Summary of Extensometric 

Measurements in El Paso, Texas 

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 03-4158 

Prepared in cooperation with the


INTERNATIONAL BOUNDARY AND WATER COMMISSION,


UNITED STATES SECTION

Summary of Extensometric 

Measurements in El Paso, Texas 

By Charles E. Heywood 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 03-4158 

Prepared in cooperation with the 

INTERNATIONAL BOUNDARY AND WATER 

COMMISSION, 

UNITED STATES SECTION 

Albuquerque, New Mexico 

2003 
Charles E. Heywood—SUMMARY OF EXTENSOMETRIC MEASUREMENTS IN EL PASO, TEXAS—

U.S. Geological Survey Water-Resources Investigations Report 03-4158 

U.S. Department of the Interior 

U.S. Geological Survey, WRD 

5338 Montgomery Blvd. NE, Suite 400 

Albuquerque, NM 87109-1311 

BOOK RATE

U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

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For additional information write to:	 Copies of this report can be purchased 

from: 

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Office using the URL http://nm.water.usgs.gov. 
CONTENTS


Page


Abstract.................................................................................................................................................................................. 1


Introduction ........................................................................................................................................................................... 1


Methods ................................................................................................................................................................................. 3


Dial-gage data.............................................................................................................................................................. 3


Strip-chart recorder data .............................................................................................................................................. 3


Digital data .................................................................................................................................................................. 7


Extensometric measurements ................................................................................................................................................ 8


Summary................................................................................................................................................................................ 8


Selected references ................................................................................................................................................................ 8


FIGURES 

1. Shaded relief map of southern Hueco Bolson showing location of extensometer site near El Paso, Texas,


and Ciudad Juarez, Chihuahua, Mexico ............................................................................................................. 2


2. Schematic showing hydrogeologic summary of borehole electrical resistivity logs, intervals of


selected core samples collected at the extensometer site, and water levels measured in piezometers on


November 16, 1992 ............................................................................................................................................ 4


3. Photograph of shallow extensometer lever arm, datum table, and selected associated instruments........................ 5


4-6. Graphs showing:


4. Regression fit of transducer output and deep extensometer displacement........................................................ 7


5. El Paso extensometer displacements ................................................................................................................ 9


6. Measured compaction in the depth interval from 100 to 335 meters ............................................................... 10


TABLE 

1. Dial-gage readings on deep and shallow extensometers .......................................................................................... 5


CONVERSION FACTORS AND DATUMS 

Multiply By To obtain 

millimeter (mm) 0.03937 inch (in.) 

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:


°F = (1.8 × °C) + 32

(NAVD 88).


Altitude, as used in this report, refers to distance above or below sea level.


Vertical coordinate information is referenced to the North American Vertical Datum of 1988

iii 
SUMMARY OF EXTENSOMETRIC 

MEASUREMENTS IN EL PASO, TEXAS 

By Charles E. Heywood 

ABSTRACT 

Two counter-weighted-pipe borehole 

extensometers were installed on the left bank of the Rio 

Grande between El Paso, Texas, and Ciudad Juarez, 

Chihuahua, Mexico, in 1992. A shallow extensometer 

measures vertical compaction in the 6- to 100-meter 

aquifer-system depth interval. A deep extensometer 

measures vertical compaction in the 6- to 305-meter 

aquifer-system depth interval. Both extensometers are 

referenced to the same surface datum, which allows 

time-series differencing to determine vertical 

compaction in the depth interval between 100 and 305 

meters. From April 2, 1993, through June 13, 2002, 1.6 

centimeters of compaction occurred in the 6- to 305-m 

depth interval. Until February 1999, most aquifersystem compaction occurred in the deeper aquifersystem interval between 100 and 305 meters, from 

which ground water was extracted. After that time, 

compaction in the shallow interval from 6 to 100 

meters was predominant and attained a maximum of 

7.6 millimeters by June 13, 2002. Minor residual

compaction is expected to continue; continued 

maintenance of the El Paso extensometers would 

document this process. 

