by a dealer-installed shield. 

C. T. Whalen (written commun.) reports that the 

DC error of 1 earth-field for the NI 002 level is 0.024 

mm/km, and AC-induced errors are insignificant at 1-5 

earth fields. This would indicate that the NI 002 is not 

very susceptible to magnetic problems, and on short lines 

a magnetic correction probably need not be applied. 

The Wild N3 level does not have a compensator 

but instead is leveled very precisely by means of a long 

level vial and a split-image level. Since the magnetic 

problem only affects compensator levels, no magnetic 

correction need be applied. 

ACKNOWLEDGMENTS 

We wish to thank Dale Benson of the National 

Mapping Division in Denver, Colorado, and Dan 

Dzurisin of the Cascades Volcanoes Observatory for 

their many helpful comments and recommendations. 

REFERENCES CITED 

Balazs, E.I., and Young, G.M., 1982, Corrections applied by 

the National Geodetic Survey to precise leveling 

observations: National Oceanic and Atmospheric 

Administration Technical Memorandum NOS NGS 34, 

12 p. 

Dzurisin, D., and Yamashita, K.M., 1986, Preliminary results 

of precise leveling and trilateration surveys in Yellowstone 

National Park, Wyoming, 1983-1984: U.S. Geological 

Survey Open-File Report 86-265-A, 24 p. 

_1987, Vertical displacements at Yellowstone Caldera, 

Wyoming 1976-1986: Journal of Geophysical Research, v. 

92, p. 13,753-13,766. 

Dzurisin, D., Yamashita, K.M., and Johnson, D.J., 1986, 

Preliminary results of precise leveling and trilateration 

surveys in Yellowstone National Park, Wyoming, 1985: 

U.S. Geological Survey Open-File Report 86-265-B. 

Federal Geodetic Control Committee, 1975, Specifications to 

support classification, standards of accuracy, and general 

specifications of geodetic control surveys: Rockville, 

Maryland, National Oceanic and Atmospheric 

Administration. 

_1984, Standards and specifications for geodetic control 

networks: Rockville, Maryland, National Oceanic and 

Atmospheric Administration. 

Floyd, R.P., 1978, Geodetic bench marks: National Oceanic 

and Atmospheric Administration Manual NOS NOS 1, 52 

p. 

Holdahl, S.R., 1985, A model of temperature stratification for 

correction of leveling refraction: National Oceanic and 

Atmospheric Administration Technical Memorandum NOS 

NGS31, 28p. 

NOAA, 1980, Classification standards of accuracy, and general 

specifications of Geodetic control surveys: National 

Oceanic and Atmospheric Administration Manual Str 

81-29, 12 p. 

Poetzschke, H., 1983, Variation of the line of sight in the NI 

002 leveling insrtument due to temperature changes: 

National Oceanic and Atmospheric Administration 

Technical Memorandum NOS NOS 38, 13 p. 

Rumpf, W.E., and Meurisch, H., 1981, Systematische 

Anderungen der Ziellinie eines parisions Compensatornivelliers-Insbesondere des Zeiss Ni 1-Durch 

Magnetitische Gleich- und Wechselfelder: paper presented 

at 16th International FIG Congress, Montreux, 

Switzerland. 

Schomaker, M.C., and Berry, R.M., 1981, Geodetic leveling: 

National Oceanic and Atmospheric Administration Manual 

NOSNGS3. 

Whalen, C.T., 1981: Results of leveling refraction tests by the 

National Geodetic Survey: National Oceanic and 

Atmospheric Administration Technical Report NOS 92 

NOS 22,19 p. 

Whalen, C.T., and Balazs, E., 1977, Test results of first-order 

class III leveling: National Oceanic and Atmospheric 

Administration Technical Report NOS 68 NOS 4, 30 p. 

Wilson, R.M., 1935, Ground surface movements at Kilauea 

Volcano, Hawaii: University of Hawaii Research 

Publication 10. 

Yamashita, K.M., 1989, Using the "HP-71B" hand-held 

computer for data entry while running first-order class II 

level surveys: U.S. Geological Survey Open-File Report 

89-179, 21 p. 

Zilkoski, D.B., and Ward, R.B., 1989, Vertical control 

workshop notes, volume 1; Vertical Network Branch, 

National Geodetic Survey, Charting and Geodetic 

Services, National Ocean Services: National Oceanic and 

Atmospheric Administration, 281 p. 

13. Using First-Order Class II Geodetic Leveling Procedures to Monitor Vertical Displacement 141 

14. Single-Setup Leveling Used to Monitor Vertical 

Displacement (Tilt) on Cascades Volcanoes 

By Kenneth M. Yamashita 

ABSTRACT 

Many different methods have been employed to detect 

tilt or other manifestations of ground deformation at Kilauea 

and other volcanoes throughout the world. This report (1) 

gives a generalized overview of the evolution of tilt 

monitoring, (2) outlines different approaches to single-setup 

tilt leveling, and (3) gives the pros and cons for the different 

methods used at the Casades Volcano Observatory to 

monitor VQicanoes of the Cascades Range for vertical 

displacement. 

HISTORY OF TILT MEASUREMENT 

The first rudimentary tilt measurements at the 

Hawaiian Volcano Observatory (HVO) were made by 

noting the wanderings of the recording stylus of the 

two-component Bosh-Omori seismograph at Kilauea 

during the early 1900's. Animprovement to this system 

was to suspend a plumb bob from a seismometer and to 

plot the departure of its point (The Volcano Letter, 

1930). 

Ground deformation associated with active 

volcanic processes was first recognized in Hawaii by 

Jaggar and Finch {1926), who noted that a horizontal 

pendulum seismometer installed in the Whitney vault 

near the summit of Kilauea volcano, Hawaii, effectively 

recorded long-term changes in ground tilt. In 1936 a 

"Clinoscope" was built at HVO. The Clinoscope 

consisted of a tripod shaped device with a heavy ring 

weight, in a cushion of oil, centered between the legs. 

When the ground tilted, the ring would move a pointer 

on a disk, marking the direction and distance from center 

(The Volcano Letter, 1936). 

In 1958, J.P. Eaton and the HVO staff installed 10 

water-tube tiltmeters around the summit of Kilauea 

(Eaton, 1959). The station installation consisted of 

sunken, triangular concrete piers to which a brass 

container (pot) was attached. The pots were connected by 

a water hose and an air hose. The water hose was used to 

fill the pots from pressurized tanks and to allow water 

from one container to flow freely into the other pot until 

an equilibrium was reached. The pressure was to flush 

the system of any air bubbles and was not meant to 

pressurize the system. The air hose assured equal 

pressure between the containers. 

A micrometer within the container was used to 

measure the height of the water in the individual pots. 

When the water heights were read simultaneously, the 

elevation differences between the piers could be 

calculated. 

These devices were simple to use but were limited 

to areas where the elevation difference between the piers 

was no greater then 1.5 em (the effective length of the 

micrometer) in an equilateral triangle with sides 25-50 m 

long. In addition, the highest precision was obtainable 

only during heavy cloud cover or at night, owing to 

thermal expansion and contraction of the water in the 

system during the daylight hours. Because water was 

used as the medium to determine the elevation 

differences between the piers, the term "wet tilt" was 

adopted for this system. 

SINGLE-SETUP LEVELING TECHNIQUE 

To circumvent the difficulties of the wet tilt 

system, D. B. Jackson and T. L. Wright began 

development of a system of precise tilt leveling at HVO. 

This procedure involved precise leveling of a 

bench-mark array using a Zeiss NI-2 level and Invar 

rods. Because no fluids were involved, the term "dry tilt" 

(Yamashita, 1981) was soon adopted. 

"Dry tilt" is not an accurate term and there has 

been controversy as to the use of this term, so it has been 

decided (by committee) that "single-setup leveling" 

(SSL) be the new name used to denote this technique of 

monitoring for vertical displacement. 

Early tests showed that significant ground tilting 

could be detected by measuring bench-mark pairs 

14. Single-Setup Leveling Used to Monitor Vertical Displacement (Tilt) on Cascade Volcanoes 143 
150-200 m apart. To shorten the base and still resolve 

meaningful tilt vectors, a Wild N-3 precision level and 

precise Invar level rods were acquired in 1969 (Kinoshita 

and others, 1974). Experience at HVO has shown that the 

accuracy of this technique when used with triangular 

arrays with 40-m sides is approximately 10 microradians 

(J.uad). 

Site Choice 

The configuration of SSL tilt stations used by CVO 

is a nearly equilateral triangle with sides of 30-40 m 

(fig. 14.1). To establish the vertices of the triangle, a 

point is marked in the approximate middle of the site 

where the tilt station is to be installed. A transit is then 

plumbed over this point and a bearing is taken in a 

southerly direction; the exact bearing depends on the 

terrain and optimum geologic orientation for the triangle. 

A distance of 23.1 m (for a 40-m equilateral 

triangle) is measured from the instrument along this 

bearing, and the resulting point is marked. The two 

remaining vertices are then located 23.1 m from the 

center point by turning successive 120° angles from the 

frrst marked point. After all the vertices are established, 

each leg of the resulting triangle is measured and 

recorded. Each leg should be roughly 40 m long. The 

elevations of the vertices are also measured to ensure 

that the elevation difference between the vertices is no 

greater than the length of the rods used. Ideally, only the 

upper 2.5 m of a 3-m rod should be used, because 

refraction close to ground level can cause appreciable 

errors (Whalen, 1981). 

z 

y 

X 

Figure 14.1. Single-setup leveling station layout. 

An alternate method is to set the transit over one of 

the vertices, establish a direction for one leg of the 

triangle, measure a distance of 40 m along this axis, and 

mark the point. The transit is "zeroed" along this azimuth 

and a 60° angle turned from one end. A distance of 40 m 

is then measured along this line and the point marked. 

The distance along the tangent determined by turning the 

60° angle should be measured to verify the accuracy of 

the angle. This distance should be 40 m. The instrument 

point is established by bisecting the 60° angle and 

measuring a distance of 23.1 m. 

When the configuration is acceptable, a bench 

mark is installed at each vertex and the central 

instrument site is marked with a nail or bar. The vertices 

are labeled X, Y, and Z counterclockwise from the 

southernmost site. 

A 40-m equilateral triangle is not a rigid 

requirement for a tilt configuration, and neither size nor 

shape is critical to the measurement of tilt. Linear, 

triangular, square, and hexagonal arrays have all been 

employed to advantage. Likewise, the station orientation 

and lettering scheme described above are standardized 

for the sake of convenience but can be modified as 

appropriate for specific installations. 

Bench Marks 

The type of bench mark currently used by CVO is 

die-cast red brass, with a 1.5 x 2.0 em nipple in the 

center. The nipple assures that the rod is always on the 

highest pan of the bench mark. This can be a problem 

with conventional flat bench marks especially if the 

bench mark is not set perfectly horizontal during 

installation. Bench marks set by CVO are either 

cemented in bedrock or fastened to the tops of rods 

driven into the ground until they can be driven no 

farther. 

Guidelines for acceptable installation of bench 

marks can be found in Floyd (1978). 

Instrumentation 

Any instrumentation system capable of producing 

frrst-order level-survey results (Federal Geodetic Control 

Committee, 1980) can be used to determine tilt to the 

precision of ±10 J.Uad. Presently at the Cascades Volcano 

Observatory, the Wild N-3 and the Wild NA2 with a 

parallel micrometer plate are the instruments used. The 

Volcano Crisis Assistance Team (VCA1) uses the Wild 

T-2000 electronic theodolite with a DI-5 electronic 

distance meter. 

144 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Three-rod System 

If three rods are available, they are labeled X, Y, 

and Z and leveled with self-supporting braces on the 

corresponding bench mark (fig. 14.2). The instrument is 

then leveled over the center mark of the triangle. The 

purpose of this scheme is to measure the elevation 

difference between each of three bench-mark pairs. 

One approach to the reading sequence (table 14.1) 

is to take the initial reading on the Y rod, followed by 

two readings on the X rod, two readings on the Y rod, 

and a final reading on the X rod. If the maximum 

difference between the three sets of readings is no 

greater than 6 x 10·3 em, the next set, X minus Z, can be 

taken. If the difference is greater then 6 x 10·3 em, 

additional readings of Y minus X should be made until 

consistency is obtained. Under extremely windy 

conditions a difference of 6 x 10·3 em or less may not be 

possible. 

The last reading taken on the X rod is carried 

down as the first reading for the set X minus Z. Two 

readings are taken on the Z rod, then two readings on the 

X rod, and finally one reading on the Z rod. Again a 

maximum difference of 6 x 1 o·3 

em is allowed. As a 

measurement of the precision of the readings taken, the 

closure leg Z minus Y is read in a similar manner (table 

14.2). 

A closure error of about 6 x 1 o·3 em is the 

accepted norm. If the closure error is greater than this, a 

determination as to which leg may be bad is made, and 

that leg is repeated until an acceptable closure is made. 

Atmospheric conditions may be a guiding force as to 

what is acceptable for that survey. 

Table 14.1. Reading sequence and example data from a 

tilt triangle 

Rod Reading Difference Rod Reading 

•1 Y..rgd Y:X.md X,rgd·2 108.273 -130.650 238.923 

•

4108.277 -130.649 238.926.3 

•5108.268 -130.648 238.916.6 

x...cml X:Z..md z....ma.1 

238.916 33.301 205.615 

•3238.914 33.301 205.613.2 

•

4238.914 33.302 205.612.5 

z....ma Z:Y..cQd Y....rs2d.1 

205.612 97.350 108.262 

•3205.614 97.351 108.263.2 

•

4205.616 97.352 108.264.5 

• Order in which readings are taken. 

Data compiled over the last 10 years from 38 tilt 

stations on Cascade Range volcanoes indicate that 6 x 

1 o·3 em is the average closure with a standard deviation 

of 6 x 10·3 em. 

Two-rod System 

If two rods are being used, they are labeled A and 

B. The A rod is set up on station Y and the B rod is set 

up on station X. Readings are similar to those of the 

three-rod system. After a consistent Y minus X reading 

is obtained, the rod on station Y is moved to station Z 

and the reading sequence repeated. If the readings are 

acceptable, the rod on station X is moved to station Y, 

and the set Z minus Y is read for the closure. 

Figure 14.2. This typical single-setup leveling station uses three-rod system. 

14. Single-Setup Leveling Used to Monitor Vertical Displacement (Tilt) on Cascade Volcanoes 145 
Table 14.2. Closure determination changes between 

surveys 

[Closure is detennined by algebraically summing the average readings 

of the Y minus X, X minus Z, and Z minus Y leg. Any closure error 

should be distributed equally among the three legs of the triangle] 

Trianaie lea A vcrage of rod Adjusted Change Previous 

difference (em) reading (em) reading 

5/18187 7/20188 

Y-X -130.649 -130.650 

X-Z 33.301 33.300 

Z-Y 97.3481 97.350 

Z-Y 22..ID.2 

+0.0033 

Thus 0.001 em ia subtracted from 

each pair. 

1 Calculated value for closing leg 2 Observed value for closing leg 3 Closure error, in centimeters 

-0.005 -130.645 

-0.010 33.310 

+0.015 97.335 

In the past we have sometimes double-run each 

station by reversing the rods on each side of the triangle 

and reading it twice. For example, the Y -X leg of the 

triangle is measured until satisfactory data are obtained, 

then both rods are interchanged and the leg remeasured. 

The intent was to eliminate any inaccuracies in the rod 

scales. 

CVO rods are calibrated by the National Bureau of 

Standards in Gaithersburg, Maryland. The rod scale 

errors are found to be less than the precision of the 

individual readings, and because we have a calibration 

table, any inaccuracies of the rod scale can be corrected 

in the office. Therefore we no longer switch the rods. 

Equation to Determine Tilt Vector 

't (N) = [( -_coscj) ) ·li (Y-X) _ XYsm(cp- 9) 

( :os 

9 ) ·& (x-Z)] . 1o,ooo XZsm(cp-9) 

't (E)= [( ~incj) ) ·li (Y-X) - XYsm(cp-9) 

( ~in 9 ) · li (X-Z)] · 10,000 XZsm (cp- 9) 

In this equation, modified from Eaton (1959), 't is 

the elevation change in microradians in the north and the 

east component. 

XY and XZ are the distances from the X -bench 

mark to the Y- and Z-bench marks, respectively, <1> and .9 

are the angles measured counterclockwise from the east 

direction to the XY and XZ directions, respectively 

(fig.14.1), and /i(Y -X) and /i(X-Z) are the height 

changes from Y to X and from X to Z between two 

surveys (table 14.2). 

Solution for Tilt Triangle 

The magnitude of the tilt vector is computed by 

taking the square root of ( 't N2 + 't Ffl ). 

The azimuth of the tilt vector is computed by 

taking the arc tangent of ( 't El't N ). 

If the sign of 'CN is positive, the vector is to the 

north, down. If the sign of 'CN is negative, the vector is to 

the south, down. If the sign of 'tE is positive, the vector 

is to the east, down. If the sign of 'tE is negative, the 

vector is to the west, down. 

When plotting the vector displacement, the length 

of the vector corresponds to the magnitude in 

microradians, and the azimuth is measured from the 

north if 'CN is positive, or from the south if 'CN is 

negative, and from the east if 'tE is positive, or from the 

west if 'tE is negative. 

Example: If XY = 40.2 m, XZ = 40.08m, 9 = 

14.5°, cp = 74.5°, li (Y-X) = -0.005, and ll (X-Z) = -0.010 

then 

't(N) = [ -0.0077 ( -0.005) -

0.0279 (-0.010)] . 10,000 = +3.18 JJ.rad 

't(E) = [ 0.0277 ( -0.005) + 

0.0072 (-0.010)] . 10,000 = -2.11 JJ.rad 

Thus the magnitude of the resultant tilt vector, 

((3.18)2 + (2.11)2

)

1

n, is 3.8 J..IIad and the azimuth, 

-l ( 2·

11 ) . N 33 6° W tan 3.lS , IS • • 

Precision 

The accuracy of an SSL tilt system as determined 

by the staff of the Hawaiian Volcano Observatory is 

about ±10 J.1I3d (Kinoshita and others, 1974). Savage and 

others (1979) have also determined that the precision of 

a tilt network is about ±10 J.1I3d. 

There are too few data to determine what our 

precision is for SSL tilt sites on Cascade volcanoes, but 

data from linear arrays at South Sister, Oregon, and level 

lines at Newberry Crater and Crater Lake, Oregon 

(Yamashita and Doukas, 1987) suggest that the 

repeatability is approximately 1-6.5 J.Uad on lines less 

than 0.5 km long, about 2 J..IIad on lines at least 1 km 

long, and on the average less then 1 JJ.rad on lines longer 

than 1 km. 

146 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
CASCADES STATIONS 

In 1975 W.T. Kinoshita and D.A. Swanson (Frank 

and others, 1977) installed three SSL stations on Mount 

Baker (fig. 14.3) in response to increased thermal 

activity in Sherman Crater on the summit of the volcano. 

Additional stations were installed on Mount Baker in 

1981. 

Since then, SSL stations have been installed at 

Mount St. Helens, on Mount Rainier (Dzurisin and 

A Mount Rainier 

Mount St. Helens 

\~ -- ( ........_. ....r--

Mount Hood A 

OREGON 

Sisters~ 

~ 

New berry Crater 

Crater Lake~ 

--.M~ Shasta/ 

CALIFORNIA J 

~ Lassen P~ak 

Flgure14.3. Locations of Cascade volcanoes with tilt 

stations. 

others, 1983), on Mount Hood, and on Lassen Peak and 

Mount Shasta, (Dzurisin and others, 1982) (fig. 14.3). 

Long-base tilt arrays (more than 400 m long) were 

installed at Crater Lake in 1983, linear arrays were 

installed at South Sister, in 1986, and leveling lines were 

established at Newberry Crater in 1986 (Yamashita and 

Doukas, 1987). 

Survey Results 

Two decades of data at the Hawaiian Volcano 

Observatory have shown single-setup leveling to be a 

reliable method of measuring ground deformation at 

Hawaiian shield volcanoes, but our experience on 

Cascade stratovolcanoes has not been as conclusive. 

SSL stations at Lassen Peak, Mount Shasta, and 

Mount Rainier have shown randomly oriented tilt 

vectors; however, tilt vectors on Mount Hood (fig. 14.4) 

are more uniform. The random tilt patterns on some 

Cascade volcanoes cannot be readily explained. One 

explanation could be that as all of our tilt stations are at 

high elevations where freeze-thaw cycles often occur, 

these conditions may affect the stability of the bench 

marks and are enough to affect the precision that we 

require for meaningful results. 

Because of the unsatisfactory results of SSL data 

on Cascade volcanoes, we have opted in favor of 

leveling lines (Dzurisin, chapter 12; Yamashita and 

Kaiser, chapter 13). These leveling lines are linear or 

L-shaped lines that range from a few hundred meters to 

as much as 190 km in length. 

One advantage of a level line is that there are 

usually more than three bench marks in the network, and 

the loss or instability of any one mark is not critical to 

the integrity of the system. In the SSL system, if one 

mark is lost or disturbed the entire station is no longer 

usable. Also, an SSL site requires a large enough area to 

accommodate an SSL station and still be level enough 

that the maximum length of the level rod is not used. 

Terrain is not so critical for a linear array. 

Another advantage of a longer base line is the 

ability of the system to tolerate slight bench-mark 

instability. On an SSL base of 40 m, a change of 0.05 em 

would give a tilt change of 12.5 J.J.rad while a change of 

0.05 em on a line 250 m long would be 2.0 J.!.rad, and 0.5 

)..IIad on a line 1 km long. 

The disadvantage of a level line when compared to 

an SSL station is that it is more labor intensive. Two 

people can measure an SSL station in about 25 minutes, 

allowing a number of tilt stations to be measured in a 

day. A level line on the other hand requires a minimum 

of four people and about 1-1/2 hours per kilometer on 

level ground, longer on steep terrain. The areal coverage 

14. Single-Setup Leveling Used to Monitor Vertical Displacement (Tilt) on Cascade Volcanoes 147 
at this rate is substantially less than that covered by the 

SSL method, but the results are much more definitive 

and worth the extra time and effort in certain 

circumstances. 

Another disadvantage of a linear array is that some 

assumptions about the plumbing system of the volcano 

have to be made. As it is not possible to compute a tilt 

component from a linear array, we assume that any 

deformation will occur at the center of the volcano, and 

therefore all of our linear arrays. are installed radially 

from the summit. The fallacy of this assumption is that 

deformation can and does occur on the flanks, and 

depending on the location of the deformation relative to 

the linear arrays, this deformation could go undetected or 

be misinterpreted. 

The best solution to this problem is to run a line 

perpendicular to the radial line, creating a cross-shaped 

or L-shaped array. Level arrays such as those at Crater 

Lake (Yam ash ita and Doukas, 1987) are probably the 

most efficient of the three different configurations. 

45" 25' 

SUMMARY 

Data from Hawaii have proved conclusively that 

SSL is a reliable and effective method of monitoring for 

deformation on basaltic volcanoes lacking freeze-thaw 

problems; our experience with SSL on Cascade 

volcanoes so far has been inconclusive. We have no 

plans to abandon the existing tilt sites, and these stations 

will be reoccupied in future years, although our 

inclination is to install more linear or L-shaped arrays. 

Ideally, a combination of tilt arrays and leveling 

lines such as those used at Kilauea has the advantage of 

using the tilt arrays as a quick monitoring tool to cover a 

large area in the shortest amount of time. Depending on 

the results of the tilt arrays, the level lines can be 

occupied for more quantitative results. 

The advantages and disadvantages of both SSL and 

linear arrays have been discussed previously. The user 

should determine which system best suits the needs for 

each situation. 

Figure 14.4. Vertical displacements at Mount Hood tilt stations, August 1983 to 

August 1984. Contours are in feet. 

148 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
REFERENCES CITED 

Dzurisin, D., Johnson, DJ., Murray, T.L., and Meyer, B., 1982, 

Tilt network at Mount Shasta and Lassen Peak, California: 

U.S. Geological Survey Open-File Report 82-670, 23 p. 

Dzurisin, D., Johnson, DJ., and Symonds, R.B., 1983, Dry-tilt 

network at Mount Rainier, Washington: U.S. Geological 

Survey Open-File Report 83-277, 9 p. 

Eaton, J.P., 1959, A portable water-tube tiltmeter: 

Seismological Society of America Bulletin, v. 49 p. 301-

316. 

Ewert, J.W., 1989, A trigonometric method for monitoring 

ground-tilt changes on composite volcanoes: U.S. 

Geological Survey Open-File Report 89-223, 12 p. 

Federal Geodetic Control Committee, 1980, Specifications to 

support classification, standards of accuracy, and general 

specifications of Geodetic Control Surveys: National 

Oceanic and Atmospheric Administration Sff 80-98, 46 p. 

Floyd, R.P., 1978, Geodetic bench marks: National Oceanic and 

Atmospheric Administration Manual NOS 1, Sff 78-391, 

sop. 

Frank, D.M., Meier, M.F., and Swanson, D.A., 1977, 

Asse~ment of increased thermal activity at Mount Baker, 

Wash10gton, March 1976: U.S. Geological Survey 

Profeusional Paper 1022-A 49 p. 

Jaggar, T.A., and Finch, R.H., 1926, Tilting and level changes 

at Pacific volcanoes: Proceedings of the 3rd Pan-Pacific 

Science Congress, Tokyo, p. 672-686. 

Kinoshita, W.T., Swanson, D.A., and Jackson, D.B., 1974, The 

measurement of crustal deformation related to volcanic 

activity at Kilauea Volcano, Hawaii, in Civetta, L., 

Gasparini, P ., Luongo, G., and Rapolla, A., eds., Physical 

volcanology: New York, Elsevier, p. 87-115. 

Savage, J.C., Prescott, W.P., Chamberlain, W.H., Usowski, M., 

and Mortensen, C.E., 1979, Geodetic tilt measurements 

along the San Andreas fault in central California: 

Seismological Society of America Bulletin, v. 69, p. 

1965-1981. 

The Volcano Letter 1930: National Park, Hawaii, Hawaiian 

Volcano Observatory, no. 288, July 3, 1930. 

_1936, National Park, Hawaii, Hawaiian Volcano 

Observatory, no. 437, July 1936. 

Whalen, C. T., 1981, Results of leveling refraction tests by the 

National Geodetic Survey: National Oceanic and 

Atmospheric Administration, Technical Report NOS 92 

NOS 22,20 p. 

Yamashita, K.M., 1981, Dry tilt: A ground deformation monitor 

as applied to the active volcanoes of Hawaii: U.S. 

Geological Survey Open-File Report 81-523, 17 p. 

Yamashita, K.M., and Doukas, M.P., 1987, Precise level lines 

at Crater Lake, Newberry Crater, and South Sister, Oregon: 

U.S. Geological Survey Open-File Report 87-293, 32 p. 

14. Single-Setup Leveling Used to Monitor Vertical Displacement (Tilt) on Cascade Volcanoes 149 

15. A Single-Setup Trigonometric Leveling Method for 

Monitoring Ground-Tilt Changes 

By John W. Ewert 

ABSTRACT 

Geodetic ground-tilt studies have been used to monitor 

volcanic activity since the 1960's. Development of more 

precise theodolites and high-quality, compact, lightweight 

electronic distance meters (EDM's) In the past 1 0 years has 

brought about a dramatic increase in the use of trigonometric 

leveling for high-order vertical control studies. A system for 

measuring ground tilt trigonometrically, employing a 

theodolite, EDM, and target/prism sets has been developed 

for use in volcanologic studies. This system enables the 

measurement of single-setup bench-mark arrays as much as 

4 times larger than is possible with a level instrument at a 

single instrument setup site. The system Is more compact 

and thus easier to transport in remote or rugged terrain than 

is the equipment for classical leveling. Data reduction Is 

slightly more Involved but Is still simple if a programmable 

calculator or portable computer is used. A comparison of the 

two methods on the same triangle shows them to be 

approximately equivalent in their ability to measure ground 

deformation. 

INTRODUCTION 

Single-setup leveling (SSL) (also called dry tilt or 

tilt-leveling) has routinely been used at the Hawaiian 

Volcano Observatory (HVO) to measure and detect 

changes in ground tilt on Hawaiian volcanoes since the 

technique was developed in 1968 (Kinoshita and others, 

1974). SSL has been used at Mount St. Helens (Lipman 

and others, 1981 ); at various volcanoes in the Cascade 

Range, including Mount Shasta and Lassen Peak 

(Dzurisin and others, 1983); at La Soufri~re volcano, 

Guadeloupe (Fiske and Shepherd, 1990); on various 

active volcanoes within the Taupo volcanic zone, New 

Zealand (Otway and others, 1984); and at Karkar 

volcano, Papua New Guinea (McKee and others, 1981). 

The technique is also currently used in Costa Rica by 

Observatorio Volcanologia y Sismologia de Costa Rica 

(OVSICORI) on Arenal and Poas volcanoes and in 

Colombia by Instituto Geologico y Minas 

(INGEOMINAS) at Ruiz and Galeras volcanoes. A 

complete description of the original spirit-level tilt 

technique can be found in Kinoshita and others (1974). 

Yamashita (1981) also presented a detailed description, 

and a Spanish translation of this paper was published by 

Vander Laat (1982). The SSL method is also described 

in chapter 14 of this volume. 

The SSL technique requires the use of two or three 

long lnvar rods and rod stays, a level, and a micrometer 

plate. The bench-mark array is typically an equilateral 

triangle, 40 m on a side. When 3-m rods are used, sites 

on which a 40-m equilateral triangle can be placed are 

limited to relatively level areas with no more than 2.5 m 

elevation difference between monuments at the triangle 

apices (1.5 m if 2-m rods are used). Sites such as these 

tend to be rare on the flanks of steep-sided volcanoes. In 

addition to site limitations, 3-m lnvar rods are difficult to 

transport on small vehicles, pack animals, and aircraft. 

These factors combine to make SSL of limited utility on 

many composite volcanoes and prompted me to find an 

alternate method that was both portable and suitable for 

use in steeply sloping terrain. A compact, precise method 

of single-setup trigonometric leveling (SSTL) was 

developed using a theodolite, electronic distance meter 

(EDM), and three target/prism pairs mounted on plumb 

poles. 

SYSTEM REQUIREMENTS 

To determine the instrumental requirements for the 

SSTL method of measuring ground tilt, the desired 

threshold of tilt detection must first be decided. Ground 

tilts of several tens to several hundreds of microradians 

(J.lrad) are common before volcanic eruptions (Newhall, 

1984 ), and in this light a detection threshold of 7 J.lrad 

was decided as appropriate. This 7 J.lrad figure represents 

a two-sigma confidence level in measurements between 

pairs of marks and is comparable to, although slightly 

less sensitive than, the 2-3 J.lrad detection capabilities of 

the SSL technique reported by Kinoshita and others 

15. A Single-Setup Trigonometric Leveling Method for Monitoring Ground-Tilt Change• 151 
(1974) and Sylvester (1978) for small-aperture (40 m) 

bench-mark arrays. 

