U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5090

In cooperation with Texas A&M University AgriLife

Direct-Current Resistivity Profiling at the Pecos River

Ecosystem Project Study Site near Mentone, Texas, 2006

EASTING, IN METERS

NORTHING, IN METERS

3,507,0 00 3,507,5 00 3,508,0 00 3,508,5 00

Site B

(control)

Site A

(treated)

less than

2 5 10 20

greater than

30

RESISTIVITY, IN OHM-METERS

630,000 630,500

0

0.5

1.6

2.8

4.0

5.5

7.0

8.8

10.7

12.8

15.0

16.3

DEPTH, IN METERS RELATIVE TO WATER SURFACE

TEXAS

Pecos Valley

aquifer

Pecos River
Front cover:

(A) Waterborne direct-current continuous resistivity profiling along Pecos River near Mentone, Texas.

(B) Treated reach (site A) along Pecos River near Mentone, Texas, 2006.

(C) Untreated reach (site B) along Pecos River near Mentone, Texas, 2006.

(D) Inverse modeling results for waterborne continuous direct-current resistivity profile along Pecos 

 River near Mentone, Texas.

(A)

(B)

(C)

(D)
Direct-Current Resistivity Profiling at the 

Pecos River Ecosystem Project Study Site 

near Mentone, Texas, 2006

By Andrew P. Teeple, Alyson K. McDonald, Jason D. Payne, and Wade H. Kress

In cooperation with Texas A&M University AgriLife

Scientific Investigations Report 2009–5090

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Teeple, A.P., McDonald, A.K., Payne, J.D., and Kress, W.H., 2009, Direct-current resistivity profiling at the Pecos River 

Ecosystem Project study site near Mentone, Texas, 2006: U.S. Geological Survey Scientific Investigations Report 

2009–5090, 11 p. 
iii

Contents

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

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

Purpose and Scope .............................................................................................................................3

Hydrogeology ........................................................................................................................................3

Direct-Current Resistivity Profiling ............................................................................................................3

Data Collection .....................................................................................................................................5

Inverse Modeling of Direct-Current Resistivity Data .....................................................................5

Distributions of Resistivity ..................................................................................................................9

Summary .........................................................................................................................................................9

References ...................................................................................................................................................10

Figures

1. Map showing location of Pecos River Ecosystem Project study site near Mentone, 

Texas ..............................................................................................................................................2

2. Photographs showing treated reach (site A) and untreated (control) reach (site B) 

along the Pecos River, Pecos River Ecosystem Project study site near Mentone, 

Texas, 2006 ....................................................................................................................................2

3. Diagram showing layout of waterborne direct-current continuous resistivity 

profiling system ............................................................................................................................3

4. Photographs showing (A) IRIS Instruments Syscal Pro direct-current resistivity 

system and (B) stainless steel electrode connected to a direct-current resistivity 

cable near the Pecos River, Pecos River Ecosystem Project study site near 

Mentone, Texas, 2006 ..................................................................................................................4

5. Map showing land-based direct-current resistivity profile at site B (control) and 

waterborne continuous direct-current resistivity profile along the river adjacent to 

site A (treated) and site B (control), Pecos River Ecosystem Project study site near 

Mentone, Texas ............................................................................................................................4

6. Diagrams showing (A) reciprocal-Schlumberger and (B) dipole-dipole arrays used 

at the Pecos River Ecosystem Project study site near Mentone, Texas, 2006 ..................5

7. Photographs showing waterborne direct-current resistivity equipment used at the 

Pecos River Ecosystem Project study site near Mentone, Texas, 2006: (A) punt boat 

with equipment and trolling motor, (B) waterborne electrode streamer with foam 

insulation for flotation, and (C) IRIS Instruments Syscal Pro resistivity meter .................6

8. Smooth inverse model results of direct-current resistivity, Pecos River Ecosystem 

Project study site near Mentone, Texas, 2006: (A) resistivity section from 

reciprocal-Schlumberger land-based profile at the control site, (B) resistivity 

section from reciprocal-Schlumberger waterborne continuous profile from 

subreach of the river adjacent to the land-based profile, and (C) resistivity section 

from dipole-dipole waterborne continuous profile from subreach of the river 

adjacent to the land-based profile ............................................................................................7

9. Smooth inverse model results of waterborne continuous direct-current resistivity 

profiles along 2.279-kilometer reach of the river adjacent to treated and control 

sites, Pecos River Ecosystem Project study site near Mentone, Texas, 2006: 

(A) resistivity section from reciprocal-Schlumberger profile, and (B) resistivity 

section from dipole-dipole profile .............................................................................................8
iv

Conversion Factors and Datums

Inch/Pound to SI

Multiply By To obtain

Length

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft)

millimeter (mm) 0.03937 inch (in)

