UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

EXPERIMENTAL INVESTIGATIONS

REGARDING THE USE OF SAND

AS AN INHIBITOR OF AIR CONVECTION

IN DEEP SEISMIC BOREHOLES

by

L. Gary Holcomb

Leo Sandoval

Bob Hutt

Open- file Report 98-362

This report is preliminary and has not been reviewed for 

conformity with U.S. Geological Survey editorial standards. 

Any use of trade names is for descriptive purposes only and 

does not imply endorsement by the U.S. Geological Survey.

Albuquerque Seismological Laboratory 

Albuquerque, New Mexico

1998
	Table of Contents

1 INTRODUCTION ................................................................................................................. 1

2 PHYSICAL SETUP ............................................................................................................... 4

3 SEQUENCE OF EVENTS .................................................................................................... 5

4 DATA PROCESSING ........................................................................................................... 6

5 BAND AVERAGED PSD RESULTS ................................................................................... 9

6 RMS LEVELS IN THE 20 TO 600 SECOND PASSBAND ................................................ 13

7 POTENTIAL SOURCES OF EXCESSIVE HORIZONTAL NOISE .................................. 20

8 CONCLUSIONS ................................................................................................................... 22

9 ACKNOWLEDGMENTS ..................................................................................................... 23

10 REFERENCES ..................................................................................................................... 24

11 APPENDIX .......................................................................................................................... 25
Table of Figures

1. Conventional and sand installation diagram ............................................................................ 3

2. Sensor 26 vertical PSD after sand ............................................................................................ 6

3. Sensor 114 vertical PSD after sand .......................................................................................... 6

4. Sensor 26 north PSD after sand ............................................................................................... 7

5. Sensor 114 north PSD after sand ............................................................................................. 7

6. Sensor 26 east PSD after sand .................................................................................................. 8

7. Sensor 114 east PSD after sand ................................................................................................ 8

8. Definition of band averaged symbols ...................................................................................... 8

9. Sensor 26 vertical band averaged signal PSD levels ............................................................... 11

10. Sensor 114 vertical band averaged signal PSD levels ........................................................... 11

11. Sensor 26 north band averaged signal PSD levels ................................................................. 11

12. Sensor 114 north band averaged signal PSD levels ............................................................... 11

13. Sensor 26 east band averaged signal PSD levels ................................................................... 11

14. Sensor 114 east band averaged signal PSD levels ................................................................. 11

15. Sensor 26 vertical band averaged noise PSD levels .............................................................. 12

16. Sensor 114 vertical and averaged noise PSD levels .............................................................. 12

17. Sensor 26 north band averaged noise PSD levels .................................................................. 12

18. Sensor 114 north band averaged noise PSD levels ................................................................ 12

19. Sensor 26 east band averaged noise PSD levels .................................................................... 12

20. Sensor 114 east band averaged noise PSD levels .................................................................. 12

21. Sensor 26 vertical RMS signal level - 20 to 600 seconds ...................................................... 14

22. Sensor 114 vertical RMS signal level - 20 to 600 seconds .................................................... 14

23. Sensor 26 north RMS signal level - 20 to 600 seconds ......................................................... 14

24. Sensor 114 north RMS signal level - 20 to 600 seconds ....................................................... 14

25. Sensor 26 east RMS signal level - 20 to 600 seconds ............................................................ 14

26. Sensor 114 east RMS signal level - 20 to 600 seconds .......................................................... 14

27. Sensor ANMO vertical RMS signal level - 20 to 600 sec ..................................................... 16

28. Sensor COLA vertical signal level - 20 to 600 sec ................................................................ 16

29. Sensor ANMO north RMS signal level - 20 to 600 sec ......................................................... 16

30. Sensor COLA north RMS signal level - 20 to 600 sec .......................................................... 16

31. Sensor ANMO east RMS signal level - 20 to 600 sec ........................................................... 16

32. Sensor COLA east RMS signal level - 20 to 600 sec ............................................................ 16

33. Definition of "HI" in sand tables ............................................................................................ 30

34. SNZO 25% minimum averaged vertical RMS noise ............................................................. 35

35. SNZO 25% minimum averaged north RMS noise ................................................................. 36

36. SNZO 25% minimum averaged east RMS noise ................................................................... 37

37. Relative noise levels at several existing sites ......................................................................... 38

38. SNZO vertical median PSD surface ....................................................................................... 39

39. SNZO north median PSD surface .......................................................................................... 40

40. SNZO east median PSD surface ............................................................................................ 41

11
Table of Tables

1. RMS signal levels at ASL borehole sites - 20 to 600 sec ........................................................ 18

2. RMS signal levels at IDA borehole sites - 20 to 600 sec ......................................................... 19

3. Sand volumes for KS-54000 .................................................................................................... 31

4. Sand volumes for KS-36000 .................................................................................................... 32

5. Potential horizontal noise reduction estimates ......................................................................... 34

6. Sensor 26 vertical band-averaged signal levels ....................................................................... 42

7. Sensor 26 north band-averaged signal levels ........................................................................... 43

8. Sensor 26 east band-averaged signal levels ............................................................................. 44

9. Sensor 26 vertical band-averaged noise levels ........................................................................ 45

10. Sensor 26 north band-averaged noise levels .......................................................................... 46

11. Sensor 26 east band-averaged noise levels ............................................................................ 47

12. Sensor 114 vertical band-averaged signal levels ................................................................... 48

13. Sensor 114 north band-averaged signal levels ....................................................................... 49

14. Sensor 114 east band-averaged signal levels ......................................................................... 50

15. Sensor 114 vertical band-averaged noise levels .................................................................... 51

16. Sensor 114 north band-averaged noise levels ........................................................................ 52

17. Sensor 114 east band-averaged noise levels .......................................................................... 53

in
1 INTRODUCTION

Tilt has been the nemesis of horizontal long period seismology since its inception. Modern horizontal long period seismometers with their long natural periods are incredibly sensitive to tilt. They can

sense tilts smaller than 10~n radians. To most readers, this is just a very very small number, so we will 

begin with an example, which should help to illustrate just how small 10"11 radians is.

Suppose we have an absolutely rigid rod which is approximately 4170 kilometers long; this just 

happens to be the Rand McNally map scaled crow flight distance between Los Angeles and Boston.

Tilting this rod 10"11 radians corresponds to raising one end of the rod 0.0000417 meters. Alas, this is 

just another very very small number! However, this corresponds to slipping a little less than one third a 

sheet of ordinary copying paper under one end of this perfectly rigid rod. To clarify, we mean, take a 

sheet of paper just like the paper this report is printed on and split it a little less than one third in the

thickness direction, then put it under the end of the 4170 kilometer long rod! This will tilt the rod 10~H 

radians.

Real world seismometers are nowhere near the length of this rod. A KS-54000 is about two 

meters long. Tilting a rod only two meters long 10~n radians corresponds to moving one end of this rod 

a mere 0.00000000002 meters or 0.02 millimicrons. As one of the authors old math teachers used to 

say, "That's PDS" (PDS = Pretty Damn Small). Unfortunately, the long period seismologist does not 

have the luxury of ignoring PDS numbers when it suits him as the mathematician frequently does. He 

must live in the real world in which tilts this small create severe contamination of long period seismic 

data.

At periods longer than 20 seconds, tilt noise contaminates the long period data from all instruments installed on or near the earth's surface. Many years of experimentation revealed that installing 

the sensors at depth in deep mines drastically reduced the level of tilt noise in long period data. 

However, low levels of tilt noise persisted even at great depth; this noise was caused by air convection 

in the vault in which the sensors were installed. Over the years, methods were developed to control the 

air motion with mechanical barriers (boxes) around the sensors and by stratifying (creating a situation in 

which the air temperature increases with height) the air in the vault near the seismometer. These methods decreased tilt noise in deep mines to very low levels. However, deep mines, that are economically 

and environmentally suitable and accessible to seismology, are not plentiful and are not evenly 

distributed over the earth's surface. Therefore, the borehole deployable Teledyne Geotech KS-36000 

and later the KS-54000 sensor systems were developed to fulfill the need for instruments that could be 

installed at depth wherever high quality long period data was desired. Early in the development program, it became evident to the Teledyne Geotech personnel that air convection within the borehole was 

going to be a significant problem in KS deployments. Experimental and theoretical investigations 

conducted by Teledyne Geotech (see Douze and Sherwin, 1975, and Sherwin and Cook, 1976) produced 

a list of recommended installation procedures for reducing the effects of air convection. These procedures consisted of wrapping the sensor in a relatively thin layer of foam insulation, filling the free space 

volume in the vicinity of the centralizer-bail assembly with foam insulation, and the installation of 

styrofoam hole plugs immediately above the cable strain relief assembly at the top of the sensor package 

and at the top of the borehole. This technology has performed quite satisfactorily for over 20 years but 

evidence of tilt noise in the system output has persisted throughout the KS deployment program (the 

evidence was that the horizontal components were usually noisier than the vertical components) even in 

deep boreholes. Some deep borehole sites have been plagued by quite high levels of horizontal noise. 

Therefore, there has been a definite need for a new technique for controlling low level tilt noise in deep 

boreholes and the use of sand has been under consideration for several years.
Figure 1 contains conceptual illustrations of both the conventional holelock installed KS sensor 

system and the same sensor installed in sand. This figure demonstrates the major differences between 

the two installation methods. The curved arrows in the borehole on the left in the figure denote possible 

air convection cells which are believed to be the source of tilt noise in some of the conventional installations. This air motion is eliminated in a sand installation by filling most of the free air volume surrounding the seismometer with sand as shown in the right hand portion of the figure. The sand actually 

performs two functions; it prevents air motion and provides a remarkably ridgid clamping of the 

seismometer in the borehole.

This report presents the results of quantitative experimental investigations into the effectiveness 

of controlling low level air convection in seismic borehole installations with sand. The main body of the 

experimental effort consisted of installing two KS-540001 sensor systems in closely spaced shallow 

boreholes, allowing the sensors to reach equilibrium operation, and then pouring sand into both boreholes to observe any changes caused by pouring sand into the holes. The hypothesis of the experiment 

was that the sand would fill up the entire free air volume between the sensor package and the borehole 

walls thereby preventing movement of the air in the vicinity of the sensor package. The validity of this 

hypothesis had been qualitatively proven by earlier experiments at ASL and by the sand installations at 

the IRIS/ASL stations ANMO in 1995 and COLA in 1996. This experiment documents the degree of 

improved noise levels to be expected if KS instruments are installed in sand instead of in the conventional manner.
TILT

(Generated by 

Hir movement)

Borehole WalIs

Rir Movement 

(Convection cells)

KS Seismometer

Hole 1ock

Sand Shoe

Sand

Figure 1. Comparison of a conventional holelock installation and a sand installation.
2 PHYSICAL SETUP

The experiment was conducted at the ASL "Snake Pit" shallow borehole facility. This site consists of two relatively shallow (approximately nine meters deep) closely spaced (about 1.5 meters apart) 

boreholes located immediately beside a shallow subsurface vault. The two boreholes penetrate about six 

meters of fractured Precambrian granite and the concrete floor of the vault is poured on the surface of 

the same granite bedrock about three meters below the surface of the alluvial fill at the site. Surface 

instruments can be operated in the vault about two and a half meters horizontally from the two boreholes.

Holelocks were installed in both boreholes near the bottom of the hole with enough room below 

each holelock to prevent the seismometer pilot from touching the bottom of the hole. Two KS-540001 

sensors (serial numbers 26 and 114) were installed in the holes in the normal manner except that neither 

sensor was wrapped with insulation, and no foam plugs were installed immediately above the sensors or 

at the top of the holes.