To meet this detection requirement, a theodolite 

capable of measuring zenith angles to 1 second of arc or 

better is required. Depending on the instrument and 

operator, a 1-second micrometer theodolite can measure 

angles accurately to :t 0. 7 to :t 1.4 seconds of arc (Ruger 

and Brunner, 1982). More precise micrometer theodolites 

and some electronic theodolites can measure angles to 

:t 0.5 second (Whalen, 1984). EDM manufacturers report 

the accuracy of their instruments as plus-or-minus a 

constant number of millimeters (usually 1-5) plus a few 

parts per million (ppm). In short-range work, such as 

SS1L where measurements are typically over distances 

less than 100 m, the ppm figure is usually not of 

consequence. Therefore, an EDM with as small a 

constant error as possible is desirable and is actually the 

limiting factor in the accuracy of the method. Finally, the 

target/prism pair must be capable of being fixed at a 

height that can be reproduced to within a few tenths of a 

millimeter with each reoccupation of the station. Several 

theodolites, EDM's and target/prism mounting systems 

meet these specifications. 

The working limits of the method were determined 

by analyzing random reading and pointing errors arising 

from angle measurements of differing precision, the 

constant error from EDM's, and a 0.2-mm uncertainty in 

establishing the target heights. Calculations were made 

following the methods outlined in Buckner (1983, p. 

209) and Davis and others (1981, p. 124), treating as 

level loops the equilateral tilt triangles with the 

(/) 

ttl 30 a: 

C) 

w 

Cl 

~ 

w· 20 ....J 

C) 

z 

<( 

w 

a.. 

g 10 (/) 

EXPLANATION 

+---+--+-+--+ZENTIH ANGLE,± 0.5 SECONDS 

~ZENITH ANGLE, ±1.0 SECONDS 

50 100 

TRIANGLE SIDE LENGTH, IN METERS 

_150 

Flgure15.1. Maximum ground slope on which 

ground-tilt can be measured to approximately ± 7 11rad 

with EDM's of ± 3 m m and ± 5 mm accuracy and 

theodolites that can measure zenith angles to ± 0.5 and 

± 1.0 second of arc. 

instrument station in the middle, and distributing the 

accumulated error over the three legs. The :t 7 J.Lrad limit 

was used to determine the approximate maximum ground 

slope over which ground tilt could be determined 

trigonometrically. Figure 15.1 demonstrates the 

topographic flexibility of this system. 

SYSTEM DESCRIPTION 

The trigonometric leveling system presently in use 

consists of a Wild T-2000 electronic theodolite, a Wild 

DI-5 EDM, and three target sets with tripods (figs. 15.2, 

15.3). Instrument specifications reported by Wild and 

confirmed by Whalen (1984) indicate that the mean of 

Flgure15.2. T-2000 electronic theodolite with Dl-5 EDM 

mounted on theodolite telescope. 

152 Monitoring Volcanoes: Technique• and Strateglee Used by the Staff of the Cascades Volcano Observatory 
the direct and reverse zenith angles measured by the 

T-2000 is accurate to z 0.5 seconds of arc (standard 

error). The DI-5 EDM measures distances reportedly 

accurate to z 3 mm, and z 2 ppm. The target sets for this 

system consist of tilting target/prism combinations, 

spaced apart the same vertical distance as the theodolite 

telescope and EDM to negate eccentricity. These target 

sets are mounted on aluminum plumb poles held upright 

by lightweight tripods. Each tripod-target set is labeled 

and placed in the same configuration on all the triangular 

bench-mark arrays. 

SITE INSTALLATION 

The ideal SSIL monument array is an equilateral 

triangle as large as practical to measure accurately. The 

exact orientation of the triangle is determined by the 

geological structure being studied. On steep-sided 

volcanoes, one side of the triangle is first laid out radial 

Flgure15.3. Complete target/reflector system, set up 

over an expansion bolt (Inset) in a lava flow. 

Target-cross and prism are set apart vertically the same 

distance as theodolite telescope and EDM. 

to the vent or summit area, and the third monument is 

positioned in the most favorable site. Although the exact 

configuration of the triangle is not important, the 

sighting distances should be approximately equal, and 

the ground slope should be nearly uniform between the 

instrument and targets to minimize refraction errors. 

The southernmost vertex of the triangle is labeled 

X, and the other vertices are labeled Y and Z in 

counterclockwise order (fig. 15.4). To calculate the tilt 

vector, the slope distances from monument X to Y and Z 

must be measured (LY and lZ in figure 15.4). This can 

be done with a tape measure directly, if the triangle is 

not too large, or indirectly by measuring the slope 

distance and horizontal angle from the instrument site in 

the center of the triangle to each station with the EDM 

and theodolite and solving the triangle. Bearings from 

east must also be taken from X to Y and X to Z with a 

compass to orient the triangle (9 and + in figure 15.4). 

The triangle parameters are assumed not to change and 

are used as constants in subsequent tilt calculations. 

The formulas to determine the components of the 

tilt vector and magnitude with the SS1L system are: 

't(N)- [(LY::(++-0)) ·A(Y-X) 

- ( LZ=(~B)) ·A (X- Z)] ·106 

[( -sin+ ) 't(E)• LYsin ( t-8) . A (Y- X) 

( sinS ) ] + LZsin(t-9) . A (X- Z) ·106 

Where L Y, LZ, A(Y -X) and A(X-Z) are in meters and 

't(N) and 't(E) are in microradians (original equations 

z 

y 

---E 

Flgure15.4. Physical layout of representative tilt 

station. 

15. A Single-Setup Trigonometric Leveling Method for Monitoring Ground-Tilt Changea 153 
modified from Eaton, 1959). Compare these equations 

with those found in Yamashita (chapter 14) for use 

with the SSL method. The two methods of measuring 

the elevation differences lead to the sign change in the 

first terms of each equation. 

Monuments 

The monuments for a single-setup leveling array 

should be placed in similar substrates to minimize 

spurious noise effects caused by differential substrate 

response to diurnal thermal or seasonal hydrologic 

effects. Monuments placed in bedrock and those 

placed in soils should not be mixed in the same SSL or 

SSTL monument array. Bench-mark stability is critical 

when leveling or trig-leveling over short distances 

(Dzurisin, chapter 12). Therefore, redundant bench 

marks should be placed wherever possible to provide 

the means for checking the stability of individual 

bench marks in the array. 

Several types of monuments can used as 

measurement points. Where bedrock is present (usually 

a lava flow), stainless steel anchor bolts or bench 

marks are placed in holes drilled in the rock (Doukas 

and Ewert, chapter 11 ). In our experience, bedrock is 

the most stable (and thus preferred) substrate for 

monument installation. Lacking bedrock, several 

methods of monument construction are possible. These 

include (1) driving a single rod straight down, 

coupling more rod as needed, until it will go no farther 

and affixing a bench mark to its top at or just below 

ground level, or (2) constructing a reinforced 

stonework cairn in which a mark can be placed (Floyd, 

1978; Doukas and Ewert, chapter 11). 

MEASUREMENT PROCEDURES 

The target sets and plumb poles are numbered 1 

to 3 and set up in the same configuration on every 

triangle; 1 on X, 2 on Y, and 3 on Z. The target 

tripods are leveled and centered over the monuments, 

and the theodolite/EDM pair is precisely leveled in the 

center of the triangle. The triangle is treated as a 

leveling loop beginning on the YX side, with the 

initial reading on the Y target followed by two 

readings on X, two readings on Y, and a final reading 

on X (table 15.1). Each reading consists of measuring 

direct and reverse zenith angles and the slope distance. 

The zenith angle is computed for each reading, and 

successive Y -X angular differences are computed. In 

our experience, if the spread of the differences of the 

three sets is less than 2 seconds, the XZ leg can be 

then be read. If the spread is larger than 2 seconds, 

additional readings should be made until consistency is 

obtained. The last reading on the X target is carried 

down as the first reading for the X and Z leg (table 

15.1). Two readings are taken on the Z target, 

followed by two readings on the X target, and 

finally one reading on the Z target. Again, a 

maximum spread of 2 seconds is allowed. As a 

measure of the accuracy of the readings taken, the 

closure leg ZY is read in a similar manner. 

When the three sides of the triangle have been 

read, the averages of the angular differences are 

summed to find the "field closure," which is not the 

closure of elevation differences but rather a measure 

of how consistently the angles were measured (table 

15.1). Ideally, the differences should sum to zero. 

In practice, if the sum is less than or equal to 1.0 

second the data are considered good. If the readings 

sum to a greater value, the triangle should be 

remeasured. 

Our experience with the T-2000/DI-5 system 

has shown that sighting distances up to 130 m can 

be used with good results if there is no strong heat 

shimmer, but that sighting distances less than 100 m 

allow for the best repeatability of measurements in a 

wide range of viewing conditions. 

Closure Determination of Error and 

Data Reduction 

Recording and data reduction for the SSTL 

method of tilt surveying are slightly more involved 

than for the original SSL method. Position I and 

position II (direct and reverse) readings must be taken 

and the average angle computed. Tern perature and 

pressure readings must be taken to correct the distance 

data. Because the sightings are generally taken parallel 

to the ground, temperatures are taken at instrument 

height. This is typically a very small correction, on the 

order of tenths of a millimeter, but could be significant 

in extreme conditions. 

Elevation differences between the instrument and 

the targets are calculated by taking the cosine of the 

zenith angle and multiplying by the corrected slope 

distance (table 15.2). Once the elevation differences 

between the targets are obtained, the data reduction 

proceeds exactly like the original method outlined by 

Yamashita (chapter 14) and reproduced as an example 

below. 

If a programmable calculator or laptop computer 

is available, a simple BASIC program can be written 

to calculate the exact closure and perform the rest of 

the data reduction while on site. To facilitate the 

computations by lessening the amount of data entry, 

154 Monitoring Volcanoea: Techniques and Strategies Uaed by the Staff of the Cascades Volcano Obaervatory 
Table 15.1. Example of successive readings of the tilt triangle legs. 

[Angles are the average of zenith direct and zenith reverse readings] 

Individual target reading 

Y target reading 

Ana Qlstanee 

1

*90• 37' 10.7" 22.g60 m 4*90• 37' og.7" 22.g60 m 5

*90• 37' 10.4" 22.960 m 

Y - average = 90• 37' 10.3" 

X target reading 

Ana Pistanee 

go• 3g' 42.5" 

3*90• 3g' 42.3" 

4• 90. 3g' 42.2" 

22.g43 m 

22.g43 m 

22.g43 m 

X - average = go• 3g' 42.3" 

Z target readiing 

An9.!2 Pistance 

go• 25 I· 50,g" 23.043 M 

3*90• 25' 51.3" 23.043 m 4

*90• 25' 51.5" 23.043 m 

Z - average = 90• 25' 51.5" 

Individual target reading 

X target reading 

Ana Distance 

90• 3g' 42,g" 22.g43 m *

2 

90• 3g' 42.7" 22.g43 m*3 

90• 3g' 42.5" 22.g43 m *

6 

X - average = 90• 3g' 42. 7" 

Z target reading 

Ana Pistance 

90• 25' 50.g" 23.043 m •t 

90• 25' 51.g" 23.043 m *

2 

90• 25' 5o.g" 23.043 m •s 

Z - average = 90• 25' 51.2" 

Y target reading 

&!al!t Distance 

90• 37' 10.2" 22.g60 m •t 

90• 37' 09.2" 22.g60 m *

2 

90• 37' 09.2" 22.g60 m •s 

Y- average= go• 37' 09.5" 

Angular difference between 

ets 

Y1-X2 = -2' 32.4" 

Y4-X3 = -2' 33.0" 

Y5-X6 = -2' 32.1" 

Average Y - X = -2' 32.4" 

Spread= 1.2" 

X6- Z1 = 13' 51.6" 

X3- Z2 = 13' 50.4" 

X4 - ZS = 13' 51.3" 

Average X - Z = 13' 51.1" 

Spread= 1.2" 

ZS- Y1 = -11' 1g.3" 

Z3- Y2 = -11' 17.g" 

Z4- Y5 = -11' 17.7" 

Average Z- Y = -11' 18.3" 

Spread = 1.6" 

Field closure= (Y- X)+ (X- Z) + (Z- Y) = +0.4" Note that 0.4" at a shot length of 23m is approximately equal to 

0.000045 m 

* Order in which readings are taken. 

the average zenith-angle reading to each target on each 

leg of the triangle is calculated (table 15.1) and the 

elevation difference found using the averages (table 15.2). 

The result is the same $ if the elevation difference for each 

set were calculated and then averaged. 

If no programmable calculator or computer is 

available, the results of the survey can be calculated in 

the field with a simple calculator, albeit laboriously. 

Exam pie Calculation 

In the following example L Y = 40.97 m, LZ = 

40.36 m, 8= 46.o·, and + = 102•. 

Triangle Side Differences (meters) 

Y -X 0.01676 m 

X-Z -0.09169 m 

Z-Y 

Z-Y 

0.07493 m (calculated) 

0.07487 m (observed) 

-0.00005 m closure error 

This closure is then distributed equally among the 

three legs of the triangle. In this case 0.00002 m would 

be added to each leg. 

Adjusted 

Reading 

6-July-88 

Y-X 0.01678 

X-Z -0.09167 

Change (m) 

aY-X -0.00009 

aX-Z 0.00014 

Z-Y o.o7489 aZ-Y -o.oooo1 

Previous 

Reading 

30-June-88 

Y-X 0.01687 

X-Z -0.09181 

Z-y -0.07488 

Reduced version of tilt equations using above 

values: 

6 't(N) = (-0.0061(aY-X)- 0.0208(aX-Z)) · 10 

't(E) = (-0.0288(aY-X) + 0.0215(aX-Z)) · 106 

't(N) = (-0:0061(-0.00009)- 0.0208(0.00014)). 106 

= -2.4 

't(E) = (-0.0288(-0.00009) + 0.0215(0.00014)). 106 

= +5.6 

15. A Single-Setup Trigonometric Leveling Method for Monitoring Ground-Tilt Changea 155 
Magnitude in micro radians = V N2 + EZ 

-v -2.42 + 5.62 - 5.8 

Bearing in 

(;::) -66.8 

If 'tN is positive, vector is in the north half, down. 

If 'tN is negative, vector is in the south half, down. If 'tE 

is positive, vector is in the east half, down. If 'tE IS 

negative, vector is in the west half, down 

The bearing is measured from the abscissa into the 

indicated quadrant. 

Thus the magnitude is 5.8 J.Lrad in a S. 66.8• E. 

direction. 

Precision 

As of this writing, the T-2000 based SSIL system 

has been in use for about a year and a half. Figure 15.5 

shows a comparison of tilt vector components from the 

same triangle, derived from measurements made with the 

SS1L system and an SSL system. The tilt triangle was 

set up in the parking lot of the Cascades Volcano 

Observatory to allow frequent measurement for 

comparative purposes. The two methods agree quite well 

in terms of sense and magnitude of the apparent 

deformation. 

The precision of this theodolite-based system is 

somewhat less than that of a level-based system, but as 

was pointed out by Savage and others (1979) and 

Sylvester (1978), factors such as bench-mark instability 

and topographic effects make the measurement of ground 

tilt with small-aperture bench-mark arrays accurate to 

60 ~A (/) 40 ~ 

~ 20 l 

o o ~ -~ ,./Jl,\o <( f ~ = ~ 

gs -20 t_ _ ___j_~____L_~ 

a: 

() 

0 

~--.---.-~---,-,g--.~ 

0 ~ 

6 ] 

._L_---L-..c~_l_~~ j 

~ ~e--~~~--.-~-~~~~~~~--~~~~j 

~ 30 ~B ~ 

~ 20 ~ ~ @ 6? 0 0 = 

i= 1 0 l io;:p 6 ~ 

-1 ~ L__~ _L___L___L__l_______L__ ~ rf - -~ 1 

JUL OCT 89 APR JUL OCT 

START DAY IS JUN 01,1988 G.M.T. -JULIAN DAY 153 

Flgure15.5. Comparison of tilt vector components from 

same triangle, measured with the level and trigonometric 

methods. A, North component. 8, East component. 

Triangles represent SSTL method, circles represent SSL 

method. 

Table 15.2. Elevation differences between Instrument 

and targets calculated by multiplying cosine of zenith 

angle by the corrected slope distance. 

[Zenith aagles aad distaaces from table lS.l] 

Elevation differences 

Y-X leg 

Y-target 

(22.960 m) · cos(90.37'10.3•) = -0.24826 m 

X-target 

(22.943 m) · cos(90.39'42.7•) = -0.26502 m 

X-Z leg 

X-target 

Y -X= 0.01676 m 

(22.943 m) · cos(90.39'42.3•) = -0.26498 m 

Z-target 

(23.043 m) · cos(90.2S'S1.2•) = -0.17329 m 

Z-Y LEG 

Z-target 

X-Z = -0.09169 m 

(23.043 m) · cos(90.2S'S1.2•) = -0.17329 m 

Y-target 

(22.960 m) · cos(90.37'09.S•) = -0.24817 m 

Z-Y = 0.07487 m 

only about :t: 10 J.Lrad regardless of the measuring 

precision. 

Field testing thus far indicates that, on a triangle 

with sides of about 100 m, results can be reproduced to 

:t: 5 J.Lrad on each of the tilt components over a period of 

months. The system has yet to be tested on a volcano 

through the course of an eruption, but the indicated 

precision lies well within the magnitude of ground tilt 

often measured on active volcanoes. Therefore, although 

slightly less precise than the instrumentation typically 

used for geodetic leveling, the trigonometric system is 

suitable for measuring ground tilt on volcanoes. 

SUMMARY 

The single-setup trigonometric leveling system is a 

compact alternative method of measuring ground-tilt 

changes on volcanoes. This leveling system permits 

volcanologists to obtain tilt data at locations previously 

unfeasible and greatly facilitates work on remote 

volcanoes where access is often by jeep or foot, and 

transportation of bulky equipment is difficult. Triangular 

SS1L benchmark arrays can be up to four times larger 

than the SSL arrays, thereby allowing a longer baseline 

to be measured, and ideally creating a more precise 

measure of vertical deformation. The relative advantages 

156 Monitoring Volcanoea: Technique• and Strategle• Uaed by the Staff of the Caacadea Volcano Obaervatory 
Table 1 5.3. Relative advantages and disadvantages of trigonometric system and the level system 

Trigonometric system 

Advantgages 

Easily portable 

Same equipment can be used for EDM octworka and triangulation 

Better topographic siting flexibility 

Larger single-setup bench mark arrays lcaacn the effects of minor 

bench-mark instability 

Disadvantages 

Slightly leas precise 

More expensive by a factor of 2 to 3 

More difficult to directly compare data from previous 

surveys while in the field 

Data reduction slightly more invoiced 

and disadvantages of the trigonometric system and the 

level-based system are listed in table 15.3. 

The system described here can also be used to 

measure level lines and small trilateration networks (the 

DI-5 EDM can measure 25 km to a single prism). If a 

more powerful EDM, capable of measuring distances of 

10 km or more, is used with the precise theodolite, the 

result would be an instrument system capable of 

performing all geodetic volcano monitoring tasks. The 

biggest drawback of the currently used T-2000 based 

system, or any trigonometric leveling system, is cost. 

Precise theodolites are expensive, and a complete system, 

such as that described herein, cost approximately 

$28,000 in 1986, whereas an SSL system such as that 

described by Yamashita (chapter 14) would have cost 

approximately $10,000 in 1986. The higher cost is partly 

offset by the versatility of the trigonometric system and 

by the fact that separate instruments need not be 

purchased to measure vertical and horizontal 

deformation. 

ACKNOWLEDGMENTS 

The development of the single-setup trigonometric 

leveling method has benefited through thoughtful 

comments and hard questions posed by Dan Dzurisin 

(USGS/CVO) and Arnold Okamura (USGS/Hawaiian 

Volcano Observatory). 

Level-based system 

Simple and caay to learn 

Equipment is widely available 

Equipment is leas expensive 

The aame equipment can be uacd for conventional geodetJc 

leveling of long traversca 

Larger aperature arrays require mucb more time to measure in steep terrain 

Long levelling roda are difficult to transport 

Topographically acceptable sites are difficult to find on stec~Hided volcanoes 

and may not be beat for monitoring purpoaca 

Small aperature single-aetup arrays are more susceptible to 

minor bench-mark instability 

REFERENCES CITED 

Buckner, R.B., 1983, Surveying measurements and their 

analysis: Rancho Cordoba, California, Landmark 

Enterprises, 275 p. 

Davis, R.E., Foote, F.S., Anderson, J.M., and Mikhail, E.M., 

1981, Surveying theory and practice: New York, 

McGraw-Hill, 992 p. 

Dzurisin, D., Johnson, DJ., Murray, T.L., and Myers, B., 1983, 

Tilt networks at Mount Shasta and Lassen Peak, California: 

U.S. Geological Survey Open-File Report 82-670, 42 p. 

Eaton, J.P., 1959, A portable water-tube tiltmeter: 

Seismological Society of America Bulletin, v. 49, p. 

301-316. -

Fiske, R.S., and Shepherd, J.B., 1990, Twelve years of 

ground-tilt measurements on the Soufriere of St. Vincent, 

1977-1989: Bulletin of Volcanology, v. 52, p. 227-241. 

Floyd, R.P ., 1978, Geodetic benchmarks: National Oceanic and 

Atmospheric Administration Manual NOS NOS 1, 55 p. 

Kinoshita, W.T., Swanson, D.A., and Jackson, D.B., 1974, The 

measurement of crustal deformation related to volcanic 

activity at Kilauea Volcano, Hawaii, in Civetta and others, 

eds., Physical volcanology: New York, Elsevier, p. 

87-115. 

Upman, P.W., Moore, J.G., and Swanson, D.A., 1982, Bulging 

of the north flank before the May 18 eruption - Geodetic 

data, in Lipman, P.W., and Mullineaux, D.R., eds., The 

1980 eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 143-155. 

McKee, C.O., Wallace, D.A, Almond, R.A., and Talai, B., 

1981, Fatal hydroeruption of Karkar volcano in 1979: 

Development of a maar-like crater, in Johnson, R.W., ed., 

Cooke-Ravian volume of volcanological papers: 

Geological Survey of Papua New Guinea Memoir 10, p. 

63-84. 

15. A Single-Setup Trigonometric Leveling Method for Monitoring Ground-TIH Change• 157 
Newhall, C.G., 1984, Short-term forecasting of volcanic 

hazards, in Williams, M.E., and Kitzmiller, C., compilers, 

Proceedings, Geologic and Hydrologic Hazards Training 

Program: U.S. Geological Survey Open-File Report 

84-760, p. 507-592. 

Otway, P.M., Grindley, G.W., and Hull, A.G., 1984, 

Earthquakes, active fault displacement and associated 

vertical deformation near Lake Taupo, Taupo volcano 

zone: New Zealand Geological Survey, Report 110, 73 p. 

Ruger, J.M., and Brunner, F.K., 1982, EDM-heigbt traversing 

versus geodetic leveling: The Canadian Surveyor, v. 36, p. 

69-88. 

Savage, J.C., Prescott, W.H., Chamberlain, W.H., Usowski, M., 

and Mortensen, C.E., 1979, Geodetic tilt measurements 

along the San Andreas fault in central California: 

Seismological Society of America Bulletin, v. 69, p. 

1965-1981. 

Sylvester, A.G., 1978, The dry tilt method of measuring crustal 

tilt, in Proceedings, Conference VII, Stress and Strain 

Measurements Related to Earthquake Prediction: U.S. 

Geological Survey Open-File Report 79-370, p. 544-568. 

Vander Laat V., R., 1982, FJ inclinometro seco: Un vigilante 

de Ia deformacion ligada a1 wlcanismo activo en Hawaii: 

Boletin de Vulcanologia de Universidad Nacional de Costa 

Rica, no. 12, p. 11-17. 

Whalen, C. T., 1984, Preliminary test results of precise 

trig-leveling with the Wild T200-D15 system: American 

Congress on Surveying and Mapping Bulletin, December, 

p. 15-18. 

Yamashita, K.M., 1981, Dry tilt: A ground deformation monitor 

as applied to the active volcanoes of Hawaii: U.S. 

Geological Survey Open-File Report 81-523. 

158 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
16. Lake-Level Monitoring as a Tool for Studies of 

Crustal Deformation 

By Jack W. Kleinman and Peter M. Otway 1 

Abstract 

A lake can be used as a large natural tiltmeter by 

measuring changes in water-surface elevation relative to 

nearby stable bench marks at two or more sites around it. 

Differential gaging systems can be either permanently 

installed or portable. The precision of either type of 

system is directly proportional to the size of the lake; tilt 

resolution increases with lake size. Permanent systems 

can accurately measure water level to within 0.5-1 mm, 

the equivalent of 0.5 to 1.0 microradian on a small (1-km 

wide) lake. Although portable systems can attain a similar 

precision on larger lakes, their precision on a small lake is 

estimated to be 1-3 microradians, depending on 

environmental conditions at the time. Factors such as 

wind conditions, seiche, instrument precision, temperature 

and barometric effects, and bench-mark stability need to 

be taken into consideration when gathering lake-level 

data. 

INTRODUCTION 

Monitoring regional crustal deformation can be 

useful in gaging volcanic restlessness. A lake located 

within an area of crustal deformation presents a unique 

monitoring opportunity. This paper presents the 

principles behind monitoring lake levels to detect crustal 

deformation, describes several different types of systems, 

and explains common sources of error associated with 

this technique. It is intended as an overview for someone 

interested in initiating a lake-level monitoring program. 

It should be stressed, however, that all lakes are unique; 

monitoring techniques will need to be adapted to each 

situation. 

1

New Zealand Geological Survey, Department of Scientific and 

Industrial Research, Private Bag, Taupo, New Zealand 

PRINCIPLES 

The operative principle in monitoring the level of a 

lake to study ground deformation is that its surface will 

remain level (horizontal) no matter how the surrounding 

crust is deformed. Therefore, relative changes in vertical 

displacement between fixed points and the surface of the 

lake can be used to determine the relative vertical 

changes between the points. In effect, the lake is used as 

a large tiltmeter. 

The elevation of the water surface is determined 

relative to two fixed points (fig. 16.1A). They can be 

either above or below water level, depending on the type 

of instrument and monitoring system used. Water-surface 

elevations at both stations are then remeasured at a later 

time (fig. 16.1B, C). The differences in relative watersurface elevation are computed for both stations. One 

station is considered fixed; any water-level change at that 

station is interpreted as a universal change. Tilt is 

computed as the change in height between stations (4 em 

in fig. 16.1C) divided by the distance between stations (1 

mm/km = 1 J!rad). 

Note that displacement at any one station can be 

measured only relative to another station on the lake. 

Thus lake-level monitoring cannot detect vertical 

deformation that affects the whole lake equally. In a 

large lake with many instrument stations, localized areas 

of uplift or subsidence may be recognized. 

LOCATING LAKE-LEVEL MEASUREMENT 

SITES 

The size and shape of the lake, the required 

measurement precision, and the extent of expected 

deformation determine the optimum network 

configuration. The most important consideration is the 

spacing between stations. The accuracy of lake-level 

measurement is limited largely by the precision to which 

the mean water-surface level can be measured. A large 

16. Lake-Level Monitoring aa a Tool for Studlea of Crustal Deformation 159 
component of the error in determining relative lake level 

is approximately the same at all measurement sites; thus 

higher resolution of ground tilt is attained with greater 

A 

_.... 

f I .a. 

X y 

10 em 10 em 

.l ! ..... 

Water surface 

B 

A. -t t A. 

X 6cm 6cm y 

+ ..... ... t ..... 

Water surface 

c 

t 

6cm 

~ 

Figure 16.1. Idealized diagram demonstrating principles 

of lake leveling. Lake bottom is rectangular in profile. 

Triangles X andY represent two fixed reference points on 

shore. A, Initial measurement of elevation difference 

between points X and Y and water surface. In this 

example, both are equal to 1 0 em. 8, Remeasurement at 

later time reveals elevation difference between points X 

and Y and water surface is now 6 em, indicating a 

universal rise in water level of 4 em. No detectable tilting 

occurred between points X and Y. C, Remeasurement at 

a later time reveals 6 em of elevation difference at point 

X and 2 em at point Y. Compared to initial measurement 

and using site X as fixed reference point, there has been 

4 em universal rise in lake level. At site Y, there is 8 em 

elevation difference of which 4 em is due to universal 

change in water level and 4 em is due to subsidence of 

Y relative to X. 

spacing between stations. For this reason, large lakes are 

generally better suited for monitoring than small ones. 

Crustal deformation is easiest to detect if the line 

between stations is oriented radially from the suspected 

deformation source. For this reason, a long, narrow lake 

oriented tangentially with respect to a volcano or other 

deformation source would not be favorable for detecting 

tilt, even if the spacing between measurement sites were 

very large. For example, if two points subside or uplift 

equally, this tilt would go undetected unless other 

stations were measured. Stations should therefore be 

located at different distances from the suspected 

deformation source . 

The number of stations on a lake should be 

increased in accordance with the extent of deformation. 

If uniaxial tilting is occurring, two to three widely 

spaced measurement sites may be sufficient. If 

deformation occurs on a scale smaller than the lake or is 

particularly complex, additional measurement stations 

enable the detection of localized areas of deformation. 

Yellowstone Lake in Wyoming, the site of a 

lake-level monitoring program since 1987, provides a 

good example of network configuration. Five stations are 

used to monitor this lake (fig. 16.2). The margin of 

Yellowstone caldera crosses through the south part of 

I~OMINJ Area ol map 

0 5 Kilometers 

Figure 16.2. Outline of Yellowstone Lake, Wyoming, 

showing location of each lake-level monitoring station. 

Promontory station lies outside caldera and is used as a 

reference for elevation changes at other stations. 

160 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Yellowstone Lake. One of the gaging stations was 

established outside the caldera. Any deformation within 

the caldera can be detected by comparison with the 

station outside the caldera. 

A long, narrow lake might require only two gaging 

stations for the computation of simple tilt, one near each 

end. As an example, San Andreas Lake in California (the 

site of a lake-level monitoring program since 1979) is 

such a lake and accordingly uses only two stations 

(Mueller and others, 1989). Additional stations between 

the end points could be established to help distinguish 

between regional and local tilts. 

I 

A third example is Lake Taupo in New Zealand, 

the site of ongoing lake-level monitoring since 1979 

(Otway, 1986a, 1986b, and 1989). In that study, 22 

different stations are now monitored 4-8 times per year 

(fig. 16.3). The crustal deformation occurring in the 

region is extensive and can be contoured given the 

relatively large number of measurement sites (fig. 16.4). 

Both the San Andreas Lake and Lake Taupo 

studies are being carried out primarily to determine 

crustal deformation associated with active fault zones, 

although deep magmatic influences are also suspected at 

Taupo. The monitoring techniques are equally 

y 

t 

~ ~ * . 

t 

i 

t 

t 

Scenic Bay 

Omori 

4Waihi 

I I Active fault 

Horomatang1 

# L4.KE TAUPO 

Motutaiko 

I / 

/ Bulli Point 

-----/ fl 

i 

Motuoapaf 

-f 

/ 

/ 

i 

Rotongaio 

/ 

i 

i 

i 

0 

t 

N 

I 

1 0 Kl LOM ETERS 

...L ..L. _ _ Concealed fault 

* Vent active in last 10,000 years • Lake level station 

Figure 16.3. lake Taupo, New Zealand, showing location of lake-level monitoring stations as well as Taupo Fault Belt 

and other local faults. Hachures on downthrown block. 

