Geophysical Analysis of the Salmon Peak 

Formation Near Amistad Reservoir Dam, 

Val Verde County, Texas, and Coahuila, 

Mexico, March 2006, to Aid in 

Piezometer Placement

By Gregory P. Stanton, Wade H. Kress, Andrew P. Teeple, Michael L. Greenslate, 

and Allan K. Clark

In cooperation with the U.S. Section, International Boundary and 

Water Commission

Scientific Investigations Report 2007–5143

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 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:

Stanton, G.P., Kress, W.H., Teeple, A.P., Greenslate, M.L., and Clark, A.K., 2007, Geophysical analysis of the Salmon 

Peak Formation near Amistad Reservoir dam, Val Verde County, Texas, and Coahuila, Mexico, March 2006, to aid in 

piezometer placement: U.S. Geological Survey Scientific Investigations Report 2007–5143, 70 p.
iii

Contents

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

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

Purpose and Scope .............................................................................................................................5

Description of Amistad Reservoir and Dam ....................................................................................5

Area Hydrogeology ..............................................................................................................................5

Background of Amistad Reservoir ....................................................................................................7

Approach ...............................................................................................................................................7

Western Embankment Investigation ........................................................................................8

Eastern Embankment Investigation .........................................................................................9

Methods ..........................................................................................................................................................9

Geodatabase .........................................................................................................................................9

Borehole Geophysics ........................................................................................................................10

Surface Geophysics ..........................................................................................................................10

Direct-Current Resistivity ........................................................................................................10

Inverse Modeling of Direct-Current Resistivity Data ..........................................................13

Borehole Drilling and Piezometer Installation ..............................................................................14

Geophysical Analysis .................................................................................................................................15

Analysis of Geologic Data and Geophysical Log Descriptions ..................................................15

Western Embankment Investigation ...............................................................................................15

Analysis of Hydrogeologic Data .............................................................................................15

Borehole Geophysical Log Analysis ......................................................................................16

Analysis of Resistivity Profiles 1 and 2 ..................................................................................17

Integration of Results ...............................................................................................................17

Eastern Embankment Investigation ................................................................................................18

Analysis of Lithologic Data ......................................................................................................18

Analysis of DC Resistivity Profile 3 ........................................................................................18

Borehole Geophysical Log Analysis ......................................................................................19

Integration of Results ...............................................................................................................20

Eastern Embankment Piezometer Placement ........................................................................................23

Summary .......................................................................................................................................................23

References Cited .........................................................................................................................................25

Appendix 1—Detailed Descriptions of Borehole Geophysical Methods ..........................................43

Electromagnetic Induction Logs .....................................................................................................45

Natural Gamma Logs .........................................................................................................................45

Electric Logs .......................................................................................................................................45

Caliper Logs ........................................................................................................................................46

Fluid Resistivity Logs .........................................................................................................................46

Acoustic Velocity Logs ......................................................................................................................46

Full-Waveform Logs ...........................................................................................................................47

Acoustic Televiewer ..........................................................................................................................47

Electromagnetic Flowmeter .............................................................................................................47
iv

Appendix 2—Detailed Descriptions of Geophysical Logs ...................................................................49

Western Embankment Piezometers ................................................................................................51

Piezometer LA–12L ...................................................................................................................51

Piezometer LA–11 .....................................................................................................................51

Piezometer LA–10 .....................................................................................................................52

Piezometer LA–16L ...................................................................................................................52

Piezometer LA–15 .....................................................................................................................53

Piezometer LA–8L .....................................................................................................................53

Piezometer LA–7 .......................................................................................................................53

Eastern Embankment Piezometers .................................................................................................54

Piezometer 105+90B .................................................................................................................54

Piezometer 105+90A .................................................................................................................54

Piezometer 106+00B .................................................................................................................55

Piezometer 106+00A .................................................................................................................55

Piezometer 106+00C .................................................................................................................56

Piezometer 106+00D .................................................................................................................56

Brite Well ....................................................................................................................................56

Appendix 3—Borehole Geophysical Logs (online only at http://pubs.usgs.gov/sir/2007/5143/

5143-app.html).................................................................................................................................59

Appendix 4—Online Positioning User System (OPUS) Solutions of Global Positioning 

System Data ...................................................................................................................................63

4-1. OPUS Base 1 ...............................................................................................................................65

4-2. OPUS Base 2 ...............................................................................................................................66

Appendix 5—Parameters Used for Inversion Process of Direct-Current Resistivity Data ............67

Figures

1. Location of Amistad Reservoir on the Rio Grande in Val Verde County, Texas; 

Coahuila, Mexico; and western and eastern embankment study sites ..............................2

2. Location of (A) western embankment study site at Amistad Reservoir, Coahuila, 

Mexico and (B) eastern embankment study site at Amistad Reservoir near Del 

Rio, Texas ......................................................................................................................................3

3. Amistad Reservoir dam construction, 1965: (A) excavation of dam foundation area 

along eastern embankment, and (B) excavation of ditch for curtain foundation 

along western embankment ......................................................................................................7

4. Elevation of Amistad Reservoir and discharge from Carmina Springs near Ciudad 

Acuna, Coahuila, 1970–94 ...........................................................................................................8

5. Near Amistad Reservoir dam, March 2006, along direct-current resistivity profile 

3: (A) IRIS Syscal Pro resistivity meter connected to multi-conductor cables, 

and (B) stainless steel electrode connected to electrode takeout built into multiconductor cable .........................................................................................................................11

6. Near Amistad Reservoir dam, March 2006, along direct-current resistivity profile 3, 

application of a saltwater solution (A) using a backpack sprayer, and (B) to an 

electrode to decrease the effect of contact resistances ...................................................11

7. Near Amistad Reservoir dam, March 2006, along direct-current resistivity profile 

2: (A) real-time kinematic (RTK) Global Positioning System base station, and 

(B) RTK rover used to survey electrode locations ...............................................................12
v