An STS-2 was installed in the vault near the wall immediately adjacent to the boreholes. This 

instrument served mainly as a wind monitor because the horizontal components of the STS-2 were considerably noisier during windy time periods than were the horizontal components of the two KS instruments in the boreholes. During calm conditions, the STS-2 produced high quality quiet horizontal data 

comparable to that obtained from the two borehole instruments.

Initially, there were concerns about whether or not the shallow boreholes would prove to be quiet 

enough to permit successful measurements of low level convection generated noise within the borehole. 

Experience gained during the course of the experiment indicates that the two holes are quite adequate 

especially during calm wind time periods.

The data were recorded on a twelve channel Quanterra 24 bit digital data system configured to 

record 80, 20, 1, 0.1, and 0.01 samples per second continuously. The continuous high rate multichannel 

data filled a 150 megabyte magnetic tape in about 2 days.
3 SEQUENCE OF EVENTS

The sensors were operated for an extended shakedown period to determine if they were behaving 

properly. During this time period, two major problems were found and corrected. First, it was discovered that one of the sensors (sensor 114) was considerably noisier than the other (sensor 26) and that this 

noise level was slowly increasing with time. After numerous checks for possible sources of noise, it was 

discovered that the cable connector at the top of the sensor package had accumulated moisture inside the 

connector and that the connector pins were slowly corroding due to current conduction between the pins. 

Careful cleaning of the connector followed by sealing it against further moisture accumulation reduced 

sensor 114 noise to levels below those for sensor 26. This experience illuminates a potential problem in 

current and future ASL KS-54000I installations in the field because up to this point in time, this connector has not been sealed; this should be standard practice in the future. Attention then focused on sensor 

26 noise levels and it's continuous "burping". At ASL, "burping" means that the data contains randomly 

distributed excursions which frequently resemble the step response of the sensor system. Burps are usually much more prominent on horizontal components and are believed to be caused by mechanical steps 

in tilt of the sensor system. We finally discovered that a modified centralizer had been mistakenly 

installed on sensor 26; for reasons unknown, the three legs had been shortened by someone in the past 

and we had unknowingly installed a nonstandard centralizer which was not capable of centering and 

holding the top of the seismometer package in the center of the hole. The top of the sensor had been left 

free to flop about in the hole at will. The installation of the proper size centralizer eliminated the burps 

in sensor 26 and reduced it's noise levels considerably.

Continuous undisturbed (undisturbed except for pouring sand into the boreholes) operation for the 

sand experiment commenced on day 219, 1996 and continued through day 260, 1996. From the beginning, both sensors were remarkably quiet considering the fact that they were not wrapped and that styrofoam plugs were not installed in the borehole. A week of operation revealed that the horizontal 

components of sensor 26 were noisier than those in sensor 114, so sensor 26 was chosen as the first 

candidate for sand installation. The borehole was unsealed and a volume of sand calculated to fill the 

annulus between the sensor and the inner borehole walls to the top of the sensor was poured into the 

sensor 26 borehole in the morning of day 225. In the afternoon after the dust in the hole settled, visual 

inspection from the top of the hole using mirror reflected sunlight revealed that more sand was needed. 

More was added and inspection on the morning of day 226 indicated that the sand had accumulated to 

near the top of the seismometer package. The borehole was then resealed. Undisturbed operation continued until day 241 when sand was added to the sensor 114 borehole. This time, enough sand was 

poured into the hole the first time as confirmed by visual inspection during the afternoon of day 241; the 

hole was then resealed. Undisturbed operation continued through day 260.
4 DATA PROCESSING

The primary method of data analysis was to estimate the signal levels from the instruments using 

standard power spectral density (PSD) analysis procedures and to estimate the instrument noise levels 

using the direct method (Holcomb, 1980). First, the time series data for the entire approximately two 

day duration of each tape was Fast Fourier Transformed using a rectangular window with a 50% overlap 

between successive segments. These segments were then converted to PSD and corrected for the 

combined seismometer and instrumentation system amplitude response. The ten percent of these segments for which the PSD level between 30 and 100 seconds were the lowest were retained to calculate a 

segment averaged PSD estimate of the minimum signal from each channel. Including only the ten 

percent quietest segments should eliminate time periods during which earthquake signals are present in 

the data and it should also eliminate time periods during which the wind was blowing. These same 

simultaneous segments were used to calculate estimates of the noise levels in each channel using the 

direct method.

I I I I I II 11 

SNAKE SAND

SELF NOISE POWER SPECTRAL DENSITY 

26 Z

STAGE XX_NS2X_00061G START 1996.246,02.45 00 STOP 1996.24B,11-15-00

PERIOD, SECONDS

I uf 19

I 1 I I I I I3E

SNAKE SAND

SELF NOISE POWER SPECTRAL DENSITY 

114 Z

STAGE XX_NS2X_00061G START 1996.246.02:45.00 STOP 1996,24B.11 15 OD 

P32

PERIOD, SECONDS

Figure 2. Power spectral density estimates for the Figure 3. Power spectral density estimates for the

total signal (PI 1) and system noise (Nl) for the total signal (P22) and system noise (N2) for the

vertical component of sensor 26 after sand was vertical component of sensor 114 after sand was

poured into the borehole. poured into the borehole.
Examples of the output produced by this analysis is shown in Figures 2, 4, and 6 for sensor 26 and 

Figures 3, 5, and 7 for sensor 114. In these figures, the thick solid line is the "new low noise model" 

from Peterson, 1993. The thin solid line (PI 1 or P22) is the PSD of the total signal (the sum of the 

coherent ground input to the sensor and the incoherent internal noise generated by the sensor) at the output of the indicated channel and the thin solid line with open circles superimposed (Nl or N2) is the 

PSD of the estimated system noise level for that channel.

The PSD plots of Figures 2 through 7 present a comprehensive picture of instrument performance 

as a function of signal period, but it is difficult to visually compare several sequential PSD plots covering an extended period of time to determine trends in PSD levels. Therefore, this report will rely quite 

heavily on plots of the numeric averages of the PSD levels over preselected bands to illustrate the time 

behavior of the PSD levels. The bands selected for calculating the averages are 40 to 70, 70 to 100, 100 

to 200, 200 to 400, 400 to 700, and 700 to 2048 seconds. The average PSD levels over each of these 

bands will be assumed to be indicative of the instrument performance for the time period under analysis. 

Plots of these averages should portray trends in overall instrument performance. Figure 8 contains a 

definition of the symbols used to plot the various bands in Figures 9 through 20.

i i i i i i 111 i i i n i 11 r i i i MM; 

F SNAKE SAND 

SELF NOISE POWER SPECTRAL DENSITY 

= 26 N !

- STAGE XX_NS2X_00061G START 1996,Z46,02:45:00 STOP 1996,248.11:15:00 '

PERIOD, SECONDS

M I I I I I I IE

SNAKE SAND

SELF NOISE POWER SPECTRAL DENSITY 

114 N

STAGE XX_NSaX_00061G START 1996,246,02.45.00 STOP 1996,248,11-15:00 - 

fZZ N2 0 a 0

PERIOD, SECONDS

Figure 4. Power spectral density estimates for the 

total signal (Pll) and system noise (Nl) for the 

north component of sensor 26 after sand was 

poured into the borehole.

Figure 5. Power spectral density estimates for the 

total signal (P22) and system noise (N2) for the 

north component of sensor 114 after sand was 

poured into the borehole.
I I I I I I IE

SNAKE SAND

SELF NOISE POWER SPECTRAL DENSITY

26 E

STAGE XX_NS2X_00061G START 1996.246.02:45.00 STOP 1996.246.11:15 00 ^

Pll

l I I I I I I I I I I I I I I I IJE

F SNAKE SAND

SELF NOISE POWER SPECTRAL DENSITY 

= 114 E

- STAGE XX_NS2X_00061G START 1996,246.02.45:00 STOP 1996.246,11 15.00 

 P22 N2 0 8 0

PERIOD, SECONDS PERIOD, SECONDS

Figure 6. Power spectral density estimates for the Figure 7. Power spectral density estimates for the

total signal (PI 1) and system noise (Nl) for the total signal (P22) and system noise (N2) for the

east component of sensor 26 after sand was poured east component of sensor 114 after sand was

into the borehole. poured into the borehole.

A = 700 TO 2048 SECONDS 

A = 400 TO 700 SECONDS

 = 200 TO 400 SECONDS

 - 100 TO 200 SECONDS 

D - 70 TO 100 SECONDS 

O = 40 to 70 SECONDS

Figure 8. Definition of the symbols 

used to plot the estimated band 

averaged PSD levels in the various 

bands.
5 BAND AVERAGED PSD RESULTS

The time dependence of the lower levels of the total signal at the seismometer output for both 

sensors 26 and 114 for the duration of the experiment are plotted in Figures 9 through 14. Each plotted 

point in these figures represents the average PSD level for the appropriate band (see Figure 8) for the 

time period beginning at approximately the time at which the point is plotted extending to approximately 

the time at which the next point is plotted. The times at which the sand was poured into each of the 

boreholes are shown in all of the appropriate figures.

The time behavior of the vertical band-averaged signal PSD of sensors 26 and 114 throughout the 

duration of the experiment are shown in Figures 9 and 10 respectively. There does not appear to be any 

general overall trend in the vertical signal levels. They remain quite constant throughout the test period 

with small apparently random day to day variations. There was no detectable change in the minimum 

vertical signal levels of either sensor at the times when sand was poured into their respective boreholes. 

Note that the minimum signal levels for sensor 114 (Figure 10) are lower than those for sensor 26 (Figure 9). This is probably indicative of lower internal electronic system noise in the vertical channel of 

sensor 114.

The time behavior of the north band-averaged signal PSD of sensors 26 and 114 throughout the 

duration of the experiment is somewhat different as shown in Figures 11 and 12 respectively. There is a 

significant drop ranging from 5.09 to 10.08 db (see Table 7 on page 42 for details) in the band averaged 

PSD signal levels for sensor 26 at the time sand was added to the sensor 26 borehole (see Figure 11). In 

contrast, there is no apparent change in the slowly increasing trend of the band-averaged PSD signal levels for sensor 114 when sand was added to the sensor 114 borehole (see Figure 12).

The behavior of the east band-averaged signal PSD of both sensors (see Figures 13 and 14) is 

quite similar to that of the north. There is an even larger decrease in the minimum east signal levels 

ranging from 8.78 to 15.64 db (see Table 8 on page 43 for details) for sensor 26 when the sand was 

poured into sensor 26's borehole. However, as was true for the 114 north component, the minimum 114 

east averaged PSD signal levels do not show a change in the overall slightly increasing trend when sand 

was poured into the sensor 114 borehole (see Figure 14).

Minimum total signals averaged over the various bands are composed of the sum of both the 

coherent true groundmotion signal and the incoherent apparent internal system noise of the seismometer. 

True grondmotion is probably not absolutely constant as a function of time thereby increasing the difficulty of assessing the performance of the instrument system itself. The apparent instrument selfnoise 

(Nl and N2 in Figures 2 through 7) is an estimate of the incoherent noise level in the combined 

instrument/borehole system. If convection in the borehole is contributing to this incoherent noise level 

and if sand suppresses convection noise, the incoherent noise level should drop if sand is added to the 

borehole. Figures 15 through 20 contain plots of the band averaged estimates of the incoherent noise 

PSD levels for all three components of sensors 26 and 114.

The vertical band-averaged incoherent noise levels for both sensors remain relatively constant 

throughout the test period (see Figures 15 and 16). There was no detectable change when sand was 

poured into either borehole and the estimated noise levels for sensor 114 are considerably lower than 

those for sensor 26.