INTRODUCTION 

Concerns about potential land subsidence 

associated with changes to the surface-water delivery 

system in the Rio Grande Valley in El Paso, Texas 

(fig. 1), were summarized by Land and Armstrong 

(1985). A primary concern was the potential for 

increased land subsidence in the area of a proposed 

extension to the American Canal, which is adjacent to 

areas of substantial ground-water withdrawals by the 

cities of El Paso, Texas, and Juarez, Mexico. Seepage 

to the underlying aquifer system in the El Paso-Juarez 

area reduces flow in the Rio Grande. Salvaging surfacewater flow by diverting it from the Rio Grande into the 

American Canal Extension (ACE) might decrease 

aquifer-system recharge in this area, exacerbating 

ground-water decline. Subsidence in this area could be 

caused either by compaction in the interval 

(approximately 100 to 200 m below land surface) from 

which ground water is extracted or by compaction in a 

shallower aquifer interval that is more hydraulically 

connected to the Rio Grande. A decrease or cessation 

of flow in the Rio Grande would cause the local water 

table to decline and possibly cause compaction in the 

shallow parts of the aquifer system. To distinguish 

between these two components of land-surface 

subsidence, two vertical extensometers were installed 

adjacent to the Rio Grande downstream from the 

terminus of the original American Canal. The 

monitoring site was selected at the location of probable 

maximum hydrologic effects from construction of the 

ACE rather than at the location of maximum 

subsidence (probably north of the Rio Grande Valley 

near areas of maximum ground-water drawdown). 

About 4 years of preliminary compaction data were 

obtained while the ACE was being constructed. Flow 

diversion from the Rio Grande into the western section 

of the ACE began on February 15, 1997; the entire 

length of the ACE was operational by February 1999 

(Rong Kuo, U.S. International Boundary and Water 

Commission, oral commun., 2002). 

This report was prepared in cooperation with the 

International Boundary and Water Commission, 

United States Section. The report documents 

extensometric measurements made adjacent to the Rio 

Grande in El Paso, Texas, between April 2, 1993, and 

June 13, 2002. Although water levels were measured at 

seven depth intervals over this period, these data are not 

included in this report but are stored in the U.S. 

Geological Survey (USGS) National Water 

Information System database. The extensometers are 

very sensitive to vertical aquifer-system strain and have 

recorded seismic surface waves (Heywood, 2000). The 

use of El Paso extensometric and piezometric data to 

determine aquifer-system compressibility has been 

documented (Heywood, 1995) and is not discussed in 

this report. 

1 
2

METHODS 

In the summer and fall of 1992, two vertical 

borehole extensometers were installed adjacent to the 

Rio Grande in El Paso, Texas, at latitude 31° 44’ 35” 

N., longitude 106° 23’ 57" W. (fig. 1). A shallow 

extensometer was completed to a depth of 100 m, 

which is just above the interval of water production in 

wells in the Rio Grande Valley. A deep extensometer 

was completed to a depth of 340 m in low-permeability 

clay underlying the freshwater-producing interval of 

the Hueco aquifer system. The construction and depth 

intervals of each extensometer, short- and long-normal 

electrical resistivity logs of the deep extensometer 

borehole, and the potentiometric heads in the screened 

intervals of piezometers adjacent to the extensometers 

are shown in figure 2. 

Sandy parts of the aquifer system, presumed to 

be relatively permeable, correlate with the higher 

resistivity depth intervals on the resistivity log. Zones 

in which low-permeability clays were cored correlate 

with lower resistivity depth intervals on the resistivity 

log. The clay zones are, in places, more than 10 m thick 

(fig. 2). 

Vertical displacement of both extensometer 

pipes was measured with respect to the same datum by 

use of a datum table in the extensometer shelter (fig. 3). 

The three legs of the datum table are each thermally 

insulated and mechanically isolated from surrounding 

alluvium to a depth of 6 m, where they are anchored in 

cement. The extensometers therefore measure aquifersystem compaction in the intervals from 6 to 100 m and 

from 6 to 340 m. By subtracting the shallow 

extensometer data from the deep extensometer data, a 

record was obtained of aquifer-system compaction in 

the 100- to 340-m interval. This depth interval includes 

the freshwater-producing interval of the Hueco aquifer 

system in the Rio Grande Valley in which water 

production wells are screened. 

Temperature variations can cause the metal 

extensometer components to change length, thereby 

creating an apparent displacement that is noise to the 

signal of interest. Five thermistor sensors were 

installed at various points on the extensometer 

apparatus to monitor temperature variations and their 

correlation to the displacement signals. The amplitude 

of the diurnal temperature variations changed with the 

seasons and was typically less than 0.07 mm. Because 

the extensometer pipes and table legs were components 

with comparable thermal expansion in a similar 

temperature environment, the temperature effects in 

each were of similar amplitude and phase. Temperature 

effects are relatively insignificant in the 100- to 340-m 

compaction record because the compaction time series 

for the 100- to 340-m depth interval was obtained by 

calculating the difference between the deep and 

shallow extensometer displacement time series. 

Riley (1969) discussed the desirability of 

minimizing frictional contact between an extensometer 

pipe and the surrounding borehole casing in order to 

accurately measure aquifer-system compaction. 