Resistivity

ohm-meter (ohm-m) 3.281 ohm-foot (ohm-ft)

Datums 

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

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.
Abstract

The U.S. Geological Survey, in cooperation with Texas 

A&M University AgriLife, did a surface geophysical investigation at the Pecos River Ecosystem Project study site 

near Mentone in West Texas intended to determine shallow 

(to about 14 meters below the water [river] surface) subsurface composition (lithology) in and near treated (eradicated 

of all saltcedar) and control (untreated) riparian zone sites 

during June–August 2006. Land-based direct-current 

resistivity profiling was applied in a 240-meter section of the 

riverbank at the control site, and waterborne direct-current 

continuous resistivity profiling (CRP) was applied along a 

2.279-kilometer reach of the river adjacent to both sites to 

collect shallow subsurface resistivity data. Inverse modeling 

was used to obtain a nonunique estimate of the true subsurface 

resistivity from apparent resistivity calculated from the field 

measurements. The land-based survey showed that the subsurface at the control site generally is of relatively low resistivity down to about 4 meters below the water surface. Most 

of the section from about 4 to 10 meters below the water 

surface is of relatively high resistivity. The waterborne CRP 

surveys convey essentially the same electrical representation 

of the lithology at the control site to 10 meters below the 

water surface; but the CRP surveys show considerably lower 

resistivity than the land-based survey in the subsection from 

about 4 to 10 meters below the water surface. The CRP 

surveys along the 2.279-kilometer reach of the river adjacent 

to both the treated and control sites show the same relatively 

low resistivity zone from the riverbed to about 4 meters 

below the water surface evident at the control site. A slightly 

higher resistivity zone is observed from about 4 to 14 meters 

below the water surface along the upstream approximately 

one-half of the profile than along the downstream one-half. 

The variations in resistivity could not be matched to variations in lithology because sufficient rock samples were not 

available.

Introduction

Saltcedar, or tamarisk (Tamarix spp.), is an invasive 

phreatophyte species that develops dense thickets in riparian 

systems (DeLoach, 1997; Weeks and others, 1987). Among 

its many known and suspected effects on stream corridors, 

water consumption in stream corridors dominated by saltcedar 

could be greater on a per-acre basis than consumption in corridors dominated by native phreatophytes (Kolb, 2001). Dense 

saltcedar stands on stream banks also accumulate sediment 

in their thick root systems, which gradually narrows stream 

channels and increases flooding (Tamarisk Coalition, 2009).

The Pecos River Ecosystem Project (PREP) is a largescale ecosystem restoration program designed to eliminate 

saltcedar and replace it with reintroduced native plants and 

grasses. Preliminary assessments of water quality indicate that 

saltcedar control will decrease salinity and increase flow in 

the Pecos River (Hart and others, 2005; Texas State Soil and 

Water Conservation Board and Texas Commission on Environmental Quality, 2006). As part of the PREP, Texas A&M 

University AgriLife initiated a saltcedar-control program in 

1999; a reduction in evapotranspiration and improvement in 

water delivery efficiency along the upper Pecos River in Texas 

were among the numerous desired effects. A paired study 

with treated and untreated riparian zones helped quantify the 

effects of saltcedar control on evapotranspiration and water 

delivery efficiency.

At the PREP study site near Mentone in West Texas 

(fig. 1) are two hydrologically similar and approximately 

equidistance riparian zones along a 1.5-kilometer (km) reach 

of the Pecos River. The downstream riparian zone (fig. 2, site 

A) has been eradicated of all saltcedar. Imazapyr (2-[4-isopropyl-4-methyl-5-oxo-2 imidazoline-2-yl] nicotinic acid), 

an herbicide in the imidazolinone chemical family (Stell and 

others, 1995) was used to kill the saltcedar. The untreated 

upstream riparian zone (fig. 2, site B) is the control site.

Direct-Current Resistivity Profiling at the Pecos River 

Ecosystem Project Study Site near Mentone, Texas, 2006

By Andrew P. Teeple1, Alyson K. McDonald2, Jason D. Payne1, and Wade H. Kress1

1

 U.S. Geological Survey, West Texas Program Office, San Angelo, Texas.

2

 Texas AgriLife Extension Service, Texas A&M University, Fort Stockton, Texas.
2 Direct-Current Resistivity Profiling at the Pecos River Ecosystem Project Study Site near Mentone, Texas, 2006

Figure 1. Location of Pecos River Ecosystem Project study site near Mentone,Texas. 

Figure 2. Treated reach (site A) and untreated (control) reach (site B) along the Pecos River, Pecos River Ecosystem Project study site 

near Mentone, Texas, 2006.