The decrease in the band-averaged incoherent noise levels for the north component of sensor 26 

when sand was added to sensor 26's borehole ranged from 5.23 db to 16.83 db (see Figure 16 and Table 

10 on page 45 for details). However, there was no discernable immediate change in the general overall 

increasing trend in the north incoherent noise levels for sensor 114 when sand was added to that borehole (see Figure 18).
The largest decrease in the band-averaged incoherent noise levels caused by adding sand occurred 

in the serial number 26 east component as shown in Figure 19. This decrease ranged from 10.8 db to 

21.74 db (see Table 11 on page 46 for details) Once again, there was no discrenable change in the east 

incoherent noise levels for sensor 114 when sand was added to the serial number 114 borehole (see Figure 20).

The reader may ask why there was no decrease in noise levels when sand was poured into the 

sensor 114 borehole. The answer to this question probably lies in the fact that the noise levels in sensor 

114 were already very low before sand was poured into the hole; they were much lower than were the 

noise levels for sensor 26. Both of these two sensors were initially installed in a conventional manner 

with a holelock, an operational centralizer, an azimuth ring, a pilot etc.; not all conventional KS installation have equal noise levels. Some conventional field sites have very quiet horizontals whereas some 

conventional field site horizontals are very noisy. Apparently, for reasons not understood even after 

over 20 years of experience with KS installations, there was little or no air convection in sensor 114's 

borehole before sand was poured into the hole. Conventional installations have always been erratic. 

Sand installations should be more reliable because they make it impossible for air to move in the vicinity 

of the sensor.

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ND IN # 26 SAND IN # 1 14

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

220 230 

TIME, DAYS

220 230 

TIME. DAYS

240 250

Figure 15. Time dependence of the vertical band 

averaged incoherent noise PSD for sensor 26.

NORTH NOISE LEVEL # 26

Figure 17. Time dependence of the north band 

averaged incoherent noise PSD for sensor 26.

EAST NOISE LEVEL # 26

Figure 19. Time dependence of the east band 

averaged incoherent noise PSD for sensor 26.

Figure 16. Time dependence of the vertical band 

averaged incoherent noise PSD for sensor 114.

> .,,-21

NORTH NOISE LEVEL # 114

SAND IN # 26 

I I I I I I I I I I I I I I I I

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Figure 18. Time dependence of the north band 

averaged incoherent noise PSD for sensor 114.

EAST NOISE LEVEL #114

SAND IN $ 26 SAND IN # 114

I I I I I I I I I I I i i i I I I I I I I I I i i i i I I I I I I I I I I I I

220 230 

TIME, DAYS

240 250

Figure 20. Time dependence of the east band 

averaged incoherent noise PSD for sensor 114.

12
6 RMS LEVELS IN THE 20 TO 600 SECOND PASSBAND

Another method of evaluating the long period performance of seismological instruments is to simply calculate the root of the mean square (RMS) value (see Bendat and Piersol, 1971) of the total signal 

in the 20 to 600 second passband for each data channel. This parameter is routinely calculated every 

half hour by the SHEAR software in all IRIS/ASL Quanterra systems and is subsequently stored as part 

of the station log. This quantity can be used to compare the relative performance of sensor systems with 

the same response functions. It provides a characterization of the signal level in the form of a single 

number; it is much easier to quantitatively compare numbers than PSD data when evaluating relative 

system performance. The data presented in this report has been slightly refined beyond the Quanterra 

procedure. Only the quietest 25% of the possible total of 48 RMS values per day has been retained and 

averaged together to provide a number representative of the quietest portions of each day. These 12 

point averages for each day from the three channels of sensors 26 and 114 are plotted for the duration of 

the experiment in Figures 21 through 26 and the same averages for the same time period from IRIS/ASL 

stations ANMO and COLA are presented in Figures 27 through 32. All four of these KS-540001 sensors 

were installed in sand.

Figures 21, 22, 27, and 28 contain the 20 to 600 second RMS signal levels for the vertical components of serial numbers 26 and 114 plus ANMO and COLA respectively. Note the great similarity in 

these four sets of data. The RMS values remain at a relatively constant value day after day. Note the 

simultaneous increase in the indicated vertical noise levels for the essentially collocated #26, #114, and 

ANMO sensor systems on day 219 and near day 250. The fact that these higher RMS levels occur on all 

three borehole systems at the same time indicates that an external source caused the higher noise levels. 

A possible external source could be increased event activity during that particular time period; if event 

generated ground motion is present in the earth's background more than 75% of the time during a particular day, a higher RMS signal level will result. This also may be the reason that the RMS signal levels 

of all three components at COLA exhibit more variation than they do at ANMO because COLA is much 

closer to a highly active earthquake source region.

The 20 to 600 second RMS signal levels for the north and east components are plotted in Figures 

23 through 26 and Figures 29 through 32. The most obvious character of these figures is that the RMS 

levels for the horizontal components of serial numbers 26 and 114 vary considerably more than those 

from ANMO and COLA. This is probably due to the fact that serial numbers 26 and 114 were installed 

in quite shallow boreholes (about 9 meters deep) whereas AMMO and COLA were installed at about 

152 and 120 meters depth respectively. Low levels of locally generated surface tilt probably contaminates the data from serial numbers 26 and 114 a significant portion of the time. Note the decreases in 

the RMS signal levels in both the north and east components of serial number 26 when sand was poured 

into the number 26 borehole. Note also that the RMS signal level for serial number 114 north component probably increased when sand was poured into the serial number 114 borehole (see Figure 24); it is 

difficult to tell for sure because the 114 RMS signal level had temporarily reached quite high levels 6 

times prior to putting sand in the 114 borehole. Therefore, the higher levels after sand was poured into 

the borehole may or may not be due to pouring sand into the hole. The RMS signal level for the east 

component of serial number 114 in Figured 26 appears to remain essentially constant as sand was added 

to the borehole.

13
#36 VERTICAL #114 VERTICAL

220 230 

TIME, DAYS

Figure 2 1 . Vertical # 26 RMS signal in the 20 to 

600 second band.

Figure 22. Vertical #114 RMS signal in the 20 to 

600 second band.

# 26 NORTH #114 NORTH

220 230 

TIME, DAYS

Figure 23. North # 26 RMS signal in the 20 to 600 

second band.

Figure 24. North #114 RMS signal in the 20 to 

600 second band.

# 36 EAST # 114 EAST

220 230 

TIME, DAYS

Figure 25. East # 26 RMS signal in the 20 to 600 

second band.

Figure 26. East # 1 14 RMS signal in the 20 to 600 

second band.

14
ANMO VERTICAL COLA VERTICAL

320 

TIME, DAYS

Figure 27. Vertical ANMO RMS signal in the 20 

to 600 second band. Data for day 243 is missing.

Figure 28. Vertical COLA RMS signal in the 20 

to 600 second band. Data for day 226 is missing.

ANMO NORTH

320 230 

TIME, DAYS

Figure 29. North ANMO RMS signal in the 20 to 

600 second band.

Figure 30. North COLA RMS signal in the 20 to 

600 second band. Data for day 226 is missing.

200

175

220 230 

TIME, DAYS

Figure 31. East ANMO RMS signal in the 20 to 

600 second band.

Figure 32. East COLA RMS signal in the 20 to 

600 second band. Data for day 226 is missing.

15
It should be very informative to compare the 20 to 600 second RMS signal levels from the four 

ASL sand installations in Figures 21 through 32 with the RMS signal levels from other borehole sites 

around the world. Table 1 summarizes the RMS signal levels at all the GTSN and IRIS/ASL borehole 

installations. The GTSN RMS values in the table were calculated from time series data recorded at the 

stations during days 001 through 080, 1995 and was corrected for the difference in the overall sensitivities between the GTSN and the IRIS/ASL recording systems. The IRIS/ASL RMS values in the table 

were calculated from time series data recorded at the stations during 1995. All of these sensors are 

installed at depths of 100 meters or greater with the single exception of BOCO which is much shallower 

(only 25 meters deep). Also included in Table 1 for comparison purposes are "eye ball" averages of the 

RMS levels for #26 and #114 in sand from Figures 21 through 26.

The RMS signal levels of the vertical components at all of the GTSN and IRIS/ASL borehole 

sites are all less than or equal to 200. That fact that the vertical RMS signal levels of the KS-36000I 

sensors tend to be larger than those for the KS-54000I sensors may be significant because the KS360001 sensors are all older than the KS-54000I instruments. The individual modules in the KS series 

are evacuated to reduce thermal convection noise generated within the module itself. Vertical modules 

have become noisy in the past when they lost vacuum. All of the KS sensor modules may be slowly 

losing their vacuum over the years; if so, the older KS-36000I verticals would tend to be noisier. Note 

the wide range in the RMS signal levels covered by the horizontal components in Table 1. There is no 

known physical reason to expect true horizontal ground motion levels at a given site to be significantly 

greater than vertical ground motion levels at that site. Therefore, horizontal RMS signal levels in the 

table which are significantly greater than their corresponding vertical level are indicative of excess noise 

in the horizontals at that station. Some of the stations have quite noisy horizontals (BDFB, CPUP, 

LBTB, and VNDA for GTSN, and CHTO, RAR, SNZO, and TATO for IRIS). Several others are somewhat noisier than they should be. On the positive side, stations such as LPAZ, ANMO, COLA, GRFO, 

and PTGA all have RMS signal levels below 100 for both horizontals. Obviously, the data in Table 1 

indicates that we (IRIS/ASL) are not controlling the noise levels in the horizontal components very well 

with current installation procedures. All of the sensors at the "AIR" type stations are installed in the 

same manner as far as is known. Yet the horizontal noise levels are quite variable.

The RMS signal levels at the IRIS/IDA borehole sites are listed in Table 2. These signal levels 

have been converted to IRIS/ASL sensitivity levels so these numbers can be compared approximately 

with those in Table 1. At most sites, the RMS levels were calculated from data recorded in early 1996; 

the single exception was MSEY for which data from 1995 was used. In general, the IRIS/IDA borehole 

sensor noise levels in Table 2 are comparable to the levels for the ASL-operated boreholes in Table 1. 

The vertical IRIS/IDA RMS signal levels are approximately equal to those found at the IRIS/ASL sites. 

It is noteworthy that the RMS signal level of the vertical at NIL is higher than those for the two horizontal sensors at that station; this may indicate a problem with the vertical but it may also be the result of 

the particular data segment selected to calculate the RMS levels. At three of the stations (BORG, 

MSEY, and SHEL), the horizontal components are quite noisy which indicates the possibility of significant air convection noise in these three boreholes. At the IRIS/IDA borehole installations, 100% of the 

horizontal RMS noise levels are greater than 100 counts, whereas 23% (5 out of 22) of the ASL 

borehole stations have horizontal noise levels below 100.