8. Configuration of current (C1 and C2) and potential (P1 and P2) electrodes used 

to collect direct-current resistivity data for (A) dipole-dipole array, (B) Schlumberger 

array, and (C) reciprocal Schlumberger array ......................................................................13

9. Section showing available lithologic data near direct-current resistivity 

(A) profile 1 and (B) profile 2 ....................................................................................................27

10. Robust inverse model results of direct-current resistivity profile 1 (A) reciprocal 

Schlumberger array and (B) dipole-dipole array; and profile 2 (C) reciprocal 

Schlumberger array (D) and dipole-dipole array .................................................................29

11. Sections showing lithologic tops from core holes and piezometers superimposed 

on direct-current resistivity profile 1 (A) dipole-dipole array, (B) reciprocal 

Schlumberger array; and profile 2 (C) dipole-dipole array, and (D) reciprocal 

Schlumberger array ..................................................................................................................31

12. Three-dimensional image of merged dipole-dipole direct-current resistivity profile 

at western embankment study site, looking (A) southeast and (B) southwest ...............35

13. Three-dimensional image of merged reciprocal Schlumberger direct-current 

resistivity profile at western embankment study site, looking (A) southeast and 

(B) southwest .............................................................................................................................37

14. Section showing available lithologic data near direct-current resistivity profile 3 .......39

15. Robust inverse model results of direct-current resistivity profile 3 (A) reciprocal 

Schlumberger array and (B) dipole-dipole array .................................................................40

16. Sections showing lithologic tops from core holes and piezometers superimposed 

on direct-current resistivity profile 3 (A) dipole-dipole array and (B) reciprocal 

Schlumberger array ..................................................................................................................41

Tables

1. Summary of lithologic and hydrologic properties of near-surface geologic units 

in the area of Amistad Reservoir dam, Val Verde County, Texas, and Coahuila, 

Mexico ...........................................................................................................................................6

2. Elevation of Amistad Reservoir and discharge from Carmina Springs near Ciudad 

Acuna, Coahuila, Mexico, 1970–94 ...........................................................................................7

3. Locations and construction information for observation wells near Amistad 

Reservoir dam, Val Verde County, Texas, and Coahuila, Mexico ......................................22
vi

Conversion Factors and Datums

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

hectare (ha) 2.471 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 264.2 gallon (gal) 

Flow rate

liter per second (L/s) 15.85 gallon per minute (gal/min) 

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Datums

Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 

88). 

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

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

Since 1992, numerous sinkholes have developed northwest of the Amistad Reservoir dam on the Rio Grande. 

Increases in the discharge of springs south of the dam, on the 

western side of the Rio Grande, in Coahuila, Mexico, have 

been documented. In 1995 the Mexico Section of the International Boundary and Water Commission (IBWC) completed a 

study of the western embankment (Coahuila, Mexico) of the 

dam that included surface geophysics, borehole geophysics, 

and installation of piezometers to learn more about subsurface 

conditions. As part of a 5-year safety inspection in 2005, technical advisors recommended that one line of similarly constructed piezometers be installed on the eastern embankment 

(Val Verde County, Texas) of the dam for comparison of water 

levels (potentiometric head) on both the western and eastern 

embankments of Amistad Reservoir dam. To provide technical assistance for the horizontal and vertical placement of 

piezometers on the eastern embankment of Amistad Reservoir 

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

Section of the IBWC, conducted a study along both the western and eastern embankments of Amistad Reservoir dam. The 

study involved an integrated approach using surface and borehole geophysical methods. In the western embankment investigation, geological and geophysical characteristics that indicate 

relatively large water-yielding properties of the Salmon Peak 

Formation were identified. The direct-current (DC) resistivity 

method was selected as the surface geophysical reconnaissance technique to correlate relatively large water-yielding 

properties of the Salmon Peak Formation, identified from 

analysis of borehole geophysical logs, with variations in subsurface resistivity. The dipole-dipole array and the reciprocal 

Schlumberger array were selected as the most applicable DC 

resistivity arrays. Two resistivity units were identified in both 

the dipole-dipole array data and the reciprocal Schlumberger 

array data along DC resistivity profiles on both embankments. 

Resistivity unit 1 generally is of relatively low resistivity, 

ranging from 45 to 150 ohm-meters compared with resistivity 

unit 2, which ranges from 120 to 345 ohm-meters (depending 

on the DC array type). The presence of mapped sinkholes in 

the reservoir north of the western embankment study area and 

the zone of increased water content (as indicated by zones of 

low neutron log count rates in nearby piezometers) leads to 

the conclusion that resistivity unit 1 is a preferential flow path 

where surface water from Amistad Reservoir is forced into the 

ground-water system (because of increased head from the reservoir). In the eastern embankment investigation, trends in the 

spatial distribution of sinkholes and the occurrence of weathered zones were identified from geologic descriptions of cores. 

The correlation of surface geophysical DC resistivity, historical lithologic data, and general trend of documented sinkholes 

along the eastern end of the eastern embankment profile were 

used to justify further exploration (drilling of piezometers) in 

the eastern expression of resistivity unit 1. The spatial location 

of the piezometers and the screened intervals were selected to 

best match the locations of the screened intervals of the western embankment piezometers. Six piezometers were installed 

on the eastern embankment and logged using borehole geophysical techniques. Surface DC resistivity sections superimposed on the resistivity logs for two piezometers indicate three 

discernible resistivity units that correlate with resistivity units 

2, 1, and 2, respectively, identified in the western embankment 

study area. Resistivity units 1 and 2 in the DC resistivity profiles generally correspond with low and high resistivity zones, 

respectively, on the normal and lateral resistivity logs collected 

in the nearby piezometers at the time of installation.