Site A Site B

Red Bluff

Lake

Pecos

Pyote

Wickett

Mentone

Orla

Toyah

Kermit

Wink

Barstow

20

285 302 18

115

17

57

NORTHING, IN METERS

3,460,000 3,480,000 3,500,000 3,520,000 3,540,000

EASTING, IN METERS

600,000 620,000 640,000 660,000 680,000

Pecos Valley aquifer (Texas Water

Development Board, 2006)

EXPLANATION

0 200 400 KILOMETERS

TEXAS

Area

 enlarged

LOCATION MAP

Pecos River

PECOS COUNTY

REEVES COUNTY

WARD COUNTY

LOVING COUNTY WINKLER COUNTY

TEXAS

NEW MEXICO

Study

site

Base from U.S. Geological Survey LandSat 7 Enhanced Thematic Mapper Plus

Universal Transverse Mercator projection, Zone 14

North American Datum 1983 0 10 20 30 KILOM ETERS
Direct-Current Resistivity Profiling 3

As a part of the PREP, Texas A&M University AgriLife 

is evaluating flows between the river and the surficial aquifer 

(Pecos Valley aquifer [formerly Cenozoic Pecos alluvium 

aquifer]) at the site, exploring the effects of saltcedar control 

on the fate of salvaged water and attempting to quantify water 

released to downstream flow and aquifer recharge (Texas 

A&M AgriLife/Texas Water Resources Institute, 2009). 

Characterization of the aquifer beneath the treated and control 

zones (sites), which could contribute to a better understanding 

of the interaction between the river and the surficial aquifer in 

response to saltcedar control, is a necessary part of this effort. 

Accordingly, the U.S. Geological Survey, in cooperation with 

Texas A&M University AgriLife, did a surface geophysical 

investigation at the PREP study site intended to determine 

shallow (to about 14 meters [m] deep) subsurface composition (lithology) in and near the treated and control sites during 

June–August 2006.

Purpose and Scope

This report presents the techniques and findings of directcurrent (DC) resistivity profiling at the PREP study site near 

Mentone, Tex., to determine the shallow subsurface composition of the Pecos Valley aquifer in and near a treated (to eradicate saltcedar) site and a control site on the Pecos River during 

June–August 2006. The report describes data acquisition and 

processing of land-based DC resistivity profiling applied in a 

240-m section of the riverbank at the control site and waterborne DC continuous resistivity profiling (CRP) applied along 

a 2.279-km reach adjacent to both sites. The resulting twodimensional sections of the shallow subsurface showing distributions of resistivity are then described. To translate variations in subsurface resistivity to variations in lithology, rock 

samples obtained from various depths (usually during well 

construction) at the site of interest are needed to match rock 

types to ranges of resistivity. Because sufficient rock samples 

were not available (wells installed in 1986 and 2003 were not 

drilled to depths below the riverbed), the specific lithology at 

the sites could not be determined from resistivity. 

Hydrogeology

According to White (1971), the 

shallow subsurface in Ward County 

to the immediate southeast of 

the PREP site (and thus assumed 

representative of the PREP site) 

comprises clay, sand, gravel, and 

caliche in beds that differ widely in 

lithology and thickness over short 

distances. The rocks are unconsolidated or poorly cemented. Most of 

the relatively small annual rainfall 

(less than 254 millimeters [10 

inches]) evaporates or is transpired 

by vegetation, thus recharge to the Pecos Valley aquifer is 

small. Groundwater occurs under water-table conditions adjacent to the Pecos River. Sheng and others (2007), as part of the 

PREP, reported that the Pecos River is hydraulically connected 

to shallow groundwater along the reach of the river adjacent 

to the treated and control sites; river stage generally is higher 

than adjacent groundwater levels.

Direct-Current Resistivity Profiling

Land-based DC resistivity profiling generally involves 

multiple electrodes put directly into the ground. The electrodes 

are configured in various geometries of current and potential 

pairs. Two current electrodes, used as a transmitter, are connected to an electrical current source to create a current field. 

An electrical potential field is created that is measured by a 

pair of potential electrodes used as a receiver. On the basis 

of the configuration of the electrodes (known as an array), 

the amount of current applied through the current electrodes, 

and the measured potential across the potential electrodes, 

apparent resistivity can be determined. Apparent resistivity 

depicts the resistivity of completely uniform (homogenous 

and isotropic) earth material (Keller and Frischknecht, 1966). 

A representation of the true subsurface resistivity is obtained 

through inverse modeling, the application of an iterative 

software program to compute the resistivity of an equivalent 

nonuniform earth material (described in the section “Inverse 

Modeling of Direct-Current Resistivity Data”). More complete 

descriptions of the DC resistivity method and typical resistivities of earth materials are in Grant and West (1965) and Zohdy 

and others (1974).