16
STATION 

ID CODE

SENSOR TYPE VERTICAL 

RMS

NORTH 

RMS

EAST 

RMS

GTSN BOREHOLE STATIONS

BGCA

BDFB

BOSA

CPUP

DBIC

LBTB

LPAZ

PLCA

VNDA

KS-54000

KS-54000

KS-54000

KS-54000

KS-54000

KS-54000

KS-54000

KS-54000

KS-54000

AIR

AIR

AIR

AIR

AIR

AIR

AIR

AIR

AIR

24

55

32

65

32

90

32

30

50

140

1200

150

5400

130

1000

50

60

3200

146

1600

320

2800

430

600

45

170

1200

IRIS/ASL BOREHOLE STATIONS

ANMO

ANTO

BOCO

CHTO

COLA

GRFO

GUMO

HNR

NWAO

PTGA

RAR

SNZO

TATO

KS-54000I

KS-36000I

KS-54000I

KS-36000I

KS-54000I

KS-36000I

KS-36000I

KS-54000I

KS-36000I

KS-54000I

KS-36000I

KS-36000I

KS-36000I

SAND

AIR

AIR

AIR

SAND

AIR

AIR

AIR

AIR

AIR

AIR

AIR

AIR

30

130

70

100

46

27

90

60

140

186

170

200

190

80

120

200

600

43

70

110

150

120

54

1500

1500

1150

55

120

300

500

52

70

130

100

120

54

1500

1400

2200

SNAKE PIT BOREHOLE TEST SENSORS

#26

#114

KS-54000I

KS-54000I

SAND

SAND

35

25

40

115

50

100

Table 1. Summary of the RMS signal levels in the 20 to 600 second 

band for all the GTSN and IRIS/ASL borehole installations plus the 

two "Snake Pit" sand test sensors. All table RMS entries are in units 

of IRIS/ASL sensitivity digital tape counts.

17
STATION 

ID CODE

SENSOR TYPE VERTICAL 

RMS

NORTH 

RMS

EAST 

RMS

IRIS/IDA BOREHOLE STATIONS

ASCN

BORG

CMLA

EFI

MSEY

MSVF

NIL

SHEL

WRAB

KS-54000I

KS-54000I

KS-54000I

KS-54000I

KS-54000I

K3-54000I

KS-54000I

KS-54000I

KS-54000I

AIR

AIR

AIR

AIR

AIR

AIR

AIR

AIR

AIR

193

258

64

31

74

103

341

66

37

317

9613

340

209

1761

238

177

519

254

276

2177

286

222

1277

249

268

680

139

Table 2. Summary of the RMS signal levels in the 20 to 600 second 

band for the indicated IRIS/IDA borehole installations. All table 

RMS entries are in units of IRIS/ASL sensitivity digital tape counts.

18
7 POTENTIAL SOURCES OF EXCESSIVE HORIZONTAL NOISE

There are several possible causes for high horizontal noise. First, the sensor electronics themselves could be excessively noisy. The data in Tables 1 and 2 argues against this possibility because in 

most cases horizontal noise appears to occur at about the same level in both horizontal components at a 

given site. It is not likely that excessive electronic noise would either always occur in both horizontal 

sensors at once or not at all; it would seem that a mixture of one quiet and one noisy horizontal would 

occur occasionally if electronic noise was the source of the excessive noise levels. In addition, note that 

none of the vertical sensors are excessively noisy. The electronic circuitry for the vertical channel in 

KS-36000I's and KS-54000I's is identical to the circuitry for the horizontal channels. It is unlikely that 

all of the vertical channels would be relatively quiet whereas several horizontal channels exhibit high 

levels of noise in pairs. The data in Tables 1 and 2 strongly suggests that the source of the noise arises 

not from electronic sources, but from tilt. Recall that the horizontal components are very sensitive to tilt 

whereas the vertical components are relatively insensitive to tilt.

Tilt of the sensor package can arise from several possible sources. First, it is possible that the 

borehole may not be cemented properly at the depth of the seismometer installation thereby allowing the 

borehole casing to "swing free" to generate tilt. Poor cementing near the bottom is highly probable if 

the hole was cemented from the top. Drillers are notorious for cutting corners if left unmonitored on the 

job for any length of time. If voids exist between the rock walls and the outside of the casing at the 

installation depth, the casing will probably tilt continuously especially if water is present in the void. 

This could explain the fact that high levels of noise only occur in pairs on the horizontals in Tables 1 and 

2 and not at all on the vertical channels. It is possible that one or more poorly-cemented boreholes are 

responsible for some of the high horizontal noise but it is not probable that all of the noisy holes are 

poorly cemented.

A myriad of possible mechanical malfunctions within the stabilizer - seismometer - holelock - 

borehole system could be sources of tilt. These possible malfunctions include poor spring tension in the 

centralizer, poor mechanical contact between the three centralizer centering legs and the borehole walls 

due to rust build up, dirt etc., poor mechanical contact between the seismometer base and the three stainless steel balls in the holelock due to debris build up on the balls, poor mechanical contact between the 

borehole walls and the casehardened holelock locking jaws due to rust build up, dirt etc., or movement 

in one or more of the many mechanical interfaces found throughout the system. If any of these possibilities is the source of tilt noise, a sand installation eliminates most of them because the centralizer and 

holelock are not utilized in a sand installation.

The most likely source of tilt noise in deep seismic boreholes lies in air convection driven by temperature gradients within the free air space around the seismometer. Personnel at Teledyne Geotech 

were well aware of air convection as a potential source of tilt noise during the KS product development 

years in the early 1970's. Numerous copies of internal company memoranda (most of them were written 

by J. C. Cook) dating from that era, which were given to ASL personnel over the years by Teledyne 

Geotech, are devoted to theoretical discussions of the effects of various gasses, the effect of varying the 

gas pressure, and the effects of varying casing diameters on the onset temperature gradient required for 

starting convection. According to these documents, the natural earth heat flow temperature gradient at 

most sites is theoretically equal to just about the gradient required to start air convection in a 7 inch 

inside diameter borehole. Adding an active component seismometer to the borehole greatly complicates 

theoretical calculations because the geometry of the seismometer package increases the difficulty of 

modeling the free air space and the active sensor electronics adds heat to the space around the package 

thereby greatly increasing the local temperature gradient in the vicinity of the package. Air convection 

in a borehole can generate tilt in the seismometer package in two ways. First, the action of the air 

motion itself acts as a wind blowing on the package thereby creating a force which moves the package. 

Second, the air convection is the result of a temperature gradient; therefore, the air motion generates 

temperature changes in the mechanical parts of the sensor - borehole system causing unequal expansion

19
and contraction of these mechanical parts which in turn tilts the sensor package. A change in the temperature of one of the approximately one inch long steel legs of a KS stabilizer of only FOUR TENTHOUSANDTHS of a degree Fahrenheit will create a tilt of the package of 10~n radians! These forces 

and temperature changes are very small but the tilt sensitivity of the horizontal sensors is extremely 

high.

20
8 CONCLUSIONS

Excessive long period noise in the horizontal components is present at many of the KS borehole 

installations around the world. Excessive horizontal noise at most sites is probably generated by air 

motion in the vicinity of the sensor which introduces tilting of the sensor package thereby generating 

noise. Current sensor installation technology has proven to be insufficient for controlling low level 

borehole noise. This experiment investigated the effectiveness of the use of sand as a means of preventing air motion near seismometer systems installed in boreholes.

Controlled experiments at ASL yielded definite decreases in horizontal noise levels when sand 

was added to the borehole. Conversion of SNZO from a conventional installation to a sand installation 

produced a significant reduction in horizontal noise levels at a station which had had noisy horizontals 

for many years. The new sand installation at COLA has the quietest horizontal components of any site 

in the world. ANMO has operated in sand for over two years with very quiet horizontals.

The combination of this evidence demonstrates that the use of sand as an installation medium 

inhibits air convection and associated tilt noise in KS installations. The reduction in horizontal noise 

levels can be very large if existing noise levels are high. Horizontal noise levels should approach the 

vertical noise level in continental sand installations; noise levels in coastal sand installations will probably be higher because of tilt noise from sources external to the borehole.

21
9 ACKNOWLEDGMENTS

The authors appreciate the assistance of several individuals during this experiment. Vern Stoup 

helped clean up the "Snake Pit" to convert it from a dark and rodent infested dungeon to a bright comfortable recording facility. In addition, Vern and Tyler Storm built a barbed wire fence around the boreholes to keep the local Isleta Pueblo "instrumentation cable eating cows" away from the boreholes. 

Alvin Garcia and Derrick Serrano performed numerous tasks to support the effort. Ken Oliver 

conducted all of the seismometer locking, unlocking, and leveling operations. Bob Woodward spent at 

least a couple of days extracting time series data from CMLA and EFI. Caryl Peterson set up the digital 

data system configuration file and assisted in interpreting the system's occasional strange operating 

behavior.

22
10 REFERENCES

Bendat, J. S., Piersol, A. G.,( 1971),"Random Data: Analysis and Measurement Procedures" WielyInterscience" p 14-15

Douze, E. J., Sherwin, John R.,(1975),"Deep Borehole Operation of the Borehole Seismometer System, 

Model 36000, Teledyne Geotech Technical Report No. 75-2

Peterson, Jon,( 1993),"Observations and Modeling of Seismic Background Noise", U.S. Geological Survey Open-File Report 93-322

Sherwin, John R., Cook, John C.,(1976),"Shallow Borehole Convection Study" Teledyne Geotech Technical Report No. 76-6

23
11 APPENDIX

SEISMOMETER REMOVAL EXPERIMENTS

Several people in the seismic community have expressed considerable concern about the possibility that a sand installed sensor system might become "stuck" in the hole if the sand becomes wet and or 

is "cemented" with an unknown cementing agent. We agree that it is quite likely that sand in most 

boreholes will become wet.

Therefore, during June, 1996 an experiment was performed at ASL to determine the difficulty of 

removing a KS sensor system from wet sand. A dummy package which was approximately the same 

size as a KS sensor was installed in a shallow borehole and completely submerged in sand to a depth of 

at least 4 to 5 inches above the top of the package. Water was then added to the hole until the sand 

remained submerged underwater and allowed to stand for three days.

The package was 5.5 inches in diameter and 65 inches long and it weighed 80 pounds. A 5000 Ib 

capacity scale was used to monitor the lifting cable tension as the dummy package was pulled from the 

sand. A total of 540 pounds of force was required to pull the package from the wet sand. This force was 

composed of the package weight plus a vacuum force plus the friction of the sand against the package 

minus the buoyancy.

Force = 540 Ibs (measured with uphole cable tension scale) 

Weight = -80 Ibs (measured with platform scale) 

Vacuum force = -349 Ibs (assuming sea level air pressure) 

Friction = -169 Ibs (calculated by summing the remaining forces) 

Buoyancy = 58 Ibs (calculated from known package volume)

For a real KS-36000 or KS-54000, these numbers are expected to vary slightly because these 

instruments weigh more than the dummy package and the real instruments have slightly different 

mechanical dimensions. The biggest change should be in the weight (a KS-54000 weighs about 145 

pounds) so the total force required to remove a real instrument should increase by about 65 pounds. For 

a real instrument in the bottom of a deep borehole (100 meters), the force will also increase by an estimated 100 more pounds due to the added weight of the soft and hard cables.

The "vacuum" and the "buoyancy" forces are undoubtedly functions of the rate at which the package is pulled from the wet sand. If the package is pulled out quite rapidly, the vacuum force should 

approach that given above whereas the buoyancy force will approach zero. If the package is pulled out 

slowly enough to allow the water to percolate downward through the sand and fill the space being 

created at the bottom of the package as the package is withdrawn, the vacuum force should approach 

zero and the buoyancy force will be near that quoted above. Therefore, slow pulling should result in the 

smallest overall removal force.

Assuming that the package is removed very rapidly thereby creating the maximum vacuum force 

and the minimum buoyancy force, the forces indicated above total to about 763 pounds of cable tension 

required to remove a deep borehole installed KS-54000 if it is in water saturated sand. This force is well 

within the capability of the hard cable (2800 pounds) and GoLo winches (1200 pounds) which are currently in use at most IRIS/ASL borehole sites. Rapid removal is not recommended! It is not prudent 

policy to try to hurry when working in boreholes.