Introduction

Since 1992, numerous sinkholes have developed northwest of Amistad Reservoir dam, which impounds Amistad 

Reservoir (fig. 1) on the Rio Grande in Val Verde County, 

Tex., and Coahuila, Mexico. An increase in discharge of 

Geophysical Analysis of the Salmon Peak 

Formation Near Amistad Reservoir Dam, 

Val Verde County, Texas, and Coahuila, 

Mexico, March 2006, to Aid in 

Piezometer Placement

By Gregory P. Stanton, Wade H. Kress, Andrew P. Teeple, Michael L. Greenslate, and Allan K. Clark
Figure 1. Location of Amistad Reservoir on the Rio Grande in Val Verde County, Texas; Coahuila, Mexico; and western and eastern embankment study sites. 

Amistad Reservoir

Rio Grande

Western embankment site (fig. 2A)

Eastern embankment site (fig. 2B)

Coahuila

Texas

VAL VERDE COUNTY 

Dam

296,000 297,000 298,000 299,000 300,000 301,000 302,000 303,000 304,000 305,000

EASTING, IN METERS

3,257,0003,261,0003,260,0003,259,0003,258,000

S NORTHING, IN METER

Brite wellOnline PositioningUser Service base stationTemporary real-timekinematic benchmarkSinkholeSpring

Direct-currentresistivity profileGeneral trend ofsinkholesSelected core holePiezometer

EXPLANATION

-500 0 500 1,000 METERS Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo Image

Universal Transverse Mercator projection, Zone 14North American Datum of 1983

UNITED STATES

MEXICO

COAHUILA

TEXAS

UNITED STATES

MEXICO

0 100 200 300 KILOMETERS

VAL VERDE COUNTY

RIO GRANDE

RIO BRAVO DEL NORTE

 GULF

OF

MEXICO

AMISTAD RESERVOIR

Area enlarged

Ciudad Acuña 

Del Rio

102o 100o

36o

34o

98o 96o 94o

32o

104o 106o

30o

28o

26o

   Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
springs south of the dam on the western side of the Rio Grande 

also has been documented by personnel at the International 

Boundary and Water Commission (IBWC) (Ken Breiton, 

U.S. Section, International Boundary and Water Commission, 

written commun., 2006). In 1995, the Mexico Section of the 

IBWC (MxIBWC) completed a study of the western embankment (Coahuila, Mexico) that included surface geophysics, 

borehole geophysics, and installation of piezometers to learn 

more about subsurface conditions. Piezometers were installed 

in three lines perpendicular to the centerline of the western 

embankment, with each line containing four piezometers 

(fig. 2A). After the 1995 study, the dam foundation along the 

centerline of a part of the western embankment was regrouted. 

As part of a 5-year safety inspection in 2005, technical advisors recommended that one line of piezometers be installed on 

the eastern embankment (Val Verde County, Tex.) of Amistad 

dam at identical land-surface altitudes and with depths similar 

to those on the western embankment. This recommendation is 

listed as task 13 in the Joint Report of the Technical Advisors 

of the International Boundary and Water Commission Regarding the Geotechnical, Electrical, Mechanical & Structural 

Safety of Amistad dam (Ken Breiton, U.S. Section, International Boundary and Water Commission, written commun., 

2006). The purpose of the piezometers is to compare water 

Figure 2A. Location of western embankment study site at Amistad Reservoir, Coahuila, Mexico.

Amistad Reservoir

Spring 1

Spring 2

Spring 3

GG15

GG16

Profile 1

Profile 2

1

2

LA-6

LA-7

LA-8L

LA-10

LA-11

LA-12L

LA-14

LA-15

LA-16L

A70E

A70W

A72E

A72W

A74E

A74W

A76E

A77E

A76W

A77W

A78E

A78W

A79E A79W

A80E

A80W

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo Image

Universal Transverse Mercator projection, Zone 14

North American Datum of 1983

298,000 298,200 298,400 298,600 298,800 299,000 299,200

EASTING, IN METERS

3,259,000

3,259,200

3,259,400

3,259,600

3,259,800

NORTHING, IN METERS

1

2

2

Direct-current resistivity profile

Selected core hole and identifier

Peizometer and identifier—Underlined

identifier indicates logged well

Sinkhole and identifier

Spring and identifier (orifices of Carmina Springs)

Online Positioning User Service base station and identifier

EXPLANATION

Spring 1

GG15

A74W

LA-7

2

0 100 200 300 METERS

Introduction 
levels (potentiometric head) on both the western and eastern 

embankments of Amistad Reservoir dam.

To provide technical assistance for the horizontal and vertical placement of piezometers on the eastern embankment of 

Amistad Reservoir dam, the U.S. Geological Survey (USGS) 

in cooperation with the U.S. Section, IBWC (USIBWC), 

conducted a study along both the western and eastern embankments of Amistad Reservoir dam. The study involved an 

integrated approach consisting of historical data review, 

geodatabase development, and surface and borehole geophysical investigations. The results of this approach were analyzed 

using the geodatabase to optimally locate piezometers along 

one line perpendicular to the centerline of the eastern embankment (fig. 2B) and install the piezometers using rotary drilling 

methods. Optimally located piezometers would allow objective comparison of water levels on the downgradient side (toe) 

Figure 2B. Location of eastern embankment study site at Amistad Reservoir near Del Rio, Texas. 