Compaction not measured because of either static 

(stick-slip) or sliding friction is termed "frictional dead 

band." During final assembly of the extensometric 

apparatus, the optimal counter weight for each 

extensometer pipe was determined by measuring and 

minimizing frictional dead band. 

A precision digital barometer monitored changes 

in atmospheric pressure to determine barometric 

effects in the piezometers. The atmosphere exerts a 

buoyant force on the extensometer counter weights and 

lever arms, which in turn exerts a tensile stress on the 

extensometer pipes. The effect of a reasonable range of 

barometric pressure on extensometer pipe length was 

calculated and determined to be negligible. 

The displacement of each extensometer pipe 

with respect to the surface datum table was measured 

with three instruments: (1) a dial gage with 5­

micrometer (0.0002-in.) resolution, (2) a 100-mm 

linear displacement transducer with 10-micrometer 

(0.0004-in.) resolution, and (3) an analog strip-chart 

recorder (modified for 13X amplification and 96-day 

chart duration) with 30-micrometer (0.001-in.) 

resolution. The dial gages were used to calibrate the 

displacement measured with the transducers and stripchart recorders. The configuration of one transducer 

and dial gage located between the shallow 

extensometer pipe and datum table is shown in figure 3. 

Dial-Gage Data 

Both dial-gage measurements were recorded 

when the extensometer site was visited and when 

analog strip charts were replaced. These measurements 

and the displacement since April 2, 1993, are 

summarized in table 1. 

Strip-Chart Recorder Data 

An analog water-level recorder provided a 

continuous analog record of displacement for each 

extensometer. A custom counter weight provided 

3 
4

Table 1. Dial-gage readings on deep and shallow extensometers 

Dial-gage reading 

for deep Displace- Dial-gage reading for 

Greenwich extensometer ment shallow extensometer Displacement 

Date mean time (inches) (millimeters) (inches) (millimeters) 

4/2/1993 2330 0.6171 0.000 0.4447 0.000 

5/5/1993 0130 0.6680 1.293 0.4631 0.467 

6/3/1993 1618 0.6804 1.608 0.4666 0.556 

6/4/1993 1304 0.6835 1.687 0.4680 0.592 

6/24/1993 2131 0.6658 1.237 0.4561 0.290 

6/25/1993 1329 0.6655 1.229 0.4591 0.366 

6/25/1993 1600 0.6659 1.240 0.4623 0.447 

8/9/1993 2200 0.6189 0.046 0.4481 0.086 

8/10/1993 1330 0.6160 -0.028 0.4487 0.102 

10/20/1993 1604 0.5859 -0.792 0.4441 -0.015 

11/4/1993 1647 0.5992 -0.455 0.4504 0.145 

1/13/1994 2032 0.6267 0.244 0.4762 0.800 

1/14/1994 2132 0.6275 0.264 0.4766 0.810 

4/19/1994 2200 No data No data 0.4502 0.140 

4/20/1994 1452 No data No data 0.4488 0.104 

5 
Table 1. Dial-gage readings on deep and shallow extensometers--Concluded 

Dial-gage reading 

for deep Displace- Dial-gage reading for 

Greenwich extensometer ment shallow extensometer Displacement 

Date mean time (inches) (millimeters) (inches) (millimeters) 