Efforts to simulate "cementing" have been less successful. First of all, it is impossible to conduct 

a truly realistic test of "cementing" because no one is sure just what "cementing" is. No one knows what 

material or materials may become mixed with the sand to create a hardened mixture or just how much 

time might be involved in this hypothetical process. During the spring of 1995, a dummy KS package 

was coated with silicone grease and cemented in a short piece of borehole casing with Portland Cement

24
Concrete. The intention was that concrete would simulate a worst case cementing situation, and that the 

silicone grease would serve as a mold release and allow the dummy package to slip out of the concrete 

after it had cured. The annulus between the dummy package and the casing wall was filled with concrete to the top of the dummy package and allowed to cure for three days. This experiment failed 

because we were not able to pull the dummy package from the concreted piece of casing. A postmortem 

disassembly of the casing revealed that the silicone grease had indeed prevented adhesion of the concrete to the dummy package. The mode of failure appeared to be radial pressure generated by the 

expansion of the concrete between the two cylinders.

Therefore, in June of 1996, a new experiment was performed in which the dummy seismometer 

package was first coated with silicone grease, then wrapped with thin rubber sheet and finally cemented 

in a short piece of casing. The purpose of the rubber sheet was to provide a compliant medium to absorb 

the expansion of the concrete and thereby reduce the radial pressure on the dummy package. This time, 

the dummy package was successfully pulled from the concrete with a peak force of about 1000 pounds.

This configuration solved the cementing problem but questions remained about whether this configuration was quiet. Therefore, on day 265, 1996, KS-54000 serial number 26, which had been coated 

with silicone grease and wrapped with a thin (1/16" thick) rubber sheet, was installed in a shallow 

borehole in a holelock and with a centralizer. Two days later on day 267, sand was poured into the 

borehole. The minimum signal levels for this experiment are tabulated in the lower portions of Tables 6, 

7, and 8, and the minimum noise levels are tabulated in the lower portions of Tables 9, 10, and 11.

Both the minimum vertical signal and minimum vertical noise levels in Tables 6 and 9 with the 

seismometer wrapped in rubber sheeting are approximately the same in all of the bands as they were 

with no rubber sheeting (days 226 through 260) both before (days 265 through 267) and after (days 267 

through 275) the sand was poured in the borehole. However, the north signal and noise minimum levels 

in Tables 7 and 10 with the rubber sheeting are much higher both in air (days 265 through 267) and after 

(days 267 through 275) the sand was poured into the borehole than they had been in the earlier installation (days 226 through 260). Similarly, the east signal and noise minimum levels in Tables 8 and 11 

with the rubber sheeting on the sensor are also greater than they had been with no rubber sheeting both 

in air and after the sand was poured into the borehole. It is not surprising that the levels are higher in air 

because the air is free to convect in this configuration, but the high levels after sand was poured into the 

hole seems to indicate that sand does not suppress air convection. Or does it?

At the time the experiment was conducted, the results were puzzling but subsequent work 

revealed an explanation for the high level noise from a sand installed sensor. Later, the same sensor, 

still wrapped in the rubber sheeting, was installed in sand without a holelock and with no centralizer. 

No holelock and no centralizer is the configuration being proposed as the standard sand installation configuration. The horizontals were quite noisy and surprisingly the noise levels increased significantly 

when the wind was blowing; the increased noise levels during windy periods were much greater than 

had been observed previously. This suggested that air pressure variations were involved; since the borehole packoff unit had not been correctly installed, therefore, it was reinstalled with great care to seal the 

borehole properly. This change eliminated most of the increased noise levels during windy time periods 

and reduced the noise levels significantly. However, the sensor was still noisier than it had been without 

the rubber sheeting.

This suggested that the probable source of the noise lies in the rubber sheeting. Remember that 

the sheeting was wrapped on a silicone greased sensor. It is highly probable that air bubbles were 

trapped between the sensor and the rubber sheeting by the grease. An air bubble will act as a "air pressure change to mechanical motion transducer" which generates lateral forces between the sensor and the 

surrounding sand thereby tending to tilt the sensor package even though it is immersed in sand. Pressure 

changes within the borehole are quite large if the packoff assembly is not properly assembled. Sealing 

the top of the borehole eliminates pressure variations due to wind but much smaller pressure changes, 

which are generated by temperature gradient driven air convection in the sand free portion of the borehole, are large enough to create tilt noise.

25
To confirm this theory, the sensor was pulled from the hole, the rubber sheeting was removed 

from the sensor, and the silicone grease was rubbed off the sensor with rags. After reinstallation in sand, 

several days of operation confirmed that the horizontal noise levels once again assumed the levels produced earlier by this sensor (days 226 through 260).

Finally, members of the seismological community had expressed concern that sand might slowly 

compact with time thereby gripping the sensor tighter and tighter as time went by. This is a difficult 

concept to address experimentally because of the long time periods necessary to conduct meaningful 

experiments. However, we were able to obtain one data point by measuring the force necessary to 

remove the ANMO KS-54000I sensor after it had been installed in sand for nearly two years. The force 

required to pull the sensor from the sand was quite small being on the order of 60 pounds or less. In this 

case at least, there was no evidence that the sand tends to compact with time.

26
FIELD SAND INSTALLATION PROCEDURES

Installing a KS instrument in sand is a relatively easy procedure compared to the installation procedure used in the past because the holelock, the holelock key, the pilot, the foam hole plugs, the foam 

wrapping, and the centralizer are not needed. A typical field sand installation would not use a holelock, 

as was done in the "Snake Pit" installation described in Section 2, because the holelock would probably 

be buried in sand and lost if the sensor system is ever removed from the hole in the future. Instead of 

utilizing the holelock to orient the sensor system in the conventional north-south east-west directions, 

the horizontal components of the sensor are installed at an arbitrary but known azimuth. The holelock 

key and pilot are not needed because there is no mating holelock. The foam hole plugs and the foam 

wrapping are not needed because we are controlling air convection generated tilt with the sand. The 

centralizer is not required because the sand holds the sensor package quite firmly within the borehole. 

In addition, the complex mechanical linkages in the centralizer are excellent possible sources of additional noise.

The following steps are major changes from previous procedures; they are things that are no longer 

required.

1. Do not install the holelock key at the bottom of the sensor.

2. Do not install the pilot at the bottom of the sensor.

3. Do not install the holelock in the borehole.

4. Do not wrap the seismometer with foam.

5. Do not install foam plugs anywhere in the borehole.

As far as is known, the type of sand to use is not critical. However, the only sand that has been 

used to date is ordinary "play ground sand", which is available at most building supply centers in the 

United States. The sand must be pure sand; impurities that might tend to harden with time such as clay, 

cement, carbonate containing minerals, etc. must be studiously avoided. To be on the safe side, it probably should be standard policy to ship all sand from ASL to the borehole site in order to maintain control 

on the quality of the sand.

The following steps are required to install a KS sensor system in a new borehole. These instructions refer to a "standard coffee can". A 39 ounce (2 pound 7 ounce) Folgers coffee can is a "standard 

coffee can". This can holds about 183.8 cubic inches of sand. If this can is not available, any container 

of known volume can be used to measure the sand.

1. Install a sand foot cup on the bottom of the sensor package (where the pilot attached 

previously). Hand tightening should be sufficient.

2. Measure the distance from the bottom of the sand foot to the top of the sensor package in 

inches (HI in Figure 33 and Tables 3 and 4). Record this distance for use later in Step 

12. Measure the inside diameter of the borehole and record this diameter for use later 

in Steps 3 and 12.

3. Pour enough DRY sand into the borehole to accumulate to about one foot deep on the 

bottom. For a 6.5" inside diameter borehole, 398 cubic inches (2.1 standard coffee 

cans) of sand are required. Note: Any foreign object at the bottom should be buried to 

a depth of about one foot above the top of the object.

4. Install the centralizer on the top of the package. Secure with the nut. Do not install the 

three springs on the centralizer. Instead, tie the three centralizer legs back with a heavy 

tie wrap to prevent them from extending.

27
5. Assemble the remaining hardware (bail, strain relief, soft and hard cables, borehole 

packoff unit, Golo winch assembly etc.) as is standard procedure.

6. Lower the sensor assembly into the borehole while clamping the soft cable to the hard 

cable at regular intervals until the sensor just rests on the sand at the bottom of the hole 

(reduced hard cable tension will be your only clue that the sensor has reached bottom).

7. Lower about 2 more feet of cable into the borehole.

8. Install the packoff assembly - do not tighten it up completely because it will need to be 

removed later.

9. Unlock and level the sensor modules.

10.Check the data output to see that the sensor system is operating properly. The horizontal components will probably be very noisy at this point.

11 .If the sensor appears to be operating, remove enough of the packoff assembly to permit 

pouring sand down the hole.

12.Pour in enough sand to completely fill the volume between the sensor and the borehole 

walls to the top of the sensor package.

Use the inside diameter of the borehole and the sensor height "HI" which were measured and recorded in Step 2 above and Table 3 to determine the volume of sand 

required for a KS-54000 installation.

Use the inside diameter of the borehole and the sensor height "HI" which were measured and recorded in Step 2 above and Table 4 to determine the volume of sand 

required for a KS-36000 installation.

DO NOT PUT IN TOO MUCH SAND!!!

Too much sand above the top of the package will tend to anchor the sensor in the 

borehole. We don't want this to happen.

13. After all of the sand is poured down the hole, shake the downhole cable to knock some 

of the sand off the cable and hopefully the strain relief. Moderate shaking only - don't 

break anything!

14. Lower about 2 more feet of cable into the borehole.

15. Reinstall the packoff assembly - tighten it up this time.

16. Relevel the sensor modules if necessary.

17. After things settle down (a few hours - preferably overnight), look at the data to see if 

everything seems to be functioning properly. The lowest VH RMS noise levels printed 

on the station log for the horizontal components for a given day should be near that 

being calculated for the vertical component (not more than two or three times the vertical level).

18. Measure the orientation of the sensor system.

28
SAND FOOT

O 

cn

Figure 33. Illustration of the measurement of the dimension "HI" which is defined as the distance from 

the bottom of the sand foot to the top of the sensor package.

29
HI

40

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

72

74

76

78

80

82

Diameter = 6.50"

VOL

420

441

462

483

504

525

546

567

588

609

630

651

672

693

713

734

755

776

797

818

839

860

CAN

2.2

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.2

4.3

4.5

4.6

4.7

LOST

12.5

13.3

13.9

14.5

15.2

15.8

16.4

17.1

17.7

18.3

19.0

19.6

20.2

20.9

21.5

22.1

22.8

23.4

24.0

24.7

25.3

25.9

Diameter = 6.75"

COL

524

550

576

602

629

655

681

707

733

759

786

812

838

864

890

917

943

968

995

1021

1047

1073

CAN

2.8

2.9

3.1

3.2

3.4

3.5

3.6

3.8

3.9

4.1

4.2

4.4

4.5

4.6

4.8

4.9

5.1

5.3

5.4

5.6

5.7

5.8

LOST

14.6

15.4

16.1

16.8

17.6

18.3

19.0

19.8

20.5

21.2

22.0

22.7

23.4

24.2

24.9

25.6

26.3

27.1

27.8

28.5

29.3

30.0

Diameter = 7.00"

VOL

623

663

695

727

758

790

821

853

884

916

948

979

1011

1042

1074

1106

1137

1168

1200

1231

1263

1295

CAN

3.4

3.6

3.7

3.9

4.1

4.2

4.4

4.6

4.7

4.9

5.1

5.2

5.4

5.6

5.8

5.9

6.1

6.4

6.5

6.7

6.9

7.0

LOST

16.4

17.2

18.1

18.9

19.7

20.5

21.3

22.2

23.0

23.8

24.6

25.4

26.3

27.1

27.9

28.7

29.5

30.4

31.2

32.0

32.8

33.7

Diameter = 7.25"

VOL

744

781

818

855

892

930

967

1004

1041

1078

1116

1153

1190

1227

1264

1301

1339

1375

1412

1450

1487

1524

CAN

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

7.2

7.5

7.7

7.9

9.1

8.3

LOST

18.0

18.9

19.8

20.7

21.6

22.5

23.4

24.3

25.2

26.1

27.0

27.9

28.8

29.7

30.6

31.5

32.4

33.3

34.2

35.1

36.0

36.9

Table 3. Sand volumes required to bury a KS-54000 to the indicated depths in 6.50, 6.75, 

7.00, and 7.25 inch inside diameter boreholes. Column HI is the height of the sand around the 

sensor measured from the bottom of the sand foot in inches. VOL is the volume of sand 

required in cubic inches. CAN is the number of "standard coffee cans" required to hold VOL 

cubic inches of sand. LOST is the length of borehole in inches which will be lost (remain 

filled with sand) when the sensor is removed from the borehole if the sand is not removed.