Amistad Reservoir

106+00A

106+00B

106+00C

106+00D

A120V

A121V

A121D

A123D

A125D

A126D

A12V A12E

A136D

A13V A13E

A14VA14E

A15VA15E

A16VA16E

A17E

A19E

A21E

A23E

A25E

A27E

A26V

A29E

A28V

A31E

A30V

A12W

A13W

A14W

A15W

A16W

A17W

A19W

A21W

A23W

A25W

A27W

A31W

A29W

A32V

Brite well

BB-41

BB-42

BB-43

BB-27

BB-45

BB-14

BB-13

BB-16

BB-17

BB-12

BB-10

BB-44

BB-19

BB-18

BB-20

BB-21

BB-22

BB-23

BB-24

BB-25

BB-26

BB46

BB-47

BB-48

BB-105

BB-11

BB-29

BB-30

BB-31

BB-32

BB-33

BB-63

Profile 3

105+90A

105+90B

A120D

301,000 301,200 301,400 301,600 301,800 302,000 302,200 302,400 302,600

EASTING, IN METERS

3,259,000

3,259,200

3,259,400

3,259,600

3,259,800

3,260,000

3,260,200

3,260,400

3,260,600

NORTHING, IN METERS

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo Image

Universal Transverse Mercator projection, Zone 14

North American Datum of 1983

A17W

Direct-current resistivity profile

General trend of sinkholes

Selected core hole and identifier

Piezometer and identifier—Underlined

identifier indicates logged well

Brite well (YR–71–40–601)—Logged well

Temporary real-time kinematic benchmark

Sinkhole and identifier

Spring

EXPLANATION

105+90A

BB-29

0 100 200 300 400 500 METERS

Service road

 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
of both the western and eastern embankments to evaluate and 

compare leakage potential on both embankments. Hydrogeologic issues considered for this project included:

Complex karstic-limestone bedrock surface geology (as 

documented in core analysis done at the time of dam 

construction) that includes several mapped sinkholes and 

other surface expressions of dissolution.

Fluctuations in Amistad Reservoir pool elevation since the 

mid-1970s have caused variations in pressure head and 

moisture in the subsurface karst openings, many of which 

are clay-filled. There is concern that lower reservoir elevations could cause the clay to become unstable and possibly mobilize during future reservoir fluctuations (Jose 

Antonio Moza, Federal Electricity Commission, written 

commun., 1995).

Springs discharging downgradient from the dam on the 

western embankment are presumably caused by water 

flow beneath the dam. On the eastern embankment, the 

only indication of water flow beneath the dam is a flowing 

well several hundred meters downgradient from the dam.

Geologic, geophysical, and hydrologic data were collected along the western embankment to understand the physical setting and near-surface hydrogeology of existing piezometers and to aid the placement of piezometers on the eastern 

embankment for objective comparison of hydraulic activity. To 

accomplish these tasks, the following factors were considered:

The land-surface altitude at prospective sites such that 

piezometers on the eastern embankment would be at 

approximately the same altitude as the piezometers on the 

western embankment (as suggested in the Joint Report of 

the Technical Advisors of the International Boundary and 

Water Commission Regarding the Geotechnical, Electrical, Mechanical & Structural Safety of Amistad Dam 

(Ken Breiton, U.S. Section, International Boundary and 

Water Commission, written commun., 2005).

The local faults and disruptions in the continuity of the 

hydrogeologic units along the embankment and their 

effect on the ability to correlate correlation between study 

areas. 

The potential for dissolution and development of secondary porosity, as detected through the use of geophysical 

methods, on both embankments.

Purpose and Scope

This report presents the results of geophysical analysis 

from a reconnaissance-level study conducted on the eastern 

(Val Verde County, Tex.) and western (Coahuila, Mexico) 

embankments of Amistad Reservoir dam on the Rio Grande 

in March 2006 to characterize the near-surface hydrogeology. 

Piezometers on the eastern embankment were to be constructed at about the same land-surface altitudes and depths as 

1.

2.

3.

1.

2.

3.

those on the western embankment, and in similar lithology on 

the basis of geophysical data and core descriptions, for objective comparison of hydraulic heads at the toe of the dam. The 

purpose of this report is to document the technical approach 

that facilitated the optimum placement of the piezometers on 

the eastern embankment. The report documents an integrated 

borehole and surface geophysical approach. Geophysical 

data are used to identify and correlate subsurface features 

(lithology and water-yielding characteristics) between the 

western and eastern embankments so that eastern embankment 

piezometers could be placed to reflect subsurface conditions 

matching, to the extent possible, those of existing western 

embankment piezometers. Reporting water-level data monitored by the IBWC was not in the scope of the study. Because 

the project was a reconnaissance-level study, the report is not a 

comprehensive geologic or geophysical evaluation of the study 

area. 

Description of Amistad Reservoir and Dam

Amistad Reservoir is located about 21 kilometers northnorthwest of Del Rio, Tex. on the Rio Grande between Val 

Verde County, Tex., and Coahuila, Mexico (fig. 1). The reservoir has a surface area of 26,300 hectares and has a volume 

of 3,886,578,000 cubic meters at conservation elevation of 

340.46 meters above NAVD 88. The total contributing drainage area to the reservoir is 327,434 square kilometers in the 

United States and Mexico (Texas Water Development Board, 

1971). 

Amistad Reservoir dam was constructed for flood control 

and water conservation storage for the benefit of the United 

States and Mexico. The dam is operated and maintained 

jointly by the USIBWC and MxIBWC. Amistad Reservoir 

dam is the largest of the storage dams and reservoirs built on 

the international reach of the Rio Grande. The dam is 10 kilometers long, stands 77.4 meters above the riverbed, and consists of a concrete gravity spillway section in the river canyon 

flanked by earthen embankments. Dam construction began in 

December 1964, impoundment of water began May 31, 1968 

(Texas Water Development Board, 1971), and construction 

was completed November 21, 1969.

Area Hydrogeology

The near-surface karstic carbonate rocks of the area 

consist of five Lower Cretaceous, predominantly limestone 

formations (oldest to youngest): the West Nueces Formation, 

the McKnight Formation, the Salmon Peak Formation, the 

Del Rio Clay, and the Buda Limestone (table 1). This study 

focused on characterizing the Salmon Peak Formation. (All 

surface and borehole geophysical data were collected in the 

Salmon Peak Formation except for data at one well that also 

penetrates the upper unit of the McKnight Formation.)

Limestone typically has very low primary porosity and 

permeability with secondary porosity and permeability commonly occurring by solution enlargement of bedding planes, 

Introduction 
Table 1. Summary of lithologic and hydrologic properties of near-surface geologic units in the area of Amistad Reservoir dam, Val Verde 

County, Texas, and Coahuila, Mexico.