16. Lake-Level Monitoring as a Tool for Studies of Crustal Deformation 161 
applicable to lakes located in areas where volcanic 

deformation is the primary concern. 

TYPES OF LAKE GAGING SYSTEMS 

Two general types of systems can measure height 

of water in lake-level monitoring: permanent and 

portable. Common to both types is a gage that enables 

accurate measurement of water level. In a permanent 

system, the gage is mounted in the lake and the water 

level is recorded at regular time intervals. In a portable 

system, the gage is set up at a station, data are recorded, 

and the gage is then moved to the next site. 

Permanent Systems 

Instrumentation 

A commonly used instrument to measure water 

level is a pressure transducer which measures the 

pressure due to the weight of the water column above it. 

+5 

t 

N 

I 0 10 KILOMETERS 

Figure 16.4. Vertical deformation (in millimeters) around 

Lake Taupo, November 7, 1987, to November 9, 1988. 

"Typical" annual deformation pattern at Lake Taupo 

indicates that center of Taupo Fault Belt is subsiding 

without associated seismicity at an average relative rate 

of nearly 10 mm/yr since a major earthquake swarm and 

deformation event in 1983 (fig. 16.11 ). Southern half of 

lake bed has undergone frequent episodes of short-lived 

deformation since then, often associated with minor 

seismicity. Hachures on downthrown block. 

It is subject to errors caused by nonlinearity, temperature 

variations, and long-term drift. A transducer must be 

carefully selected on the basis of manufacturer 

specifications and then tested under conditions 

appropriate for field use. Another instrument is an 

ultrasonic sensor, which is placed in a stilling well to 

record the vertical distance between the sensor head and 

the water surface. 

The monitoring at San Andreas Lake and 

Yellowstone Lake is done with permanent systems. The 

San Andreas Lake study uses Model 8020 Paroscientific 

Digiquartz pressure transducers chosen for their 

long-term stability and accuracy (Mueller and others, 

1989). Each transducer has a 20-m dynamic range of 

water level, which is needed because San Andreas Lake 

is an artificial reservoir and can have large fluctuations 

in volume. The Yellowstone study presently uses 

V alidyne Model P307 submersible pressure transducers 

with a 3-m dynamic range and an estimated ±0.5% 

accuracy. Workers from the Cascades Volcano 

Observatory (CVO) plan to enhance the current 

Yellowstone program by installing Paroscientific 

transducers with a ±0.01% accuracy and ±0.005% 

repeatability (the equivalent of 1 mm error when 

measuring 10 m of water), similar to those used in the 

San Andreas Lake study. Pressure transducers for 

monitoring lake levels need to be of sufficient quality to 

make repeatable, stable measurements to an accuracy of 

a few millimeters. Several significantly less expensive 

transducers with a lower accuracy (±2.0% linearity and 

±0.15% repeatability, the equivalent of 200 mm error in 

a 10-m measurement) have also been used but were 

found to be unsuitable for lake-level monitoring, even 

after stringent calibration. 

Testing and calibration 

The transducers should be tested before 

installation. After placing a transducer in water and 

reading the output signal, the depth should be altered so 

that the resulting difference in output signal corresponds 

with the change in depth of water. If possible, 

differential tests should be done throughout the depth 

range of the transducer and over a range of water 

temperature appropriate for the field application. If more 

than one transducer is used, all should be run through 

these tests simultaneously. If the output curves are 

widely different, one or more of the transducers is 

malfunctioning. Minor variations between the curves can, 

however, be calibrated and corrected. Most high-quality 

transducers are calibrated in the factory, but this 

procedure is strongly recommended to confrrm that they 

·are functioning properly. 

162 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Installation 

Once the sites for the permanent stations have been 

selected, the first step in establishing a 

permanent-installation lake-level monitoring program is 

to install the pressure transducers. The transducers will 

most likely be left at the site for long periods and hence 

will be subjected to seasonal lake-level fluctuations. The 

water level of Yellowstone Lake, for example, fluctuates 

by approximately 1-2 m annually. The pressure 

transducer should be mounted (1) so that it remains 

below water level throughout the low water period, (2) 

below wave base so that it is not damaged by breaking 

waves, and (3) below the common winter freezing depth 

so that it is not moved or damaged by ice. Transducers at 

Yellowstone Lake are installed about 1 m below the 

low-water level. 

The method used for attaching the sensors to the 

bottom depends on the slope and material properties of 

the shoreline. The methods described here do not require 

the use of hydraulic drills or other heavy machinery, 

which is commonly prohibited by cost or environmental 

concerns. The transducers must be both immobile and 

below water-surface effects. Transducers are most stable 

if they can be attached to bedrock at a sufficient depth at 

the intended site. A perfect scenario would be a vertical 

bedrock cliff at the shoreline where holes could be 

drilled under water and the transducer attached using 

anchor bolts. If holes cannot be drilled under water, the 

transducer could be affixed to one end of a length of 

angle iron, and the other end (projecting out of the 

water) could be affixed to the bedrock. Another method 

' is to attach the transducer to the bedrock using cement. 

A quick-setting cement works best when pouring under 

water. Cracks or other irregularities in the rock help 

cement adhere to the rock. Common sense and ingenuity 

need to be used, as every site is different. Lakes located 

in volcanic areas may be highly acidic; the anchoring 

and instrument housing materials used should be 

appropriate for the chemical conditions known to exist in 

the lake. 

Lake bottoms are most commonly covered with 

sediment, as at both Yellowstone and San Andreas 

Lakes. At several sites on Yellowstone Lake, a single 

fencepost was driven almost 2 m into the sediment and 

the transducer strapped to it using hose clamps. This 

method is adequate for short-term studies but leaves 

some doubt as to the long-term stability of the mounting. 

The data from one of the transducers on Yellowstone 

Lake show an abrupt offset believed to have been caused 

by movement of the transducer on the fencepost or, more 

likely, movement of the whole fencepost. 

A preferred method is to use anchor rods driven as 

far as possible, rather than to arbitrary depth. At San 

Andreas Lake, three rods were hammered into the lake 

bottom as far as they would go (typically more than 4 

m) at each transducer site. A plate connecting the 

three rods was cemented in place for additional 

stability, and the transducer was then affixed to this 

baseplate. The rods commonly used are either 1.5-m 

lengths of copper-clad grounding rods with threaded 

ends or any convenient diameter of galvanized pipe. 

Copper grounding rods have the advantage of being of 

fairly small diameter (1.25 em) and therefore easier to 

drive. One rod can be joined end to end with another 

rod using a threaded couple sleeve (see also Doukas 

and Ewert, chapter 11 ). 

The battery power and data recorder or telemetry 

for the transducer are located on shore, connected by a 

power cable and a data cable to the transducer (fig. 

16.5). All cables should be buried in a conduit to 

protect them from damage by waves, debris, wildlife, 

and vandals. Some types of transducers use 

atmospheric pressure as a reference and may also 

require a "dry" conduit hose connecting the transducer 

to the atmosphere. 

Data collection 

. Data from permanent lake-level gaging systems 

can be collected using an on-site data logger or can 

be telemetered to a central recording station. 

Telemetry systems can use GOES satellite telemetry 

(as is currently used at San Andreas Lake) or a 

dial-up receiver (as is currently used at Yellowstone 

Lake). A more detailed description of applicable 

telemetry systems can be found in other chapters of 

this volume. 

Portable Systems 

Two types of portable gages for monitoring lake 

levels, one used by CVO scientists and one used at Lake 

Taupo, New Zealand, are presented as two different 

approaches to portable monitoring. The CVO system is 

experimental and uses an electronic linear-position 

sensor to measure lake levels. The Taupo system relies 

more heavily on manual readings. 

CVO System 

Instrumentation 

The CVO portable system employs a 

linear-displacement transducer instead of the pressure 

transducer used in permanent systems. The 

Temposonics linear-displacement transducer claims a 

linearity of 0.05%, a repeatability of 0.001%, and no 

16. Lake-Level Monitoring as a Tool for Studies of Crustal Deformation 163 
instrument drift. This device precisely measures the time 

interval between an interrogating electromagnetic pulse 

transmitted through an attached rod (the transducer 

waveguide) and a return electromagnetic pulse generated 

by a floating external permanent magnet (fig. 16.6). This 

time interval directly correlates with the position of the 

external magnet along the transducer waveguide. The 

dynamic range of the instrument depends upon the length 

of the transducer waveguide, 60 em in this case. The 

linearity and repeatability of this model are 0.305 mm 

and 0.0061 mm, respectively. 

Several adaptations were made to apply the 

linear-displacement transducer to portable lake-level 

measurements (fig. 16.6). The external magnet was 

mounted on a floating ring to represent water level 

accurately. A stilling well, needed to minimize wave 

effects and other small fluctuations in water level, was 

added to surround the transducer waveguide rod. A 

unit for vertically mounting the whole assembly 

(linear-displacement transducer, external magnet, and 

stilling well) from a standard surveying tripod was 

designed and fabricated. Care was taken to orient the 

transducer vertically, because a 3-4• deviation could 

cause an error of about 1 mm. A tripod with a domed 

bead (the Kern system) was used so that the whole 

assembly can be rotated and tightened in the desired 

vertical position using a level bubble affixed to the 

horizontal base plate of the mounting unit. 

Calibration 

The linear-displacement transducer, like the 

pressure transducer, should be calibrated before and after 

each field survey. One method of calibration requires a 

tall container in which changes in water depth can be 

accurately measured. Tubing allows water to flQW 

between the container and a transparent graduated buret 

permanently affixed to the outside of the container. The 

water level in the container is precisely read (to the 

nearest 0.5 mm) using the graduated buret, and the full 

range of the transducer can be tested. The electronics box 

of the transducer bas adjustment potentiometers that can 

be used to adjust the null point and full-scale output of 

the transducer. Another calibration technique is to 

fabricate two different lengths of tube that can be used as 

spacers between the transducer head and the magnet ring. 

All the sensors used in the study can then be adjusted to 

have the same output with each spacer. 

Measurement procedure 

In the procedure for determining lake level with a 

portable system, the tripod is set up and the transducer 

and stilling well are mounted vertically using the 

fabricated mounting unit. The depth of water is not 

critical as long as the water surface is within the 60-cm 

length of the transducer waveguide. Three threaded bars 

(see fig. 16.6) connecting the lower end of the stilling 

Battery power supply 

Buried conduit for protection of cables 

"Dry" hose to atmosphere 

(if required for transducer) 

"'-Anchor rod 

Figure 16.5. Sketch of a permanently installed lake-level monitoring system. 

164 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
well to the legs of the tripod help stabilize the unit 

beneath the tripod. 

Power is supplied to the transducer from a 12-volt 

battery on shore. A portable laptop computer is 

programmed to record the output data from the 

transducer and can also regulate the sample frequency. 

Alternatively, transducer-output data could be recorded 

manually at regular intervals with a voltmeter. The time 

Optical level 

bubble 

Linear displacement transducer: 

Flgure16.6. Instrumentation used for CVO portable 

system. Standard surveying tripod is used to hold gage. 

Section of 

fiberglass level rod 

mounted onto tripod 

l 

t 

3 

needed and sample interval used to obtain an output 

voltage from the transducer that is representative of lake 

level depends on the specific application and the period 

of seiche oscillations. The final lake level is an average 

of transducer-output values through one to three 

complete seiche periods. When beginning a monitoring 

program, initial measurements must be made frequently 

over long time periods until the dynamics of the lake at 

each site are understood well enough to chose the most 

efficient sampling period and frequency required for 

accurate measurement. 

Two separate measurements must be made with 

this portable monitoring system. The level of the lake 

surface is measured with the displacement transducer, 

and the vertical position of the transducer is measured 

relative to a reference point on the shore (fig. 16.7). The 

height of the transducer head above the water surface 

(the level of the floating magnet ring) is the averaged 

output signai from the transducer (1 in fig. 16.7). The 

output value of the calibrated displacement transducer 

(averaged for one to three seiche periods as discussed 

above) correlates with this height. The vertical distance 

from the transducer head to the top of the tripod (2 in 

fig. 16.7) should be constant for every setup. A 1.5-m 

section of fiberglass level rod is mounted onto the top of 

the tripod. Standard leveling techniques can then be used 

Level gun 

Standard 

level rod 

Flgure16.7. Water-surface elevation relative to a fixed point on land is equal to sum of height 1 (water-surface 

elevation relative to transducer) and heights 2 and 3 (position of transducer itself relative to bench mark). Height 1 is 

measured output from transducer. Height 2 is constant mounting-unit length. Height 3 is measured height to bench 

mark using standard leveling techniques. In this example, height 3 is equal to height 3a (height of level gun relative 

to tripod-mounted level rod) minus height 3b (height of level gun relative to bench-mark level rod). 

16. Lake-Level Monitoring as a Tool for Studies of Crustal Deformation 165 
to determine the height between the top of the tripod and 

the reference mark (3 in fig. 16.7). This measurement is 

made easier if the reference mark, preferably a bench 

mark cemented into bedrock or a large boulder, is as 

close to shore as possible and within the height range of 

the level rod on the tripod. 

This procedure is repeated at each station around 

the lake. Ideally each station would have a gage, so that 

data could be collected from all sites synchronously. 

Alternatively, two sets of equipment can be used, one set 

in operation throughout the measurement period and the 

other moving to the different sites. This approach would 

minimize errors due to filling or emptying of the lake 

basin during the measurement period. If budget or 

personnel constraints prohibit such an approach, changes 

in basin filling can be found by reoccupying the origin at 

the beginning and end of each measurement day. Any 

ground tilting would most likely be negligible during 

such a short time period, and any changes found in lake 

level at the origin station would most likely be 

attributable to water inflow or outflow from the lake. 

Accuracy may be sacrificed, however, when conducting 

a monitoring program with only one instrument. 

The measurements themselves can often be 

completed by one person while the portable computer is 

recording output from the transducer or between manual 

readings, so that the total time at each site is not much 

more than that needed to make a water-level 

measurement. Depending on the number of stations, their 

access, and the proximity of associated bench marks, a 

lake-level survey could be completed in one to four 

transducer days (that is, fewer days if two or more 

transducers are used concurrently). 

Taupo System 

Instrumentation 

The water-level gage consists of a galvanized iron 

cylinder 0.6 m long and 50 mm wide (fig. 16.8). The 

cylinder hangs vertically by a chain from a hook and fills 

with water through a 1.5-mm orifice, effectively acting 

as a stilling well to damp out normal wave action (less 

than 0.5 m amplitude). When the tap is closed, the gage 

can be lifted and the inside water level read at eye level 

to within 1 mm by means of a clear viewing tube set 

against a scale on the outside of the gage. As an 

improvement, a pressure transducer (Honeywell Micro 

Switch 163PC) was mounted on top of the gage with one 

port extending below water level and the other open to 

the atmosphere; this provides an electrical readout 

capable of being logged automatically. 

Attached to the gage is a motorcycle chain used for 

hanging the unit at each station. A station consists of a 

hook and reference mark set into a wharf or bridge piling 

or into a sheer rock face. A bench mark on shore can 

also be used if the gage is suspended from a temporary 

tripod set up in shallow water, similar to the setup with 

the linear-displacement transducer. 

Procedure 

The gage is hung from the hook located at each 

station or from a tripod in shallow water. The gage is 

positioned by choosing the appropriate link in its 

suspension chain so as to be approximately 

half-submerged. The tap is opened and the gage allowed 

to stabilize for several minutes. Readings are then made 

optically every 5 minutes or electronically every 2.5 

minutes for a 30-minute period (one cycle of the 

predominant seiche on Lake Taupo). The readings are 

scanned for obvious anomalies before moving on to the 

next site. The height difference between the horizontal 

surface on top of the gage and the reference mark on the 

piling, rock face, or ground above is obtained either by 

direct taping or by precise leveling. The sum of this 

height difference and the manual-gage reading equals the 

elevation difference from the water surface to the 

reference mark. The reference marks are connected, 

where possible, to nearby bench marks by precise 

leveling and checked periodically for possible instability. 

Reference mark 

on land -......-- 0 ·'-'---~· Hook set in piling or rock 

Height difference 

determined by 

tape measure or 

precise leveling 

Fixed vertical 

offset (250 mm) 

Clear plastic tube 

set against scale 

• / Housing (galvanized iron) 

Optional pressure transducer 

Electrical cord (input/output 

"-.. between battery, transducer, 

\ 

and voltmeter/logger through 

air hole) 

Tap handle 

El--l--- Air tube ( 2 mm diam.) from 

transducer port to 10-mm "bell" 

L ~--~'""'""'-"--V'--_.A._~__...__-"--' Lake level 

~Tap and opening (2 mm) 

to gage 

Flgure16.8. Portable water-level gage used at Lake 

Taupo. Modified from Otway (1989). 

166 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
As with the eva system, the observer both starts 

and finishes observations at the origin station. Before the 

introduction of the continuous recording gage in 1987, 

the manual gage was used to detennine the water level at 

the start and finish of each day's work. Lake level at the 

time of each observation around the lake was then 

derived by interpolation that assumed a linear response 

during the day. Now the continuous recording gage is 

used at the origin, and lake level is computed by fitting a 

linear regression to recorded data. 

POTENTIAL SOURCES OF ERROR 

Data from ongoing lake-level studies demonstrate 

one particularly notable natural characteristic of lakes: 

their level continually fluctuates. In addition to natural 

fluctuations, measurement errors lead to inaccurate 

estimates of lake level. With reasonable care, patience, 

and experience, most of the errors can be either avoided 

or minimized, and the fluctuating water surface 

accounted for. 

Natural Errors 

Wind 

Wind and associated waves are an obvious natural 

problem. Small waves can be adequately filtered, either 

electronically when permanently mounted transducers are 

used, or physically by a stilling well used with the 

portable systems. Large waves, however, may prevent 

reliable measurements of water-surface elevation. 

Wind-driven waves may appreciably raise lake level, 

especially in shallow bays; the effect in deep water is 

less pronounced. For example, a 12-knot wind (starting 

to fonn small whitecaps) on Lake Taupo, approximately 

40 km long and more than 100 m deep, has been 

modeled and observed to produce a surface slope of 

about 0.07 J.trad, or a height differential of 3 mm along 

the length of the lake at cliff faces or in deep sheltered 

bays. In shallow water the effect is commonly much 

greater because strong winds cause water level to rise on 

the exposed shore, which could erroneously be 

interpreted as ground tilt. Mueller and others (1989) 

found that strong wind effects also increase seiche 

amplitudes. Resolution of crustal tilt during periods of 

high wind is degraded by these effects. 

Seiche 

The seiche of a lake can be likened to the effect of 

slopping water in a saucer and takes the form of a 

relatively rapid, small tide whose spatial and temporal 

characteristics can vary greatly (fig. 16.9). A seiche is 

usually initiated by strong winds or a rapid pressure 

change and may last for days before either dying out or 

being superseded by a new seiche. Its amplitude and 

frequency are dependent upon the dimensions, shape, and 

depth of the lake. Most lakes, particularly if complex in 

shape, have multiple seiche modes; seiches travel along 

various axes, all with different characteristics. The 

resultant seiche at any point depends on the degree of 

exposure to each mode. In practice, recording lake level 

at each proposed site can identify those sites with unduly 

amplified or complex seiche. The periodicity of the most 

significant modes can be determined by analyzing the 

records with accuracy obviously increasing as more data 

are collected. From this preliminary work, the optimum 

observing time and frequency of sampling (if not using a 

continuous recording system) can be determined. The 

most effective method to filter the seiche can be derived 

30 Rainbow Point 12 Dec 1985 - calm 

20 

10 

(/) 

cr: 0 

w 

rw 70 ~ 

:::i 60 .....J 

~ 50 

z 40 

_j 

w 30 

> w 20 .....J 

w 10 ~ 

< 0 .....J 

w -10 

> 

~ 60 .....J 

w 

cr: 50 

40 

30 

20 

10 

0 

-10 

0 2 3 

HOURS 

Figure 16.9. Lake-level records from Lake Taupo. The 

30-minute seiche mode is clearly visible at Rainbow 

Point, but develops an unusual 1 0-minute resonance at 

Kinloch. 

16. Lake-Level Monitoring aa a Tool for Studlea of Cruatal Deformation 167 
by experimentation and refined by mathematical 

modeling. For Lake Taupo, a wide heart-shaped lake, all 

but the southern arm of the lake is influenced primarily 

by a mode with a period of 30 minutes and 9 seconds. At 

the southern end a mode of 35 minutes and 35 seconds 

predominates (Gilmour and Butcher, 1987). Computing 

the mean of readings taken at regular intervals 

throughout one or more full seiche periods usually 

produces reliable data (within 1 mm of the 

mathematically modeled mean) for seiche amplitudes 

between 3 and 30 mm. For example, at Lake Taupo six 

readings are taken at 5-minute intervals for all but the 

southern stations, where an additional reading is taken. 

In complex lakes, however, more refined techniques may 

be required to filter other significant seiche modes. 

Barometric Gradient 

A differential in atmospheric pressure should result 

in a predictable response in water level across the lake. 

For example, personal experience has shown that a 

gradient of 0.02 mbar/km (common in middle and high 

latitudes in association with frontal weather systems) 

should produce a lake surface gradient .of 0.2 1-1rad (a 

2-mm height differential across a 10-km lake)1

• Stress 

from the resulting wind may, however, greatly 

complicate the resulting lake level, particularly because · 

the airflow tends to be at right angles to the pressure 

gradient owing to the Coriolis and frictional forces 

(Ahrens, 1982). Less well understood are the effects of 

localized weather systems, such as thunderstorms and 

squalls, with their associated low-pressure cells and 

sudden winds. It is usually much easier to avoid making 

measurements during unstable weather systems than to 

calculate their influences. 

Variations In the Rates of Inflow and Outflow 

Variations occur during and after heavy rainfall or 

when river-control structures are adjusted on lakes used 

for hydroelectric storage, irrigation, or flood control. 

Water levels may rise erratically during and immediately 

after heavy rain and consequently be difficult to predict. 

The rate of change becomes more regular as time passes 

following the rain, but the minimum waiting time 

required for any lake can best be determined only by 

observation. Lakes that are controlled artificially require 

consultation between the observer and the controlling 

authorities to determine the optimum periods when 

inflows and outflows will be most stable and, if the rates 

must be varied during this time, the effect this is likely to 

1

Calculated with the approximates of 1mb., 0.75 mm Hg., 10.2 

mm HzO. 

have on the lake level. Again, it is best to avoid such 

water-level variations if possible. 

Other Natural Influences 

Currents, internal seiches, lunar tides, and density 

differences are all potential sources of error. Their effects 

are likely to be minimal in all but the largest lakes (more 

than 1,000 km2 surface area) by exercising basic 

precautions. Currents can be produced by persistent 

winds that cause a slow rotation of some or all of the 

water in the lake. Currents tend to be strongest on 

promontories or in shallow water, where they may result 

in a significant gradient of the lake surface. Inlets and 

outlets are sites of local currents that should also be 

avoided. Internal seiches can occur along the boundary 

between a dense bottom layer and lighter surface layer of 

water but are thought to contribute generally small errors 

in lakes smaller than 1,000 km2

• Disturbances can also 

be caused by the Coriolis force and lunar tidal influences 

(Gilmour and Heath, 1989), but are not a significant 

source of error compared with wind, seiche, and 

barometric effects. Significant density differences owing 

to large volumes of meltwater or geothermally heated 

water may also cause lake-level anomalies, and should 

be avoided. It is possible that large gravity changes 

associated with tectonic or magmatic activity may also 

effect lake levels, but this effect has not been adequately 

researched or documented. For more precise work, 

therefore, further research is required on such effects, 

especially in large lakes. Until more is known, the 

observer should be on guard against any unusual 

systematic errors. 

Measurement Errors 

Calibration and Instrument Reliability 

The instrument for measuring water level, whether 

it is read optically (for example a manual gage), 

mechanically (such as the floating ring of the linear 

pressure transducer), or electronically (such as the output 

from pressure transducers), must be calibrated and be 

capable of producing repeatable data. Each individual 

water-level reading should have a resolution and 

accuracy to within at most 1 mm, and preferably better. 

Thermal Effects on Equipment 

The accuracy of lake-level measurement is 

decreased if there are large temperature differences 

between (or during) readings. Electronic output and the 

relative dimensions of some materials can vary owing to 

changes in temperature. Calibration is recommended 

168 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
throughout the expected temperature range in which the 

equipment will be used. Most precision electronic 

instruments (including pressure transducers and linear 

position sensors) have manufacturer's specifications for 

temperature effects. The effects of variations in 

temperature depend on the type of instrumentation and 

on the climate in which the system is used, hence each 

monitoring program will have its own corrections applied 

to each application. 

Instrument Setup 

Errors associated with instrument setup are 

obviously not applicable to stable permanent 

installations, but could cause significant problems in 

portable systems. When using a tripod to support the 

gage, care should be taken to make sure that the legs of 

the tripod are stable with respect to the lake bottom. If 

the tripod sinks at all during the measurement, lake level 

will appear too high. The stilling well mounted beneath 

the tripod must hang vertically to ensure that all 

measurements are made at right angles to the water 

surface. 

Leveling or Taping Errors 

Adequate care and self-checking procedures must 

be taken to ensure that errors from leveling between the 

instrument and the bench mark remain less than 1 mm. 

Such errors will be negligible with strict adherence to 

precise leveling procedures and standards. In the CVO 

portable system, the leveling procedure deviates from 

first-order standards by using a fiberglass level rod 

mounted on the tripod instead of an lnvar first-order 

quality rod (Yamashita and Kaiser, chapter 13). Errors in 

reading a fiberglass level rod can easily be kept under 1 

nim. Likewise, great care and several readings must be 

taken to reduce error and eliminate blunders when using 

a tape to determine the height difference between the 

gage and a reference mark directly above it. 

Bench-mark Instability 

The permanent bench mark used as a reference for 

determining instrument height must be reliable. If there 

is doubt about its stability, the mark should be connected 

by leveling to at least one other bench mark as a check. 

When using a hook as a reference mark on a structure, it 

must be referenced to a bench mark where possible to 

prove stability. Alternatively, hooks and reference marks 

should be established on two independent nearby 

structures. 

REDUCTION AND ASSESSMENT OF DATA 

The recommended method of analyzing lake-level 

data begins with choosing one station as the origin. This 

station is considered "fixed" (that is, not undergoing any 

vertical displacement) throughout the study. Any 

changes in lake level found by reoccupation of this 

station, whether reoccupation occurs later in the day or 

over a year later as part of an annual survey, would by 

convention be attributed to changes in water volume of 

the lake, not to deformation. 

The surface elevation of the lake relative to the 

origin must be determined at the time of each 

measurement of the other stations around the lake shore. 

To achieve this, the seiche effect is removed from the 

average lake level by computing the mean of regular 

readings over one or more complete seiche cycles, as 

described above. Then either a linear regression is fit to 

the record (if obtained continuously by electronic or 

mechanical means) or simple interpolation is used (if 

lake level at the origin is measured at the beginning and 

end of each day) to determine the lake level at the times 

of measurement at other sites around the lake. In either 

case, the lake level is assumed to respond linearly to 

steady inflow and outflow during the day. 

Data from the other stations are then compared to the 

origin. An elevation is chosen for the origin either 

arbitrarily or derived from leveling to a bench mark. The 

water-surface elevation relative to that mark is then 

computed. The elevation difference between the bench 

mark or reference mark and the water surface at each 

measuring site is then added to this water-surface elevation. 

The elevation of each bench mark relative to the ch~ 

elevation of the origin is then determined. This elevation 

can then be compared with any earlier set of data to 

determine apparent height differences relative to the origin. 

Only the relative changes in water-surface elevation 

are measured. Therefore, changes in lake level owing to 

changes in water volume in the lake basin as well as to 

differential tilting can be ascertained only after 

reoccupation of the stations. 

The reliability of the results can be assessed only 

after spending a considerable period carrying out frequent 

monitoring surveys of a particular lake under a wide range 

of conditions in order to determine means and standard 

deviations. A simple empirical formula can then be derived 

to estimate standard errors for future observations carried 

out by similar methods. As an example, the formula 

adopted at Lake Taupo since 1986, based on initial repeat 

observations and upgraded using more recent repeat 

observations at numerous stations, is as follows: 

Standard deviation in mm for a single survey is 

1.2 + (0.1 X d) 

where d is the straight line distance, in kilometers, from 

the origin. For a station 4 km from the origin, standard 

16. Lake-Level Monitoring as a Tool for Studies of Crustal Deformation 169 
deviation is 1.6 mm (0.4 J.lrad); for a station 40 km from 

the origin, it is 5.2 mm (0.13 J1rad). This provisional 

formula was derived from Lake Taupo data only; it may 

be a useful guide but is not necessarily applicable to 

other lakes because each has its own unique 

characteristics. 

As a precaution, stations should be reoccupied 

when elevation differences indicate possible vertical 

crustal deformation. Despite precautions taken to 

minimize error, unexpected systematic influences related 

to localized pressure gradients, wind, internal seiches, 

and currents can still sometimes cause larger than usual 

errors. The best rule is to be vigilant and prepared to 

repeat observations whenever an anomaly becomes 

apparent. 

EXAMPLES OF RESULTS 

Yellowstone Lake 

The Yellowstone Lake monitoring program is 

carried out by Wayne Hamilton from the Research Office 

of Yellowstone National Park in cooperation with the 

Cascades Volcano Observatory. Permanent lake-level 

systems have been established at five sites around the 

lake (fig. 16.2). The dashed line of figure 16.2 

corresponds with the inferred rim of Yellowstone caldera 

(Christiansen, 1984); the Promontory station lies outside 

of the caldera. Figure 16.10 displays the data from 

stations Stevenson and Promontory from May 1988 to 

May 1989. In the top two curves, showing the transducer 

depth for the two sites, the seasonal variation in level is 

apparent. These data by themselves are not useful in 

detecting ground tilt. The bottom curve in figure 16.10 

depicts the difference in depth between Stevenson and 

the Promontory reference station. The net difference over 

the year was about 1 em. This difference is within the 

error of the transducers used in the study. Any ground 

tilt in the area of Yellowstone Lake over the year was 

below the detection level of the transducers. The 4 em 

amplitude in the difference curve has not yet been fully 

explained. It may be due to poor calibration of the two 

transducers at different water depths, seasonal water 

currents as a result of geothermal activity, or possibly a 

seasonal ground-deformation pattern. 

San Andreas Lake 

The San Andreas Lake monitoring program shows 

a long-term tilt rate of 0.020±0.08 J1rad/yr over the last 

decade (Mueller and others, 1989). This tilt is within the 

errors of the instrumentation and means that no 

long-term crustal tilting can be identified in the area. 