30
HI

40

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

72

74

76

78

80

82

Diameter = 6.50"

VOL

542

569

596

623

650

677

705

732

759

786

813

840

867

894

921

948

975

1003

1030

1056

1083

1110

CAN

2.9

3.0

3.2

3.3

3.5

3.6

3.8

3.9

4.1

4.2

4.4

4.5

4.6

4.8

4.9

5.1

5.2

5.4

5.5

5.8

5.9

6.0

LOST

16.3

17.1

18.0

18.8

19.6

20.4

21.2

22.0

22.9

23.7

24.5

25.3

26.1

26.9

27.8

28.6

29.4

30.21

31.0

31.8

32.7

33.5

Diameter = 6.75"

COL

646

678

711

743

775

807

840

872

904

937

969

1001

1034

1066

1098

1130

1163

1195

1227

1259

1291

1324

CAN

3.5

3.6

3.8

4.0

4.2

4.3

4.5

4.7

4.8

5.0

5.2

5.4

5.5

5.7

5.9

6.1

6.2

6.4

6.6

6.9

7.0

7.2

LOST

18.1

19.0

19.9

20.8

21.7

22.6

23.5

24.4

25.3

26.2

27.1

28.0

28.9

29.8

30.7

31.6

32.5

33.4

37.0

35.2

36.1

37.0

Diameter = 7.00"

VOL

754

792

829

867

905

942

980

1018

1056

1093

1131

1169

1206

1244

1282

1319

1357

1395

1433

1470

1507

1545

CAN

4.0

4.2

4.4

4.6

4.8

5.1

5.3

5.5

5.7

5.9

6.1

6.3

6.5

6.7

6.9

7.1

7.3

7.5

7.7

8.0

8.2

8.4

LOST

19.6

20.6

21.6

22.5

23.5

24.5

25.5

26.4

27.4

28.4

29.4

30.4

31.3

32.3

33.3

34.3

35.3

36.2

37.2

38.2

39.2

40.2

Diameter = 7.25"

VOL

866

909

952

996

1039

1082

1126

1169

1212

1256

1299

1342

1385

1429

1472

1515

1559

1602

1645

1688

1731

1775

CAN

4.6

4.9

5.1

5.3

5.6

5.8

6.0

6.3

6.5

6.7

7.0

7.2

7.4

7.7

7.9

8.1

8.4

8.6

8.8

9.2

9.4

9.7

LOST

21.0

22.0

23.1

24.1

25.2

26.2

27.3

28.3

29.4

30.4

31.5

32.5

33.6

34.6

35.7

36.7

37.8

38.8

39.9

40.0

42.0

43.0

Table 4. Sand volumes required to bury a KS-36000 to the indicated depths in 6.50, 6.75, 

7.00, and 7.25 inch inside diameter boreholes. Column HI is the height of the sand around 

the sensor measured from the bottom of the sand foot in inches. VOL is the volume of 

sand required in cubic inches. CAN is the number of "standard coffee cans" required to 

VOL cubic inches of sand. LOST is the length in inches of borehole which will be lost 

(remain filled with sand) when the sensor is removed from the borehole if the sand is left in 

the hole.

31
THE SNZO PROOF OF CONCEPT EXPERIMENT

The results of the snake pit sand installation experiment and the COLA station installation were 

presented at the IRIS Standing Committee meeting at Harvard on November 12-13, 1996. The Standing 

Committee requested that the technique be demonstrated at an existing borehole site that had been producing relatively noisy horizontal data for a long time. The station at South Karori, New Zealand 

(SNZO) was chosen as the site because the horizontals had been noisy since the station was first 

installed in the mid 1970's.

The necessary equipment and supplies were shipped to New Zealand and AlliedSignal Field Engineer Chuck Cazier arrived at the site in late January, 1997. The existing KS-36000 seismometer was 

removed from the borehole, converted to a sand installation configuration, and reinstalled in sand 

February 8 (day 39) 1997. A few days of data system software modification and station calibration followed with routine undisturbed operation beginning on about day February 11 (day 42) of 1997.

The results of installing the sensor in sand are presented in Figures 34, 35 and 36. The RMS 

noise level for the vertical component remained unchanged by the sand (see Figure 34). However, the 

RMS noise levels for both the north and east components decreased approximately 15 db after the sand 

was put in the hole (see Figures 35 and 36 respectively). With sand in the hole, the horizontals are only 

slightly noisier than the vertical component (See SNZO in Figure 37). These results are exactly what 

should be expected if air convection generated tilt was causing the horizontal noise because tilt should 

not cause nearly as much noise in the vertical component; therefore, it was quiet to begin with. Filling 

up the volume around the seismometer with sand prevents air convection thereby decreasing tilt generated horizontal noise.

The results of adding sand to the SNZO borehole are also shown in the frequency domain PSD 

surfaces in Figures 38, 39, and 40. These surfaces are constructed from daily median PSD estimates 

calculated as follows. First, a given day of 1 sample per second time domain data from a given channel 

was divided into 2048 second segments with a 50% overlap between segments. The FFT of each segment was then converted to PSD and the FFT bin wise median of these 84 PSD segments was evaluated. 

This yielded the "daily median" PSD estimate for that channel and day; these spectra are plotted on a 

logrithemic acceleration PSD scale between 30 and 2048 seconds over a 125 day period to create the 

PSD surfaces in the figures. At this point, it should be mentioned that the source of the peak near 500 

seconds in all three figures both before and after sand was added to the borehole is unknown.

To assist in determining the levels of the PSD surfaces, the daily PSD levels at 127 and 256 seconds have been highlighted in all three figures and straight line segment fits have been "eyeballed" 

through these PSD levels at these two periods.

Note the essentially constant PSD level across the long period band which is evident in the PSD 

surface for the SNZO vertical in Figure 38 both before and after sand was put in the borehole. The performance of the vertical component was not influenced by the sand. However, note the decrease in the 

north and east horizontal PSD levels between 30 and 600 seconds produced by the sand installation 

(Figures 39 and 40 respectively). Also note that the horizontal long period PSD levels before the sand 

installation were very constant and smooth whereas the horizontal PSD levels after the sand installation 

show some variation and roughness with time. The constant PSD levels before the sand installation 

were probably due to steady state air convection generated tilt noise within the borehole in the vicinity 

of the sensor. The variation of the PSD level after the sand installation probably arises from tilt sources 

external to the borehole system; the chief source of this variation is quite probably due to changes in the 

state of the sea which is only about 4 kilometers from the site.

The decrease of the PSD level after the sand was put in the hole in the horizontal PSD surfaces in 

Figures 39 and 40 at 128 and 256 seconds is 12.4 and 14.4 db for the north component and 11.5 and 

12.5 db for the east component. The PSD decrease at 128 seconds was determined by calculating the db 

decrease between the average of the PSD level at 128 seconds (circles) before sand was poured into the 

borehole to the average of the PSD level after sand was poured in. The PSD decrease at 256 seconds

32
was determined in the same manner. The PSD surface estimated changes in the noise level are approximately equal to the 15.3 db decrease for both components as calculated by the 25 minimum averaged 

RMS noise level method. In contrast, the same calculation for the vertical channel yields a change of 

only 0.4 db at 128 seconds and 0.3 db for 256 seconds; vertical noise levels were essentially unchanged 

by the sand installation.

The results of the SNZO experiment were presented to the IRIS Standing Committee at their 

meeting at Disney World on March 5-6, 1977. The committee agreed that the experiment was a success 

and that in the future ASL could install KS sensors in sand at it's own discretion.

The data in Figure 37 provides an estimate of how much improvement that should be expected by 

converting several of the existing borehole sites to sand installations. Reinstalling the sensors in sand 

should produce horizontal noise levels equal to or slightly greater than the noise level for the vertical 

component at that site. At BDFB, CPUP, LBTB, VNDA, SHEL, and MSEY the vertical noise levels are 

rather low so a sand installation should result in a large improvement in the horizontal noise levels. The 

potential reductions of horizontal noise levels are tabulated in Table 5 for all of the non sand borehole 

stations in Figure 37. Smaller but quite significant decreases in noise levels should be expected at additional sites not shown in Figure 37 and Table 5.

CHTO

14.3

RAR

19.1

TATO

20.8

BDFB

28.9

CPUP

38.0

LBTB

20.0

VNDA

36.0

BORG

31.4

SHEL

20.0

MSEY

27.2

Table 5. Potential horizontal noise reduction figures in db which could be 

produced by converting these stations to sand installations.

33
c O-H

a' f

3 CD

S to

CD 

i-i 

P5

CTQ

CD

SAND INSTALLATION RESULTS

25% MINIMUM AVERAGED RMS NO

SNZO VERTICAL

GO

CD 

^. o' 

EL

GO

CDCD"

£L 

PJrtGO

N 

O

0"

CD

(71 

H

O

CJ

STN FLAT 

DOWN LINE

0 

300 320 340

1996

TIME, JULIAN DAYS

FLAT

 »-

LINE

SAND

IN

HOLE

NO

« *-

DATA
in 

> 

< 

Q

SJLNHOO SPVH

Figure 35. The 25% minimum averaged RMS north noise level at SNZO before and after the sensor 

was installed in sand.

35
5 31

o> 

to

SAND INSTALLATION RESULTS

25% MINIMUM AVERAGED RMS NO

SNZO EAST

00

{/a( K

3 

{/a

ciT

a

00

N 

O 

cr

P-

>

N0 .-NO 

DATA 1 HOLE

TIME, JULIAN DAYS

{/a 

3 

O

3
CO

w 

co

N ( I N

O CO

^

CO

<^ H ^^

<

w

lO

SlNflOD

Figure 37. Selected 25% averaged RMS noise levels from the three currently existing sand installations 

(COLA, ANMO, and SNZO), from several noisy borehole sites (stations CHTO through MSEY), and 

from five conventional surface installations (KIEV through ADK). The Horizontal noise levels at 

SNZO before the sand installation are shown as empty bars under N and E. The Horizontal noise levels 

at SNZO after sand are shown as the shaded portion of the same bars.

37
SNZO SAND INSTALLATION RESULTS 

MEDIAN PSD SURFACE 

30 TO 2048 SECONDS 

SNZO VERTICAL

SAND IN HOLE

PERIOD, SECONDS

Figure 38. Median power spectral density surface for the vertical component before and after sand in 

the borehole at SNZO. The circles are the PSD levels at 256 seconds on each day and the squares are 

the PSD levels at 127 seconds.