Waterborne CRP involves electrodes deployed in water 

so that electrical contact is maintained without the electrodes 

being driven directly into the ground (Ball and others, 2006). 

The floating array is pulled at a constant speed to collect 

continuous data (fig. 3). Multiple receiver channels on the 

resistivity meter allow for multiple potential electrode pairs 

to be used simultaneously with a single electrode pair serving 

as the current source (Ball and others, 2006). The waterborne 

CRP technique greatly reduces electrode installation time for 

large survey areas. More complete descriptions of waterborne 

CRP are in Ball and others (2006) and Masterson and others 

(2006).

Figure 3. Diagram showing layout of waterborne direct-current continuous resistivity 

profiling system (modified from Ball and others, 2006).

River bottom

Resistivity meter

Global Positioning System unit

Depth sounder

5 meters Water surface

Electrode streamer
4 Direct-Current Resistivity Profiling at the Pecos River Ecosystem Project Study Site near Mentone, Texas, 2006

Figure 4. (A) IRIS Instruments Syscal Pro direct-current 

resistivity system and (B) stainless steel electrode connected to a 

direct-current resistivity cable near the Pecos River, Pecos River 

Ecosystem Project study site near Mentone, Texas, 2006. 

Figure 5. Land-based direct-current resistivity profile at site B 

(control) and waterborne continuous direct-current resistivity 

profile along the river adjacent to site A (treated) and site 

B (control), Pecos River Ecosystem Project study site near 

Mentone, Texas. 

B

A

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

90

950

1,000

1,050

1,100

1,150

1,200

1,250

1,300

1,350

1,400

1,450

1,500

1,550

1,600

1,650

1,700

1,750

1,800

1,850

1,900

1,950

2,000

2,050

00

2,150

2,200

2,250

2,279

841

850

900

950

1,000

1,050

1,079

0

2,1

0 250 500 METERS

Surface direct-current resistivity profile with distance

in meters

Waterborne continuous direct-current resistivity profile

with distance in meters

Well location

EXPLANATION

Site A (treated)

Approximate location

of treated section

Site B (control)

Pecos

River

Base of U.S. Department of Agriculture

National Agricultural Imagery Program

Digital Ortho Photo Image

Universal Transverse Mercator, Zone 14

North American Datum 1983

EASTING, IN METERS

630,000 630,500

NORTHING, IN METERS

3,508,500 3,508,000 3,507,500 3,507,000
Direct-Current Resistivity Profiling 5

Figure 6. Diagrams showing (A) reciprocal-Schlumberger 

and (B) dipole-dipole arrays used at the Pecos River Ecosystem 

Project study site near Mentone, Texas, 2006.

Data Collection

All resistivity data were collected using the IRIS Instruments Syscal Pro system (IRIS Instruments, 2008) (fig. 4A). 

For the land-based survey, the system was deployed along a 

240-m section of the bank to collect shallow subsurface resistivity data (fig. 5). The IRIS system incorporated 48 stainless 

steel electrodes (fig. 4B) spaced 2 m apart. The reciprocal Schlumberger array (fig. 6A) was applied to collect the 

land-based data. Because of dry conditions during surveys, 

saltwater was applied to each electrode to ensure good electrical contact with the ground. To obtain the 240-m profile, the 

system was advanced along the profile line three times. Each 

time the system was advanced, 48 m of duplicate points were 

collected to ensure accuracy of the data and minimize data 

gaps in the deeper parts of the profile. Precise coordinates 

of each electrode were collected using a Trimble 5800 dual 

frequency kinematic Global Positioning System (GPS) unit 

(Trimble, 2008).

For the CRP surveys, a punt boat (fig. 7A) was used to 

tow a waterborne electrode streamer of 13 stainless steel electrodes with 5-m electrode spacing along the river reach adjacent to both sites (fig. 5). The streamer was built with a Kevlar 

support cord to attach the streamer to the boat. Pipe insulation 

(foam floats) were attached to the electrode streamer (between 

electrodes) to keep it at the surface of the river while keeping 

each electrode in the water (fig. 7B). The electrode streamer 

has 20 m of lead between the first electrode and the leading 

end of the streamer. Two oars and a trolling motor were used 

to maneuver the boat up and down the river.

A Garmin GPSMAP 188 Sounder (Garmin, 2008) was 

used to obtain geographic coordinates and water depth data 

near the back of the boat. These values were used to georeference the electrode locations and to define the approximate 

water depth below the electrodes. Sysmar version 04.08.01 

(IRIS Instruments, 2008) software was used on a laptop computer to acquire the CRP data.