Lake Taupo 

Monitoring lake levels at Lake Taupo with a 

manually read portable gage has produced interesting 

results in the first 10 years the program has been in 

operation. Recorded changes between surveys are 

generally systematic both in time and space, with 

short-termanomalies generally within the range 

estimated by the standard deviation formula, indicating 

that the larger, long-term apparent changes are 

probably real. The annual trend since 1982 indicates 

subsidence of the northern shoreline, which crosses the 

Taupo Fault Belt (figs. 16.4 and 16.11). The rate of 

subsidence in the deepest part of the trough averaged 

10±3 mm/yr (0.6±0.2 J.lrad/yr) relative to stations in 

the central and southern part of the lake. The 

movement has occurred without shallow seismicity and 

is interpreted to be due to tectonic extension affecting 

the whole volcanic zone. An even greater tectonic tilt 

rate (0.9±0.2 J.lrad/yr) was recorded during the same 

period by three permanent stations at Lake Tarawera, 

70 km northeast of Lake Taupo (Scott, 1989). In 

addition, several other areas, ranging from 5 to 30 km 

in diameter, have exhibited episodes of uplift and 

140 

120 

100 

80 

60~~~~~~~~~~~~~~~~~~ 

30 

28 \~~ j 26 ~ _J _,.~~,.,.__ __ _ 

:;[, I 11 I 11 I ~ 1 I 1 l1 1 l1 1 I 1 I 11 I 

MAY JUN JUL AUG SEP OCT NOV DEC 89 FEB MAR APR 

START DAY IS MAY 1, 1988 G.M.T. -JULIAN DAY 122 

Flgure16.10. One year of lake-level. data from two 

permanent sites on Yellowstone Lake. Top two plots are 

of transducer depth; seasonal fluctuation in transducer 

depth Is readily apparent. Bottom plot depicts difference 

between Promontory and Stevenson sites. 

170 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
subsidence of 5-20 mm over periods of up to 6 months; 

an example is in the southwestern area in figure 16.4. 

Shallow minor seismicity frequently occurs in the same 

general area (Otway, 1987). 

The most dramatic example of vertical deformation 

associated with seismicity was recorded in 1983, when a 

swarm of frequent earthquakes of less than magnitude 

4.3 occurred in the center of the Taupo Fault Belt, 

between Kinloch and Rangatira stations. Preseismic 

deformation over a 3-month period was recorded in the 

form of widespread uplift exceeding 40 mm at Kinloch 

(Otway, 1986a; fig. 16.12). Coseismic deformation 

during the month-long swarm consisted of rapid 

subsidence west of the Kaiapo Fault (between Whakaipo 

and Rangatira) and uplift to the east. A 50-mm 

displacement (down to the west) occurred on the fault a 

week after the swarm commenced (Hull and Grindley, 

1984). Postseismic adjustment continued in the same 

sense but at a diminishing rate. 

CVO System Results 

The portable gaging system that uses 

linear-displacement transducers to measure water height 

relative to a stable point on shore has only recently been 

developed. Lake-level monitoring is concerned only with 

temporal changes in water height, so that deformation 

can be detected only after the gaging sites have been 

measured at least twice. The eva system was initially 

used in September 1988 at Yellowstone Lake, Wyoming, 

c 

U5 c ctS 

~ U5 

CIS ctS 0 ...:.:: 

ctS .r:. w 

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~ an independent check on the permanent system already 

established. Bench marks were placed on shore near the 

five permanent gaging sites, and the water-surface elevation 

relative to the bench marks was measured using the 

portable gaging procedure. Many improvements resulted 

from that survey. For example, the apparent water level 

fluctuated as much as several millimeters from one reading 

to the next, because the stilling well (and therefore the 

whole mounting unit assembly) was buffeted too much by 

small waves. 1be stabilizer bars now used were developed 

to eliminate this problem. 

1be gaging sites were reocrupied in June 1989 and 

the data compared with those from 1988. Differences in 

lake level were detected of ~ much as several centimeters. 

We suspect that the 1988 data set is not reliable. The data 

in 1988 were taken during windy conditions that were 

simply unacceptable for lake-level mt3urements. Because 

Yellowstone Lake is shallow, the water surface must be 

relatively calm for the data to be meaningful. One site is on 

the lee side of the lake, where conditions were calm at that 

location at that time, but the computed lake-level surface 

was not accurate because high winds elsewhere were tilting 

the lake surface. This problem emphasizes the need to 

mt3ure lake level only on nearly calm days when portable 

equipment is used. 

Despite this somewhat disappointing result, we are 

confident of the potential of this type of portable lake-level 

monitoring system. The linear-displacement transducer 

continues to function exceptionally well; its stability is 

extremely high and it exhibits no detectable instrument 

drift. 

1:: 1:: c c c U5 U5 U5 U5 U5 c E 8. ;g ctS U5 ~ ~ "(5 cu ·c;; '- ...:.:: ~ ~- 8._ a.. 

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0 5 10 15 

KILOMETERS FROM KAWAKAWA STATION 

Flgure16.11. Profile (AA' of figure 16.3) of apparent height changes across Taupo Fault Belt, consisting of cumulative 

change at 1 0 stations along north shore of Lake Taupo over a 3-year period from November 1985 to November 1988 

(see figure 16.3 for locations). 

16. Lake-Level Monitoring •• a Tool for Stucllea of Cruatal Deformation 171 
Standard 

deviation 

0 0.6mm • • 

Kaiapo Earthquakes start 1 

faulting 

(16 Jun) : : (23 Jun) 

I 

•: Rotongaio 

Small earthquake Small earthquakes 

Kai~po South~m L. Taupo 

I 

• • :a 

-10 ~--------~------------~·------------~--------~----~:~~------------~r---------~~--------~~r-~ I I 

0 1---3._a_m_m_--4_t-___ H_o_r_o_m_a_ta_n-lltg'-li -~---~~~ :: 

I I 

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10 

0 

-10 

5.8mm 

I I 

I I 

I 

I 

00 -20 ~--~------~----~----~----~~~~----~~----~----~~ a: 

w 

1-

w 0 

3.6mm • 

I I 

Scenic Bay 

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~ 40 

30 u) 

w 20 

C) 

z 

z 

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I 

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II 

C) 

w 

10 

0 

-10 

I 30 4.0mm 

20 

10 

0 

-10~--~------~----~----~--~~~~~~----~----~~ 

10 

0 

-10 

3.6mm Whakaipo ••---••-· 

-20L_ __ _L ____ ~----L---~--~~~~~~~L_--~~ 

30 2.9mm 

I 

20 I :Acacia_ 

I B .............-: ~ - 10 : ay : I I I 

0 ~--------~------------~------------~--------~--------~~~~------------~~------~------------~~ 

FEB MAR APR MAY JUN JUL AUG SEP 

Figure 16.12. Record of height changes on northern shore of Lake Taupo, determined by lake 

leveling relative to Rainbow Point from February through September 1983 (see figure 16.3 for 

locations). A clear example of preseismic, coseismic, and postseismic deformation is 

associated with a major earthquake swarm located between Kinloch and Rangatira stations, 

which started on June 16, 1983. 

172 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
DISCUSSION 

There are advantages and disadvantages to both the 

permanent and portable types of lake-level monitoring 

systems. A permanent system has the obvious advantage 

of providing continuous data from all sites around the 

lake. Given continuous data, one can much more easily 

determine the periodicity and amplitude of the seiche and 

accordingly filter the seiche from the record. One also 

can more easily identify meteorological effects. Once 

established, permanent systems require field crews only 

for maintenance of the station, not for data collection 

unless a logger, rather than telemetry, is used and can in 

theory track rapid displacement events. 

The use of portable gaging systems enables 

different lakes to be monitored with the same equipment. 

The goal of the portable lake-level program at CVO is to 

monitor crustal deformation using lakes near several 

different volcanoes. Except for the bench mark and any 

reference marks at each site, nothing needs to be 

permanently established around the lake, a desirable 

condition in wilderness areas or national parks. The short 

dynamic range of the displacement transducer used in the 

CVO system prohibits long-term installation, but the 

system can be easily converted to telemetered data 

collection; such conversion could be useful during a 

period of volcanic unrest. 

Another advantage of a portable lake-level system 

is the ability to check and correct for instrument drift. 

The pressure transducers used in permanent installations 

are expensive, because they are designed to have a low 

instrument drift, but any possible instrument drift could 

be interpreted erroneously as ground tilt unless the 

transducer were removed from its permanent installation 

and recalibrated. The instrument used in a portable 

system, whether a pressure transducer or a 

linear-displacement transducer, can be checked and 

recalibrated in the office before and after each use in the 

field. 

Permanent systems generate a larger data set than 

do portable systems, but portable systems have the 

advantage of being used only in known and suitable 

weather, assuming that observers don't push their luck in 

marginal weather. Consequently, data from a portable 

system should not be contaminated by strong wind 

effects or other meteorological noise as are data from a 

permanent system. On the other hand, the required good 

weather limits the days on which a portable system can 

be used. In deep lakes, such as Lake Taupo, meaningful 

data can still be gathered even in moderatly windy 

conditions. In shallow lakes, however, such winds can 

cause the lake surface to tilt. At Lake Myvatn in 

Iceland, the most reliable lake-level observations are 

made only if calm conditions prevail for 24 hours before 

the measurement (I'ryggvason, 1987). Errors associated 

with measurements taken in adverse conditions can be 

unusually large. This is a problem for which there is 

only one simple yet exasperating solution: wait for a 

calm day. Such delays could add significantly to the 

time needed to complete a lake-level survey using the 

portable monitoring technique. 

The reliability of data from permanent systems 

may be improved with the addition of meteorological 

observations to the data set. Weather observations (wind 

velocity in particular) could be arranged with volunteers 

in populated areas, or a weather station could be installed 

near a more remote lake. Transducer data from times of 

unfavorable conditions could then be filtered out. 

The precisions of the two systems are comparable 

for large lakes. Both systems incur errors from climatic 

effects, variations in instrument precision, and 

bench-mark stability. Portable systems also have errors 

owing to instrument setup, seich effects and inaccurate 

leveling or taping, but these errors can be kept well 

within tolerance with proper care and adherence to 

standards. The imprecision associated with the smaller 

data set from portable systems decreases as experience 

increases. Permanent systems have a precision of 0.1 to 

1.0 J.Lrad, depending on the specific application (lake size 

and shape) and the precision of the transducer. Portable 

systems rival the permanent systems in terms of 

precision on larger lakes (greater than 100 km2 in area), 

although the tilt precision decreases for smaller lakes. If 

measurements are made in favorable environmental 

conditions, the resolution of a portable system is about 

1-3 mm. This corresponds to a tilt resolution of 1-3 J.Lrad 

across a 1-km lake, or several tenths of a microradian 

across a 10-km lake. Precision is restricted by leveling 

and stability errors as well as those involved in the 

determination of water level, but these have the greatest 

effect on portable systems. Even on days when the 

environmental conditions seem perfect for measuring 

lake level (no winds, stable weather system), the lake 

surface may not be completely level owing, for example, 

to recent adverse wind conditions or unusually strong 

currents. As experience increases, reliability of the data 

will improve. 

ACKNOWLEDGMENTS 

We thank all those who have assisted in the Taupo 

lake-level monitoring project, in particular numerous 

colleagues in the Department of Scientific and Industrial 

Research (DSIR) for technical and scientific expertise in 

development work and interpretation of the results. Bill 

Chadwick and Don Swanson from CVO maintained 

continuity of monitoring during critical periods relating 

16. Lake-Level Monitoring aa a Tool for Studies of Crustal Deformation 173 
to Taupo seismic activity in 1983 and 1984. Thanks also 

go to those who helped in developing the eva 

lake-level monitoring program, in particular Gene 

Iwatsubo and Tom Murray, as well as those who were 

forced awake and onto the lake at ridiculously early 

hours to make measurements. We are especially grateful 

for the advice, patience, and encouragement of Dan 

Dzurisin. 

REFERENCES CITED 

Ahrens, C.D., 1982, Meteorology today: St. Paul, Minnesota, 

West Publishing Co., 514 p. 

Christiansen, R.L., 1984, Yellowstone magmatic evolution: Its 

bearing on understanding large-volume explosive 

volcanism, in Explosive volcanism: inception, evolution, 

and hazards: Washington, D.C., National Academy Press, 

p. 84-95. 

Gilmour, A.B., and Butcher, C.N., 1987, Free oscillations in 

Lake Taupo - A triangular lake: Department of Scientific 

and Industrial Research, New Zealand Geological Survey, 

DMFS report 1, p. 11-14. 

Gilmour, A.B., and Heath, R.A., 1989, Barotropic and 

baroclinic waves in Lake Taupo, New Zealand: Journal of 

Marine and Freshwater Research, v. 21, p. 353-357. 

Hull, A.G., and Grindley, G.W., 1984, Active faulting near 

Taupo: Eos (American Geophysical Union Transactions), 

v. 65, no. 7, p. 51-52. 

Mueller, R.J., Johnston, MJ.S., Myren, G.D., and Murray, T., 

1989, Lake level observations to detect crustal tilt: San 

Andreas Lake, California, 1979-1989: Geophysical 

Research Letters, v. 16, p. 661-664. 

Otway, P.M., 1986a, Vertical deformation associated with the 

Taupo earthquake swarm, June 1983: Royal Society of 

New Zealand Bulletin, v. 24, p. 187-200. 

__1986b, Monitoring of vertical deformation in Taupo 

Volcanic Centre; Volcanic hazards assessment in New 

Zealand: New Zealand Geological Survey Record, v. 10, p. 

41-47. 

__1.987, Taupo Volcanic Centre: Deformation survey: New 

Zealand Volcanological Record, v.15, p. 68-70. 

__1.989, Vertical deformation monitoring by periodic water 

level observations, Lake Taupo, New Zealand, izJ. Latter, 

J.H., ed., Volcanic hazards, IAVCEI Proceedings in 

Volcanology 1: Heidelberg, Springer-Verlag, p. 561-574. 

Scott, B.J., 1989, Geodetic and geophysical monitoring of the 

1886 Tarawera rift, in Latter, J.H., ed., Volcanic hazards, 

IAVCEI proceedings in Volcanology 1: Heidelberg, 

Springer-Verlag, p. 575-584. 

Tryggvason, E., 1987, Myvatn lake level observations 

1984-1986 and ground deformation during a Krafla 

eruption: Journal of Volcanology and Geothermal 

Research, v. 31, p.131-138. 

174 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
17. Techniques for Continuous Monitoring of Surface 

Water at Active Volcanoes: Examples From Loowit 

Drainage, Mount St. Helens, and Kelut Crater Lake, 

Indonesia 

By Kenneth A. McGee, Andrew J. Sutton, David E. Wieprecht, and Mark E. Ivan 

ABSTRACT 

Surface water associated with active or potentially 

active volcanoes can exhibit physical and chemical changes 

related to renewed volcanic activity. Measuring variations in 

parameters such as temperature, pH, conductivity, and water 

level on a regular basis can provide Insight into the 

processes expected to accompany volcanic unrest. 

Continuous telemetered monitoring Installations at Loowit 

Creek, Mount St. Helens, and Kelut crater lake, Indonesia, 

are examples of the techniques and equipment that can be 

used to monitor two different kinds of surface water at 

volcanoes. 

INTRODUCTION 

Springs, lakes, and streams associated with active 

volcanoes can exhibit both physical and chemical 

changes prior to renewed volcanic activity (Bercy and 

others, 1983, Giggenbach, 1983). Monitoring specific 

water parameters such as temperature, pH, conductivity, 

turbidity, concentration of dissolved gases, and water 

level can provide insight into the processes expected to 

accompany unrest or renewed volcanic activity, 

especially when these parameters are correlated with 

coincident seismic and other geophysical observations. 

Lakes within volcanic craters may hide many of 

the physical and thermal features of interest. Fumaroles 

are likely to be actively discharging into a crater lake. 

During the days or weeks of unrest that might precede an 

eruptive episode, fumarolic discharge and heat transfer 

into the lake might increase and raise the temperature of 

the water. In addition, the pH would likely fall owing to 

an increase in discharge of acid gases into the lake, and 

the conductivity of the lake water would increase owing 

to the presence of more dissolved ions. The water level 

might change because of deformation of the lake bottom. 

Thermal springs originating from a volcanic edifice 

can also show changes that reflect volcanic unrest. The 

extent of these changes depends upon the extent to which 

the hydrothermal system communicates with the 

subsurface springs. As with crater lakes, changes in the 

water chemistry, pH, temperature, conductivity, and 

discharge of springs can be expected to accompany the 

intrusion of magma into the shallow crust. A 

comprehensive monitoring program for volcanoes that 

have springs or crater lakes should include geochemical 

and hydrologic monitoring of these parameters. 

DESCRIPTION OF EQUIPMENT 

In summer 1989, a water-monitoring station was 

established on Loowit Creek approximately 2 km north 

of the active lava dome of Mount St. Helens. Loowit 

Creek is a small stream with a late summer discharge of 

approximately 7 ft3 /s containing surface-water runoff 

from the crater of Mount St. Helens as well as thermal 

water from at least three groups of hot springs. Steep 

topography and unstable ground prevent the hot springs 

from being monitored directly, so the monitoring site 

was located several tens of meters downstream from the 

most downstream hot spring. The monitoring station was 

established to record changes in water chemistry that 

might correlate with changes or unrest in the volcanic 

system. 

Robust sensors for temperature, pH, and 

conductivity are mounted in a perforated polyvinyl 

chloride (PVC) housing 10 em in diameter and 

suspended in the stream by cable (fig. 17.1). Signal 

output from each of the sensors is transmitted to a 

water-quality monitoring instrument called the "USGS 

Minimonitor." This device, developed by the U.S. 

Geological Survey's Hydrologic Instrumentation Facility, 

17. Techniques for Continuous Monitoring of Surface Water at Active Volcanoes 175 
is designed to measure and record various water-quality 

properties (Ficken and Scott, 1988). The battery-operated 

(12 volts) minimonitor consists of a timer, signal 

conditioners for each parameter, and output circuitry. 

The signal conditioners allow measurement of 

water temperatures in the range 0 to 50 •c (or 0 to 100 

•c with a resistor change) and pH from 0 to 10 (or 2-12 

pH units by changing a switch setting). Conductivity can 

be measured in any of four switch-selectable ranges from 

0-100 to 0-100,000 micromhos. Sensor readings are 

each converted to output values in the range 0-5 volts. 

The output from the minimonitor is connected to a 

low-data-rate telemetry system (Murray, 1988). Every 10 

minutes, the telemetry system triggers the minimonitor to 

scan its sensor inputs and provide readings to the 

telemetry system. These data are then transmitted via 

VHF radio to the Cascades Volcano Observatory for 

analysis. Although the minimonitor was readily available 

to the authors for this application, any device capable of 

reading the outputs of water-quality sensors and 

converting these values to voltages usable by a telemetry 

system can be used. Typical data from the monitoring 

station on I..oowit Creek are shown in figure 17 .2. 

LAKE MONITORING 

Kelut volcano in east Java, Indonesia, has a history 

of ejecting water from its crater lake during eruptions 

and forming damaging lahars (Suryo and Clarke, 1985). 

Because of such behavior and the many fumaroles 

currently discharging into the lake, Kelut was chosen as 

a test site to develop the techniques and technology for 

monitoring crater lakes. 

Flgure17.1. Schematic drawing of water-monitoring 

station at Loowit Creek: 1, temperature, pH, and 

conductivity sensor assembly in PVC housing; 2, 

suspension cable; 3, electrical lead cables; 4, 

minimonitor; 5, radio-telemetry box; 6, battery; 7, 

antenna; 8, optional solar panel. 

In July 1986, a telemetered water-monitoring 

station was established at Kelut (Sutton and others, 

1987). Sensors for temperature, pH, and conductivity are 

suspended from a floating platform in the lake to a depth 

of 2.5 m (fig. 17 .3). The floating platform is located 

about 4 m from the edge of the lake and is stabilized 

with guy wires to prevent movement around the lake. 

The lake depth below the platform is 5-6 m. Lake level 

is monitored by a technique described in detail in the 

following section. Uke the installation at I..oowit Creek, 

signals from the sensors at Kelut are transmitted to a 

minimonitor for signal conditioning. At 10-minute 

intervals, these data are transmitted by radio telemetry 

through a signal repeater to Margomulyo Observatory, 6 

km to the west and operated by the Volcanological 

Survey of Indonesia. A small computer records the 

transmitted data and computes both hourly and daily 

averages for all of the monitored parameters. These 

averages are then displayed on a printer and stored on 

cassette tape. Solar panels provide power for the 

monitoring site, repeater, and receiving station. 

Representative data from Kelut are shown in figure 17 .4. 

A SIMPLE TECHNIQUE FOR MEASURING 

WATER LEVEL 

It is sometimes necessary to monitor water level in 

a crater lake or a stream. The depth of water can change 

in response to volcanic activity as well as other factors 

such as rainfall. The bottom of the crater lake at Kelut, 

for example, has undergone remarkable changes during 

several eruptive episodes (Hadikusumo, 1974). Although 

deformation of a crater floor covered by water is difficult 

C/) 

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START DAY IS OCT 3, 1989 G.M.T.- JULIAN DAY 276 

Flgure17.2. Representative data from monitoring 

station at Loowit Creek. 

176 Monitoring Volcanoes: Techniques and Strategle• Used by the Staff of the Cascade• Volcano Observatory 
to detect directly, lake-level monitoring can provide 

some insight into this problem. Care should be exercised 

in interpreting lake-level data, however, because 

evaporation will occur in crater lakes heated above 

ambient temperatures as in the case of the crater lake at 

La Soufri~re volcano on St. Vincent in 1971-72 

(Aspinall and others, 1973). 

We describe here a simple technique that can be 

used for monitoring water level of a crater lake or other 

bodies of water. It resembles the bubble-gage sensor 

described by Rantz (1982a) except that the mercury 

manometer is replaced by a pressure transducer. The 

apparatus consists of a cylinder of compressed gas (02, 

N2, or air) with a standard regulator, a constant-flow 

regulator, a needle valve, a 0-200 kPa (0-30 psi) 

pressure transducer with 0-5 volt output, and a length of 

teflon tubing anchored at a fixed depth in the water (fig. 

17 .5). Gas is bubbled at a constant low flow rate through 

the tubing into the water. Changes in water level are 

reflected by changes in pressure measured by the 

pressure transducer. For example, as the water level 

rises, backpressure increases and pressure read by the 

transducer increases. The system may be calibrated 

before installation by inserting the end of the teflon tube 

to different depths in a water-filled pipe and reading the 

pressure-transducer output. The system can be 

normalized to any convenient zero point after installation 

in the lake. The lake-level system at Kelut can resolve 

changes in lake level to about 1 em and has an absolute 

range of over 6 m. A typical pressure transducer 

calibration curve is shown in figure 17 .6. Other pressure 

transducers with different response characteristics can be 

used to adapt the system to other depth ranges and 

resolution values. In areas with large diurnal or seasonal 

temperature changes, special care should be taken to 

Flgure17.3. Schematic drawing of water-monitoring 

station at Kelut volcano: 1, temperature, pH, and 

conductivity sensor assembly In PVC housing; 2, floating 

platform; 3, electrical sensor leads; 4, minimonitor; 5, 

radio-telemetry box; 6, battery; 7, antenna; 8, solar panel. 

Water-level apparatus is shown in figure 17.5. 

select a pressure transducer with low temperaturecoefficient characteristics. If a solar panel powers the 

monitoring station, a voltage regulator can be used to 

minimize cycling of the pressure-transducer output as the 

charge-discharge cycles of the solar-panel power supply 

cause swings in power supply voltage. 

For long-term telemetered monitoring installations, 

a tank-pressure transducer with 0-5 volts output can be 

installed in parall~l with the tank pressure gage. Data on 

the amount of gas remaining in the cylinder can then be 

transmitted through the telemetry system to reduce the 

number of required visits to the field site (fig. 17.4). 

Besides the technique described here, many other 

techniques for measuring water level can be used. These 

include suspending a pressure transducer directly in the 

water from a fixed point (Kleinman and Otway, chapter 

16), mounting an ultrasonic ranging device above the 

surface of the water, constructing a stilling well with 

float, or installing a load sensor in a stream bed (Rantz, 

1982a). We believe, however, that the technique 

presented here is one of the simplest and easiest to 

implement. 

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w 

0 

t "I. I 1.11 I I 'I II I 'I' I iii II II ill ii.l 11 Ill ill lll.il ~.II. I l.i II I Ill 

~~G~;~~~~ .. : .... , ..... , .. , ......... , ... '····'··· .. '" 1 

a r:l'."l' "I." I I "I" 1""1 "'l"':l' •• 'l·'··'·l'l ~kp~ ~' ' ' ' ' ' ' ' ' "' J 

t. I:' :I"" I".' I. "I '".'I." I'" I "I'" I'' •• ~."'" I 'l I l:~~t~·~~~~.. . ... . . . ... . . . c .... J 

i i~[,;~.,····~········· ······,····~····,···········,] !I !:Ef~~~~;p.;.~:::::;;;;:::::::::,::~ JUL5 10 15 20 25 AUG 5 10 15 20 25 SEP 

START DAY IS JUL 1, 1986 G.M.T.- JULIAN DAY 182 

Flgure17.4. Representative data from crater lake at 

Kelut volcano. Lake-level data are corrected for swings in 

battery voltage owing to charging and discharging of 

solar panel. 

17. Technique• for Contlnuoua Monitoring of Surface Water at Active Volcanoea 177 
DISCUSSION 

Besides volcanic activity, other factors likely to 

have an effect on streams draining hot-spring areas or 

crater lakes are rainfall and melting of snow packs. For 

crater lakes, significant rainfall will tend to cool the lake, 

drive the pH toward neutral, and raise the lake level. The 

magnitude of the effect of rainfall on the monitored 

parameters depends upon the amount of rain and the size 

of the drainage system discharging into the lake as well 

as to the ratio of the volume of rain to the volume of the 

lake. The effect of rainfall on conductivity is uncertain. 

The addition of pure rainwater to the lake would lower 

the conductivity; however, an increase in water flow over 

soluble materials in the drainage system might increase 

the amount of dissolved ions present in the lake and 

might encourage mixing, thus disturbing preexisting 

thermal and chemical density gradients. 

The effect of rainfall or melted snow on streams 

draining hot-spring areas is similar to that for lakes. 

Depending on local subsurface conditions, the direct 

effect of rainfall on hot springs will likely be less 

immediate and of smaller magnitude. 

In any case, rainfall should be measured and 

recorded regularly when monitoring a crater lake or other 

surface water associated with a volcano in order to better 

interpret the water-property data. Rainfall can be 

recorded continuously with a tipping-bucket rain gage 

connected to the telemetry system, or can be measured 

once a day with a standard rain gage. In many areas, 

0-SV 

to telemetry 

\\ 

,, 8 

,, 5 

2 

rainfall data are also available from other sources such as 

meteorological agencies, local radio and television 

stations, and forestry agencies. 

Measuring stream discharge is another tool to 

provide information for interpreting the impact of rainfall 

upon water-monitoring data. Discharge measurements are 

a direct indication of the amount of water flowing past a 

particular point and are useful in assessing the effects of 

dilution on other parameters being monitored. Discharge 

measurements are particularly important for streams 

draining craters or hot-spring areas, as well as for 

streams flowing into and out of crater lakes. The 

techniques for measuring discharge are well known and 

are described in detail in Rantz (1982a, 1982b). 

Conditions that can damage equipment or disrupt 

data must also be considered when establishing a 

water-monitoring station. Volcanic tephra and easily 

eroded deposits contribute to high suspended-sediment 

loads in streams, which can damage or destroy sensors. 

Thermal water commonly contains algae that can grow 

on sensors and inhibit response. Chemical precipitates 

can also form on sensors under certain conditions. 

Telemetry and electronic equipment should be located on 

high ground away from areas subject to damage from 

lahars or flooding. Lightning can be a problem at remote 

monitoring sites. Sensitive equipment should be 

thoroughly grounded and shielded to minimize damage 

from lightning (see Lockhart and others, chapter 3). 

Many other observations and measurements can be 

made in conjunction with, or in place of, continuous 

6 

Conoflow 

regulator 

3 

~-----~0- 30 psig....,_ __ __..,. __ _ 

0-5 V out 

1 4 

0-SV 

to telemetry 

Flgure17.5. Schematic drawing of water-level apparatus: 1, cylinder of compressed gas; 2, tank regulator; 3, 

constant-flow ~~gul.ator; 4, needle valve; 5, feedback loop; 6, pressure transducer; 7, teflon or other Inert tubing 

anchored to piling 1n water; 8, optional tank-pressure transducer. Tubing from cylinder to pressure transducer (6) is 

6.35-mm-diameter (1/4 inch) copper tubing. 

178 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
telemetered monitoring stations. Measuring temperatures 

manually as often as possible, sampling for water 

chemistry, using portable instruments to measure pH and 

conductivity, studying the deposits around volcanic bot 

springs, and taking repeated photographs over time from 

established photo points are just some of the things that 

can be done to help provide a picture of current activity 

or inactivity. The important point is to be observant and 

document all changes. 

CONCLUSIONS 

Monitoring volcanic bot springs and crater lakes 

for changes that might accompany unrest or renewed 

volcanic activity is an important and challenging task. 

Relatively simple monitoring systems for pH, 

temperature, conductivity, and lake level can be 

established using off-the-shelf equipment. Crater lakes 

and volcanic hot springs commonly occur at high 

elevations and in difficult terrain, where the 

establishment of continuous-monitoring stations is 

difficult. Short of continuous monitoring, many periodic 

measurements or observations can be made to assess 

activity. The importance of integrating geochemical and 

4 

(/) 

~3 0 

> 

2 

2 3 4 5 6 

WATER DEPTH, IN METERS 

Figure 17 .6. Plot of pressure-transducer output versus 

water depth for lake-level sensor. 

hydrologic data from crater lakes and hot springs with 

seismic and other geologic and geophysical data in order 

to construct a complete model of volcanic activity cannot 

be emphasized enough. 

REFERENCES CITED 

Aspinall, W.P., Sigurdsson, Haraldur, and Shepherd, J.B., 

1973,Eruption of Soufri~re volcano on St. Vincent Island, 

1971-1972: Science, v. 181, p. 117-124. 

Bercy, Claude, Sabroux, J.C., and Suparwoto, Gatot, 1983, 

Underwater noise survey in the crater lake of Kelut 

volcano (Indonesia), in Tazieff, Haroun, and Sabroux, J.C., 

eds., Forecasting volcanic events: New York, Elsevier, p. 

529-543. 

Ficken, J.H., and Scott, C.T., 1988, Operating manual for the 

U.S. Geological Survey minimonitor, 1988 revised edition; 

punched-paper-tape model: U.S. Geological Survey 

Open-File Report 88-0491, 76 p. 

Giggenbach, W.F., 1983, Chemical surveillance of active 

volcanoes in New Zealand, in Tazieff, Haroun, and 

Sabroux, J.C., eds., Forecasting volcanic events: New 

York, Elsevier, p. 311-322. 

Hadikusumo, D., 1974, The rise and drop of Mt. Kelut crater 

bottom after paroxysmal eruptions, in Green, R., ed., 

Recent crustal movements and associated seismic and 

volcanic activity: Tectonophysics, v. 23, p. 341-347. 

Murray, T.L., 1988, A system for telemetering low-frequency 

data from active volcanoes: U.S. Geological Survey 

Open-File Report 88-201, 41 p. 

Rantz, S.E., 1982a, Measurement and computation of 

streamflow: Volume 1; Measurement of stage and 

discharge: U.S. Geological Survey Water-Supply Paper 

2175, p. 1-284. 

_1982b, Measurement and computation of streamflow: 

Volume 2; Computation of discharge: U.S. Geological 

Survey Water-Supply Paper 2175, p. 285-631. 