38
SNZO SAND INSTALLATION RESULTS 

MEDIAN PSD SURFACE 

30 TO 2048 SECONDS 

SNZO NORTH

SAND IN HOLE

PERIOD, SECONDS

Figure 39. Median power spectral density surface for the north component before and after sand in the 

borehole at SNZO. The circles are the PSD levels at 256 seconds on each day and the squares are the 

PSD levels at 127 seconds.

39
SNZO SAND INSTALLATION RESULTS 

MEDIAN PSD SURFACE 

30 TO 2048 SECONDS 

SNZO EAST

SAND IN HOLE

PERIOD, SECONDS

Figure 40. Median power spectral density surface for the east component before and after sand in the 

borehole at SNZO. The circles are the PSD levels at 256 seconds on each day and the squares are the 

PSD levels at 127 seconds.

40
Days

219-221

221-223

223-225

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

3.51

3.07

2.54

3.04

70-100

4.24

5.09

3.12

4.15

100-200

8.27

11.36

8.44

9.36

200-400

23.96

18.46

17.00

19.81

400-700

84.72

68.45

77.76

76.98

700-2048

1331

842

1124

1099

Sand poured into serial number 26 borehole on day 225

226-228

228-231

231-233

233-236

236-238

238-241

241-243

243-246

246-248

248-250

250-253

253-255

256-258

258-260

Averages

2.47

1.52

1.53

1.60

2.31

1.96

2.16

1.90

2.29

3.14

1.65

1.54

1.95

2.21

2.02

Delta dbj -1.78

2.89

2.58

2.04

2.00

3.05

2.75

3.08

2.35

3.45

3.48

2.66

1.84

2.77

2.56

2.68

-1.90

7.51

4.97

3.71

4.79

5.70

5.61

5.06

3.75

6.06

7.29

4.86

3.52

4.66

5.12

5.19

-2.56

15.33

12.47

11.01

11.19

13.92

17.28

13.14

14.74

15.32

30.96

13.74

10.84

13.88

11.99

14.70

-1.29

60.95

65.42

67.89

63.94

59.72

59.68

42.38

77.53

70.27

77.32

84.72

54.97

80.93

65.51

66.52

-0.63

951

662

1228

1149

726.

731.

247

384

968

837

984

578

854

645

782

-1.48

Remove sensor, wrap in rubber sheeting, reinstall in air

265-267 1.44 2.02 3.13 9.20 53.19 750

Sand poured into serial number 26 borehole on day 267

267-270

270-272

272-275

1.74

1.43

1.89

2.32

2.20

2.46

3.39

3.77

3.81

8.26

9.29

9.71

67.46

63.77

49.74

1451

737

619

Table 6. Tabulation of the vertical band-averaged signal levels in units of 

W~l9m 2/s 4/Hz for sensor 26 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the signal level 

(in db) resulting from introducing sand into the borehole for the indicated band.

41
Days

219-221

221-223

223-225

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

11.33

12.61

10.65

11.53

70-100

42.83

36.26

31.31

36.80

100-200

79.23

70.25

61.94

70.47

200-400

159

111

116

129

400-700

336

577

382

432

700-2048

3310

3973

3745

3676

Sand poured into serial number 26 borehole on day 225

226-228

228-231

231-233

233-236

236-238

238-241

241-243

243-246

246-248

248-250

250-253

252-255

255-258

258-260

Averages

Delta db

2.58

1.85

1.52

1.61

2.23

1.77

2.62

2.56

2.36

3.04

2.02

1.64

1.79

2.41

2.14

-7.31

5.88

3.51

3.19

2.98

3.92

3.13

4.10

3.94

3.74

4.04

3.45

2.52

2.85

3.33

3.61

-10.08

11.98

6.90

7.39

6.80

7.99

7.39

9.38

7.11

6.75

10.02

8.10

5.61

5.35

8.05

7.77

-9.67

32.37

14.53

16.99

13.50

18.31

17.93

25.42

17.30

12.69

23.86

19.32

18.33

9.33

23.35

18.80

-8.35

120

109

130

111

106

151

83.89

90.00

81.71

116

113

80.79

176

144

115

-5.74

738

1068

2723

940

983

1830

715

631

853

759

1190

738

1540

1246

1140

-5.09

Remove sensor, wrap in rubber sheeting, reinstall in air

265-267 4.14 11.19 22.56 67.04 195 1258

Sand poured into serial number 26 borehole on day 267

267-270

270-272

272-275

4.33

4.37

4.57

6.28

8.55

6.57

16.77

22.96

13.25

87.46

118

40.58

781

500

125

6450

6211

1406

Table 7. Tabulation of the north band-averaged signal levels in units of 

lQ~l9m 2/s 4/Hz for sensor 26 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the signal level 

(in db) resulting from introducing sand into the borehole for the indicated band.

42
Days

219-221

221-223

223-225

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

36.58

44.20

36.39

39.06

70-100

105

92.98

85.04

93.34

100-200

296

280

264

280

200-400

767

705

618

697

400-700

2387

3238

1618

2414

700-2048

8326

8923

12790

10013

Sand poured into serial number 26 borehole on day 225

226-228

228-231

231-233

233-236

236-238

238-241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

Averages

Delta db

2.67

2.20

1.95

1.88

2.53

2.04

2.91

2.51

2.47

3.50

2.52

1.86

2.03

2.59

2.40

-12.11

5.44

4.15

3.29

3.60

4.87

3.51

5.09

3.56

4.03

3.78

3.80

3.09

3.36

4.24

3.99

-13.69

10.79

6.30

7.36

5.12

10.02

7.70

8.37

6.50

6.20

8.68

7.05

7.67

6.18

8.99

7.64

-15.64

27.15

15.49

19.23

20.14

24.02

19.12

30.98

17.13

15.42

23.70

22.17

34.44

24.48

37.83

23.66

-14.69

117

107

132

154

104.

131

256

101

88.83

114

123

117

292

259

150

-12.08

873

1512

1808

1421

1263.

1002

1408

830

1199

1786

897

1000

1371

2196

1326

-8.78

Remove sensor, wrap in rubber sheeting, reinstall in air

265-267 1 3.96 5.39 20.72 82.62 262 3032

Sand poured into serial number 26 borehole on day 267

267-270

270-272

272-275

6.94

5.65

4.52

10.77

9.23

6.36

22.24

26.52

16.27

89.85

168

39.49

577

1367

238

6652

10280

2156

Table 8. Tabulation of the east band-averaged signal levels in units of 

W~l9m2/s 4/Hz for sensor 26 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the signal level 

(in db) resulting from introducing sand into the borehole for the indicated band.

43
Days

219-221

221-223

223-225

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

2.38

2.09

1.11

1.86

70-100

3.38

3.78

2.09

3.08

100-200

6.04

9.10

5.78

6.97

200-400

16.11

15.36

12.18

14.55

400-700

42.72

45.88

48.02

45.54

700-2048

941

802

621

788

Sand poured into serial number 26 borehole on day 225

226-228

228-231

231-233

233-236

236-238

238-241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

Averages

1.08

0.91

0.81

0.77

1.25

0.84

0.83

0.73

1.10

1.26

0.88

0.74

0.72

0.85

0.92

Delta db || -3.07

1.61

1.41

1.19

1.24

1.63

1.43

1.30

1.18

1.60

2.08

1.50

1.00

1.14

1.31

1.40

-3.42

4.66

3.29

2.49

3.35

3.98

3.40

2.89

2.25

3.59

4.60

3.08

1.81

2.23

3.21

3.20

-3.38

12.55

9.65

8.66

9.20

11.14

14.29

9.40

12.36

12.52

21.17

9.95

8.73

9.68

8.75

11.29

-1.10

32.64

49.65

31.90

34.93

41.33

46.05

25.77

57.05

49.51

53.72

56.81

34.91

45.96

34.13

42.45

-0.30

1206

526

717

610

421

381

940

263

992

737

1141

325

1265

342

705

-0.49

Remove sensor, wrap in rubber sheeting, reinstall in air

265-267 0.35 0.58 1.70 6.59 33.37 1288

Sand poured into serial number 26 borehole on day 267

267-270

270-272

272-275

0.52

0.44

0.56

0.82

0.86

0.96

1.52

1.60

2.09

5.36

5.97

7.90

33.87

39.60

31.28

1451

907

683

Table 9. Tabulation of the vertical band-averaged noise levels in units of 

W~l9m 2/s 4/Hz for sensor 26 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the noise level 

(in db) resulting from introducing sand into the borehole for the indicated band.

44
Days

219-221

221-223

223-225

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

12.87

11.57

9.36

11.27

70-100

37.36

32.87

28.07

32.77

100-200

60.28

57.43

49.61

55.77

200-400

135

82.72

88.23

102

400-700

202

308

192

234

700-2048

1756

1900

2509

2055

Sand poured into serial number 26 borehole on day 225

226-228

228-231

231-233

233-236

236-238

238-241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

Averages

Delta db

0.38

0.28

0.18

0.34

0.23

0.15

0.23

0.42

0.33

0.36

0.32

0.33

0.13

0.25

0.28

-16.04

0.56

0.44

0.17

0.57

1.11

0.60

0.97

0.53

1.09

0.51

0.69

1.05

0.62

0.60

0.68

-16.83

2.40

1.96

2.04

2.22

1.73

1.68

3.73

1.06

2.74

3.44

3.04

1.74

3.49

1.12

2.31

-13.82

4.33

5.71

2.76

5.92

4.65

4.42

11.63

7.99

3.60

7.57

5.87

3.67

2.46

8.96

5.68

-12.54

12.25

14.05

15.94

11.35

19.00

28.94

22.19

17.48

31.59

16.68

24.34

61.89

66.81

55.12

28.40

-9.16

508

578

988

173

393

1132

517

157

369

423

1333

478

1066

512

616

-5.23

Remove sensor, wrap in rubber sheeting, reinstall in air

265-26?J| 2.53 9.49 15.05 54.28 92.11 673

Sand poured into serial number 26 borehole on day 267

267-270

270-272

272-275

2.32

2.18

2.30

3.43

3.91

3.29

9.67

13.56

9.19

53.91

65.60

18.69

323

132

29.51

6275

4233

1050

Table 10. Tabulation of the north band-averaged noise levels in units of 

10~19m2/s 4/Hz for sensor 26 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the noise level 

(in db) resulting from introducing sand into the borehole for the indicated band.

45
Days

219-221

221-223

223-225

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

37.97

38.97

29.69

35.54

70-100

102

92.15

80.46

91.54

100-200

244

255

211

237

200-400

632

586

449

556

400-700

1525

2437

921

1628

700-2048

5652

8283

10940

8292

Sand poured into serial number 26 borehole on day 225

226-228

228-231

231-233

233-236

236-238

238-241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

Averages

Delta db

0.51

0.39

0.48

0.34

0.43

0.30

0.52

0.38

0.55

0.27

0.48

0.26

0.16

0.31

0.38

-19.66

0.44

0.26

0.53

1.01

0.49

0.91

0.83

0.93

0.78

0.71

0.27

0.18

0.56

0.69

-761

-21.74

4.02

1.64

2.89

1.02

2.98

4.16

1.14

1.56

1.50

4.63

1.44

1.54

3.75

2.11

2.46

-19.84

7.28

9.96

12.50

8.27

6.24

8.78

9.44

2.31

2.26

5.92

4.78

13.80

7.12

12.17

7.92

-18.46

234

34.53

42.72

46.19

21.27

53.24

47.19

36.80

14.39

41.72

21.40

28.65

124

33.36

55.68

-14.66

1441

815

1252

336

466

583

1045

559

416

916

539

123

951

186

688

-10.81

Remove sensor, wrap in rubber sheeting, reinstall in air

265-267 1.07 3.27 11.83 70.58 200 1630

Sand poured into serial number 26 borehole on day 267

267-270

270-272

272-275

2.72

2.13

1.26

6.28

5.52

2.46

12.97

10.46

7.98

59.53

57.83

21.78

362

825

135

9727

9848

1611

Table 11. Tabulation of the east band-averaged noise levels in units of 

\(Tl9m 2/s4/Hz for sensor 26 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the noise level 

(in db) resulting from introducing sand into the borehole for the indicated band.