[Groups, formations, and members modified from Greenwood (1956), Lozo and Smith (1964), Reeves and Small (1973), Humphreys (1984); lithology modified 

from Dunham (1962)] System

Group, formation,

unit/member

Thickness

(meters) Lithology Fieldidentification

Cavern

development

Porosity/

 permeability

Lower Cretaceous

Washita Group

Buda Limestone 12–15 Buff, light-gray, dense mudstoneHard, porcellaneous 

and marly limestones

Minor Low porosity/low permeability

Del Rio Clay 12–18 Blue-green to yellow-brown 

clay

Blue-green to 

medium-brown 

shale; Ilymatogyra

None Low porosity/low permeability

West prong beds of 

Del Rio Clay

0–15 Yellowish-brown, argillaceous limestone with thin 

clay partings; Wacoensis 

waconella and Ilymatogyra 

areintina common

Yellowish-brown, argillaceous limestone; 

appears similar to 

Georgetown Formation on San Marcos 

Platform

Minor, associated 

with collapse from 

caverns developed 

in Salmon Peak 

Formation

High porosity/high permeability along fractures 

and areas of collapse, 

also some along bedding 

planes

Salmon Peak Formation

Upper unit 0–9 Wackestone to packstone, 

abundant echinoid fragments, calcispheres, 

foraminifera; glauconite, 

limonite and hematite

Light-gray mudstone, 

with abundant fossil 

fragments, stringers 

of shale; contains 

chert, nodules, and 

pyrite

Numerous, karst 

appear to develop 

in trends along 

fractures, also 

development along 

bedding planes

High porosity/high permeability associated with 

caverns and fractures

Lower unit 0–130 Mudstone to wackestone; 

pelagic foraminifers, 

calcispheres, and scattered 

shell fragments, occasional organic matter and 

burrows

Massive, gray mudstone, stringers 

of shale; contains 

nodular and bedded 

chert, and pyrite

Numerous, karst 

appear to develop 

in trends along 

fractures, also development along 

bedding planes

High porosity/high permeability associated with 

caverns and fractures

McKnight Formation

Upper unit 0–36 Anhydrites, dark-brown, 

fecal pellet packstone and 

organic-rich, laminated 

mudstone; fecal pellets, 

miliolids, shell fragments 

and globogerinids, oysters

Alternating light- to 

dark-grey, laminated 

and thin-bedded 

fecal pelleted mudstone to grainstone

Negligible High porosity and permeability where evaporite 

dissolution has occurred

Fredericksburg Group

Middle unit 0–16 Brown to black, laminated, 

organic rich mudstone; 

fecal-pellet packstone to 

grainstone, thin anhydrite 

and sulfur beds

Strong petroliferous 

odor, laminated 

mudstone; vegetative band on aerial 

photo

None Low porosity/low permeability

Lower unit 0–35 Anhydrite, alternating with 

laminated, black, shell 

fragment mudstone and 

wackestone; pelleted, shell 

fragmented, interclastic 

grainstone

Thin-bedded mudstone to grainstoneNegligible Low to high porosity/low 

permeability

West Nueces Formation

Undivided 0–91 Gray, thick-bedded, burrowed, shell-fragment 

wackestone, packstone, 

and grainstone; dolomitic

Hard, massive, cherty, 

gray wackestone, 

some dolomite, miliolids, gastropods, 

and Texigryphaea 

Minor, associated 

with fracture solutioning

Low porosity/low permeability

Basal nodular 

member

6–30 Shaly, nodular limestone; 

mudstone and miliolid

grainstone

Massive, nodular and 

mottled; abundant 

gastropods and 

Exogyra texana

Large lateral caves at 

surface

Porosity stratigraphically 

controlled/large conduit 

flow at surface; negligible permeability in 

subsurface

   Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
A B

fractures, and faults (Ford and Ewers, 1978). The intensity 

of these features is dependent on the intersection of faults 

or joints where dissolution can cause the local formation 

of caverns, sinkholes, and conduits for ground-water flow 

(Palmer, 1984). The rate at which dissolution features occur 

usually depends on the amount of water moving through the 

system, the pH of the water, the degree of calcium carbonate 

saturation (Fetter, 1994), and the availability of carbon dioxide 

in the water. Dissolution, as described above, has altered the 

permeability of the Salmon Peak Formation, which constitutes most of the bedrock beneath the Amistad Reservoir dam 

(Jose Antonio Moza, Federal Electricity Commission, written 

commun., 1995). 

Background of Amistad Reservoir 

During design of the Amistad Reservoir in the early 

1960s, dissolution features such as depressions, sinkholes, 

and caverns in the Rio Grande valley walls, river terraces and 

adjacent flood plains were documented (Jose Antonio Moza, 

Federal Electricity Commission, written commun., 1995). 

Preventative measures taken during construction (1964–65) 

to minimize dam seepage included the excavation of the dam 

foundation area (fig. 3A), filling of sinkholes in the reservoir 

impoundment area, excavation and concrete filling of caverns 

and elongated solution conduits in the ditch excavated for the 

curtain foundation (fig. 3B), and the construction of a grout 

curtain to a depth of about 50 meters in the bedrock foundation along the centerline of the dam (Jose Antonio Moza, 

Federal Electricity Commission, written commun., 1995).

As the reservoir began to fill in 1968, an increase in 

discharge of local springs was observed. Early discharge data 

were collected from orifices of Carmina Springs near the western embankment (fig. 2A), beginning in 1969 (Jose Antonio 

Moza, Federal Electricity Commission, written commun., 

1995). By 1970 Amistad Reservoir pool elevation had reached 

approximately 322 meters above sea level, and Carmina 

Springs discharge was 0.4 cubic meter per second. Measurements in later years (table 2) indicate that there is a direct correlation (fig. 4) between the pool elevation and the discharge 

at Carmina Springs. Thus, despite efforts to minimize seepage, 

data indicate a substantial hydraulic connection between the 

reservoir and the shallow subsurface immediately downstream 

from the dam, which facilitates seepage.