Suryo, 1., and Clarke, M.C.G., 1985, The occurrence and 

mitigation of volcanic hazards in Indonesia as exemplified 

at the Mount Merapi, Mount Kelut and Mount Galunggung 

volcanoes: Quarterly Journal of Engineering Geology 

London,v. 18,p. 79-98. 

Sutton, A.J., McGee, K.A., Sutawidjaja, I.S., Pardyanto, L., and 

Casadevall, T.J., 1987, A monitoring system for active 

volcanoes with crater lakes: Hawaii Symposium on How 

Volcanoes Work, Hilo, 1987, abstracts, p. 245. 

17. Technique• for Contlnuoua Monitoring of Surface Water at Active Volcano•• 179 

18. Fundamental Volcanic-Gas-Study Techniques: An 

Integrated Approach to Monitoring 

By A. Jefferson Sutton, Kenneth A. McGee, Thomas J. Casadevall, and J. Barry Stokes 

ABSTRACT 

Fundamental gas-study methods for volcano 

monitoring Include field sampling with laboratory analysis, 

measurements of volcanic-plume emission rates, and 

continuous on-site monitoring. Field sampling with laboratory 

analysis provides detailed chemical information about a 

specific location for a specific time. Gas-emission 

measurements reveal an average output commonly for S02 

and C02. Continuous monitoring provides an understanding 

of temporal changes in gas concentrations. Each method 

has advantages and disadvantages, and a strong 

gas-studies effort for monitoring purposes is synthesized 

using all three approaches, along with conventional 

geophysical-monitoring methods. 

INTRODUCTION 

Gases dissolved in magmas are an important 

driving force of eruptive activity, even though they 

.generally make up less than 5 weight percent of the melt 

for more silicic magmas, and 0.5 weight percent for 

basaltic magmas (Greenland, 1987). The solubility of a 

specific volcanic gas in a magma is a complex function 

of pressure, temperature, and bulk composition of the 

magma; moreover, different gases in the same magma 

have different solubilities and reactivities. As magma 

ascends, changes in subsurface conditions will likely be 

reflected as changes in emission rate or composition of 

gases issuing from surface vents and fumaroles, provided 

that the rock between the magma and the surface is 

permeable. Under these conditions, the ascent of magma 

may be detected by studying gases escaping from 

fumaroles, active vents, porous ground surfaces, and the 

volcanic-gas plume. Recently, Tilling (1989) cited gas 

geochemical studies as a promising, though still 

experimental, approach to volcano monitoring and 

eruption forecasting. Volcanic gas studies in the United 

States go at least as far back as the early 1900's in 

Hawaii (Jaggar, 1940). More recent volcano monitoring 

reviews that feature gas-studies techniques include a 

UNFSCO volume (1972) and a special issue of Bulletin 

Volcanologique on volcanic gases (volume 45, no. 3, 

1982). Gas geochemical studies require careful 

interpretation and evaluation of collected data 

(Casadevall and others, 1987; Giggenbach, 1989). In this 

paper we suggest that current volcanic gas study 

techniques for monitoring purposes are logically divided 

into three classifications. We emphasize the importance 

of each classification and how the synthesis of a 

comprehensive gas monitoring effort involves using 

these methods together. 

Volcanic gas studies for monitoring purposes are 

conducted in several ways (fig. 18.1). Field sampling of 

fumaroles with laboratory analysis provides detailed 

information about vent-gas composition. Emission-rate 

studies of gases such as S02 and C02 can yield 

estimates of supply rate and other attributes of the 

magma system. Continuous on-site monitoring with gas 

sensors gives valuable information about temporal 

changes in gas composition in and around fumaroles, 

fissures, and porous volcanic soils. We discuss each of 

these gas-study techniques, and how they tie in with 

other volcano-monitoring methods. 

Gas Studies Effort 

I 

I 

Sampling/Lab Emission Rates Continuous 

Analysis Monitoring 

I 

I Evacoated Bottle I ><~me I 

I 

Fumarole 

I Flow-through Air 

bottle ~round-based Soil 

Flgure18.1. Fundamental gas studies for volcanomonitoring purposes should consist of integrated field 

and laboratory Investigations Involving periodic and 

continuous sampling and emission-rate measurements. 

18. Fundemental Volcanic-Gas-Study Techniques: An Integrated Approach to MonHorlng 181 
GAS STUDIES TECHNIQUES 

Field Sampling with Laboratory Analysis 

Two primary methods are used to collect and 

analyze fumarolic gases: evacuated bottles and 

flow-through bottles. The evacuated-bottle method was 

standardized by Giggenbach (1975; Giggenbach and 

Goguel, 1988). This method has the advantage of 

providing a detailed gas analysis using a minimum 

amount of equipment. It has been used to study many 

types of volcanoes and geothermal systems. In 

practice, a titanium, alumina, mullite, or silica 

sampling tube (chemically inert and physically 

durable) is inserted into the fumarole and allowed to 

heat until condensation in the tube reaches 

equilibrium, usually 5 minutes or less (fig. 18.2). 

Equilibrium is indicated by visible gas-flow from the 

exit of the sampling tube. Fumarolic gas is passed 

through a short length of teflon tubing to the sampling 

flask, which consists of a borosilicate glass bottle with 

a high-vacuum stopcock. The bottle is partially filled 

with concentrated ( 4N) aqueous sodium hydroxide and 

has been evacuated and weighed. The stopcock is 

opened, and gas bubbles into the bottle through the 

alkaline solution. Water, C02, H2S, S02, HCl, and HF 

dissolve in the aqueous fraction, while N2, 02, H2, 

CO, He, and Ne bubble through the solution and 

collect in the headspace. Many liters of fumarolic gas 

can be collected in a single bottle, because volcanic 

gas is typically composed mostly of water and 

Headspace gas containing 

collected H2, He, CH4, 02. 

CO, N2, Ar, Ne 

1---------r= .._---r--T- Septum port 

Alumina, mullite, quartz, 

or titanium sampling tube 

inserted into fumarole 

Solution containing 

4N NaOH and collected 

H20, H2S, S02, HCI, 

HF, and C02 

Flgure18.2. Evacuated-bottle sampling scheme, 

showing sampling tube and sample bottle. 

condensable acid gases. This method concentrates gases 

in the solution and headspace, and thereby promotes 

better analytical precision. The headspace "permanent" 

gases are analyzed by gas chromatography on a 

molecular-sieve column with thermal-conductivity 

detection and argon carrier gas. Dissolved gases are 

analyzed by wet chemical and gravimetric techniques for 

C02, S02, H2S, 1120, and HCl and ion-selective 

electrode methodology for HF. Other evacuated-bottle 

sampling and analysis techniques are described by 

Piccardi and Cellini-Legittimo (1983) and Greenland 

(1986). 

Flow-through-bottle collections are made more 

rapidly than the evacuated-bottle collections. They are 

used where a complete gas analysis is unnecessary, or 

where field conditions are too hazardous to safely make 

an evacuated-bottle collection. The sampling-tube 

arrangement is similar to that for the evacuated-bottle 

method, but the bottle has two stopcocks (fig. 18.3). A 

small hand-operated pump is used to flush and fill the 

sample bottle with gas. Samples are analyzed by gas 

chromatography for the gases listed in the preceding 

paragraph except water, HCl, and HF. Sulfur gases 

present in flow-through samples are stable for only about 

6 hours, so analyses for these gases must be completed 

promptly after collection. Other gases present are stable 

for months. Hydrogen and helium are exceptions to this, 

owing to their small molecular size and high diffusivity. 

Permanent gases are analyzed as described above. Acid 

gases are also analyzed on a second chromatograph, 

using a helium carrier and a porous polymer such as 

Poropak-Q or Chromosorb 107 with a silica-gel 

precolumn to eliminate water-vapor interference during 

chromatography. This method, discussed by Greenland 

(1984), is used in Hawaii to measure carbon/sulfur ratios 

of Kilauea's summit-fumarole gases. These ratios have 

shown sympathetic behavior with inferred magmatic 

intrusions and changes in magma supply rates 

(Greenland and others, 1985). 

I _...., / .-.. 

Septum port 

/ 

/' 

Flow-through sample 

bottle with dual stopcocks 

<E-----_ Alumina, mullite, quartz or 

titanium sampling tube 

inserted into fumarole 

Hand-operated 

sampling pump 

Flgure18.3. Flow-through sampling scheme, showing 

sampling tube, sample bottle and pump. 

182 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Emission-Rate Studies 

Emission rates of gases are studied to estimate the 

quantity of S02 and C(h being emitted from a volcano, 

and are carried out by making measurements in the 

volcanic plume. Sulfur dioxide emission rates have been 

used to infer the volume of degassing magma 

(Casadevall and others, 1983) and magma supply rates 

(Casadevall and others, 1987). Sulfur dioxide and carbon 

dioxide measurements are both reported as fluxes with 

units of metric tonnes per day, although the two gases 

are measured in different ways. 

Sulfur dioxide is measured remotely using a 

correlation spectrometer (COSPEC) (Barringer Research 

Ltd., Canada). Scattered solar ultraviolet energy of 

specific wavelengths is absorbed in proportion to S02 

concentration and plume thickness. This absorption, with 

units of pathlength-concentration, is measured by the 

COSPEC and calibrated with the absorption of an S02 

gas standard in the instrument. The product of the plume 

light-absorption profile and wind speed yields the S02 

emission rate. The technique is described by Casadevall 

and others (1981; 1983; 1987), Stoiber and others 

(1983), and Millan and others (1976). This method is 

used routinely and worldwide to make volcanic S02 

measurements. Measurements can be made from the 

A Plume ~ II 

II 

II 

Volcano 

/ \ \ 

B 

Plume 

c 

TRAVERSE 

Figure 18.4. Ground-based (vehicle-mounted) COSPEC 

measurement of sulfur dioxide. A, Side view. 8, Front 

view. C, Typical data 

ground or the air. Wind speed is determined by using a 

hand-held anemometer for ground-based measurements. 

Airborne S(h and C(h measurements use wind speeds 

measured by a variety of methods including those 

reported by local airports. Alternatively, wind speed is 

determined by comparing true air speed, flying with and 

against the wind, with true ground speed. Accurate 

determination of wind speed is critical for reliable 

emission-rate determinations (Casadevall and others, 

1987). Airborne measurements of S(h tend to be more 

reliable than ground-based measurements, because wind 

speed is determined at the site of measurement. 

Ground-based COSPEC measurements can be 

made from a vehicle by traversing beneath the plume 

while pointing the instrument up through it (fig. 18.4). 

Alternatively, the COSPEC can be mounted on a tripod 

near the vent (Chartier and others, 1988) to scan either 

vertically or horizontally through the plume (fig. 18.5). 

Airborne S02 measurements are made by flying 

below and at right angles to the plume trajectory (fig. 

18.6). Multiple traverses for either ground-based or 

airborne measurements are averaged to calculate S02 

emission rates, which are then scaled up to a daily rate. 

Carbon dioxide emission measurements are also 

made spectroscopically, but require measurements within 

A 

/ 

B 

c 

(/L -

/ \ \ 

Volcano 

\ 

Plume 

TRAVERSE 

' ' ' ' 

Plume 

' ' ' ' 

I I 

II 

II 

II 

',',,·~ 

~ 

Flgure18.5. Ground-based (tripod-mounted) COSPEC 

measurement of sulfur dioxide. Dotted lines represent 

field of view for fixed angle of COSPEC. A, Side view. 8, 

Front view. C, Typical data. 

18. Fundemental Volcanlc-Gaa-Study Technlquea: An Integrated Approach to Monitoring 183 
the plume. This is done by flying repeatedly through the 

plume at right angles to its trajectory (fig. 18. 7). Infrared 

(IR) absorption measurements are made by pumping 

plume gas through a spectroscopically-tuned Miran IR 

spectrophotometer (Foxboro Company, Foxboro, 

Massachusetts) mounted in a fixed-wing aircraft. This 

method generates a concentration profile for the plume 

and is used along with wind speed to calculate C02 

emission rates. The method, discussed by Harris and 

others (1981), bas been used to make measurements at 

Mount St. Helens and in Hawaii (Casadevall and others, 

1983, Greenland and others, 1985). 

Continuous Gas Monitoring 

Some gas-emission events have durations as short 

as a few minutes, and thus may not be detected by 

collection and analysis techniques involving periodic 

sampling or emission-rate measurements. Continuous 

measurements, however, can track fleeting concentration 

changes. Continuous gas monitoring involves using one 

or more sensors to measure gas concentration in situ. The 

technique was originally developed and tried in the 

mid-1970's by Sato and his colleagues (Malone and 

Frank, 1975; Sato and others, 1976) to monitor the 

activity of Mount Baker, Washington (Frank and others, 

A (i_L - Plume 

/ lj = \ \ ~ 

/ Volcano 

\ \ 

B 

Plume 

I ,, 

<1-1 

I Volcano 

I \~ 

c 

TRAVERSE 

Figure 18.6. Airborne COSPEC measurement of sulfur 

dioxide. A, Side view. 8, front view. C, Typical data. 

1977). Data are typically taken from the sensor(s) every 

10 minutes and telemetered by radio, satellite, or 

telephone to a receiving site where they are available in 

near-real time (McGee and others, 1987). Continuous in 

situ gas-monitoring studies by scientists of the U.S. 

Geological Survey have used a fuel-cell type sensor, 

developed by the USGS, that is sensitive to reducing 

gases including Fb, H2S, SOl, CO, COS, HCl, and HF. 

The sensor, which is resilient under rough and corrosive 

field conditions, was described by Sato and McGee 

(1981). More recently, detailed studies of this sensor 

were reported by Sutton and McGee (1989) and McGee 

and Sutton (1990). Species-selective, rugged, 

commercially available gas sensors at reasonable cost 

have been difficult to find until recently. Chemical gassensing techniques and potentially useful commercially 

available gas sensors were summarized by Sutton (1990). 

Commercial sensors are now available for H2, S02, C02, 

H2S, CO, COS, HCl, and HF. Nearly all of these sensors 

require some modification before they can be used in 

volcanic environments, and species selectivity is a 

frequent problem that requires careful deployment. For 

example, an SOl sensor may show cross-sensitivity to 

H2S. By deploying an H2S sensor that has no S02 

sensitivity along with the original S02 sensor, one can 

monitor both gases and construct an interference 

correction algorithm for the S02 sensor output. 

A 

B 

c 

TRAVERSE 

Figure 18.7. Airborne operation of Mlran Instrument for 

measurement of carbon dioxide. A, Side view. 8, Front 

view. C, Typical data. 

184 Monitoring Volcanoes: Techniques and Strateglea Uaed by the Staff of the Caacadea Volcano Observatory 
Continuous gas-monitoring measurements can be 

made in fumaroles, in the air near active fumaroles, and 

in soils near structural features (fig. 18.8). Continuous 

fumarolic measurements are made by placing gas sensors 

directly in the vent. A widespread gas event at Kilauea 

volcano, Hawaii, was detected before an East Rift 

eruption using gas sensors deployed in low-temperature 

fumaroles (McGee and others, 1987). This mode of 

deployment is useful when fumaroles are thought to be 

in good communication with the main conduit of the 

magma system. 

Air monitoring is carried out by mounting gas 

sensors 1 to 2 m high on a strategically located wooden 

post near an area of active fuming. This method of 

sensor deployment has advantages over fumarolic 

monitoring. It avoids the possible error of selecting a 

rootless fumarole and provides an integrated 

gas-emission signal for multiple vents within an area, 

although it is subject to anomalous sensor readings as the 

wind direction varies. In addition, air monitoring permits 

the us~ of a larger variety of sensors, as many sensors 

cannot withstand harsh fumarolic conditions. Air 

monitoring on the south flank of Mount St. Helens was 

described by Sato and McGee (1981 ), and air monitoring 

on the active Mount St. Helens lava dome is in progress 

(McGee and others, 1986). 

Continuous monitoring of soil gases involves 

burying a sensor at least one meter deep in an area of 

suspected faulting or in other gas-permeable zones. 

Favorable soil-gas monitoring sites may be located by 

surveillance methods. For instance, Allard and others 

(1989) used a portable infrared C02 detector to obtain 

information about geologic structure on the volcanic 

edifice of Mount Etna, Sicily. They determined that the 

integrated C02 emission from the soil gases was 

approximately equal to that of the summit craters at 

Instrumentation and 

telemetry housing 

Soil sensor 

~(@1 ,_, 

Air sensor 

Zone of inferred 

~ geologic structure 

~ (fi~ure \""~) 

Flgure18.8. Continuous gas monitoring with air, 

fumarole, and soil sensors. Telemetry can be 

conventional ground-based radio, satellite relay, or 

telephone. 

Table 18.1. Generalized Advantages and 

disadvantages for fundamental volcanic gas studies 

techniques 

Advantages 

Sampling/ 

Analysis 

Emission Rates Continuous 

Monitoring 

Very apecific. Remote technique. 

Much data about Ovenll output of 

aingle point in 802 and 002. 

apace and time. Magma aupply rate 

calculable. 

Excellent 

temporal 

control on 

outgauing. 

Continuoua 

dataaet. True 

monitoring. 

Disadvantages Temporal, apatial Two gaaea only. Small number 

bolea in dataaet. Requirea aite viait of durable and 

Labor intenaiv~ for each data point. aeloctive 

pouibly Labor intenaive. 1en110ra. 

hazardoua. Weather dependent. Requirca 

telemetry. 

Mount Etna. Thomas (1987) determined that radon 

concentration in shallow ground gases changed 

substantially with both volcanic and seismic activity at 

Kilauea volcano. Continuous soil-gas monitoring 

studies have also been carried out at Long Valley 

Caldera, California (McGee and others, 1982; McGee 

and Sutton, 1990). Soil-gas sampling and continuous 

monitoring of helium gas were described by Friedman 

and Reimer (1987). 

DISCUSSION 

The fundamental gas-studies techniques for 

monitoring volcanic activity have advantages and 

disadvantages, summarized in table 1. Fumarolic 

sam piing with laboratory analysis gives detailed 

chemical information, but only about a specific 

gas-vent location for a specific instant in time. 

Emission-rate studies reveal overall gas output for a 

volcano during a given measurement period but only 

for two gases, S02 and C02, using present-day 

techniques. Furthermore, high S02 emission values for 

certain volcanoes with renewed activity may indicate 

release of originally magmatic gases accumulated 

within an extensive but shallow hydrothermal system 

(Giggenbach, 1989). So, taken by themselves, high 

S02 values do not necessarily indicate a direct 

magmatic source. Continuous monitoring provides 

information about temporal changes in relative 

concentration of a few chemical species for fumaroles, 

air, and soils; however, currently used sensors are not 

very selective and hence rigorous interpretation is 

difficult. By using the three approaches together, a 

more complete picture of volcanic gas emissions can 

be obtained. Sampling with laboratory analysis 

provides fundamental information about the total 

com position and phase relations of the gas and checks 

the analytical accuracy of continuous monitoring 

studies. Emission-rate studies focus on overall 

18. Fundemental Volcanlc-Gaa-Study Technlquea: An Integrated Approach to Monitoring 185 
gas-emission rates, which are checked qualitatively by 

continuous monitoring, and sampling with laboratory 

analysis. 

Other direct-gas-study methods not descnbed here 

include portable gas chromatographs for on-site gas 

analysis (LeGuern, 1982), and potentially useful isotopic 

gas studies to determine if gas species are of meteoric or 

magmatic origin (Evans and others, 1981). Indirect 

gas-study methods include ash-leachate studies, which 

are useful where direct gas sampling is not possible 

(Williams and others, 1986; Hinkley, 1987). Monitoring 

chemical and physical parameters of crater lakes and 

streams may indicate renewed movement of magma into 

these systems (McGee and others, chapter 17). 

Results from gas studies can be used with other 

time-series data to help assess volcanic activity. Figure 

18.9 shows monitoring data from dome-building activity 

at Mount St. Helens in October 1986. A reducing-gas 

sensor, tiltmeter, ground-temperature sensor, and 

seismometer located in the crater recorded the 

dome-building episode in real time. Sulfur dioxide 

emission rates were measured intermittently and show a 

similar pattern. 

~CJ) 80 :>wen 

t;(~2 60 

a:t!l(J) ww...J 40 n.ow 20 ::2 (.) w~ 0 fCJ) 

~ ~ 6,000 Crater seismic amplitud~ 

i u H~~L.,., '' _, .. L. '"' ,,.A. I"' 

Figure 18.9. Comparison of telemetered gasgeochemical, tilt, ground temperature, RSAM, and 

Intermittent airborne sulfur dioxide data for time period 

Including October 22, 1986 dome-building eruption at 

Mount St. Helens. 

CONCLUSION 

Gas studies for volcano-monitoring purposes are 

conducted using three basic techniques: fumarole 

sampling with laboratory analysis for major and minor 

volatile species, emission-rate measurements of S02 and 

C02, and continuous on-site monitoring of gases using 

chemical sensors. Fumarole sampling and laboratory 

analysis techniques are well established and give the 

most complete information about specific vent 

compositions. Such techniques have also been used to 

infer magmatic gas compositions and volatile budgets 

(Gerlach and Casadevall, 1986; Gerlach and Graeber, 

1985). Emission-rate measurements for S02 and C02 are 

similarly well established and can record changes in 

magma supply rate and the general level of activity of a 

volcano. Continuous monitoring establishes a temporal 

continuum for selected gas species in fumaroles, air, and 

soils and is also the monitoring technique most likely to 

experience growth in the near future, especially as 

commercial sensors for a wider variety of gas species 

become more readily available. 

The gas-studies techniques described here are most 

valuable when used together, especially if in concert with 

time-series geophysical data. A successful volcanomonitoring effort should incorporate a balance of 

geophysical and geochemical methods to measure 

physicochemical changes in a magmatic system. 

REFERENCES CITED 

Allard, P., Baubron, J.C., Carbonelle, J., Dajlevic, D., 

LeBronec, J., Robe, M.C., and Zettwoog, P., 1989, Diffuse 

soil degassing from volcanoes: Geochemical and 

volcanological implications: IAVCEI Continental 

Magmatism General Assembly, Santa Fe, New Mexico, 

abstracts p. 3. 

Cssadevall, TJ., Johnston, D.A., Harris, D.M., Rose, W.I. Jr., 

Malinconico, L.L., Stoiber, R.E., Bornhorst, T.J., 

Williams, S.N., and Woodruff, Laurel, and Thompson, 

J.M., 1981, S02 emission rates at Mount St. Helens from 

March 29 through December, 1980, in Lipman, P.W., and 

Mullineaux, D.L., eds., The 1980 eruptions of Mount St. 

Helens, Washington: U.S. Geological Survey Professional 

Paper 1250, p. 193-200. 

Cssadevall, T.J., Rose, W.I., Jr., Gerlach, T.M., Greenland, 

L.P., Ewert, J., Wunderman, R., and Symonds, R., 1983, 

Gas emissions and the eruptions of Mount St. Helens 

through 1982: Science, v. 221, p. 1383-1385. 

Casadevall, TJ., Stokes, J.B., Greenland, L.P., Malinconico, 

L.L., Casadevall, J.R., Furukawa, B.T., 1987, 802 and C02 

emission rates at Kilauea Volcano, 1979-1984, in Decker, 

R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism in 

Hawaii: U.S. Geological Survey Professional Paper 1350, 

p. 771-780. 

186 Monitoring Volcanoes: Techniques and Strateglea Used by the Staff of the Cascades Volcano Observatory 
Chartier, T.A, RO§e, W.I., Stokes, J.B., 1988, Detailed record 

of SOl emissions from Pu'u 'O'o between episodes 33 and 

34 of the 1983-86 eruption, Kilauea, Hawaii: Bulletin of 

Volcanology, v. SO, p. 215-228. 

Evans, W.C., Banks, N.G., and White, L.D., 1981, Analyses of 

gas samples from the summit crater, in Lipman, P.W., and 

Mullineaux, D.L., eds., The 1980 eruptions of Mount St. 

Helens, Washington: U.S. Geological Survey Professional 

Paper 1250, p. 227-232. 

Frank, David, Meier, M.F., and Swanson, D.A., 1977, 

Assessment of increased thermal activity at Mount Baker, 

Washington, March 1975-March 1976: U.S. Geological 

Survey Professional Paper 1022-A, 49 p. 

Friedman, I.F., and Reimer, G.M., 1987, Helium at Kilauea 

Volcano, in Decker, R.W., Wright, T.L., and Stauffer, 

P.H., eds., Volcanism in Hawaii: U.S. Geological Survey 

Professional Paper 1350, p. 809-820. 

Gerlach, T.M., and Casadevall, T J., 1986, Fumarole emissions 

at Mount St. Helens volcano, June 1980 to October 1981: 

Degassing of a magma-hydrothermal system: Journal of 

Volcanology and Geothermal Research, v. 28, p. 141-160. 

Gerlach, T.M., and Graeber, E.J., 1985, Volatile budget of 

Kilauea volcano: Nature, v. 313, p. 273-277. 

Giggenbach, W.F., 1975, A simple method for the collection 

and analysis of volcanic gas samples: Bulletin 

Volcanologique, v. 39, p.132-145. 

__ 1989, The El Ruiz magmatic-hydrothermal system: 

IAVCEI Continental Magmatism General assembly, Santa 

Fe, New Mexico, abstracts, p. 106. 

Giggenbach, W. F., and Goguel, R. L., 1988, Methods for the 

collection and analysis of geothermal and volcanic water 

and gas samples: New Zealand Department of Scientific 

and Industrial Research, Chemistry Division, report no. 

CD 2387,53 p. 

Greenland, L.P., 1984, Gas composition of the January 1983 

eruption of Kilauea volcano, Hawaii: Geochimica et 

Cosmochimica Acta, v. 48, p. 193-195. 

__ 1986, Gas analyses from the Pu'u O'o eruption in 1985, 

Kilauea volcano, Hawaii: Bulletin of Volcanology, v. 48, 

p. 341-348. 

__ 1987, Hawaiian eruptive gases, in Decker, R.W., 

Wright, T.L., and Stauffer, P.H., eds., Volcanism in 

Hawaii: U.S. Geological Survey Professional Paper 

1350, p. 759-770. 

Greenland, L.P., Rose, W.I., Stokes, J.B., 1985, An estimate of 

gas emissions and magmatic gas content from Kilauea 

volcano: Geochimica et Cosmochimica Acta, v. 49, p. 

125-129. 

Harris, D.M., Sato, Motoaki, Casadevall, T.J., Rose, W.I. Jr., 

and Bornhorst, T.J., 1981, Emission rates of C02 from 

plume measurements, in Lipman, P.W., and Mullineaux, 

D.L., eds., The 1980 eruptions of Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 

1250, p. 201-207. 

Hinkley, T.K., ed. 1987, Chemistry of ash and leachates from 

the May 18, 1980 eruption of Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 

1397,64 p. 

Jaggar, T.A., 1940, Magmatic gases: American Journal of 

Science, v. 238, p. 313-353. 

LeGuern, G., Gerlach, T.M., and Nohl, A, 1982, Field gas 

chromatograph analyses of gases from a glowing dome at 

Merapi Volcano, Java, Indonesia, 1977, 1978, 1979: 

Journal of Volcanology and Geothermal Research, v. 14, p. 

223-245. 

Malone, S.D., and Frank, David, 1975, Increased heat emission 

from Mount Baker, Washington: Eos, v. 56, p. 679-685. 

McGee, K.A, Casadevall, T.J., Sato, Motoaki, Sutton, AJ., and 

Clark, M.D., 1982, Hydrogen gas monitoring at Long 

Valley Caldera, California: U.S. Geological Survey 

Open-file Report 82-930, 12 p. 

McGee, K.A., and Sutton, AJ., 1990, A detailed study of the 

USGS reducing-gas sensor with field tests at Long Valley, 

California: U.S. Geological Survey Open-File Report 

90-61,68 p. 

McGee, K.A., Sutton, AJ., and Sato, Motoaki, 1987, Use of 

satellite telemetry for monitoring active volcanoes, with a 

case study of a gas-emission event at Kilauea volcano, 

December 1982, in Decker, R.W., Wright, T.L., and 

Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological 

Survey Professional Paper 1350, p. 821-825. 

McGee, K.A., Sutton, A.J ., and Sa to, Motoaki, 1986, 

Continuous gas monitoring in volcanic systems: lA VCEI 

workshop on volcanic gases, Usu, Japan, June 1985, 

abstracts, p. 10. 

Millan, M.M., Gallant, AJ., and Turner, H.E., 1976, The 

application of correlation spectroscopy to the study of 

dispersion from tall stacks: Atmospheric Environment, v. 

10, p. 499-511. 

Piccardi, G., and Cellini-Legittimo, P., 1983, A sampling and 

chemical analysis procedure for fumarolic gases: 

Mikrochimica Acta [Wien], v. 11, p. 159-167. 

Sato, Motoaki, Malone, S.D., Moxham, R.M., and 

McLane, J.E., 1976, Monitoring of fumarolic gas at 

Sherman Crater, Mount Baker, Washington : Eos, v. 

57, p. 88-89. 

Sato, Motoaki, and McGee, K.A., 1981, Continuous monitoring 

of hydrogen on the south flank of Mount St. Helens, in 

Lipman, P.W., and Mullineaux, D.L., eds., The 1980 

eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 209-219. 

Stoiber, R.E., Malinconico, L.L., and Williams, S.N., 1983, Use 

of the correlation spectrometer at volcanoes, in Tazieff, H., 

and Sabroux, J.C., eds., Forecasting volcanic events: 

Amsterdam, Elsevier, p. 425-444. 

Sutton, A.J., 1990, Chemical sensors for volcanic gases with a 

compilation of commercial availability: U.S. Geological 

Survey Open-File Report 90-44, 27 p. 

Sutton, A.J., and McGee, K.A., 1989, A multiple-species 

volcanic gas sensor-Testing and applications: lA VCEI 

Continental Magmatism General assembly, Santa Fe, New 

Mexico, abstracts, p. 262. 

Thomas, D.M., 1987, An interpretation of the response of 

ground-gas radon concentration to seismic and volcanic 

activity at Kilauea volcano: Hawaii Symposium on How 

Volcanoes Work, abstracts, p. 251. 

18. Fundamental Volcanic-Gas-Study Techniques: An Integrated Approach to MonHorlng 187 
Tilling, R.I., 1989, Volcanic hazards and their mitigation: 

Progress and problems: Reviews of Geophysics, v. 2, p. 

237-269. 

UNESCO, 1972, The surveillance and prediction of volcanic 

activity, a review of methods and techniques: United 

Nations Educational, Scientific and Cultural Organization, 

Earth Sciences v. 8, 166 p. 

Williams, S.N., Stoiber, R.E., Garcia, P. N., Londono, C. A., 

Gemmel, J.B., Lowe, D.R., and Connor, C.B., 1986, 

Eruption of the Nevado del Ruiz wlcano, Colombia, on 13 

November 1985: Gas flux and fluid geochemistry: 

Science, v. 133, p. 964-967. 

188 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
19. Video Surveillance of Active Volcanoes Using 

Slow-Scan Television 

By Bruce T. Furukawa, Thomas L. Murray, and Kenneth A. McGee 

ABSTRACT 

VIdeo monitoring using slow-scan television has 

proved to be a valuable monitoring tool at Mount St. 

Helens. The system provides a permanent video record 

of events at the volcano and Is also an important aid In 

planning field logistics and operations. Such a system is 

particularly valuable at remote volcanoes, where 

continuous observations are otherwise not feasible. 