5/17/1994 2000 0.5909 -0.665 0.4871 1.077 

8/5/1994 1540 0.5992 -0.455 0.4682 0.597 

9/13/1994 2008 0.5951 -0.559 0.4632 0.470 

9/14/1994 2033 0.6019 -0.386 0.4651 0.518 

9/15/1994 1425 0.6043 -0.325 0.4649 0.513 

12/1/1994 2338 0.6784 1.557 0.4871 1.077 

12/2/1994 1502 0.6774 1.532 0.4868 1.069 

1/12/1995 2027 0.6861 1.753 0.4998 1.400 

3/7/1995 1651 0.7076 2.299 0.4890 1.125 

6/6/1995 1541 0.7079 2.306 0.4742 0.749 

9/6/1995 1930 0.7213 2.647 0.4632 0.470 

9/25/1995 1630 0.6660 1.242 0.4490 0.109 

10/26/1995 1819 0.6568 1.008 0.4481 0.086 

1/31/1996 1752 0.6799 1.595 0.4559 0.284 

3/7/1996 2357 0.7016 2.146 0.4642 0.495 

6/11/1996 1545 0.8156 5.042 0.4709 0.665 

11/19/1996 1438 0.7923 4.450 0.4471 0.061 

5/30/1997 1537 0.7835 4.227 0.4656 0.531 

5/27/1998 No data No data No data 0.4850 1.024 

3/3/1998 No data No data No data 0.4628 0.460 

5/19/1999 No data No data No data 0.5562 2.832 

11/5/1999 1942 No data No data 0.5128 1.730 

11/5/1999 2000 No data No data 0.5141 1.763 

2/9/2000 2056 0.9618 8.755 0.5483 2.631 

2/9/2000 2125 0.9618 8.755 0.5545 2.789 

5/4/2000 2108 1.0520 11.046 0.6369 4.882 

7/27/2000 1724 1.1249 12.898 0.6440 5.062 

10/18/2000 1955 1.1100 12.520 0.5978 3.889 

1/19/2001 1840 1.0921 12.065 0.6009 3.967 

6/21/2001 2100 1.1839 14.397 0.6962 6.388 

9/25/2001 2123 1.1340 13.129 0.6321 4.760 

9/25/2001 2134 1.1340 13.129 0.6303 4.714 

12/13/2001 2050 1.1389 13.254 0.6492 5.194 

12/13/2001 2100 1.1389 13.254 0.6490 5.189 

3/19/2002 2115 1.1611 13.818 0.7120 6.789 

6/13/2002 1600 1.2338 15.664 0.7500 7.755 

6 
torque on the recorder drum to prevent dead band from 

gear backlash. From April 2 to June 24, 1993, the stripchart clock-motor gear was set so that one chart 

encompassed a 32-day period. The drive-gear 

assembly was then further modified to increase the 

chart time span to 96 days. A field visit to the 

extensometer site was scheduled approximately every 

3 months to replace charts and download digital data. 

A complete analog record of extensometer 

displacement from April 2, 1993, to March 15, 2002, 

has been recorded on 82 paper charts (41 for each 

extensometer), which are archived at the USGS office 

in Albuquerque, New Mexico. 

Digital Data 

Zero displacement values were assigned to the 

beginning of the digital and analog record at 17:30 

Greenwich mean time on April 2, 1993; displacement 

data were measured from these initial positions. The 

vertical displacement of each extensometer pipe was 

recorded every 30 minutes with the displacement 

transducer and digital data logger. The ratiometric 

resistance of each transducer in an AC Half Bridge 

configuration was an analog for displacement. The 

digital data logger provided 2,500-millivolt excitation 

for two single-ended measurements with opposite 

polarity for ion depolarization. The measurement was 

recorded on a digital storage module. Dial-gage 

readings (table 1) and corresponding transducer 

measurements were fit with linear least-squares 

regression to obtain a conversion factor for translating 

the ratiometric transducer output to vertical 

displacement. The displacement transducers have a 

nominal 10-cm (4-in.) range. The regression fit of 

transducer output to dial-gage measurement of 

extensometer displacement for the deep extensometer 

is shown in figure 4. The slope of the regression line is 

-3.8928, the y intercept is 2.4433, and the coefficient of 

determination (r2

) is 0.99956. Because of several 

malfunctions, the fit of transducer data from the 

shallow extensometer to all dial-gage readings (table 1) 

was poor. The strip-chart records for the shallow 

extensometer were digitized for problematic periods. 

7 
After the raw digital transducer output was 

downloaded, the data were converted to vertical 

displacements (in millimeters) and smoothed with a 

24-hour moving average filter to remove the small 

diurnal temperature effects. The 30-minute data were 

then resampled at a daily interval (fig. 5). Calculating 

the difference between the shallow and deep 

extensometer displacement time series yielded a 

measure of compaction in the aquifer-system interval 

between 100 and 340 m deep (fig. 6). 

EXTENSOMETRIC MEASUREMENTS 

From April 1993 through January 1999, minor 

(less than 1 mm) compaction occurred within the 6- to 

100-m aquifer-system interval spanned by the shallow 

extensometer (fig. 5). The similarity of the total 

compaction record (6 to 340 m) to the compaction 

record in the deeper (100 to 340 m) aquifer-system 

interval (fig. 6) indicates that most aquifer-system 

compaction occurred in the deeper interval from April 

1993 through January 1999. Ground-water 

withdrawals within the United States part of the Rio 

Grande Valley decreased after 1995 because of 

increased chloride concentration in production wells 

(Heywood and Yager, 2003). The consequent 

decreased change in ground-water-level variations is 

evident as decreased amplitude of monthly and 

seasonal compaction variations in the deep compaction 

record (fig. 6). 

From February 1999 through the end of the 

record in June 2002, 6.7 mm of net compaction 

occurred in the shallow aquifer-system interval, with 

seasonal elastic rebound during the winter months. 