Approach

The approach was designed to facilitate the optimum 

placement of piezometers that were to be constructed on the 

eastern embankment at the same land-surface altitude and 

depth as those on the western embankment and in similar 

lithology on the basis of geophysical data and core descriptions. These piezometers were to be used for the objective 

comparison of hydraulic heads at the toe of the dam. Specifically, the approach involved the following steps: 

Figure 3. Amistad Reservoir dam construction, 1965: (A) excavation of dam foundation area along eastern embankment, and 

(B) excavation of ditch for curtain foundation along western embankment. 

Table 2. Elevation of Amistad Reservoir and discharge 

from Carmina Springs near Ciudad Acuna, Coahuila, Mexico, 

1970–94 (modified from Jose Antonio Moza, Federal Electricity 

Commission, written commun., 1995).

Year

Elevation, 

Amistad Reservoir 

(meters above sea level)

Discharge, 

Carmina Springs 

(cubic meters per second)

1970 322 0.4

1971 328 .9

1973 343 2.0

1974 346 2.1

1984 334 1.5

1986 329 .8

1990 332 1.6

1994 331 1.1

Introduction 
Review existing literature to create a geodatabase containing geologic, geotechnical, and geophysical data that 

could be used for analysis and interpretation.

Document geologic and hydrogeologic characteristics of 

the Salmon Peak Formation near the western embankment 

piezometers. 

Collect and analyze new and existing surface and borehole geophysical data along the western embankment. 

Develop preliminary borehole geophysical interpretations to identify the geologic and hydrogeologic 

characteristics contributing to the hydraulic connection between the reservoir and the shallow subsurface immediately downstream from the dam. 

Correlate surface geophysical data with preliminary 

interpretations of borehole geophysical data from the 

western embankment to identify subsurface features 

that could be used to correlate data collected from 

the eastern embankment.

4. 

5. 

Analyze the geodatabase to determine the best starting 

point for surface geophysical survey.

Collect, continuously process, and analyze surface geophysical data collected from the eastern embankment to 

identify a location for one piezometer transect.

1.

2.

3.

a.

b.

6. Present preliminary data, interpretations, and the location 

identified for the installation of the eastern embankment 

piezometer transect to USIBWC staff.

7. Guide the drilling and installation of the transect of 

piezometers along the eastern embankment.

8. Obtain and log drill cuttings and borehole geophysical 

data for each new piezometer. Evaluate borehole geophysical logs to determine optimum vertical placement of well 

screens for each piezometer. 

Western Embankment Investigation

Geologic and geophysical characteristics that indicate 

relatively large water-yielding properties of the Salmon Peak 

Formation were identified. Borehole geophysical logs (natural 

gamma, gamma-gamma, caliper, and neutron) were collected 

during the installation of the western embankment piezometer 

transects in 1995 (Paul Gibson, U.S. Section, International 

Boundary and Water Commission, written commun., 2006). 

The direct-current (DC) resistivity method (Zohdy and others, 

1974) was selected for the reconnaissance study as the surface 

geophysical technique to correlate relatively large wateryielding properties of the Salmon Peak Formation, identified from analysis of existing borehole geophysical logs, with 

variations in subsurface resistivity. To aid in the correlation 

Figure 4. Elevation of Amistad Reservoir and discharge from Carmina Springs near Ciudad Acuna, Coahuila, 1970–94 (modified from 

Jose Antonio Moza, Federal Electricity Commission, written commun., 1995). 

Spring discharge

Pool elevation

1970

1971

1972

1973

1974

1975

1976

1977

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1978

1979

1980

1981

1982

1983

YEAR

350

345

340

335

330

325

320

315

310

2,500

2,000

1,500

1,000

500

0

DISCHARGE, IN LITERS PER SECOND

ELEVATION, IN METERS ABOVE NAVD 88

 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
between surface and borehole geophysical data, borehole 

induction resistivity logs, as well as natural gamma and fluid 

conductivity logs, were collected. Borehole resistivity logs 

were collected to relate geophysical variations in gamma, 

caliper, and neutron logs, identified as changes in wateryielding properties, to changes in the electrical stratigraphy of 

the subsurface along DC resistivity profiles.

Eastern Embankment Investigation

Historical data from the USIBWC were analyzed to 

identify trends in the spatial distribution of sinkholes near the 

eastern embankment and the occurrence of weathered zones 

identified from geologic descriptions of cores collected by 

the IBWC along the eastern embankment. By evaluating the 

occurrence of these features, an area was selected for further 

exploration using DC resistivity. DC resistivity data collected 

along the eastern embankment were continuously processed 

and inverted. Inversion involves estimation of subsurface physical properties from measured geophysical field data using 

computer-modeling software. Inversion results in the form of 

images representing sections of subsurface resistivity produced from the DC resistivity data were evaluated to identify 

subsurface electrical signatures similar to those obtained from 

the data collected along the western embankment. Similar signatures were identified and correlated with geologic properties 

from the geodatabase to select potential sites for piezometer 

installation. As the piezometers were installed, drill cuttings 

were used to construct geologic logs to complement and refine 

borehole geophysical logs. The resistivity data used to site the 

new piezometers were compared to geologic and geophysical log data collected after installation of the piezometers to 

verify similarities to lithologies at piezometers on the western 

embankment. 

Methods

The combination of historical data, applied geophysics, 

and ground-truthing methods can be used to contribute to the 

solution of most geotechnical engineering and environmental problems (U.S. Army Corps of Engineers, 1995). Most 

geophysical techniques do not measure the physical properties 

needed to solve the problem under consideration; however, 

most geophysical techniques can measure contrasts in the 

physical properties that can be used to infer contrasts in geology or hydrogeology. The correlation of measured geophysical 

contrasts that reflect changes in geology or hydrology of the 

subsurface primarily is empirical and depends on the quality of the results. There is no substitute for specific geologic 

or engineering observations (such as borings, test pits, and 

trenches) to aid in the empirical correlation. Borings or other 

tests are used to validate and calibrate geophysical results and 

ultimately to improve the accuracy of the interpretation.