INTRODUCTION 

Video monitoring of Mount St. Helens volcano 

using slow-scan television (SSTV) began on September 

4, 1987. The system consisted of a video camera, a scan 

converter, a radio transmitter, and a power system, all 

located on a ridge 8.5 km north of Mount St. Helens; a 

radio repeater on a high point west of the volcano; and a 

scan converter, video monitor, and recording equipment 

at the Cascades Volcano Observatory (CVO) in 

Vancouver, Washington (fig. 19.1). After several months 

of operation, the system was modified to tum off at night 

and back on again in the morning in order to conserve 

power. A later enhancement included the capability to 

remotely control various other functions, such as zoom 

and pan. 

Video monitoring of active volcanoes has several 

attractive benefits. First, there is the potential to 

significantly reduce the cost of monitoring a volcano. 

Adverse weather conditions as well as steam and dust 

conditions can be observed and considered before 

planning daily field activities. Second, the need for 

aircraft observation flights is reduced. Third, a reliable 

video system can significantly reduce hazards to field 

personnel during periods prior to and during eruptive 

activity, by supplementing on-site observations of field 

personnel. Remote variable-power zoom can allow 

observers to remain a safe distance from the activity. 

Fourth, video information available during daylight hours 

is easily correlated with other telemetered monitoring 

data to assess current conditions. At night, an SSTV 

monitoring system could utilize an infrared camera for 

monitoring hot spots. Or a video cassette recorder (VCR) 

could be connected at the output of the video camera in 

parallel with the output of the scan converter and 

triggered to tum on in response to either a control signal 

from a base station or a signal from a monitoring device 

in the field such as a seismometer or a trip wire, in order 

to continuously record events such as eruption plumes, 

debris avalanches, or lahars for subsequent detailed 

analysis. It is even possible to design a transportable 

SSTV system that could be rapidly deployed at 

volcanoes threatening to erupt. Finally, video information 

can be stored on magnetic tape for future analysis. 

The earliest video system at Mount St. Helens was 

a closed-circuit television system installed in July 1980 

in response to the onset of eruptive activity earlier that 

year (Miller and Hoblitt, 1981). This system was 

extremely valuable to scientists monitoring the ongoing 

eruptive activity at the time, but was hastily assembled 

under emergency conditions and operated for less than 

one year before succumbing to problems related to its 

complex microwave technology and large power 

consumption. In January 1984, an idea to use a 

microprocessor-controlled digital video camera with 

radio modem to obtain images from Mount St. Helens 

was proposed, but not acted upon. In the winter of 

1985-86, eva seriously considered a proposal to 

establish a permanent microwave link to Mount St. 

Helens that would carry telemetry and video signals from 

field monitoring stations. The idea was later abandoned 

because of the high equipment cost and system 

maintenance. Finally, in 1986, the idea of employing 

slow-scan television for video monitoring of Mount St. 

Helens was adopted as a goal for CVO. The system 

described in this paper is the result. 

Slow-scan television, first developed by amateur 

radio experimenters in the 1950's, is a system whereby 

video images are converted line by line to a varying tone 

for transmission through a narrow-band radio system. 

Conventional television requires a bandwidth of 6 MHz 

for transmission of video signals, whereas SSTV requires 

19. VIdeo Surveillance of Active Volcanoea Ualng Slow-Scan Televlalon 189 
only a standard 3 kHz voice channel. This critical 

difference in RF spectrum requirements results in many 

significant advantages for SSTV over conventional 

television systems (lower cost and power requirements, 

less complex radio-telemetry systems, and ease of 

servicing). Furthermore, because of the narrow 

bandwidth, SSTV images can be stored on standard 

audio magnetic tape rather than video tape. A 

disadvantage of SSTV is that the video information is 

discontinuous, because it is displayed as a time series of 

video snapshots. However, for volcano observatories 

which must monitor one volcano or several volcanoes 

over periods of years, slow-scan television may be the 

method of choice. 

DESCRIPTION OF SYSTEM 

The current slow-scan television system at Mount 

St. Helens consists of a camera, scan converter, and 

controller at the field site; a scan converter, remote 

controller, TV monitor, and recorder at the Cascades 

Volcano Observatory; and a radio system, including a 

repeater, for transmitting video and control signals (fig. 

19.1). Appendix 1lists the equipment used by CVO. 

Any good video camera with a 12-volt DC 

operating voltage, or specifications similar to those 

listed in appendix 1, can be used in this application. 

CVO's camera, located 8.5 km north of Mount St. 

Helens, can be zoomed in to view only the interior of 

the crater or zoomed out to view the entire volcanic 

edifice and nearby terrain through the use of the remote 

control system described below. The camera is mounted 

on a pan and tilt servo-controller unit. When fully 

implemented, this pan-tilt unit will allow the camera to 

pan 150 degrees to the left or right from center and to 

tilt 30 degrees up or down from horizontal. 

Video output from the camera is fed directly into 

a slow-scan video color converter that has been 

modified to operate from a 12-V DC power source. The 

rll))))) ooooaa)~• ((((((11 RADIO ~ A ~ RCVR 1----~:; 

:-:.:' ........ ' . : ......... ~; ' .... . 

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RELAY 

RADIO 

TRANSCEIVER 

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RADIO 

LINK 

TATTLETALE 

CONTROLLER 

RADIO 

TRANSCEIVER 

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MONITOR 

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LN:S 

SOLO ARROWS = 

HARDWIRED 

LiteS 

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ROBOT 1200C 

CASSETTE SLOW-SCAN VIDEO 

CONVERTER 

DECK fE--

ROBOT 1200C I-E- CASSETTE SLOW-SCAN VIDEO 

CONVERTER DECK 

Figure 19.1. Flowchart illustrating components of CVO SSTV system. 

190 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
SSTV color converter is actually an integrated 

transceiver capable of transmitting or receiving full color 

images over any voice-grade communications link. 

The converter digitizes the video signal from the 

camera and, after conversion to audio tones, transmits 

the image line by line using a process called 

time-multiplexed component color in 72 seconds. At 

the highest resolution setting, the converter can 

produce a digitized image composed of 61,440 picture 

elements in a 256 pixel by 240 line array. When 

operated unattended at a field location, the functions 

of the SSTV converter can be controlled by a 

computer through an RS-232 serial port. Two different 

control systems have been developed at eva and are 

described below. 

The digitized and encoded video signal from the 

SSTV converter is fed into a low-powered VHF or 

UHF radio telemetry transmitter. The use of standard 

radio transmitters similar to those used for seismic or 

low-frequency data telemetry represents one of the 

biggest advantages of SSTV over other types of video 

transmission. The antennas used throughout the system 

are standard radome-protected, UHF band, 

five-element yagis with 10 dB gain. 

All of the equipment at the field site, except for 

the antenna and camera, is housed in a small building. 

Power for the equipment is provided by combining 

two sets of batteries. One set consists of four Delco 

2000 high-capacity deep-cycle 125 ampere-hour (Ah) 

lead-acid batteries designed for use in solar-charging 

applications. The other set is com posed of six Model 

KCP-SA-13, 2-V 560-Ah high-capacity wet-cell 

batteries, manufactured by C & D Batteries, Plymouth 

Meeting, Pennsylvania. Both sets are simultaneously 

charged during daylight hours by a bank of two 

30-watt solar panels. 

Video information from the field-monitoring site 

is received at the observatory via a standard telemetry 

repeater west of Mount St. Helens. At CVO, the audio 

output from the SSTV radio receiver is fed into 

another SSTV converter, where the signal is decoded, 

converted into a standard color video signal, and fed 

into a standard color monitor for viewing. The 

converter also feeds a signal into a standard audio 

cassette recorder. A separate SSTV converter with 

cassette player and color monitor is available nearby 

for off -line viewing of the tapes. 

As of this writing (September 1990), a slow-scan 

television monitoring system for Mount St. Helens has 

been in operation for 3 years. The earliest version of 

the system operated in a continuous mode. Owing to 

excessive power consumption, a controller was added 

to turn the system off at night. However, power 

consumption was still a serious problem, and the 

program was later changed to transmit a single picture 

every 15 minutes during daylight hours. These were 

marginally acceptable, temporary solutions. Finally, a 

remote controller was developed and installed that allows 

all functions to be controlled from eva. 

SSTV CONTROLLER 

Two controllers have been utilized with the SSTV 

system. The first, an on-site battery-powered laptop 

computer, turned the system on and off at programmed 

times (fig. 19.2). 

Typically, the computer activated the system 

every 15 minutes to transmit a single picture of 

2-minute duration. Turning the system on and off in 

this manner decreased the average power consumption 

of the system dramatically and allowed solar panels to 

substitute for the propane-fueled, thermoelectricgenerator charging system. 

The second controller' developed at eva, allows 

users to remotely control the system by issuing 

commands from eva to the controller over a radio 

link (fig. 19.1). This technique allows turning the 

system on and off, controls the picture resolution, 

monitors the battery voltage, and adjusts the zoom, 

pan, and tilt of the camera. 

Battery-Powered Computer Controller 

Figure 19.2 shows how a small computer at the 

camera site connects to the system as a controller. Pin 

4 (RTS) of the computer's serial port controls the 

relay that switches power on and off to the SSTV 

converter, camera, and radio transmitter. Commands 

are transmitted from the serial port of the computer to 

the graphics input port of the SSTV converter. 

A BASIC program running on the computer 

controls the sequence of events and the transmit times. 

A typical program is listed in appendix 2. With this 

program, the computer powers up the system every 15 

minutes between 0600 and 2000 hours, sends 

commands via the serial port to the controller to 

transmit a single picture at the highest resolution, 

waits 2 minutes (long enough for the picture to be 

transmitted), and then powers down the system. 

Thirteen minutes later the process repeats. 

Of the two controllers, utilizing a 

battery-powered computer is the simpler, less 

expensive method. Its major drawback is that changes 

to the program cannot be done remotely; the site must 

be visited to change the program. At sites accessible 

by vehicle this may not be a problem, and this system 

may suffice. 

10. VIdeo Surveillance of Active Volcanoea Ualng Slow-Scan Televlalon 1 01 
Remote Control System 

The remote control system allows users to control 

the SSIV system from the observatory. A menu-driven 

program running on a laptop computer allows users to 

radio messages to the controller in the field (appendix 

3). The messages instruct the controller to execute any 

of several desired options, so site visitation is 

unnecessary. 

Observatory staff run the menu-driven BASIC 

program to indicate the desired action to be performed 

by the controller. User input initializes the computer, 

which then generates the appropriate instructional 

command for the desired controller action. This message 

is sent to the radio-interface circuit via the RS-232 

serial port. The interface circuit reads the command, 

puts the radio into transmit mode, sets the modem to 

originate mode, and transmits the message in standard 

Bell 103 format. After the command is sent, the radio 

returns to receive mode, and the modem to answer 

mode. Messages and acknowledgments sent to the base 

station frol)l the field are then received by the computer. 

An Onset Computer Tattletale Model III 

single-board controller, a radio, and a CVO-designed 

interface circuit (fig. 19.3) make up the controller at the 

camera site. Messages sent from the observatory are 

received by the radio, demodulated by a single chip 

modem in the interface circuit, and sent to a serial port 

on the Tattletale. A BASIC program running on the 

Tattletale decodes the message and performs the correct 

action. The SSTV controller is directed by sending 

commands from the Tattletale's second serial port to the 

graphics input of the SSTV controller. MOSFETs 

attached to the Tattletale's digital 1/0 lines provide the 

contact closures necessary for the zoom, pan, tilt, and 

system on/off controls. The entire package is housed in 

a watertight case. 

CONCLUSIONS 

The use of video systems has the potential to 

reduce the operational costs and risks involved in 

monitoring active volcanoes, particularly dangerous 

volcanoes in rugged or remote terrain. Slow-scan 

L ROBOT 1200C ~ VIDEO ~ SLOW-SCAN ~ RADIO )))~ 

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RADIO SHAO< MODEL 100 COtvPUTER 

USED AS A SSTV SYSTEM CONTROLLER 

BROKEN AFR:>WS ARE CONTROLLER 

Lit-ES FROM TI-E IVOCeL 100 

TO TI-E SYSTEM 

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+6 VOLT 

REGLLATOR 

SOLD /iiFPCWS AFe 1-W=DWF£0 POWER JlK) S1<?NAL LN:S 

FOR TI-E SYSTEM 

192 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
RADIO XMIT 

RADIO RECEIVE 

us 

SLOW-SCAN CONTROLLER INTERFACE 

FOR TATTLETALE Ill 

TOM MURRAY 

U .S . GEOLOGICAL SURVEY 

CASCADES VOLCANO OBSERVATORY 

1989 

Figure 19.3. CVO-designed Interface circuit. 

2 .2K 

television, with its simplicity and modest RF spectrum 

requirements, is an attractive low-cost alternative to 

conventional television systems. Although its power 

requirements are greater, a slow-scan television 

monitoring station is no more complex to install and 

operate than a seismic or low-frequency monitoring 

station, and can be optimized or enhanced for a 

particular application. 

The slow-scan television system in operation at 

Mount St. Helens has evolved in design since its 

initial installation and has proved to be a useful and 

economical addition to the monitoring capability at the 

Cascades Volcano Observatory. Similar SSTV 

installations at other potentially active volcanoes 

should be considered. 

REFERENCES CITED 

Miller, C.D., and Hoblitt, R.P., 1981, Volcano monitoring by 

closed-circuit television, in Lipman, P.W., and 

Mullineaux, D.R., eds., The 1980 eruptions of Mount St. 

Helens, Washington: U.S. Geological Survey Professional 

Paper 1250, p. 335-341. 

+5 

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APPENDIX 1 

Equipment List 

Video camera JVC Model TK-860U color video 

camera head, with a Sanyo TV 

Autofocus, f 1.2, motorized zoom lens 

with a focal length of 12.5 to 75 mm. 

Pan/tilt controller Daiwa Model MVH-2D pan and tilt 

unit, with pan of 150 degrees taft and 

right from center and tilt of 30 degrees 

up and down from horizontal. 

Slow-scan video Robot Model 1200C Color Scan 

converter Converter, modified to operate from a 

12 VDC power source. 

Telemetry radios Monitron T40F-1 UHF radio telemetry 

transmitter. 

Computers 

Field controller 

Antennas 

Monitrori R40F UHF radio receiver. 

Synthex Model LM4400 VHF radio 

tranceiver. 

Radio Shack Model100 (or 102) 

Onset Computer Tattletale Model III 

single-board controller. 

SCALA, model RAS-450, radomeprotected, five-element yagi, with 10 

dB gain. 

19. Video Surveillance of Active Volcanoes Using Slow-Scan Television 193 
APPENDIX2 

Typical BASIC program on Radio Shack llodel1 00 

A program for the Radio Shack Model 100 to control the 

slow-scan video system. ne program transmits a picture every 15 

minutes between 0600 hours and 2000 hours. 

set the automatic running of this program on reset 

20 IPL "SLOSCN.BA" 

21 PRINT "THE SLO-SCAN WILL BE TURNED ON EVERY" 

22 PRINT "15 MINUTES (XX:OO, XX:15, XX:30, AND XX:45) AND • 

23 PRINT "TRANSMIT ONE PICTURE. CURRENT TIME IS" 

24 PRINT TIME$ : FOR 1::1 TO 2000: NEXT I 

26' always transmit a pidure on start-up 

30 GOSUB 200 

now just loop around waiting for a 15 minute mark 

between 06:00 and 20:00 

40 IF (MIDS(TIME$,1,2)<"061 OR (MIDS(TIME$,1,2)>"191 THEN GOTO 40 

41 IF (MID$(TIME$,4,2):"00") OR (MIDS(TIME$,4,2)="30") THEN GOSUB 200 

42 IF (MID$(TIME$,4,2):•15") OR (MIDS(TIME$,4,2)=•45") THEN GOSUB 200 

45 GOTO 40 

50' the subroutines for delays between commands (a short and a long) 

180 FOR 1::1 TO 2000:NEXT I:RETURN 

185 FOR 1=1 TO 300:NEXT I:RETURN 

190' the tum-on subroutine 

191 ' opening the com port turns on the relay to power the system 

200 BEEP : PRINT "POWER ON" : OPEN "COM:77N2D" FOR OUTPUT AS 1 

205' power up delay 

210 GOSUB 180 :GOSUB 180 

215' issue the commands to put in the correct configuration to 

216' transmit 1 (one) pidure 

220 PRINT "36n~ : CALL 28210,25 : GOSUB 180 

230 PRINT "TRANSMIT": CALL 28210,16: GOSUB 180 

235' do a double-push on the "camera• button 

240 PRINT "CAMERA": CALL 28210,42 :GOSUB 185: CALL 28210,42: GOSUB 180 

245' wait until 2 minutes later and turn off. the tum-off has the retum 

250 SCS=MIDS(TIME$,7,2) 

251 MN$:CHR$(48+((2+VAL(MID$(TIME$,5,1))) MOD 10)) 

2521F (MID$(TIME$,5,1)=MN$) AND (MID$(TIME$,7,2)>=SC$) THEN GOTO 300 

255' go back to the wait loop 

260 GOTO 252 

270' the tum-off subroutine 

275' closing the port deac:livates the relay that powers the system 

300 BEEP :PRINT "POWER DOWN AT "+TIME$: CLOSE 

305' go back 

320 RETURN 

APPENDIX3 

Command• available In the BaM Station Computer Program 

Picture transmission control instructions 

Power down the system 

Transmit a single picture 

Transmit continuously 

Transmit one picture at intervals of either 5, 10, 15, 30, 

or 60 minutes 

Set the picture resolution 

Miscellaneous instructions 

Set the Tattletale's clock 

Transmit the battery voltage 

Instructions for controlling the camera 

Zoom in or out from the current position 

Zoom to an absolute position 

Pan left or right from the current position 

Pan to an absolute position 

Tilt up or down relative to the current position 

Tilt to an absolute position 

Note: Zoom, pan, and tilt positioning is measured in the 

length of time (in hundredths of seconds) the motor drive is 

engaged. For example, an absolute zoom position of 125 

indicates the zoom was driven completely out and then driven 

back in for 1.25 seconds. A relative zoom in of 25 means the 

zoom is driven in for 0.25 second. Pan and tilt positions are 

measured in the same fashion. 

194 Monitoring Volcanoes: Techniques and Strateglea Used by the Staff of the Cascades Volcano Observatory 
20. Basic Photography at Mount St. Helens and Other 

Cascades Volcanoes 

By Lyn Topinka 

ABSTRACT 

During the last decade, researchers at the Cascades 

Volcano Observatory (CVO) in Vancouver, Washington, 

have taken thousands of photographs of Mount St. Helens 

and the surrounding area, and hundreds more of other 

Cascades volcanoes. They have used a great many types of 

cameras. Cameras are one of the most versatile and useful 

tools available to document changes around volcanoes. Still 

cameras take one picture at a time and can be divided on 

the basis of film size into small format, medium format, and 

large format. There is also an instant-processing still camera 

which produces a single image within 30 seconds. Video and 

movie cameras are used for many of the same applications 

as still cameras and are especially. useful in oblique and 

illustrative terrestrial photography. Video or movie footage is 

valuable when studying dynamic events such as ash plumes 

or pyroclastic flows, or calculating the speed of lahars or 

floods. Vertical and oblique aerial photography, repeat and 

illustrative terrestrial photography, and time-lapse 

photography are all techniques available for documenting 

changes occurring on or around volcanoes. The resulting 

photographs and footage can be used for interpretation, 

illustrations in publications, scientific talks and public slide 

shows, quantitative measurements, and historical 

documentation of volcanic processes. 

INTRODUCTION 

Active volcanoes are capable of producing great 

changes in themselves and the surrounding landscape. 

To illustrate, document, and to better understand these 

changes, photographic records should be kept. 

On May 18, 1980, over 400 m of Mount St. Helens 

collapsed as a series of great landslides, releasing 

pressure which produced a 20 km-high plinian ash 

column, and leaving behind a crater 1.5 km wide and 

600 m deep. Over 650 km2 of recreation, timber, and 

private lands were damaged. Approximately 60 km2 of 

the Toutle River valley was buried by 2.8 km3 of debris 

from the collapsing cone (Tilling, 1984). The magnitude 

of change is clearly illustrated by comparing photographs 

taken from a site 10 km northwest of Mount St. Helens 

on May 17, 1980, the day before the devastating eruption 

(fig. 20.1A), and from the same location on September 

10 (fig. 20.1B). The same view taken on March 30, 1987, 

illustrates the continuing changes to the landscape after 7 

years of dome growth and river channel erosion (fig. 

20.1C). 

During the last decade, personnel at the Cascades 

Volcano Observatory (CVO) have taken thousands of 

photographs of Mount St. Helens and the surrounding 

area, and hundreds more of other Cascades volcanoes. 

These photographs are used for illustration in 

publications, scientific talks and public slide shows, 

quantitative measurements and interpretation, and 

historical documentation of volcanic processes. Many 

camera types are used including still cameras, video 

cameras, and movie cameras. Many techniques are 

employed including vertical and oblique aerial 

photography, repeat and illustrative terrestrial 

photography, and time-lapse photography. The type of 

equipment and the purpose for which the equipment or 

technique is used largely depend upon the scientific 

need, budget limitations, location accessibility, and the 

scientist's personal preference. This chapter covers 

possible combinations of cameras and techniques and 

cites examples of what has been useful at Mount St. 

Helens and other Cascades volcanoes. There is no correct 

or universal way to photograph volcanoes; how little, 

how much, or of what quality depends upon variables 

unique to each volcano and to the scientists involved. 

STILL PHOTOGRAPHY CAMERAS 

Cameras are one of the most versatile and useful 

tools available to scientists monitoring volcanoes, and 

still cameras are the most commonly used. Still cameras 

take one picture at a time resulting in either positive or 

negative images, and can be divided on the basis of film 

size into small format, medium format, and large format. 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 195 
A 

8 

Figure 20.1. Mount St. Helens as viewed from 1 0 km to the northwest, showing development of lava dome and 

drainage channels. A, One day before devastating May 18 eruption. 8, Four months after May 18 eruption. C, 

Seven years after May 18 eruption. 

196 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
There is also an instant-processing still camera, which 

produces a single image within 30 seconds. The 

scientists at CVO use all four. 

Small-Format Cameras 

The most common small-format cameras use 

35-mm-size roll film, producing 12-36 frames per roll. 

These cameras are readily available, economical, 

convenient to use, and offer a wide variety of lenses and 

accessories. Film is available in a wide range of types 

and speeds, and processing is quick, with certain films 

able to be processed in the office. For a detailed 

scientific study, however, the small images may not have 

sufficient resolution to show the detail needed to 

compare landscape changes, and high-quality 

enlargements are often difficult and expensive to obtain. 

Two styles of small-format cameras used at CVO are the 

35-mm lens/shutter and the 35-mm single-lens reflex 

(SLR). 

The lens/shutter is a camera where the user looks 

through a viewfinder, not the lens, resulting in a photo 

that is slightly offset from what is actually viewed. The 

lens/shutter cameras at CVO are usually auto-load, 

auto-focus, and auto-rewind, and are often referred to as 

"point-and-shoots." Lenses are either a single fixed focal 

length or zoom between two focal lengths. The cameras 

c 

Figure 20.1. Continued 

are generally small and lightweight, and are easily 

carried in pockets or packs. Because the cameras are 

typically cheaper than SLR's, they are often considered 

"disposable" cameras, and are used while stream-gaging, 

surveying drainage-channels, working in dusty or ashy 

areas, or doing activities when there is a chance of the 

camera (or the scientist) falling into a river or becoming 

covered with dust and ash. 

The camera most commonly used at CVO is the 

35-mm SLR. In the SLR, the image coming through the 

lens is reflected to the viewfinder by a mirror and a 

prism, so the user sees exactly what the film records. 

These cameras are relatively compact, lightweight, and 

can use a large selection of interchangeable lenses and 

accessories. Many different brands of SLR' s are used, as 

each researcher buys camera equipment to fit his or her 

scientific needs, project budget restrictions, and personal 

taste. For example, many use the Olympus OM2 or the 

Nikon FE, both of which have proved dependable in 

Mount St. Helens's ashy environment. Others use the 

Minolta X700 with its optional multifunction 

programmable data back. This data back can record 

hours, minutes, and seconds on the film, a valuable 

feature when documenting dynamic events such as ash 

plumes, pyroclastic flows, lahars, and floods. The 

Minolta data back can also be programmed to trigger the 

camera every 1 second to every 99+ hours, or an 

optional motor drive can be set to trigger 2 or 3.5 frames 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 197 
per second, options that have been important in 

time-lapse photography sequences. 

The standard 50-mm focal-length lens is most 

commonly used with the SLR's. This lens is ideal, as it 

provides approximately the same magnification as the 

human eye. The wide-angle 28-mm focal-length lens is 

also popular and forms an image of a subject which is 

approximately half the size of that formed by the 

standard lens. 

Only a few scientists use medium-range telephotos 

(100 to 300-mm focal-lengths), long-range telephotos 

(300 to 1000-mm focal-lengths), or zoom lenses, as these 

lenses are expensive and bulky. In the field, most 

scientists usually carry only one camera body and lens, 

as extra bodies and lenses take up room and add extra 

weight; moreover, it is often unwise or difficult to 

change lenses in ashy, dusty, or winter environments. 

Occasionally field crews carry a variety of cameras and 

lenses, resulting in different scale photographs of the 

same subject on the same day, a convenience when 

choosing photographs for publication or slide shows. For 

A 

example, one member of CVO's deformation monitoring 

crew routinely uses a Nikon with a 28-mm lens, while 

another uses a Nikon with a 50-mm lens. 

The CVO staff photographer uses a Nikon FE with 

a Vivitar 17-mm lens, and Nikkor 28-mm, 55-mm, and 

200-mm lenses. The 17-mm is a super-wide-angle lens 

with a field of view of approximately 100• (as measured 

from corner to corner) and is useful when photographing 

scientists working in their environment (fig. 20.24). The 

28-mm is a wide-angle lens with a field of view of 

approximately 75• and is useful when photographing 

aerial views of the crater or dome (fig. 20.28). The 

standard 55-mm lens has a field of view of 

approximately 45• and is an excellent all-around lens, 

useful for all types of photography (fig. 20.2C). The 

200-mm lens is a medium-range telephoto lens with a 

field of view of approximately 12• and is useful when 

photographing the dome from a distance (fig. 20.2D). 

During critical periods, the staff photographer also 

carries a Minolta X700 with its hours-minutes-andseconds data back, a motor drive, and an MD 

Figure 20.2. Mount St. Helens photographed using different focal-length lenses. A, A 17-mm lens was used 

to photograph researchers making measurements to dome. 8, A 28-mm lens was used to photograph an aerial 

view of Mount St. Helens crater and dome. C, A 55-mm lens was used from 8 km away to photograph a small 

steam and ash burst. D, From same location, a 200-mm lens was used to photograph steamy lava dome and 

remnants of a rockslide down its north face. 

198 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
8 

c 

Figure 20.2. Continued 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 199 
50-135-mm zoom lens, or an extra Nikon FE body and 

Nikkor 35-105-mm zoom and 100-300-mm zoom lenses. 

Multiple cameras allow the simultaneous use of different 

types of films and provide an extra camera should the 

primary camera be inoperable. 

A wide selection of film types is available to users 

of 35-mm small-format cameras. At CVO, most scientists 

participate in scientific talks and slide shows and thus 

prefer color transparency (slide) films over color or 

black-and-white (b/w) negative films. Transparencies are 

also economical, easy to view, convenient to store, and 

for most applications can easily be made into b/w or 

color prints. Kodak brand .film is generally used, with 

Kodachrome 64 and Ektachrome 200 transparency films 

being the most popular. Both of these films provide good 

color and show little grain. The faster speed Ektachrome 

400 transparency film is often used when photographing 

during overcast or rainy periods. 

Very little black-and-white film is used by CVO 

researchers, as they usually carry only one camera and 

have it loaded with color. The staff photographer, 

however, routinely carries two cameras, one loaded with 

color slide film and the other loaded with a fine-grained 

b/w negative film. The b/w film is used when shooting 

specifically for publication photographs or historical 

D 

Figure 20.2. Continued 

documentation. Black-and-white film is very important 

for historical archiving. According to Eastman Kodak 

Co. (1979, p. 32): 

When black-and-white negatives on acetate or 

polyester film base are processed properly. and are 

protected in storage from the effects of heat. 

moisture. oxidizing gases. and reactive storage 

materials, they are extremely stable articles. They 

will outlast most other photographic records and, 

since they can be duplicated with very little loss of 

quality by a comparatively simple process. they 

constitute the best material for archival purposes. 

Medium-Format Cameras 

Medium-format cameras typically use 120-size roll 

film, produce 12-15 frames per roll, and provide 57-mm 

(21!4 in.) square or 60x70-mm rectangular transparencies 

or negatives. These cameras offer handling convenience 

similar to that of the 35-mm cameras, some have 

interchangeable lenses and film backs, and the larger 

film provides for greatly improved image detail. 

However, medium-format cameras are generally more 

200 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
costly, larger in bulk and weight, and lack an extensive 

or readily available selection of lenses and films. 

Few medium-format cameras are used at CVO. 

One scientist uses a Pentax 6x7 with a standard Takumar 

105-mm lens. The staff photographer uses a Hasselblad 

500C with a standard Planar 80-mm lens and a 

wide-angle Distagon 50-mm lens. Since most 

medium-format photographs are used for publication or 

historical documentation, fine-grained b/w films are 

generally used. A second Hasselblad film back is carried 

loaded with either color slide or color negative film. 

Large-Format Cameras 

Large-format cameras typically use 102x127 or 

203x254-mm (4x5 or 8x10 in.) sheet film or 241-mm 

(9\1 in.) size roll film, and are heavy (some weigh in 

excess of 130 kg), bulky, and not readily available. The 

cameras require a tripod or are mounted in an aircraft, 

and are generally used for vertical aerial photography. 

Film processing is specialized and expensive, but film 

resolution is excellent. The 241-mm size roll film also 

has the convenience of more than 300 frames per roll, 

although smaller rolls are available. 

At CVO, large-format cameras are used for vertical 

aerial photography and most CVO requirements are 

contracted to professional aerial photography companies. 

In 1985, "in-house" large-format vertical aerial 

photography was attempted with a borrowed Fairchild 

camera. A local aircraft company was contracted to cut a 

camera port for the camera in the belly of its airplane. 

The project was abandoned after a short time, however, 

owing to the poor quality of the camera and the resulting 

photographs. 

Instant-Processing Cameras 

The instant-processing cameras used at CVO are 

the Polaroid SX -70 instant cameras. The Polaroid SX -70 

Land Camera Model-2 folds to approximately 

100x200x40 mm and is convenient to carry. Final print 

size is approximately 80x80 mm. Although the film is 

expensive, the advantage of seeing prints in 30 seconds 

to 8 minutes (depending upon the film used) outweighs 

the disadvantage of expense for some uses. Instant prints 

allow scientists the luxury of photographing an outcrop 

or stratigraphic section and within minutes writing field 

notes pertaining to that feature directly on the print. The 

print is then secured in the field notebook and becomes a 

part of the field records. Polaroids, like other color 

prints, will fade with time. 

VIDEO AND MOVIE CAMERAS 

Video and movie cameras are used for many of the 

same applications as still cameras and are especially 

useful in oblique and illustrative terrestrial photography. 