Compaction in the deep aquifer-system interval 

appears to have continued until August 2000, then 

abated or recovered somewhat. Possibly decreased 

total geostatic stress resulting from the declining water 

table has compensated for decreased aquifer-system 

pore pressure in this depth interval in a way that the 

resulting intergranular effective stress has remained 

constant or decreased somewhat. 

The electrical resistivity log (fig. 2) suggests that 

clay beds may be about 10 to 20 m thick. Clay and silt 

sequences as thick as 13 m were penetrated during 

drilling (Arthur Clark, U.S. Geological Survey, written 

commun., 1992). Measured hydraulic-conductivity 

values of clay samples from cores ranged from 1 x 

10-6 to 4 x 10-3 m/day during laboratory consolidation 

testing at effective stresses equivalent to in situ burial 

depths (Harold Olsen, Colorado School of Mines, 

written commun., 1997). If these values are 

representative of in situ clay vertical hydraulicconductivity values, the time required for clay porepressure equilibration may be calculated (Scott, 1963; 

Riley, 1969) assuming reasonable values of clay-unit 

specific storage. For a 13-m-thick clay bed with a 

vertical hydraulic conductivity of 1 x 10-6 m/day and 

specific storage of 3 x 10-4 m-1, the time required to 

reach 93 percent of total consolidation is 32 years. 

SUMMARY 

Compaction of aquifer-system sediments 

between 6 and 340 m in depth resulted in land 

subsidence of 1.6 cm at the El Paso extensometer site 

for the period of record between April 2, 1993, and 

June 13, 2002. As much as 9 mm of compaction 

occurred in the deeper aquifer interval (100 to 340 m) 

before July 2000. This compaction likely was a 

response to ground-water withdrawals. As much as 7.6 

mm of compaction has occurred in the shallow aquifersystem interval (6 to 100 m). Because individual clay 

unit thicknesses of about 10 m exist within the aquifer 

system at this location, the time required for pore 

pressures within low-permeability clays to equilibrate 

with those in the surrounding, more permeable sands 

may be tens of years. Residual compaction of several 

cm will likely continue because of this equilibration 

process, even if water levels measured in the permeable 

sandy parts of the aquifer system stabilize. Continued 

maintenance of the El Paso extensometers would 

document this process. 

SELECTED REFERENCES 

Galloway, D.L., Jones, D.R., and Ingebritsen, S.E., eds., 

1999, Land subsidence in the United States: U.S. 

Geological Survey Circular 1182, 175 p. 

Heywood, C.E., 1993, Monitoring aquifer compaction and 

land subsidence due to ground-water withdrawal in the 

El Paso, Texas - Juarez, Chihuahua, area, in Prince, 

K.R., Galloway, D.L., and Leake, S.A., eds., U.S. 

Geological Survey Subsidence Interest Group 

Conference, Edwards Air Force Base, Antelope Valley, 

California, November 18-19, 1992—Abstracts and 

summary: U.S. Geological Survey Open-File Report 

94-532, 84 p. 

———1995, Investigation of aquifer-system compaction in 

the Hueco Basin, El Paso, Texas, USA, in Proceedings 

of the Fifth International Symposium on Land 

Subsidence, The Hague: International Association of 

Hydrological Sciences Publication 234, IAHS Press, 

Oxfordshire, UK, p. 35-45. 

8 
9

10

Heywood, C.E., 2000, Extensometers as seismometers: 

American Geophysical Union Transactions, v. 81, no. 

48, abstract S11B-16. 

Heywood, C.E., and Yager, R.M., 2003, Simulated groundwater flow in the Hueco Bolson, an alluvial-basin 

aquifer system near El Paso, Texas: U.S. Geological 

Survey Water-Resources Investigations Report 02­

4108, 73 p. 

Land, L.F., and Armstrong, C.A., 1985, A preliminary 

assessment of land-surface subsidence in the El Paso 

area, Texas: U.S. Geological Survey Water-Resources 

Investigations Report 85-4155, 96 p. 

Riley, F.S., 1969, Analysis of borehole extensometer data 

from central California, in Tison, L.J., ed., Land 

subsidence: Proceedings of the International 

Symposium on Land Subsidence, Tokyo, International 

Association of Scientific Hydrology Publication no. 89, 

p. 423-431.

———1986, Developments in borehole extensometry, in 

Johnson, A.I., Carbognin, L., and Ubertini, L., eds., 

Land subsidence: Proceedings of the Third 

International Symposium on Land Subsidence, Venice, 

Italy, 1984, International Association of Scientific 

Hydrology Publication no. 151, p. 169-186. 

Scott, R.F., 1963, Principles of soil mechanics: Palo Alto, 

Calif., Addison-Wesley Publishing Company, 550 p. 

11