CRP data were collected three times along the 2.279-km 

reach of the river, beginning about 850 m upstream from site 

B and continuing downstream past site A (fig. 5). The first 

dataset was collected using the reciprocal Schlumberger array 

(fig. 6A) while moving downstream. Throughout data collection, water-depth data from the depth sounder were compromised by the presence of moss and algae in the river. Because 

of this, the first dataset was not used for interpretation. For 

the two subsequent datasets, a polyvinyl chloride pipe marked 

at increments of 0.25 m was used to manually measure depth 

approximately every 20 m to verify the accuracy of the water 

depth data. The second dataset was collected using the dipoledipole array (fig. 6B) in the downstream direction. After data 

were collected in the final subsection, the boat was turned 

around and the software and system reconfigured to collect 

data using the reciprocal Schlumberger array. The boat was 

then navigated upstream until the electrode streamer snagged 

debris along the side of the river. Multiple upstream runs were 

necessary to collect the data; the flow of the river tended to 

have a greater influence on pushing the electrode streamer 

into the riverbank when towed upstream than when towed 

downstream. The final dataset was collected in the downstream direction using the reciprocal Schlumberger array. 

Inverse Modeling of Direct-Current Resistivity 

Data

Apparent resistivity, as calculated from the field measurements, is the electrical resistivity of an equivalent electrically homogeneous and isotropic subsurface and is used to 

represent the average resistivity of the heterogeneous subsurface (Loke, 2004). To estimate the true subsurface resistivity, 

an inverse modeling program develops a model consisting of 

rectangular blocks of individual resistivities. The inversion 

program then determines the calculated system response of 

that model—the calculated apparent resistivities—on the basis 

of the field data. The root mean square (RMS) difference 

between the measured and calculated apparent resistivities is 

used to determine the accuracy of the model. The inversion 

program attempts to reduce the RMS difference by altering 

the simulated resistivity values and recalculating the apparent 

resistivities in a series of iterations until the RMS difference 

between the calculated and measured apparent resistivity no 

longer improves appreciably between iterations (more than 

1 percent between iterations). The final model represents 

a nonunique estimate of the true distribution of subsurface 

resistivity. The inverse modeling process is described in detail 

by Loke (2004) and Yang (2006). 

A

B

Transmitting electrodes Receiving electrodes

Transmitting electrodes

Receiving electrodes

RECIPROCAL SCHLUMBERGER ARRAY

DIPOLE-DIPOLE ARRAY
6 Direct-Current Resistivity Profiling at the Pecos River Ecosystem Project Study Site near Mentone, Texas, 2006

Apparent resistivity data were inverted using EarthImager 2D version 2.1.6 Resistivity and Induced Polarization 

Inversion Software (Advanced Geosciences Inc., 2008). 

To obtain the best-fit model, EarthImager 2D was used to 

derive smooth model (Occam’s) inversions (Constable and 

others, 1987; deGroot-Hedlin and Constable, 1990). Data 

were inverted using a maximum eight-iteration inversion, 

but each profile reached an acceptable RMS difference 

between iterations before the eighth iteration. Unprocessed 

apparent-resistivity data were initially filtered to remove 

poor-quality data points using the outlier analyses feature of 

the EarthImager 2D software. Data also were filtered during 

the inversion process on the basis of the resistivity inversion 

parameters. These include but are not limited to minimum 

resistivity (1 ohm-meter) and maximum resistivity (1,000 

ohm-meters).

EarthImager 2D provides several graphical displays 

(scatter plot, convergence curve, and data-misfit histogram) 

to visualize the inversion results; these tools were used to 

evaluate the accuracy of the smooth-modeling results as 

well as the quality of the apparent-resistivity data. The 

data-misfit histogram allows the user to identify data points 

that are markedly different from the majority of the data. 

If there were data points that exceeded about 15 to 20 percent 

relative data misfit after the multiple iteration inversion, 

the points were removed and the section was re-inverted. 

The final inversion results were combined in Oasis montaj 

(version 6.4 [9m] HF1) (Geosoft Inc., 2008) to create 

Figure 7. Waterborne direct-current resistivity equipment used at the Pecos River Ecosystem Project study site near Mentone, Texas, 

2006: (A) punt boat with equipment and trolling motor, (B) waterborne electrode streamer with foam insulation for flotation, and (C) Iris 

Instruments Syscal Pro resistivity meter. 

Foam

floats

Submerged

electrode

A

B

C
Direct-Current Resistivity Profiling 7

Figure 8. Smooth inverse model results of direct-current resistivity, Pecos River Ecosystem Project study site near Mentone, 

Texas, 2006: (A) resistivity section from reciprocal-Schlumberger land-based profile at the control site, (B) resistivity section 

from reciprocal-Schlumberger waterborne continuous profile from subreach of the river adjacent to the land-based profile, and 

(C) resistivity section from dipole-dipole waterborne continuous profile from subreach of the river adjacent to the land-based 

profile.