Video or movie footage is valuable when studying 

dynamic events such as ash plumes or pyroclastic flows, 

or for calculating the speed of lahars or floods. When 

shown at talks or seminars, video or movie footage 

allows the scientific and public community to feel what 

it is like to be at an active volcano. 

CVO has made only limited use of video or movie 

cameras as an illustrative tool, and numerous 

public-education opportunities have been lost simply 

because video or movie cameras were not taken into the 

field to take appropriate· footage of the crater, dome, or 

scientists working. Such cameras have been and are 

being used in individual scientific projects, however. 

One project was the 1980-82 closed-circuit 

television/video system installed on Harrys Ridge (Miller 

and Hoblitt, 1981), and a similar project currently in use 

employs a slow-scan video system to relay real-time 

images of the crater to the offices at CVO (Furukawa 

and others, chapter 19). In 1986-87, a low-light 

television/video system was installed on the Toutle River 

for viewing the river channel during lahars or floods (Jon 

Major, oral common., 1990), and from 1981-86, a 

night-vision system consisting of video and still cameras 

was used to monitor changes in the system of hot cracks 

on the lava dome (Robin Holcomb, oral common., 1990). 

A 16-mm movie camera was used in 1981-83 to study 

debris flows originating from Shoestring Glacier on 

Mount St. Helens {Tom Pierson, oral common., 1990), 

and in 1988 and 1989, 8-mm movie cameras were used 

to monitor glacial outburst floods originating from South 

Tahoma glacier on Mount Rainier (Joe Walder, oral 

common., 1990). Movie cameras were also successfully 

used to photograph sequences of active dome growth 

during the 1982 and 1986 dome-building episodes at 

Mount St. Helens. 

Video cameras are available in l-inch, 3/4-inch, 

\1-inch, and 8-mm tape formats. They offer the luxury of 

convenience; footage shot on videotape can be instantly 

reviewed and studied back at the office, thereby offering 

quick interpretation and response to possible hazards. · 

Prints can be made from a single video frame, a 

technique that has been used to compare hot cracks on 

the dome preceding an eruptive episode. Such prints are 

generally of much lower resolution than either still or 

movie film however. Most video cameras also record 

sound simultaneously with the video, so that the 

photographer can narrate the scene and describe exactly 

what is being observed. 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 201 
There is no one ideal video camera system among 

the many types of video cameras available; cost and 

personal preference are major factors in deciding which 

camera to purchase. One popular video-camera design is 

the "camcorder," a system where the recording tape is 

mounted in the camera body. In 1989, four camcorder 

systems in use at CVO were: Jn-inch Panasonic AG-160 

ProLine VHS, JVC GF-S550 SuperVHS, and JVC 

GR-S77 SuperVHS "Compact," and an 8-mm Sony 

Handycam "Environmental Quality" system. 

One-half-inch VHS and SuperVHS systems are 

economical, readily available, and offer good quality 

images; their tapes can be purchased at nearly every 

camera store or supermarket (a useful feature when in 

the field). The Jn-inch JVC "Compact" and the 8-mm 

Sony "EQ" systems are small and lightweight and fit 

easily into backpacks. The Sony "EQ" camcorder is 

weather resistant and ashproof. Its 8-mm blank tapes are 

not as readily available as are Jn-inch format tapes, 

however. This camera is popular with the scientists, as 

they do not have to carry a bulky camera around yet can 

still enjoy viewing the scene immediately when back in 

the office. 

CVO uses two formats of movie cameras, a 16-mm 

and an 8-mm-size format. The 16-mm offers better 

quality, but the 8-mm is lightweight, economical, and 

more convenient to use. In 1989, three types of movie 

cameras in use at Mount St. Helens were a 16-mm 

Canon Scoopic, an 8-mm Canon Auto Zoom 518 

Super-8, and three 8-mm Eumig 128 XL Super-8 movie 

cameras, modified for time-lapse by Timelapse, Inc. The 

Eumigs are in weatherproof housings that can be easily 

mounted on a tripod, and the time-lapse feature is 

programmable from one frame every 0.5 second to one 

frame every 99.5 minutes. 

CVO has used movie cameras to obtain 

"one-of-a-kind" footage. On May 18, 1980, a CVO 

geologist shot 700 ft of 16-mm movie film of Mount St. 

Helens's eruption plume. This footage has been 

repeatedly requested by video and movie companies and 

television producers during the past 10 years. For 

security and protection against damage to the film, the 

original is now kept under lock and key and a copy 

negative is kept on file. Duplicate films and videotapes 

are sent to prospective users, and all reproductions of the 

footage are made from the file negative. This procedure 

is highly recommended to anyone obtaining irreplaceable 

footage. In 1982 and 1986, one-of-a-kind dome-eruption 

footage was filmed using the modified Eumig 8-mm 

movie cameras. The cameras photographed growth of 

emerging lobes on the lava dome. 

Unfortunately, movie-camera formats are becoming 

more difficult to effectively find and use, owing to the 

popularity of video camera systems with the general 

public. Some camera companies that manufactured 8-mm 

movie cameras are simply no longer doing so, and some 

film laboratories are phasing out the processing of movie 

film. 

TECHNIQUES 

Vertical and oblique aerial photography, repeat and 

illustrative terrestrial photography, and time-lapse 

photography are all techniques available for documenting 

changes occurring on or around volcanoes. Any and all 

types of cameras can be used. To ensure a successful 

photographic trip, pay attention to four main points. 

Camera Protection 

Volcanic ash and gases destroy cameras. Ash 

scratches camera lenses and film and can jam the 

camera's internal mechanisms, while volcanic gases 

corrode the camera body and the delicate circuits of 

electronic cameras. To protect the camera against ash 

and gas, keep it in a case or camera bag when not in use 

and load film in a protected environment. Use clear 

filters (such as "UV" or "Skylight" filters) to protect the 

lens surface. Filters are expendable and more economical 

to replace than the camera lens. Have the camera system 

cleaned regularly. Take extreme care when using video 

systems in the ashy volcano environment, as even the 

slightest dusting of ash can interfere with the recording 

heads. 

Low-temperature Photography 

Volcanic activity occurs during cold winter months 

as well as warm summer ones, creating a problem with 

camera operation in freezing weather. Cameras 

continually exposed to freezing temperatures become 

sluggish as mechanical reaction time increases (shutter 

speeds lengthen). Film freezes and becomes brittle, and 

camera batteries lose their efficiency at low 

temperatures. To alleviate these problems, keep camera 

and extra film warm by carrying it near the body under 

outer garments, and if available, install fresh silver oxide 

or lithium batteries before venturing into the field. 

Silver oxide and lithium batteries perform better in cold 

temperatures than the regular alkaline types (Gillsater, 

1985, p. 33). Always carry extra batteries. Frozen film 

should be thawed at least an hour before using. 

202 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Bracketing Exposures 

When photographing volcanoes, film is usually the 

cheapest factor involved, so take many frames of the 

same subject using different exposure settings. Shoot one 

frame as the camera lightmeter indicates, then bracket 

exposures by increasing exposure (overexposing) and 

decreasing exposure (underexposing) for the next frames. 

In extreme conditions, increase or decrease exposures by 

one and two stops, depending upon the subject. For 

example, the correct exposure for dark objects (as Mount 

St. Helens's dome) surrounded by brightly-lit snow will 

generally be one or two stops of increased exposure from 

that which the lightmeter indicates. Conversely, photos 

of a snow-covered peak surrounded by blue sky and 

green forest often are washed-out if shot at the exposure 

the lightmeter indicates. Therefore, decrease exposure by 

one or two stops to properly expose for the snowy peak. 

Ash, mud, and snow are difficult subjects to photograph, 

for they have very little inherent color and contrast, and 

in these situations, most camera lightmeters have 

difficulty indicating the correct exposure. By bracketing 

exposures, the proper exposure-not necessarily the one 

the camera meter indicates-may be obtained. To 

bracket exposures on automatic cameras, use the 

"exposure-compensation dial" and adjust to the plus side 

to increase exposure and to the minus side to decrease 

exposure. 

Accurate Notes 

Keeping accurate notes may be the most important 

aspect of volcano photography, other than the actual 

shooting of the photograph. Keep notes on cameras, 

lenses, and filters used, film type, exposure, date and 

time, weather and lighting conditions, the height of the 

tripod, the subject being photographed, and the location 

from where the photograph was taken. Keep notes on 

everything and anything that might be important, for they 

may prove useful years later. 

VERTICAL AERIAL PHOTOGRAPHY 

Vertical aerial photographs (figs. 20.3, 20.4) are 

taken when the camera's optical axis is pointing 

downward, within 20• of vertical (Ray, 1960, p. 2). The 

cameras (usually large format, occasionally medium 

format, and rarely small format) are mounted in the belly 

of an aircraft and shoot through a camera port. The 

aircraft flies in straight lines back and forth across an 

area, following predetermined flightlines. A flightline 

consists of three or more photographs taken in measured 

succession, with each photograph overlapping adjacent ones 

by 30 to 60 percent. Vast areas of ground can be 

photographed in a few minutes. The flightlines at Mount St. 

Helens were designed to document the impact of volcanic 

eruptions on the peak and the subsequent changes occmring 

within the crater and surrounding river valleys after the 

May 18 eruption. 

The photographic scale of vertical aerial photography 

varies and is determined by the focal length of the lens 

used divided by the altitude of the aircraft above the 

ground, expressed in a ratio. The lower the altitude or the 

longer the focal length, the larger the photographic scale is 

and the more detail that can be seen (fig. 20.3). 

Photographic scale is only an approximation at best, 

as topographic relief of the terrain photographed and the 

inclination of the optical axis of the camera are two major 

factors that cause variation in scale (Miller and Miller, 

1961, p. 8). For example, at Mount St. Helens a flightline 

over the crater and north to Spirit Lake is flown at an 

altitude of 7.6 km (25,000 ft). A 152-mm (6-in.) 

focal-length lens is used, and approximately six frames are 

photographed. A single frame can cover the entire dome, 

crater, and flanks of the volcano (fig. 20.3A). Photographic 

scale of that frame ranges from 1:42,000 around the flanks 

of the volcano, to 1:40,000 across the crater floor, to 

1:34,000 at the crater rim. Another flightline is flown at an 

altitude of approximately 2.7 km (9,000 ft) and photographs 

are taken using the same lens. A single frame covers the 

dome (fig. 20.3B}. This photograph has a larger scale than 

the previous photograph (hence more detail can be seen), 

and its scale ranges from approximately 1:8,000 at the base 

of the dome to 1:6,000 on top of the dome. 

By 1990, the flightline over Mount St Helens crater 

and dome had been rephotographed over 150 times. All 

photography was contracted to professional aerial 

photography companies, altitudes and photographic scale 

varied, and b/w and color negative film was used. In June 

1980, over 100 flightlines specifically designed to 

document drainage-channel development around Mount St. 

Helens were established (fig. 20.4), and these flightlines 

were rephotographed yearly during the low water season 

and after major hydrologic events. Scale was 1:9,600, and 

b/w film was used (with color fum being used occasionally 

in 1980). Flightlines varied in length from 3 to over 20 

frames depending upon the straight length of the drainage 

reach. Large drainages would typically have many 

flightlines, whereas small drainages were covered by one 

flightline (fig. 20.5). 

The individual photographs from flightlines can be 

pasted together and read like a map. The overlapping areas 

of two adjacent photographs form stereoscopic pairs that 

can be used to directly see elevation differences in the 

photographed terrain. With the proper support 

equipment, stereoscopic pairs can be used to create 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 203 
Figure 20.3. L a r g e - f o r m a t 

vertical aerial photographs, 

reduced 50 percent, illustrating 

different photographic scales·. A, 

Mount St. Helens crater and dome 

photographed from 7.6 km 

(25,000 ft), showing crater floor at 

a scale of approximately 

1:40,000. 8, Lava dome 

photographed from 2. 7 km (9,000 

ft), showing top of dome at a scale 

of approximately 1 :6,000. 

A 

204 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 

B 
Figure 20.4. L a r g e - f o r m a t 

vertical aerial photographs, 

reduced 50 percent, illustrating 

drainage channel development 

around Mount St. Helens at a 

scale of approximately 1:9,600. A, 

Section of debris avalanche 

photographed September 15, 

1980, shows absence of drainage 

channel development. 8, Same 

area photographed September 

28, 1985, shows establishment of 

major drainage channels. 

A 

B 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 205 
206 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
topographic contour maps. At Mount St. Helens, 

photography flown after each dome-building episode or 

after major hydrologic events has been used to produce 

numerous topographic maps of the crater, dome, and 

debris avalanche. 

The value of vertical aerial photography was 

proved at Mount St. Helens in 1980 (Moore and Albee, 

1981). A topographic map was made from photography 

flown across the volcano on April 12, 23 days after 

earthquake activity began. This map was compared with 

a published 1958 contour map (based on 1952 

photographs) and significant topographic differences 

were noted. To eliminate possible changes due to glacial 

action during the 28-year span covered by the maps, a 

new topographic map was "hastily prepared" from 

photography of August 15, 1979, and compared with the 

April 12, 1980 map. Differences in elevations of the 

summit graben and bulge area on the north side of 

Mount St. Helens were apparent. According to Moore 

and Albee (1981, p. 127): 

The photogrammetric data *** in conjunction with 

ground geodetic measurements *** were of prime 

importance in understanding the processes acting 

within the volcano and in assessing the hazard due 

to oversteepening of the north slope. In retrospect, 

the photogrammetric data set was acquired at low 

risk, and consequently should be an important 

element in monitoring future activity of potentially 

explosive volcanoes of similar type. 

OBLIQUE AERIAL PHOTOGRAPHY 

Oblique aerial photographs (fig. 20.28) are taken 

when the camera's optical axis is considerably inclined 

from the vertical, generally 20• or more (Ray, 1960, p. 2). 

Krimmel and Post (1981, p. 31) believe that this 

technique is one of the more useful photographic 

techniques used at volcanoes: 

Oblique aerial photographs offer many advantages 

over vertical photographs for documentation of 

volcanic events; these include more natural 

perspective, greater aerial coverage from low altitude, 

greater flexibility in aircraft operation, coverage of 

specific localities during periods of partial cloud or 

ash obscuration, and lower cost. 

Figure 20.5. (facing page) Section of a location map 

showing layout of flightlines over drainage channels 

southeast of Mount St. Helens. Contours in feet. 

Numbers on the flightlines are indexed to the CVO photo 

library. Circled numbers within bold arrows refer to 

interpretive features described on original base map 

(U.S. Geological Survey, 1981 ). 

Small-format 35-mm cameras loaded with color 

slide film are the most common camera format and film 

used at CVO for oblique aerial photography of the 

Cascades volcanoes. A medium-format camera loaded 

with b/w film is also used by the staff photographer. 

(Although not common, a large-format camera can also 

be used for oblique aerial photography; see Krimmel and 

Post, 1981). Video or movie cameras are occasionally 

used. The cameras are always hand-held (except for the 

large-format camera used by Krimmel and Post, which is 

mounted in the airplane). The most useful lenses for 

oblique aerial photography are the standard and wide 

angle, as the telephoto lens magnifies aircraft vibrations, 

making it difficult to use with complete success. 

The following are a number of helpful points to 

remember when photographing from an aircraft. 

1. Set the camera focus on infmity. Since virtually 

all aerial photographs will be taken at infinity, tape the 

focus ring at infinity before the aircraft leaves the ground. 

This will ensure that the focus is not accidently bumped 

during the excitement of volcano photography. Vinyl 

plastic electrical tape works well, as it is easily removed 

without breaking or leaving residue. 

2. The shutter speed of the camera used in aerial 

photography should be at least l/250 second, and 

preferably 1/500 or 1/1000 second A slower shutter speed 

may introduce motion, resulting in blurred photographs. 

Motion during exposure is one of the main problems of 

good aerial photography and is caused by the camera being 

hand-held, the forward speed and mechanical vibration of 

the aircraft, and the smoothness of the flight relative to how 

much turbulence is in the air (Eastman Kodak Company, 

1985, p. 8). To reduce the effects of motion, use as fast a 

shutter speed as possible and keep the camera from 

touching the aircraft Use a faster film instead of a slower 

shutter speed if the day is overcast (at CVO, 200 or 400 

speed films are used on overcast days). To obtain a fast 

shutter speed with an automatic camera, use as fast a fum 

as possible. 

3. Shoot through open windows or remove the 

helicopter door before takeoff. Aircraft windows have 

distorting curves and hundreds of scratches, so that 

shooting through plastic windows severely limits the quality 

of the photographs. If possible, shoot through an open 

window or have the helicopter pilot remove the doors, 

thereby leaving an unobstructed view. If you must shoot 

through windows, hold the camera as close to and as 

perpendicular to the window as possible. This will 

minimize the possibility of unpleasant reflections from the 

windows. With auto-focusing cameras, shooting through an 

open window is a must, as many auto-focusing cameras 

may focus on the window instead of the volcano. 

4. Keep airplane wings and helicopter blades out 

of the view. Photographs with airplane wings and 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 207 
helicopter blades are less pleasing than those without 

Airplane wings and helicopter blades also can cause 

auto- focus cameras to focus on them instead of the 

view. 

5. The location of the sun is an important factor 

when shooting aerial photographs. Aerial photographs 

taken with a sun angle of 20-30• above the horizon are 

the most satisfactory (Hackman, 1967, p. B157); 

photographs taken when the sun is 1 o· or less above the 

horizon produce long shadows. In areas of high relief, 

such as volcanic terrain, shadows of large topographic 

features often obscure too much of the area unless the 

sun is well above the horizon. This has proved to be a 

major problem when photographing the Mount St. 

Helens dome, as the crater wall casts its shadow on the 

dome during winter months, or early or late in the day. 

In the summer, photography taken at midday produces 

little shadow and shows almost no relief at all, making 

details harder to see. 

REPEAT TERRESTRIAL PHOTOGRAPHY 

Repeat terrestrial photographs are taken over a 

period of time from a permanent mark and are used to 

document long-term landscape change, such as biological 

(fig. 20.6), erosional (fig. 20.7), and geological (figs. 

20.1, 20.8) development around volcanoes. Repeat 

photographs can also be combined into one composite 

diagram to illustrate years of cumulative landscape 

evolution (fig. 20.9). Since 1980, more than 100 repeat 

terrestrial photography stations (photo stations) have 

been established at Mount St. Helens and the other 

Cascades volcanoes. 

Ideally, repeat terrestrial photographs are taken 

from the same point, using the same camera and lens (or 

at least the same camera format and focal-length lens), 

with the camera at the same height above the ground, 

and aimed in exactly the same direction. This results in 

the same photographic view each time. At Mount St. 

Helens, this goal has not been fully achieved. Many 

photo stations are sporadically visited by different people 

with different camera, lens, and film combinations, and 

who stand in slightly different locations. Harrison (1974, 

p. 469) stated: "Retrieval of information from old 

photographs of geological features will be enhanced if 

the photographs are repeated from the same site. 

Reoccupying the exact site may not be essential, but it 

makes the interpretation of the photographs more 

convenient." What has resulted at CVO is a collection of 

photographs that are not all perfectly repeated views, yet 

nonetheless document changes around the volcano (fig. 

20.7). 

When establishing or reoccupying a photo station, 

a number of ideas should be kept in mind: 

1. Numerous photo stations should be established. 

According to Veatch (1969, p. 51), "it is better to 

establish too many rather than too few photographic 

stations. Some stations can always be dropped, but once 

any photographic records are missed they are lost 

forever." Over the years at Mount St. Helens, many 

different photo stations have been established, often 

within sight of each other, but not all stations have 

survived. Some have been destroyed by volcanic activity 

and river channel migration; the locations of others have 

been forgotten. A few were established for short-term 

A 

B 

Figure 20.6. Two repeat photographs of Lassen Peak 

illustrate biological development occurring around 

volcanoes. A, Lassen Peak after May 22, 1915 blast. 

B.F. Loomis photograph, courtesy of Loomis Museum 

Association, Mineral, California (loomis, 1926). 8, View 

from same location 69 years later. 

208 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
projects, occupied for a few days, weeks, months, or 

even years, and then abandoned when the project ended. 

Some stations have turned out to be good photo stations 

and have been reoccupied many times over the past 10 

years, and others, while being visually and 

informationally good, have not been reoccupied 

consistently because of cost or difficulty of access. 

2. Each photo station should be a permanent 

mark on the ground. Permanent marks should be 

established so photographers know exactly where to 

place the tripod to repeat the photographs. At Mount St 

Helens, fence posts, steel towers, large rocks, bridge 

supports, established USGS bench marks, and, as a last 

resort, spots on the ground located a known number of 

paces from a permanent object have been used as 

permanent markers. During one dome-building episode, a 

A 

8 

Figure 20.7. Although taken from slightly different 

positions, two repeat photographs of Upper Muddy River 

at Mount St. Helens illustrate erosional development 

around volcanoes. A, October 1980, five months after 

May 18, 1980, eruption. 8, Same area photographed one 

year later. 

short-term permanent mark was established using a 

55-gallon drum as a camera mount (fig. 20.10). 

Long-term permanent marks at other Cascades volcanoes 

are often established CVO bench marks (fig. 20.11). 

3. Tripods are a must A tripod facilitates aiming 

the camera, helps to ensure that the camera is correctly 

aligned over the permanent mark, and is necessary to 

maximize photo sharpness by reducing camera 

vibrations. Very few people can hold a camera 

sufficiently still during an exposure longer than 1/60 

second. Camera movement results in blurry photographs. 

At CVO, Bogen 3020 "Professional" tripods with Bogen 

3025 "3-D" or 3028 "Super 3-D" three-way-tilt heads are 

used. At other Cascades volcanoes, the staff 

photographer uses a Bogen 3058 "Super Pro" tripod with 

a 3047 "Deluxe" heavy-duty three-way-tilt pan head, or 

A 

8 

Figure 20.8. Repeat photographs of Mount St. Helens 

lava dome illustrating importance of choosing correct focallength lens. A, In 1981, dome was photographed using a 

standard 55-mm lens from a photo station approximately 

1 km away. 8, When photographed 4 years later, dome 

had outgrown field of view of 55-mm lens. 

20. Basic Photography at Mount Sl Helens and other Cascade Volcanoes 209 
the head is mounted on the Kern tripod used in the 

deformation network (lwatsubo and Swanson, chapter 

10). 

4. Take along a copy of the photo taken 

previously at the photo station to aid in exact framing of 

the same scene. 

5. To ensure successful repeat photography, keep 

accurate notes on cameras and lenses used, film type, 

date and time, the height of the tripod, the angle of tilt of 

the camera if not level, and the azimuth of the camera. 

Record detailed descriptions on how to get to the photo 

site and how to identify the permanent mark. These will 

all be important when repeating the photograph at a later 

date. If the photo station cannot be reoccupied on a 

particular visit, note why. 

6. Take a photograph of the camera setup at the 

photo station. This greatly facilitates reoccupying the 

exact site and is an invaluable aid to the next person sent 

to occupy the site. Many of the photo stations at other 

Cascades volcanoes are official U.S. Geological Survey 

bench marks that do not protrude above the surface of 

the ground. Photographs showing where the tripod is, 

with nearby rocks, vegetation, or outcrops visible (fig. 

20.11), have saved considerable time when searching for 

the station. 

7. Shoot the scene with both color slide and b/w 

negative film, so that the same scene can be 

photographed in slide format for talks and shows and in 

b/w format for publications and historical archiving. 

Al~ough b/w prints may be derived from color slides, 

they are inferior to a print from an original b/w negative 

and therefore not as desirable for historical purposes. 

8. If possible, use the same focal-length lens each 

time a photo station is occupied. This helps to facilitate 

quick comparison of images without having to 

manipulate the photographs, but it is not absolutely 

necessary. According to Malde (1973, p. 198) "views 

made with cameras of different focal lengths from the 

same lens position will always match exactly, provided 

that both are printed and cropped to the same size." 

~1986 

EXPLANATION l: :; : l 1984/1985 

[::::::::::::::::::11983 

l ~ ~ 7

] 1982 

f.•2 1981 

-1980 

Figure 20.9. Composite diagram of dome growth, made 

from 6 years of repeat terrestrial photography, taken from 

a photo station approximately 1 km north of dome. 

During eruptions and floods, however, there is no time to 

print and crop photographs. Also, when different people 

are reoccupying the photo stations, having one "project 

camera" available to give to the field crews results in 

more consistent photographs. For example, many CVO 

photo stations were established at cross-section survey 

points along the stream drainages. Repeat photography is 

shot upstream, downstream, and across the drainage 

while hand-holding the camera and standing at the 

channel edge near the cross-section marker, or near the 

edge over the instrument stake. The field crews doing the 

Figure 20.10. A 55-gallon drum filled with rocks was 

used as a camera mount and a permanent mark for a 

short-term photo station. Drum was filled with rocks for 

stability, a tripod head was secured to iid, and camera's 

protective metal housing was attached to tripod head. 

210 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
surveying (and the photography) are seldom the same 

each time. By eliminating one variable, that of the 

different fields of view of different cameras and lenses, 

the resulting images have been easier for researchers to 

study later. 

9. Lenses of different focal lengths should be 

used on the same subject at each photo station during 

each visit. Shooting a photo station at only one focal 

length might present irreversible difficulties in the future. 

On August 22, 1981, the dome was approximately 163m 

high and 400 m wide when photographed from a station 

1 km north of the dome. The dome fit within the frame 

of the 35-mm camera using a standard 55-mm 

focal-length lens (fig. 20.8A). By August 12, 1985, the 

dome was 230 m high and approximately 800 m wide, 

and no longer fit in the frame (fig. 20.88). Unfortunately, 

it was too late to capture the 1981 scene with the 

wider-angle 28-mm focal-length lens. 

10. When possible, the photographs should be 

taken about the same time and date each year. "In this 

way, because the shadows and highlights are then 

faithfully reproduced, the old and new photographs can 

be better compared." (Rogers and others, 1984, p. xxvi). 

At Mount St Helens this often is not possible; many of 

the photo stations are reoccupied only after eruptions or 

floods, regardless of the time of the year. Other photo 

stations visited several times a year offer better 

opportunities for getting photographs at nearly the same 

date and time. 

11. If the weather or lighting conditions are not 

perfect, take the photograph anyway. Because film is 

inexpensive compared to the cost of getting to a photo 

station, a large number of photographs are taken at 

Mount St Helens photo stations even if the weather or 

lighting conditions are not perfect. That way, if the 

station cannot be reoccupied during better conditions for 

some reason, at least something has been recorded. At 

the other Cascades volcanoes, photographs are always 

taken when a photo station is reoccupied because it may 

be years before we visit it again. 

12. Consider the future. Try to imagine what the 

photo station will look like in the years to come. Will 

there be any small trees in the foreground that will one 

day grow up to block the view (fig. 20.6)? Is the station 

so close to the edge of an active channel that it may be 

eroded away? Is the station directly in the path of 

Figure 20.11. Tripod setup at South Sister volcano facilitates relocating that site years. later. At South Sister, 

same tripod is used for photo station and deformation network's surveying instruments. 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 211 
potential lahars, pyroclastic flows, or rock avalanches? If 

so, perhaps consider establishing another photo station in 

a more secure spot. 

ILLUSTRATIVE TERRESTRIAL 

PHOTOGRAPHY 

Illustrative terrestrial photography is perhaps the 

most common photographic technique employed by 

scientists. Illustrative terrestrial photographs (figs. 20.2A, 

B, D, and 20.12) are taken from the ground of any 

subject, with no plans to ever reoccupy that exact 

photographic spot again, and with no other purpose 

except to document and illustrate some natural or 

man-made feature. Every year at Mount St. Helens 

hundreds of illustrative terrestrial photographs are taken 

of the crater, dome, drainage channels, erosion, rockfalls, 

returning plant life, scientific equipment, researchers at 

work, and other topics. 

Occasionally place a "ruler" in the scene when 

in a roll of spectacular footage of the lobe's active 

spreading center. 

Minolta X700 cameras with programmable data 

backs were also used during the same two eruptive 

episodes. On August 19, 1982, a camera was mounted in 

photographing close-in subjects and include- a A 

recognizable object in the scene when photographing a 

distant subject. Measurements can then be calculated 

from the photograph, or at least a general idea of scale of 

the image can be obtained. Rock hammers, shovels, ice 

axes, camera lens caps, or film boxes are useful when 

photographing close-in subjects (fig. 20.12A), and fellow 

researchers, helicopters, and automobiles work well in 

wider views (fig. 20.128). 

TIME-LAPSE PHOTOGRAPHY 

Time-lapse photography is another technique 

useful when photographing volcanoes. For purposes of 

this report, time-lapse photography is defined as repeat 

terrestrial photography taken in a short period of time 

with programmable cameras. The cameras are 

programmed to photograph an important geologic or 

hydrologic event by shooting a roll of film in 

predetermined time intervals. At Mount St. Helens, 

time-lapse photography has been successfully used to 

photograph dome growth during dome-building episodes 

and debris flows originating at Shoestring Glacier. 

In August 1982, an 8-mm Eumig movie camera, 

modified for time-lapse, was installed at a photo station 

approximately 1 km from the dome and used to 

photograph an emerging lava lobe. A frame was shot 

every 5 minutes, resulting in nearly 5 days of lobe 

growth condensed on one 50-ft roll of film. In October 

1986, the camera was placed on top of the dome and 

aimed at an actively extruding lobe. The camera was 

programmed to shoot a frame every 10 seconds, resulting 

B 

Figure 20.12. "Rulers" help illustrate size in 

photographs. A, Film box (circled) shows relative size of 

boulder. 8, Two scientists (circled) show relative size of 

trees devastated on May 18, 1980. 

212 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
a protective housing and set on a tripod approximately 

0.5 km north of the dome. Throughout the next two days, 

five rolls of fllm were shot, at different time intervals. 

The site was then reoccupied and photographed 

sporadically during the next two weeks. Figure 20.13A 

shows ttacings of the August 19 lava lobe taken from the 

time-lapse frames, and figure 20.13B shows ttacings of 

the new lobe on August 19 and 20 and September 1. 

On October 21, 1986, two Minolta X700's were 

installed 8 km north of the dome in an attempt to 

photograph the predicted emergence of a new lobe 

during the night One camera was programmed to take an 

exposure 10 minutes long every 2 hours, and the other 

camera was programmed to take a 20-minute exposure 

every hour. A 20-minute exposure taken at 0500 hours 

A 

'i~~-j~t;, 

8 

•• ·~.-·. :,. 0 

: -::~· .. :~·. 

EXPLANATION 

8-19-82 

· · · • · 09:57 P.D.T 

----- 15:44 

-20:44 

EXPLANATION 

..• • . 8-19 09:57 

----- 8-20 11:19 

--9-1 11 :20 

/ New lobe 

""" New lobe 

Figura 20.13. Between August 19 and September 1, 

1982, a 35-mm time-lapse programmable camera was 

used to photograph dome growth. Resulting photographs 

were later used to sketch growth. A, Tracings of lobe 

during August 19. 8, Tracings of lobe on August 19 and 

20 and September 1. 

(local time) the morning of October 22 shows no hint of 

a glowing lobe while the next frame taken at 0600 hours 

clearly shows glow from a hot dome. The camera 

documented the emergence of the lobe, consistent with 

the interpretation of seismic records (Elliot Endo, oral 

commun., 1986). The next night, October 22, the 

cameras were programmed to take 30-minute exposures 

of the new lobe (fig. 20.14). 