46
1 AVERAGING BAND LIMITS IN SECONDS

Days

219-221

221-223

223-225

226-228

228-231

231-233

233-236

236-238

238-241

[

Averages

40-70

1.15

1.52

1.38

1.72

0.88

0.56

1.04

1.27

1.42

1.22

70-100

1.37

1.77

1.97

1.75

1.30

0.57

1.44

1.54

1.64

1.48

100-200

2.86

2.98

3.77

2.77

2.13

1.21

2.57

2.51

3.39

2.69

200-400

11.78

5.95

6.78

5.58

5.17

3.37

5.71

5.23

7.58

6.35

400-700

39.48

40.70

38.75

46.25

37.71

18.83

39.69

42.66

36.04

37.79

700-2048

380

434

512

564

581

233

451

675

527.

484

Sand poured into serial number 1 14 borehole on day 241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

260-262

262-265

265-267

267-270

270-272

272-275

Averages

Delta db

1.67

1.40

1.47

1.61

1.27

1.06

1.22

1.48

1.77

2.22

1.38

1.49

1.34

1.61

1.50

0.91

1.98

1.46

1.77

2.06

1.32

1.09

1.57

1.77

1.88

2.01

1.87

1.85

1.43

2.10

1.73

0.66

3.21

2.12

2.64

2.49

2.48

2.43

2.63

3.00

3.83

3.05

2.95

2.67

2.91

3.11

2.82

0.21

7.80

4.95

5.91

11.97

5.59

5.51

5.59

7.69

5.31

7.17

6.44

7.72

6.57

6.95

6.80

0.30

54.80

29.17

43.85

47.05

40.23

32.15

45.56

32.41

36.59

44.09

29.89

54.42

45.56

47.43

41.66

0.42

742

487

849

874

547

648

828

490

1025

428

253

508

693

437

629

1.14

Table 12. Tabulation of the vertical band-averaged signal levels in units of 

lQ~l9m2/s 4/Hz for sensor 114 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the signal level 

(in db) resulting from introducing sand into the borehole for the indicated band.

47
Days

219-221

221-223

223-225

226-228

228-231

231-233

233-236

236-238

238-241

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

2.00

2.47

2.13

3.41

2.54

1.41

1.67

3.33

2.75

2.41

70-100

4.41

4.88

5.64

7.02

4.66

2.57

2.85

5.83

4.87

4.75

100-200

11.82

8.32

13.64

17.59

11.29

6.78

7.47

14.11

11.40

11.38

200-400

52.03

24.15

53.13

74.04

31.55

21.78

23.24

90.50

44.32

46.08

400-700

247

364

256

356

287

312

293

517

414

338

700-2048

2232

3223

2255

2576

2738

2317

2484

3843

6710

3153

Sand poured into serial number 114 borehole on day 241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

260-262

262-265

265-267

267-270

270-272

272-275

Averages

Delta db

4.24

6.37

5.60

6.55

5.69

5.13

7.13

6.48

6.85

8.60

6.10

7.04

5.92

5.68

6.31

4.18

10.72

11.46

10.94

11.32

13.02

9.76

13.32

10.70

13.87

13.52

10.26

10.88

13.59

11.72

11.79

3.95

21.56

49.98

45.66

40.96

45.93

40.76

48.76

40.48

50.81

54.29

36.70

42.09

42.98

37.75

42.77

5.75

94.52

252

150

162

135

178

233

248

253

230

164

199

185

140

187

6.09

1072

1391

626

897

771

993

1059

1004

1102

1592

696

814

1246

555

987

4.65

6396

7185

4644

5137

6826

6943

7241

7459

8762

7704

4651

7601

8219

6049

6772

3.32

Table 13. Tabulation of the north band-averaged signal levels in units of 

lQ~l9m2/s 4/Hz for sensor 114 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the signal level 

(in db) resulting from introducing sand into the borehole for the indicated band.

48
Days

219-221

221-223

223-225

226-228

228-231

231-233

233-236

236-238

238-241

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

3.57

7.69

6.76

4.91

3.35

3.81

3.40

8.49

7.59

5.11

70-100

6.78

12.22

13.18

10.10

7.65

10.10

7.61

13.26

14.75

10.63

100-200

28.36

23.30

33.41

23.26

16.28

24.46

19.50

30.62

67.07

29.58

200-400

86.97

84.36

157

82.10

64.81

80.78

105

144

162

107

400-700

624

1156 ^

1036

380

594

557

838

1134

1449

896

700-2048

3962

15630

13310

2670

5063

6969

8299

10740

26500

10349

Sand poured into serial number 114 borehole on day 241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

260-262

262-265

265-267

267-270

270-272

272-275 

Averages

Delta db

4.01

4.74

4.99

6.77

5.98

5.35

6.07

7.36

7.56

9.35

6.09

7.34

6.79

6.58

6.36

0.62

9.74

8.86

9.21

11.98

11.09

10.59

11.98

10.74

14.86

13.28

10.08

12.26

10.54

11.88

11.22

0.24

24.48

28.36

33.73

41.34

33.78

39.27

45.70

41.20

40.39

35.12

36.18

35.49

43.62

41.18

37.13

0.99

86.35

128

114

188

134

166

229

161

180

147

129

131

130

115

146

1.32

713

600

583

794

641

563

1005

920

950

537

516

570

584

523

679

-1.21

7038

3437

3835

6765

3434

3506

4980

6772

4228

2622

4162

2589

5244

3576

4442

-3.67

Table 14. Tabulation of the east band-averaged signal levels in units of 

W~l9m 2/s 4/Hz for sensor 114 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the signal level 

(in db) resulting from introducing sand into the borehole for the indicated band.

49
I AVERAGING BAND LIMITS IN SECONDS

Days

219-221

221-223

223-225

226-228

228-231

231-233

233-236

236-238

238-241

Averages

40-70

0.31

0.40

0.22

0.23

0.28

0.25

0.26

0.21

0.40

0.28

70-100

0.79

0.46

0.30

0.10

0.29

0.30

0.26

0.22

0.53

0.36

100-200

0.69

0.81

0.34

0.54

0.32

0.84

1.00

0.82

0.85

0.69

200-400

2.26

2.70

2.96

2.91

2.29

1.62

1.91

2.98

2.67

2.48

400-700

23.43

16.39

17.55

19.36

13.04

10.31

15.77

18.51

20.65

17.23

700-2048

387

454

219

880

216

334

287

427

674

431

Sand poured into serial number 14 borehole on day 241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

260-265

265-267

267-270

270-272

272-275

Averages

Delta db

0.26

0.29

0.31

0.35

0.38

0.41

0.31

0.32

0.25

0.25

0.20

0.57

0.55

0.52

0.40

0.53

0.77

0.83

0.70

0.63

0.91

0.89

0.64

1.03

3.20

1.40

2.31

4.89

2.16

1.86

3.59

4.52

19.08

13.21

15.16

18.99

7.73

14.13

13.59

17.23

307

79.31

769

691

296

594

1096

833

No noise level estimates - #26 was not operating

0.28

0.35

0.33

0.31

0.33

0.59

0.54

0.50

0.34

0.50

0.43

0.75

1.46

1.05

0.70

1.13

0.89

1.13

3.69

5.16

4.05

4.79

3.47

1.46

16.98

30.77

19.62

33.06

18.30

0.26

198

1635

650

329

623

1.60

Table 15. Tabulation of the vertical band-averaged noise levels in units of 

\Q~l9m2/s4/Hz for sensor 114 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the noise level 

(in db) resulting from introducing sand into the borehole for the indicated band.

50
Days

219-221

221-223

223-225

226-228

228-231

231-233

233-236

236-238

238-241

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

0.42

1.33

1.09

0.94

0.91

0.89

0.54

1.37

1.35

0.98

70-100

1.14

1.51

3.16

1.59

2.25

1.76

0.93

2.37

2.00

1.86

100-200

5.22

3.76

6.51

7.55

3.37

3.93

3.53

7.10

5.92

5.21

200-400

17.16

9.37

34.18

34.18

16.46

13.21

12.24

43.49

35.47

23.97

400-700

117

93.47

93.25

215

127

184

170

297

214

168

700-2048

1520

523

1215

2517

1783

1580

1646

2917

6333

2226

Sand poured into serial number 114 borehole on day 241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

260-265

265-267

267-270

270-272

272-275

Averages

Delta db

2.38

4.05

3.36

3.86

4.17

4.05

4.94

4.35

6.17

9.02

8.96

9.70

11.48

8.63

12.58

9.05

13.90

38.84

37.69

31.17

37.66

33.55

36.71

31.22

61.72

194

134

145

107

136

189

212

723

1088

512

509

543

757

772

659

4446

6687

3736

5050

5604

5320

4318

6105

No noise level estimates - #26 was not operating

3.76

4.17

3.55

4.28

5.09

7.15

8.05

9.07

7.20

10.94

9.24

6.97

28.46

35.90

26.22

23.34

31.22

7.78

134

167

59.82

76.61

135

7.50

534

441

760

269

631

5.75

4802

5801

6632

1675

5015

3.53

Table 16. Tabulation of the north band-averaged noise levels in units of 

lQ~l9m2/s 4/Hz for sensor 114 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the noise level 

(in db) resulting from introducing sand into the borehole for the indicated band.

51
Days

219-221

221-223

223-225

226-228

228-231

231-233

233-236

236-238

238-241

Averages

AVERAGING BAND LIMITS IN SECONDS

40-70

1.68

2.75

3.64

2.71

1.46

3.04

2.19

5.46

5.73

3.18

70-100

1.48

4.18

4.74

5.41

3.42

8.18

4.88

9.76

11.62

5.96

100-200

12.43

9.76

11.85

15.67

10.24

19.75

14.80

22.37

46.44

18.15

200-400

31.46

31.62

92.29

64.42

42.98

68.61

88.35

107

107

70.41

400-700

30.01

81.64

315

248

339

330

546

732

583

356

700=2048

4460

15560

9548

2472

10460

7359

7199

10460

21590

9901

Sand poured into serial number 1 14 borehole on day 241

241-243

243-246

246-248

248-250

250-253

253-255

255-258

258-260

260-265

265-267

267-270

270-272

272-275 

Averages

Delta db

1.65

2.54

3.06

3.78

3.72

4.07

4.35

4.27

5.08

5.86

7.26

9.01

9.09

8.68

10.18

9.05

13.67

21.75

27.48

30.02

26.79

31.78

36.51

32.03

58.23

106

90.52

159

111

134

183

120

349

459

518

600

492

449

704

581

6475

3035

2902

5914

3762

3249

3280

5041

No noise level estimates - #26 was not operating

3.81 8.80 27.53 81.01 318 2243

No reliable noise level estimates - #26 too noisy

4.04

3.55

3.53

0.45

10.59

7.97

8.32

1.45

29.46

22.69

27.25

1.77

85.20

55.93

107

1.84

392

217

462

1.13

3006

2231

3831

-4.12

Table 17. Tabulation of the east band-averaged noise PSD levels in units of 

lQTl9m 2/s 4/Hz for sensor 114 averaged over the indicated bands for the duration 

of the experiment. The row "Delta db" contains the change in the noise level 

(in db) resulting from introducing sand into the borehole for the indicated band.

52