Outline of grounded electrode profile

Outline of grounded electrode profile

Grounded

 electrode

 profile

EXPLANATION

Riverbed Water level of river

less than

2 5 10 20

greater than

30

RESISTIVITY, IN OHM-METERS 0 25 50 METERS

800 825 850 875 900 925 950 975 1,000 1,025 1,050 1,075 1,100

DISTANCE, IN METERS

800 825 850 875 900 925 950 975 1,000 1,025 1,050 1,075 1,100

DISTANCE, IN METERS

800 825 850 875 900 925 950 975 1,000 1,025 1,050 1,075 1,100

DISTANCE, IN METERS

4

2

0

-2

-4

-6

-8

-10

4

2

0

-2

-4

-6

-8

-10

4

2

0

-2

-4

-6

-8

-10

DEPTH, IN METERS RELATIVE TO WATER SURFACE

A

B

C
8 Direct-Current Resistivity Profiling at the Pecos River Ecosystem Project Study Site near Mentone, Texas, 2006

Figure 9. Smooth inverse model results of waterborne continuous direct-current resistivity profiles along 2.279-kilometer reach of the river adjacent to treated and control sites, Pecos River Ecosystem Project study site near Mentone, Texas, 2006: (A) resistivity section from reciprocal-Schlumberger profile, and (B) resistivity section from dipole-dipole profile.

Approximate location

of treated section

Outline of grounded

electrode profile

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500 1,600 1,700 1,800 1,900 2,000 2,100 2,200 2,300

Area of profile

in figure 6

Location of

Site B

Location of

Site A

EXPLANATIO

N

Extent of riverbed 2 5 10 20 30

Approximate location

of treated section

Outline of grounded

electrode profile

0 100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500 1,600 1,700 1,800 1,900 2,000 2,100 2,200 2,300

0

-2-4-6-8

-10-12-1

42

4

Area of profile

in figure 6 Area of profile

in figure 6

Location of

Site B

Location of

Site A

Water level of river

A

B

Area of profile

in figure 6

DISTANCE, IN METERSDISTANCE, IN METERS

ACE TER SURF WA TIVE TO DEPTH, IN METERS RELA

less than greater than

RESISTIVITY, IN OHM-METERS 0 100 200 METERS

0

-2-4-6-8

-10-12-1

42

4
Summary 9

continuous two-dimensional resistivity sections of the reciprocal Schlumberger and dipole-dipole arrays. 

Distributions of Resistivity

The smooth-model inversion results of each array collected produced generally similar resistivity structure; similar 

structure with RMS differences less than 1 percent engenders 

confidence in the results. The resistivity sections from the 

land-based DC survey and the CRP survey from the adjacent 

subreach of the river are shown in figure 8.

The land-based survey showed that the subsurface 

generally is of relatively low resistivity down to about 4 m 

below the water surface, with the exception of a subsection 

immediately below land surface about 25 m long between 

distances 875 and 900 m (fig. 8A). Most of the section from 

about 4 to 10 m below the water surface is of relatively high 

resistivity. The reciprocal Schlumberger (fig. 8B) and dipoledipole (fig. 8C) configurations of the waterborne CRP surveys 

convey essentially the same electrical representation of the 

lithology (relatively low resistivity immediately below the riverbed but higher with depth) to 10 m below the water surface; 

but the CRP surveys show considerably lower resistivity than 

the land-based survey in the subsection from about 4 to 10 m 

below the water surface. 

Figure 9 shows the results of the CRP surveys along the 

2.279-km reach of the river adjacent to both the treated and 

control sites. The same relatively low resistivity zone from the 

riverbed to about 4 m below the water surface evident at the 

control site (fig. 8) appears over the entire length of the profile. Along the upstream approximately one-half of the profile, a slightly higher resistivity zone is observed from about 

4 to 14 m below the water surface than along the downstream 

one-half. The downstream one-half of the profile shows the 

same zone but at a slightly lower resistivity.

The variations in resistivity could not be matched to 

variations in lithology because sufficient rock samples were 

not available; wells installed in 1986 and 2003 were not 

drilled to depths below the riverbed. Generally, relatively high 

resistivity zones correspond to more coarse-grained material 

or relatively fresh water, or both, and relatively low resistivity zones correspond to finer-grained silts and clays or more 

saline water, or both. To provide a definitive characterization of the shallow subsurface, additional data in the form of 

lithologic cores from new wells that would extend below the 

riverbed and water-quality samples from new wells screened 

at depths equivalent to those of the geophysical methods used 

in this study are needed.