Surface and borehole geophysical methods are used to 

measure the physical properties of the subsurface, such as 

electrical conductivity or resistivity, dielectric permittivity, 

magnetic permeability, density, or acoustic velocity (American 

Society for Testing and Materials, 1999). Such methods provide a relatively quick and inexpensive means to characterize 

the subsurface. The results (measurements) can be influenced 

by chemical and physical properties of soils, rocks, and pore 

fluids. Interpretations from these measurements can be used to 

image the distribution of physical properties in the subsurface 

(American Society for Testing and Materials, 1999). Surface 

and borehole geophysical methods and their application to 

ground-water and environmental investigations are described 

in detail in Zohdy and others (1974), Keys (1990), and U.S. 

Army Corps of Engineers (1995). The geophysical surveys 

used in this investigation were conducted in accordance 

to applicable American Society for Testing and Materials 

(ASTM) guides (1999; 2001; 2004a, b, c). 

Drilling in karstic limestone can be challenging because 

of the high potential for intersecting permeable dissolution 

cavities, which can cause circulation loss and lack of returned 

drill cuttings. Drilling methods used to complete the boreholes 

in this investigation were a combination of tri-cone air-rotary 

and down-hole pneumatic hammer, which have several advantages: (1) ability to drill in partly consolidated and consolidated geologic materials, (2) cuttings returned in air stream for 

accurate correlation, (3) drilling is rapid and readily accomplished, and (4) facilitates geophysical logging of the borehole 

(Lapham and others, 1997). Some disadvantages include the 

potential for circulation loss in fractured or karstified rock 

and the potential for altering the formation near the borehole 

by causing or opening new fractures. Well construction and 

completion was performed in accordance with ASTM standards (American Society for Testing and Materials, 2004d), 

Geodatabase

A geodatabase was developed to consolidate geological, 

geotechnical, and geophysical data collected during previous 

(early 1960s to 1995) and current (2006) investigations in a 

comprehensive temporal and spatial database (Shah and 

Quigley, 2005). The geodatabase was developed using Oasis 

montaj (Geosoft Inc., 2006). (Oasis montaj is the software 

used to create, manage, and visualize the geodatabase.) The 

geodatabase includes geologic-log data from foundation 

test borings along the centerline of Amistad Reservoir dam; 

geologist descriptions of borings from piezometers installed 

along the western embankment; borehole geophysical natural 

gamma, resistivity, and neutron logs; and DC resistivity data. 

These data were entered into the geodatabase in an American 

Standard Code for Information Interchange (ASCII) format. In 

addition, data collected during construction of the piezometers 

also were archived in the geodatabase. Existing borehole geophysical logs for the western embankment piezometers were 

digitized and imported into the geodatabase. The raw geophysical data that were collected also were modified to an ASCII 

format and archived in the geodatabase. Surface information, 

such as sinkhole and spring locations, also was obtained and 

Methods 
entered into the geodatabase. The geodatabase was used during fieldwork to produce lithologic and resistivity sections and 

to perform various types of spatial analyses useful to understand and visualize the subsurface and to aid in placement of 

the new eastern embankment piezometers.

Borehole Geophysics 

Thirteen piezometers (seven on the western embankment 

and six on the eastern embankment) and one existing open 

hole (Brite well) were logged using borehole geophysical 

methods. The seven piezometers (fig. 2A; LA–12L, LA–11, 

LA–10, LA–16L, LA–15, LA–8L, LA–7) on the western 

embankment, owned by the MxIBWC, are located below 

the western end of the dam. These piezometers were logged 

with electromagnetic (EM) induction conductivity, natural 

gamma, and fluid resistivity tools. The selected piezometers 

on the western embankment are completed with polyvinyl 

chloride (PVC) casing, which prevented the collection of some 

additional logs. The six eastern embankment piezometers 

(fig. 2B; 105+90B, 105+90A, 106+00B, 106+00A, 106+00C, 

106+00D) and one existing open hole (Brite well) owned by 

the USIBWC were logged using the previously mentioned 

methods and also with open-hole tools such as caliper and 

casing collar locator. In wells with sufficient static water level, 

normal resistivity logs, spontaneous potential (SP) logs, acoustic televiewer (ATV) logs, acoustic velocity logs, full-waveform sonic variable intensity logs (VIL), and EM flowmeter 

logs also were collected if possible.

Pertinent information for wells such as spatial location, 

elevation, well identifier, total depth, and casing and wellconstruction record was provided by the MxIBWC. In addition 

to the logs collected for this study, the MxIBWC piezometers 

were logged by a contractor in 1995 at the time of drilling. 

The logs collected at that time were neutron, density, caliper, 

and natural gamma. These previously collected logs were 

digitized for use in this study because only photocopies of the 

logs were available formatted with compressed depth scales 

and very little detail. This study included the collection of EM 

induction conductivity logs to correlate borehole geophysical 

data with surface geophysical resistivity data. All geophysical 

probes interfaced to either a Mount Sopris MGXII or Century 

System VI log-acquisition system in the logging truck by way 

of 0.25-inch- (6-millimeter-) diameter four-conductor wireline. Individual borehole geophysical methods are described 

in appendix 1; logs are described in appendix 2; and logs are 

contained in appendix 3.

Surface Geophysics

Three profiles, (two on the western embankment 

and one on the eastern embankment) were surveyed using 

DC resistivity methods. The two profiles surveyed on the 

western embankment (fig. 2A) were done to determine the 

effectiveness of using DC resistivity profiling methods to 

image changes in subsurface resistivity that correlate with 

increases in the water-yielding properties of the Salmon Peak 

Formation.