In 1981-83, the Minolta X700 camera was also 

used to photograph sequences of debris flows originating 

from Shoestring Glacier. The camera's motor drive was 

set to shoot 3.5 frames per second when remotely 

triggered by an infrared beam {Tom Pierson, oral 

commun., 1990). This setup was successful three times. 

Time-lapse cameras left at photo stations create a 

new set of photographic problems. Rain, snow, ice, and 

ash can coat camera lenses or housing windows, clouds 

can obscure views, wind can blow cameras over, and 

eruptions, lahars, or floods can destroy cameras. Camera 

batteries can die with no warning; cold weather is the 

major reason for the failures. Numerous attempts at 

photographing emerging lobes on the dome have been 

ruined by blowing ash during the summer, cold, snow, 

and ice during the winter, and clouds obscuring the view 

at all times of the year. Attempts to photograph 

drainage-channel development during major storms have 

been unsuccessful due to wind and rain. One of the 

Eumig 8-mm movie cameras was swept away at Mount 

Rainier during a glacial outburst flood. 

To ensure the best possible chances of obtaining 

good time-lapse volcano photography, mount the 

cameras at secure locations in environmentally sealed 

camera housings, keep the cameras . warm during colder 

months, and provide enough power to keep the system 

running. At Mount St. Helens, an unheated plexiglass 

box and an unheated metal box with an inset glass 

window (fig. 20.10) are used to protect the Minolta 

X700's. The Eumig movie cameras have their own 

weatherproof housings, which still need to be sealed with 

duct tape to prevent blowing ash from entering the 

system (fig. 20.15). All cameras are generally mounted 

low to the ground, and rocks or sandbags are placed 

around them to provide stability in winds (which can 

exceed 100 km/hr). Although the Eumig movie cameras 

have not been heated, they have usually remained 

running even during the winter. Ice quite often forms on 

them, however, making the footage unusable. The 

Eumigs run on one 6-volt "lantern" battery, and the 

cameras have been modified to accept a parallel battery 

setup. The Minolta X700's will not stay running during 

periods of cold, and they have not been modified to 

accept parallel batteries. In the warmth, at 2o·c, one 

lithium battery (3 volts) will operate a Minolta X700 for 

approximately 6 hours of exposure time. 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 213 
The Minolta X700's used to photograph the night 

scene of the growing new lobe (fig. 20.14) were mounted 

in an unheated observation building, with camera ports 

cut into the walls. Winter temperatures in the building 

have been recorded as low as -ts•c, necessitating an 

inexpensive method to keep the cameras warm. A 

camera-heating system using electric socks (such as 

those worn by hunters and fishermen) was devised. 

Electric socks have a heating element sewn into the sock 

toe and are powered by a 1.5-volt "D"-cell battery. The 

socks are modified to accept up to four batteries in 

parallel, therefore providing over 30 hours of heat. Two 

socks are used per camera. The socks are turned 

inside-out and the heating elements placed next to the 

camera, data back, and motor-drive batteries. The rest of 

the sock is then wrapped around the camera to provide 

extra insulation, and the entire setup is covered with 

waterproof nylon. Only the front of the camera lens is 

exposed. 

QUANTATATIVE MEASUREMENTS 

The capability to make quantitative measurements 

from photographs is an important application of 

photography. The researchers at CVO have used this 

capability extensively with vertical aerial photography 

and occasionally with terrestrial or oblique aerial 

photography. The dimensions of the dome (Swanson and 

others, 1987) are routinely calculated with the help of 

photographs. 

Quantitative measurements and calculations are 

based on four variables: h, the subject size; h', the image 

size (size of the subject as measured on the negative or 

transparency); F, the lens focal length; and v, the 

lens-to-subject distance (Blaker, 1976, p. 343). Their 

relationship is 

h'/F = h/v. _ 

This formula can be used not only for obtaining 

measurements from photographs but also to determine 

what focal-length lens is needed, or how far away a 

photo station must be from a subject in order to have that 

subject fill the frame. 

Measurements are usually obtained from 

large-format vertical aerial photographs. The lens focal 

length and lens-to-subject distance (altitude of the 

aircraft above the ground surface) are routinely recorded, 

image size is easily measured off the large negative or 

transparency, and subject size is then computed. Image 

size can also be determined from photographic prints if 

Figure 20.14. A programmable 35-mm time-lapse camera was used to shoot this 30-minute evening exposure 

of the October 22, 1986, dome growth, resulting in this moonlit view of a 15-hour-old lobe. 

214 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
the relationship is known between the print and the 

original negative or transparency (amount of enlargement 

or reduction). Image size is then calculated as if 

measured from the negative or transparency. 

If the scale of a vertical aerial photograph is 

known, two variables can be deduced. Since scale is 

represented as a ratio against 1, the image size (h') is the 

"1", and the subject size (h) is the other number. If either 

the altitude of the aircraft or the focal length of the lens 

is known, the remaining variable can then be solved for. 

This is useful in determining at what altitude the aircraft 

should fly to obtain the desired scale. If a 1:10,000-scale 

photograph of the dome is desired and the camera has a 

152-mm (6-in.) lens, the aircraft will need to fly 

approximately 1.5 km (5,000 ft) above the top of the 

dome to obtain the desired scale 

1/152 = 10,000/v = 1,520,000 mm = 1.5 km. 

Measurements can also be obtained from oblique 

aerial or terrestrial photography, although the process is 

more difficult and not as exact. In an oblique photo the 

distances in the photo generally increase from the bottom 

(nearest the camera) to the top (farthest from the 

camera), thus making it difficult to calculate subject size. 

Image size is readily obtainable from the negatives or 

transparencies, and lens focal length should be written in 

the notes. Unfortunately, both the subject size and the 

lens-to-subject distance are variable across the film and 

are often unknown. One of these needs to be obtained to 

complete the equation. 

Figure 20.15. For camera protection, this time-lapse 

movie camera has its housing edges taped for ash 

protection and tripod sandbagged for wind protection. 

An easy method to obtain lens-to-subject distance for 

terrestrial photography calculations is to measure the 

distance between the photo station and the subject using 

topographic maps or vertical photography. Another method 

is to use surveying equipment and accurately measure the 

distance from the camera to the subject. When surveying 

cannot be done, rough estimates of the lens-to-subject 

distance can be made if the distances of at least two known 

objects (control points) roughly in the same plane as the 

main subject are obtained. The distance to the original 

subject can then be estimated geometrically between the 

control points. The equation can also be solved if the 

subject size or the control-point sizes are known or can be 

measmed or estimated, leaving the lens-to-subject distance 

to be calculated. The surveying or measwing of the control 

points does not have to be done when the photo station is 

initially established and it does not have to be done each 

time a photograph is taken, so long as it is completed 

sometime during the life of the photo station (Malde, 1973, 

p. 197). 

Once the distances to, or sizes of the control points 

are known, distances and sizes of many features within any 

photograph taken from that photo station can be calculated, 

although the exactness of the calculations depends on the 

number and location of the control points. The greater the 

nmber of control points measured, the more exact will be 

the calculated subject size. Approximate size can still be 

determined if only one control point is available and often 

this rough estimate is all that is needed to help illustrate an 

idea. For example, on April 16, 1983, 35-mm photography 

from a photo station was used to calculate the rate of rise of 

a small plume of steam and ash (fig. 20.16). This particular 

photo station was chosen because of its location of 

approximately the same elevation as the dome, thus 

offering a nearly perpendicular view of the dome providing 

one known control point The photo station was also close 

enough to be able to measure the plume images on 35-mm 

film, yet far enough away to minimize the effect of 

varying distances between the plumes and the camera. A 

Minolta X700 camera with motor drive and data back was 

programmed to shoot a picture every 5 seconds and was 

manually turned on at the first sign of a plume. For this 

situation, the focal length of the lens was known, the image 

size was measured from each frame. The lens-to-subject 

distance of the "center-of-dome" control point was 

measured from topographic maps. The unknown variable, 

the height of the plume, was then calculated and the rate of 

rise was plotted. 

CONCLUSIONS 

Personnel of the Cascades Volcano Observatory have 

used a variety of cameras and techniques to document changes 

20. Basic Photography at Mount St. Helens and other Cascade Volcanoes 215 
and events at Mount St. Helens and other Cascades 

volcanoes. The selection of camera equipment has been 

largely decided by scientific need, budget limitations, 

and personal preference. Still photography, especially 

small-format, has been used most extensively, while 

video and movie photography has been tmderutilized. A 

variety of techniques has been used involving aerial and 

terrestrial photography, and numerous photo stations 

have been established. Use of photographic products 

A 

16:21 :30 145m 16:21 :35 205m 

16:21 :45 350m 16:21:50 410 m 

16:22:00 535 m 16:22:05 595 m 

i 

eral~

16:22:15 680 m 16:22:20 720 m 

\ 

~- ~ 

16:22:30 805 m 16:23:00 1030 m 

16:21:40 290m 

16:21 :55 475 m 

('( 

'\ f 

1 

crate;-wan 

16:22:10 640 m 

16:22:25 760 m 

\ 

\, 

\ 

I 

\ 

\_, 

16:23:30 

Out of frame 

include illustrations for publications, transparencies for 

slide shows, maps made from vertical aerial 

photography, and diagrams made from repeat terrestrial 

or time-lapse photography. We have learned that 

photography in any form is better than no.photography at 

all. 

REFERENCES CITED 

Blaker, A.A., 1976, Field photography, beginning and advanced 

techniques: San Francisco, W.H. Freeman, 451 p. 

Eastman Kodak Company, 1979, Preservation of photographs: 

Rochester, Eastman Kodak Company, 61 p. 

__ 1985, Photography from light planes and helicopters: 

Rochester, Eastman Kodak Company, 33 p. 

Gillsater, S., 1985, Antarctica: Photomethods, v. 28, no. 11, p. 

33. 

Hackman, R.J ., 1967, Time, shadows, terrain, and 

photo interpretation: U.S. Geological Survey Professional 

Paper 575-B, p. B155-B160. 

Harrison, A.E., 197 4, Reoccupying unmarked camera stations 

for geological observations: Geology, v. 2, p. 469-471. 

Krimmel, R.M. and Post, Austin, 1981, Oblique aerial 

photography, March-October 1980, in Lipman, P.W., and 

Mullineaux, D.R., eds., The 1980 eruptions of Mount St. 

Helens, Washington: U.S. Geological Survey Professional 

Paper 1250, p. 31-51. 

Loomis, B.F., 1926, The pictorial history of Lassen Volcano: 

Mineral, California, Loomis Museum Association, 96 p. 

Malde, H.E., 1973, Geologic bench marks by terrestrial 

photography: U.S. Geological Survey Journal of Research, 

v. 1, p. 193-206. 

Miller, C.D., and Hoblitt, R.P., 1981, Volcano monitoring by 

closed-circuit television, in Lipman, P.W., and Mullineaux, 

D.C., eds., The 1980 eruptions of Mount St. Helens, 

B 

(/) 

a: 

~ 1000 w 

~ 

~ 800 r: 

I 

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w 

I 600 w 

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_J 

a.. 400 0 

w 

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i= 

200 

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0 0 

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C\i c\i c\i c\i c\i cY:i C\1 C\1 C\1 C\1 C\1 

TIME, IN FIVE SECOND INTERVALS 

Figure 20.16. Two minutes of time-lapse photography help illustrate rate of rise of a small plume of steam and ash. 

A, Height of rising plume sketched and calculated from oblique photographs. 8, Plume height plotted against time. 

216 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Washington: U.S. Geological Survey Professional Paper 

1250, p. 33S-341. 

Miller, V.C., and Miller, C.F., 1961, Photogeology: New York, 

McGraw-Hill, 248 p. 

Moore, J.G., and Albee, W.C., 1981, Topographic and 

structural changes, March-July 1980-Photograrnrnetric 

data, in Lipman, P.W., and Mullineaux, D.C., eds., The 

1980 eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 123-134. 

Ray, R.G., 1960, Aerial photographs in geologic interpretation 

and mapping: U.S. Geological Survey Professional Paper 

373,230 p. 

Rogers, G.F., Malde, H.E., and Turner, R.M., 1984, 

Bibliography of repeat photography for evaluating 

landscape change: Salt Lake City, University of Utah 

Press, 179 p. 

Swanson, D.A., Dzurisin, D., Holcomb, R.T., lwatsubo, E.Y., 

Chadwick, W.W., Jr., Casadevall, T.J., Ewert, J.W., and 

Heliker, C.C., 1987, Growth of the lava dome at MolDlt St. 

Helens, Washington (USA), 1981-1983, in Fink, J.H., ed., 

The emplacement of silicic domes and lava flows: 

Geological Society of America Special Paper 212, p. 1-16. 

Tilling, R., 1984, Eruptions of Mount St. Helens: Past, present, 

and future: U.S. Government Printing Office, 46 p. 

U.S. Geological Survey, 1981, Mount St. Helens and vicinity: 

United States 1:100,000-Scale Series, 4S121-H8-TM-100. 

Veatch, P.M., 1969, Analysis of a 24-year photographic record 

of Nisqually Glacier, Mount Rainier National Park, 

Washington: U.S. Geological Survey Professional Paper 

631, S2 p. 

20. Basic Photography at Mount SL Helens and other Cascade Volcanoes 217 

21. The Importance of Field Observations for 

Monitoring Volcanoes, and the Approach of 

"Keeping Monitoring as Simple as Practical" 

By Donald A. Swanson 

ABSTRACT 

Geologic field observations form an Important part of 

any volcano-monitoring program yet are often overlooked 

in lists of monitoring techniques. Such observations 

provide the opportunity to Integrate many different kinds 

of data on the spot and to design simple measurements 

to test key questions resulting from the observations. 

Field observations go hand In hand with more 

sophisticated equipment and techniques to form a 

complete system for monitoring volcanoes. Monitoring 

programs should explicitly include provisions for geologic 

field observations and Instill In field workers, scientists 

and technicians alike, the need to be flexible and clever 

In designing simple experiments and measurements to 

test Important field observations on the spot. 

INTRODUCTION 

Other chapters in this bulletin describe the many 

different methods of monitoring volcanoes that have 

been used by the on-site and affiliated research staff of 

the Cascades Volcano Observatory (CVO). Most of these 

methods inv-olve relatively sophisticated equipment, 

training, or data analysis. Many of the methods have 

been used successfully at a number of volcanoes around 

the world in various environmental conditions. They are 

largely proven commodities that can be put to use for 

study of active volcanoes anywhere. They refl~ the 

remarkable ingenuity and significant advancement in 

monitoring volcanoes that characterize the past decade in 

volcanology. 

My aim in this essay is to highlight another means 

of monitoring that is the oldest that volcanologists have 

and yet is commonly overlooked in establishing a 

monitoring program. It cannot take the place of the ~ore 

sophisticated techniques but can augment them and help 

guide their use. It is the only means that allows instant 

on-the-spot integration of a number of parameters that 

characterize the volcano, and instant reevaluation of that 

integration. It is old-fashioned yet up-to-date, operates in 

all but the most extreme environments, and is as durable 

as the remarkable machine that runs it. The means is 

direct field observation, and the machine is the observer. 

The simplicity of geologic field observations belies 

their significance, and I provide three examples to 

demonstrate that fact. I also try to make the point that 

even simple measurements made on the basis of the 

observations can be sufficient, and that making the 

measurements more sophisticated than required does not 

necessarily improve their value--and in fact can even 

undermine their utility. 

EXAMPLES 

Three examples from Mount St. Helens illustrate 

my thesis that observations and simple measurements can 

be effective under the right circumstances. They are 

representative but not exhaustive examples of the 

importance of this thesis at Mount St. Helens. The 

examples are some of those with which I am most 

familiar. 

Spirit Lake as a Tiltmeter 

The first explosion from Mount St. Helens took 

place on March 27, 1980, and immediately thereafter 

volcanologists began to wonder if a magma body were 

rising beneath the volcano and, if so, what were its depth 

and volume. The volcano was covered by deep snow, but 

it was possible to drive to its base and a short ·distance 

up its northeast flank. Hence plans were quickly made to 

initiate tilt measurements by both single-setup leveling 

and electronic tiltmeters. But it would take time to 

acquire the necessary equipment, wait for new bench 

marks and foundations to stabilize, and obtain data. 

Moreover, the thick pumice blanket at the base of the 

21. The Importance of Field Observations for Monitoring Volcano•• 219 
volcano near Spirit Lake made it difficult to find stable 

sites for instruments and bench marks. 

Why not use Spirit Lake as a giant carpenter's 

level? This idea had several advantages. The lake was far 

enough from the foot of the volcano to be an effective 

monitor of deep-seated changes under the volcano. The 

lake was oriented with its long direction radial to the 

volcano, and quick calculations suggested that tilts on the 

order of 2 J!rad would be detectable with reasonably 

careful measurement procedures. The lake was covered 

with ice, so that wind-related waves would be damped 

out. Boat docks and partly submerged stumps in small 

areas of open water provided sites to install measurement 

devices. 

But what devices could be obtained quickly? We 

couldn't afford the time to obtain normal water-level 

sensors and install them. An inexpensive and readily 

available alternative was immediately apparent. A visit to 

a local lumber yard resulted in an outright gift of several 

wooden yardsticks (metersticks), which were easily 

nailed to piers and stumps. Within a day the level of the 

lake was monitored by individuals visiting each site, 

reading the level of the lake on the partly submerged 

meterstick, recording the time of their visit, and closing 

back on a station chosen as a reference point. Later we 

used several observers with synchronized watches, but 

the results were the same: The shore of Spirit Lake was 

not tilting. This told us that a large volume of magma 

was not intruding under the volcano, so that we could 

concentrate our attention on the edifice of the volcano 

itself (Lipman and others, 1981 ). 

The North-Flank Bulge 

In mid-April, 10-12 days after monitoring of the 

lake began, several different geologists viewing the 

volcano noticed that its upper north flank had apparently 

deformed relative to its pre-1980 shape. Earthquakes 

recorded on portable seismographs were located beneath 

the north flank, so it seemed reasonable that the 

deformation was related to shallow intrusion of magma 

into the edifice. But the observed deformation was 

obvious only on glaciers, which apparently were 

buckling and bulging upward. Was this deformation 

confined to the glaciers and the result of rapid advance 

caused by subglacial melting, or was the underlying 

edifice also taking part in the deformation? 

The obvious way to test for deformation of the 

edifice was to make repeated measurements to targets on 

snow-free parts of the volcano. Quickly a standard 

theodolite used by most surveyors was obtained and 

angles measured to prominent natural features on the 

volcano. The data suggested outward movement of the 

north flank, but aiming on natural features was difficult 

and subject to considerable error. Within a day or two an 

electronic distance meter (EDM) arrived, and reflectors 

were placed on the mountain. The EDM is a 

sophisticated instrument, but the targets were nothing 

more than clear plastic highway reflectors screwed to 

boards that were lashed to steel fence posts hammered 

into the ground. A combination of the theodolite and 

EDM measurements to these targets soon indicated that 

the north flank was moving northward at a steady rate, so 

that clearly the volcanic edifice was deforming in 

response to intrusion of magma (Upman and others, 

1981). The theodolite measurements to the wooden 

targets provided sufficient data by themselves to define 

the rapid displacements of as much as 2.5 m/d. The EDM 

data were useful adjuncts to the theodolite measurements 

but were not necessary to trace the movements at all but 

the least sensitive targets. 

Measurements of Cracks and Thrust Faults 

Used for Prediction of Dome Growth 

Geologists first observed cracks on the crater floor 

in mid-September 1980. Would they continue to widen, 

or had they formed quickly in response to some 

short-lived but unknown event? Simple measurements 

provided the answer. Short stakes of steel reinforcement 

rod (rebar) were pounded into the ground on either side 

of several cracks, and the distances between the stakes 

were measured with a carpenter's steel tape. Repeated 

measurements showed that some of the cracks widened 

with time, and that the rate of widening accelerated 

before the explosive event of October 16-18 and the 

dome-growth event of December 27, 1980-January 3, 

1981 (Swanson and others, 1981). 

During the December-January event, geologists 

were surprised to find two thrust faults on the crater floor 

north and northwest of the dome. The thrust faults faced 

outward, away from the dome. The same questions arose 

with these faults as with the cracks: were they still 

moving and, if so, might they provide an indication of 

future eruptive activity? Again rebar stakes were driven 

into the upper and lower plates of each thrust, and the 

distance between them was measured with the steel tape. 

The distance between each pair of stakes shortened with 

time and indicated that the upper plate was moving 

across the lower plate. Repeated measurements of the 

thrusts and associated radial cracks soon showed a 

distinctive pattern. Rates of displacement were slow after 

a dome-building episode but increased nearly 

exponentially as the onset of the next dome-growth event 

neared (Chadwick and others, 1983). This pattern and 

various other details of the entire deformation process of 

the crater floor were nicely traced with the simple 

rebar-and-tape method and formed the basis for a series 

220 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
of predictions of dome growth in 1981-1982 (Swanson 

and others, 1983). Moreover, the simple measurements 

with a steel tape provided the primary data for two 

interpretative papers about how and why the crater floor 

deformed (Chadwick and others, 1988; Chadwick and 

Swanson, 1989). 

DISCUSSION 

These three examples each show the value of 

on-the-spot field observations and the application of 

simple measurement techniques to resolve important 

questions and stimulate significant interpretations. 

Within hours to several days the basic questions raised 

by each set of field observations were answered, 

although of course refining the details took much longer. 

The answers came quickly, in part because the questions 

were easy (yet very important), but in part because the 

measurements could be started quickly owing to the 

simplicity of the methods used. Moreover, the simple 

methods were reliable and didn't depend on a complex 

chain of equipment, any link of which could fail 

unexpectedly. Total reliance on sophisticated equipment 

can backfire if the means is unavailable for its rapid 

repair. In general, a good guide to follow for devising a 

monitoring method is to "keep it as simple as practical." 

Since 1980 notable improvements have been made 

in portable electronic-monitoring equipment, as several 

chapters in this bulletin show. Nonetheless, given the 

same circumstances as those in the three examples, I 

believe that the answers would still come faster with the 

simpler methods, largely because the sophisticated 

equipment requires significant installation and 

"settling-in" time. There is no question that the more 

sophisticated techniques fill important gaps, because 

most have the capability to acquire data almost in real 

time, 24 hours a day under any weather condition. But 

one of the principal limitations of these techniques is one 

of the principal strengths of field observation and simple 

measurements: flexibility and the ability to integrate 

several observations on the spot and to design a 

measurement to test that integration. Continuous 

measurements often can proxy for an on-site observation, 

but even the most broadly based-television 

monitors-are less adaptable and ingenious than a human 

observer in the field. 

The flexibility provided by field geologic 

observations cannot be overemphasized. Most electronic 

sensors are designed to detect and report one parameter, 

such as tilt, displacement, seismicity, or changes in a 

particular species of gas. If some unmonitored parameter 

changes, the sensor either doesn't detect it or may 

provide spurious information. On-the-spot geologic 

observation clearly is not restricted to a single parameter, 

although of course it is restricted to what can be 

observed with the eyes, ears, and nose. Small changes 

may go unnoticed by field observers, but large changes 

may go unnoticed by electronic sensors not monitoring 

the proper parameter. Only on-the-spot observers can 

quickly assess the situation and determine which 

parameters are likely to provide vital information. 

Measuring an unimportant quantity accurately and 

continuously using sophisticated equipment can be a 

waste of time and resources; it is much better to get to 

the heart of the matter by the simplest means possible. 

Another important point is that even the best 

remote-monitoring techniques require verification by 

on-site field observations and integration with other data. 

For example, an electronic tiltmeter may indicate a 

change, but this change could reflect an electronic 

problem, an unstable installation, or real deformation of 

the volcano. Only independent information can determine 

which interpretation is most likely. As a rule of thumb, 

never blindly accept data obtained remotely until on-site 

verification can be made. Accept no substitute! 

Many, colleagues emphasize the importance of 

sophisticated equipment and telemetered data during 

times of great hazard to field workers, and I agree with 

their reasoning. It is far better to lose a piece of 

equipment than a life. No reasonable person would 

advocate fieldwork under conditions that he or she feels 

are life-threatening. But there are many times in the 

activity of a volcano when conditions are not so 

hazardous, and it is these times that are suitable for field 

observation and indeed would benefit from such 

observations. Moreover, those of us who study active 

volcanoes must admit that certain risks exist, just as they 

do for firefighters and police officers. An integral part of 

our job is to assess those risks and establish personal 

guidelines for the relatively safe conduct of our research. 

I see no point in providing further discussion of 

putative advantages and disadvantages of geologic field 

observations versus electronic measurements. The real 

point to be made is that we should think not in those 

terms (although many of us do), but instead in terms of 

an integrated monitoring effort that incorporates the best 

of all observations into unified interpretations. No one 

approach is inherently superior. There is a tool for every 

job, and the trick is to find the best tool or set of tools, 

whether they are close-in observations, complex 

equipment, sophisticated telemetry systems, or a 

combination of all three. 

Nothing is new in this discussion, but the emphasis 

on sophistication and telemetered data in this bulletin 

seems to me to require a counterpoint, even though an 

obvious one. Field observations and related measurements 

are a vital component of volcano monitoring, just as are 

electronic sensors, radios, and high-priced surveying 

equipment. In fact, the two approaches commonly merge. 

21. The Importance of Field Observations for Monitoring Volcano•• 221 
For example, sophisticated (and expensive) EDM's 

measure to painted wooden boards with plastic reflectors 

screwed on them-a real marriage of the aristocracy and 

the proletariat! 

Indonesia provides a remarkable example of how a 

combination of simplicity and complexity has saved 

lives. In 1988, Indonesia experienced seven explosive 

eruptions, all of which had seismic precursors. As a 

result of the warnings and preliminary, closely observed, 

minor eruptive activity, 33,000 people were evacuated 

from their homes, and only four lives were lost (all on 

Banda Api, where those who were killed were knowingly 

evading the evacuation order) (f. J. Casadevall, written 

commun., 1989). Indonesia has about 150 active 

volcanoes and 50 volcano observatories. Each 

observatory has from two to four observers, generally 

only one seismometer with a smoked-drum seismograph, 

and typically only radio contact (a few have telephones) 

to central headquarters in Bandung. Nonetheless, the 

observers are well trained to note changes in seismicity 

and many other parameters (such as the presence of new 

ejecta, changes in plume behavior, fumarole 

temperatures, characteristics of crater lakes, and other 

factors), and they are from the region around the 

observatory, know the local people well, and have their 

trust. Perhaps there are lessons here for all of us. One 

lesson is that the simplest kind of monitoring and 

warning system that works is the best one to use; in this 

example, the monitoring system consisted of the simplest 

kind of complex instrument-a single seismometer and 

smoked-drum seismograph, and the warning system 

involved the observers who were intimately familiar with 

the monitored volcanoes and with the people of the 

region. Another lesson, not germane to the theme of this 

chapter yet of great importance, is that the familiarity 

(and even closeness) of the observers to the local 

populace is a key that we should consider more in our 

efforts to save lives. 

During some volcanic crises, many scientists and 

technicians converge on the scene (or to a field 

observatory) but may stay for only short periods of time, 

returning to other duties as required. Continuity of 

observations and data gathering is difficult to maintain 

with such a rotation of staff, especially for field 

observations that commonly involve relatively 

subjective descriptions. One way to minimize such 

problems is to develop and use a check list of the kinds 

of field observations that should be made routinely. 

Such a list would in detail be specific to each volcano 

and would be susceptible to modification as the activity 

of volcano developed. Another key element in 

maintaining continuity of field observations is the 

thorough transfer of information between departing and 

arriving observers, preferably by visits to the volcano 

together. 

CONCLUSIONS 

My purpose in writing this essay is not to minimize 

in any way the modem monitoring techniques that are 

helping make volcanoes safer and better understood. I 

have used many of these techniques myself and have 

championed their implementation by others. Instead I 

simply want to point out that any monitoring effort is 

incomplete if trained observers are not made a part of it, 

and if the opportunity is not given for these observers to 

influence the gathering and interpretation of the data 

acquired by sophisticated techniques. In my view a 

volcano observatory or monitoring program of any sort 

starts with the people on its staff, not with the equipment 

in its locker or planned in its budget. Each geologist or 

geophysicist who spends much time on a volcano should 

be trained to observe field conditions, to think about 

those observations while in the field, and to be flexible 

and clever in devising simple measurements that can be 

made quickly and definitively once changes are noted. 

Those scientists and technicians should not have tunnel 

vision for only their specialty but instead should 

integrate all available information and be ready to 

respond to observable changes of any significant 

parameter. These individuals should be given the field 

time to spend on active volcanoes, and in fact such time 

should be an integral part of the monitoring program. 

Nature is too complex for us to learn enough about a 

volcano by monitoring it only remotely; we must also 

observe and monitor it personally and closely. 

A volcano is too complex to be understood by 

individuals working alone. Free communication and 

exchange of data, observations, and ideas among all 

workers involved in monitoring are essential. Integration 

of field observations with telemetered data is vital to 

preparing a unified assessment of the volcano and its 

hazards. The stakes are so high that the data and 

observations, and the resulting development of 

interpretations, must be shared in an atmosphere of 

intellectual curiosity and social responsibility, rather than 

compromised in an atmosphere of competition or one of 

conflict among senior and junior scientists and 

technicians. 

REFERENCES CITED 

Chadwick, W.W., Jr., Archuleta, RJ., and Swanson, D.A., 

1988, The mechanics of ground deformation precursory to 

dome-building extrusions at Mount St. Helens 1981-1982: 

Journal of Geophysical Research, v. 93, p. 4351-4366. 

Chadwick, W.W., Jr., and Swanson, D.A., 1989, Thrust faults 

and related structures in the crater floor of Mount St. 

Helens volcano, Washington: Geological Society of 

America Bulletin, v. 101, p. 1507-1519. 

222 Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory 
Chadwick, W.W., Jr., Swanson, D.A., lwatsubo, E.Y., Heliker, 

C.C., and Leighley, TA, 1983, Deformation monitoring at 

Mount St. Helens in 1981 and 1982: Science, v. 221, p. 

1378-1380. 

Upman, P.W., Moore, J.G., and Swanson, D.A, 1981, Bulging 

of the north flank before the May 18 eruption-Geodetic 

data, in Upman, P. W., and Mullineaux, D. R., eds., The 

1980 eruptions of Mount St. Helens, Washington: U.S. 

Geological Survey Professional Paper 1250, p. 143-155. 

Swanson, D.A, Upman, P.W., Moore, J.G., Heliker, C.C., and 

Yamashita, K.M., 1981, Geodetic monitoring after the 

May 18 eruption, in Upman, P. W., and Mullineaux, D. 

R., eds., The 1980 eruptions of Mount St. Helens, 

Washington: U.S. Geological Survey Professional Paper 

1250, p. 157-168. 

Swanson, D.A., Casadevall, TJ., Dzurisin, D., Malone, S.D., 

Newhall, C.G., and Weaver, C.S., 1983, Predicting 

eruptions at Mount St. Helens, June 1980 through 

December 1982: Science, v. 221, p. 1369-1376. 

21. The Importance of Field Observations for. Monitoring Volcanoes 223 

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