Summary

At the Pecos River Ecosystem Project (PREP) study site 

near Mentone in West Texas are two hydrologically similar 

and approximately equidistance riparian zones along a 1.5-km 

reach of the Pecos River. The downstream (treated) riparian zone has been eradicated of all saltcedar. The upstream 

(untreated) riparian zone is the control site. As a part of the 

PREP, Texas A&M University AgriLife is evaluating flows 

between the river and the surficial aquifer at the PREP 

study site, exploring the effects of saltcedar control on the 

fate of salvaged water and attempting to quantify water 

released to downstream flow and aquifer recharge. Characterization of the aquifer beneath the treated and control zones 

(sites) is a necessary part of this effort. Accordingly, the U.S. 

Geological Survey, in cooperation with Texas A&M University AgriLife, did a surface geophysical investigation at the 

PREP study site intended to determine shallow (to about 14 

m below the water [river] surface) subsurface composition 

(lithology) in and near the treated and control sites during 

June–August 2006.

Land-based direct-current (DC) resistivity profiling was 

applied in a 240-m section of the riverbank at the control site, 

and waterborne DC continuous resistivity profiling (CRP) 

was applied along a 2.279-km reach of the river adjacent to 

both sites to collect shallow subsurface resistivity data. All 

resistivity data were collected using the IRIS Instruments 

Syscal Pro system. For the land-based survey, the system 

incorporated 48 stainless steel electrodes spaced 2 m apart. 

The reciprocal Schlumberger array was applied to collect the 

land-based data. For the three CRP surveys, a punt boat was 

used to tow a waterborne electrode streamer of 13 stainless 

steel electrodes with 5-m electrode spacing along the 2.279-

km reach. The reciprocal Schlumberger arrays (two surveys) 

and dipole-dipole array (one survey) were used to collect the 

CRP data.

Inverse modeling was used to obtain a nonunique 

estimate of the true subsurface resistivity from apparent resistivity calculated from the field measurements. The land-based 

survey showed that the subsurface at the control site generally is of relatively low resistivity down to about 4 m below 

the water surface. Most of the section from about 4 to 10 m 

below the water surface is of relatively high resistivity. The 

waterborne CRP surveys convey essentially the same electrical representation of the lithology at the control site (relatively 

low resistivity immediately below the riverbed but higher 

with depth) to 10 m below the water surface; but the CRP surveys show considerably lower resistivity than the land-based 

survey in the subsection from about 4 to 10 m below the water 

surface.

 The CRP surveys along the 2.279-km reach of the river 

adjacent to both the treated and control sites show the same 

relatively low resistivity zone from the riverbed to about 4 

m below the water surface evident at the control site. Along 

the upstream approximately one-half of the profile, a slightly 

higher resistivity zone is observed from about 4 to 14 m below 

the water surface than along the downstream one-half. The 

downstream one-half of the profile shows the same zone but at 

a slightly lower resistivity. The variations in resistivity could 

not be matched to variations in lithology because sufficient 

rock samples were not available.
10 Direct-Current Resistivity Profiling at the Pecos River Ecosystem Project Study Site near Mentone, Texas, 2006

References

Advanced Geosciences, Inc., 2008, AGI EarthImager 2D 

inversion and modeling software: accessed November 17, 

2008, at http://www.agiusa.com/agi2dimg.shtml

Ball, L.B., Kress, W.H., Steele, G.V., Cannia, J.C., and 

Andersen, M.J., 2006, Determination of canal leakage 

potential using continuous resistivity profiling techniques, 

Interstate and Tri-State Canals, western Nebraska and 

eastern Wyoming, 2004: U.S. Geological Survey Scientific 

Investigations Report 2006–5032, 27 p.

Constable, S.C., Parker, R.L., and Constable, C.G., 1987, 

Occam’s inversion—A practical algorithm for generating 

smooth models from electromagnetic sounding data: Geophysics, v. 52, no. 3, p. 289–300.

deGroot-Hedlin, C., and Constable, S.C., 1990, Occam’s 

inversion to generate smooth, two-dimensional models 

from magnetotelluric data: Geophysics, v. 55, no. 12, 

p. 1,613–1,624.

DeLoach, J., 1997, Effects of biological control of saltcedar 

(Tamarix ramosissma) on endangered species: Temple, Tex., 

U.S. Department of Agriculture, Biological Assessment 

Report.

Garmin, 2008, GPSMAP 188/188C Sounder: accessed 

December 31, 2008, at https://buy.garmin.com/shop/

shop.do?pID=160

Geosoft, Inc., 2008, Oasis Montaj version 6.4 (9m) HF1: 

accessed January 2, 2009, at http://geosoft.com/pinfo/

oasismontaj/index.asp

Grant, F.S., and West, G.F., 1965, Interpretation theory in 

applied geophysics—Part 3, Electrical conduction and electromagnetic induction methods: New York, McGraw-Hill, 

p. 384–572.

Hart, C.R., White, L.D., McDonald, Alyson, and Sheng, 

Zhuping, 2005, Saltcedar control and water salvage on the 

Pecos River, Texas, 1999–2003: Journal of Environmental 

Management, v. 75, no. 4, p. 399–409. 

IRIS Instruments, 2008, Principles of geophysical methods for 

groundwater investigations: accessed December 31, 2008, at 

http://www.iris-instruments.com

Keller, G.V., and Frischknecht, F.C., 1966, Electrical methods in geophysical prospecting: Oxford, United Kingdom, 

Pergamon Press, 519 p.

Kolb, T.E., 2001, Water use of tamarisk and native riparian trees, in Tamarisk Symposium, Grand Junction, 

Colorado, September 26–27, 2001, Proceedings: accessed 

January 2, 2009, at http://www.tamariskcoalition.org/

tamariskcoalition/pdf/tamarisk%20ecology%20and%20

impacts.pdf

Loke, M.H., 2004, Tutorial—2-D and 3-D electrical imaging 

surveys: Geotomo Software, accessed November 1, 2006, at 

http://www.geoelectrical.com

Masterson, J.P., Sorenson, J.R., Stone, J.R., Moran, S.B., and 

Hougham, Andrea, 2006, Hydrogeology and simulated 

ground-water flow in the Salt Pond region of southern 

Rhode Island: U.S. Geological Survey Scientific Investigations Report 2006–5271, 56 p.

Sheng, Zhuping, McDonald, A.K., Hart, C., Hatler, W., 

and Villalobos, J., 2007, Quantity and fate of water salvage as a result of saltcedar control on the Pecos River 

in Texas: Texas A&M University, Texas Agricultural 

Experiment Station/Texas Cooperative Extension, TR–304, 

42 p.

Stell, S.M, Hopkins, E.H., Buell, G.R., and Hippe, D.J., 1995, 

Use and occurrence of pesticides in the ApalachicolaChattahoochee-Flint River Basin, Georgia, Alabama, and 

Florida, 1960–91: U. S. Geological Survey Open-File 

Report 95–739, 65 p.

Tamarisk Coalition, 2009, Tamarisk ecology and impacts: 

accessed February 9, 2009, at http://www.tamariskcoalition.

org/tamariskcoalition/pdf/tamarisk%20ecology%20and%

20impacts.pdf

Texas A&M AgriLife/Texas Water Resources Institute, 2009, 

Pecos River Basin assessment program—Subtask 3.3 

Quantity and fate of water salvaged as a result of saltcedar 

control: accessed January 30, 2009, at http://pecosbasin.

tamu.edu/subtasks.php?view=3.3

Texas State Soil and Water Conservation Board and Texas 

Commission on Environmental Quality, 2006, 2006 Annual 

report—Managing nonpoint source pollution in Texas: 

accessed January 3, 2009, at http://www.tceq.state.tx.us/

assets/public/comm_exec/pubs/sfr/066_06.pdf

Texas Water Development Board, 2006, Major aquifers of 

Texas: accessed November 19, 2008, at http://www.twdb.

state.tx.us/mapping/maps/pdf/aqu_maj_8x11.pdf

Trimble, 2008, Trimble 5800 GPS receiver: accessed December 31, 2008, at http://www.trimble.com/5800.shtml

Weeks, E.P., Weaver, H.L., Campbell, G.S., and Tanner, B.D., 

1987, Water use by saltcedar and by replacement vegetation 

in the Pecos River floodplain between Acme and Artesia, 

New Mexico: U. S. Geological Survey Professional Paper 

491–G, 33 p .

White, D.E., 1971, Water resources of Ward County, Texas: 

Texas Water Development Board Report 125, 219 p.

Yang, Xianji, 2006, Instruction manual for EarthImager 

2D version 2.1.0—Resistivity and IP inversion software: 

accessed November 1, 2006, at http://www.agiusa.com/files/

pub/softwareproducts.shtml
References 11

Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, Application of surface geophysics to ground-water investigations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 2, chap. D1, 116 p.

Publishing support provided by

Lafayette Publishing Service Center

Information regarding water resources in Texas is available at 

http://tx.usgs.gov/
Teeple, McDonald, Payne, and Kress—Direct-Current Resistivity Profiling at the Pecos River Ecosystem Project Study Site near Mentone, Texas, 2006—SIR 2009–5090

Printed on recycled paper