Direct-Current Resistivity

Electrical surface geophysical methods can be used to 

detect changes in the electrical properties of the subsurface 

(Zohdy and others, 1974). The electrical properties of soils 

and rocks are determined by water content, mineralogical clay 

content, salinity, porosity, and the presence of metallic minerals. Typically the resistivity of the water has a large effect on 

the bulk resistivity. One of the most commonly used methods 

for measuring earth resistivity is the DC resistivity method 

(Keller and Frischknecht, 1966). Generally the DC resistivity 

method uses two electrodes as a transmitter connected to a 

source of electric current 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 transmitter, and the measured potential 

across the receiver, the apparent resistivity can be determined. 

The underlying physical principle used to calculate resistivity for the DC resistivity method is embodied in Ohm’s law. 

According to Ohm’s law, the resistance (R) of earth material 

can be determined by 

R= ΔV/I, (1)

where ΔV is the potential difference (voltage drop) measured 

by the receiver, and I is the current (amperes) applied by the 

transmitter. The resistance calculated from DC resistivity 

measurements is a specific measurement of the ability of earth 

material to transmit electrical current that is directly dependent on the geometry and electrode spacing of the array used 

to obtain that measurement. To obtain a measurement that is 

independent of the geometry and electrode spacing resistance, 

R measurements are multiplied by a geometric factor (K), 

unitless, to calculate apparent resistivity (ρa) represented in the 

following equation:

ρa= K ΔV/I. (2)

Apparent resistivity represents the resistivity of completely uniform (homogenous and isotropic) earth material 

(Keller and Frischknecht, 1966). To determine the resistivity of nonuniform earth material, inverse modeling software 

is used. A description of the DC resistivity method as well 

as electrical tables of the properties of earth materials can 

be found in Zohdy and others (1974), Sumner (1976), and 

Sharma (1997).

Apparent resistivity data were collected along three 

profiles (fig. 2A–B) at the Amistad study site using an 

IRIS Syscal Pro 10-channel 96-electrode resistivity meter (fig. 

5A). The IRIS Syscal Pro was configured with eight sets of 

10 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
multi-conductor cables, each cable having 12 electrode terminals with 5-meter spacing. Stainless steel electrodes were 

installed in the ground and connected to electrode terminals 

built into the multi-conductor cables (fig. 5B). After the 

initial section of resistivity data was collected, the first two 

cables of 24 electrodes were moved ahead of the survey line. 

A partial section of data then was collected using the previous 72 electrodes (electrodes 25–96) and the 24 electrodes 

(electrodes 97–120) just moved. This process, known as the 

roll-along technique, was continued until all data along the 

desired line length were collected. The data from the rollalongs can be combined into a single apparent-resistivity data 

set during processing or processed individually. Appropriate 

quality assurance/quality control procedures such as testing 

contact resistance before data collection was performed for 

each segment of each profile. Contact resistance measures the 

resistance to current flow at electrodes caused by imperfect 

electrical contact with the earth (Boyd, 2006). Poor data quality or anomalous data can result from high or highly variable 

electrode contact resistances along a profile. To decrease the 

effect of contact resistances along each profile, a saltwater 

solution was applied using a backpack sprayer (fig. 6A) to 

each electrode (fig. 6B) before the contact resistance test was 

performed. If upon testing, contact resistance was anomalously high, salt­water was reapplied; if the problem persisted 

then the electrodes were moved slightly, saltwater was reapplied, and then contact resistance was remeasured until acceptable values were obtained. The final contact resistance values 

Figure 5. Near Amistad Reservoir dam, March 2006, along direct-current resistivity profile 3: (A) IRIS Syscal Pro resistivity meter 

connected to multi-conductor cables, and (B) stainless steel electrode connected to electrode takeout built into multi-conductor cable.

A B

Figure 6. Near Amistad Reservoir dam, March 2006, along direct-current resistivity profile 3, application of a saltwater solution 

(A) using a backpack sprayer, and (B) to an electrode to decrease the effect of contact resistances. 

A B

Methods 11
collected using the dipole-dipole array configuration prior to 

apparent resistivity data collection for each profile is on file at 

the Texas Water Science Center, West Texas Program Office, 

San Angelo.

Trimble 5800 Global Positioning System (GPS) units 

were used to establish two temporary GPS survey benchmarks referred to as Online Positioning User Service (OPUS) 

(National Oceanic and Atmospheric Administration, 2006) 

base stations in the western embankment study site (fig. 2A). 

The GPS units collected static GPS observations at base 

stations 1 and 2 for approximately 3 and 4 hours, respectively. 

The GPS static observations were processed using OPUS, 

provided by the National Geodetic Survey, to derive geographic and elevation coordinates for each base station. 

The OPUS solutions for each base station are provided in 

appendix 4. Three Trimble 5800 GPS units, one configured 

as a base station (fig. 7A) and two configured as rovers (fig. 

7B), were used in real-time kinematic (RTK) mode to survey 

electrode locations along each profile to derive geographic 

and elevation coordinates. A GPS base station was set up 

on OPUS base station 1 using the OPUS-derived solution 

to establish the geographic and elevation coordinate for that 

point. The GPS at base station 1 was used to collect GPS 

observations and determine corrections, then radio these 

corrections to the rover GPS units for real-time corrections. 

To verify the proper set-up of base station 1 and each rover 

GPS, both rovers were set up on base station 2 to collect RTK 

GPS data. The RTK solution for each rover was compared to 

the results of the OPUS solution to verify that the GPS units 

were configured correctly. The RTK-GPS derived elevations 

were incorporated in the apparent-resistivity data sets during 

post-processing to allow for topographic corrections during 

inverse modeling. Geographic coordinates were input into 

the geodatabase to geospatially reference each DC resistivity 

profile and the eastern embankment piezometers. To extend 

data collection into the eastern embankment study site a 

temporary RTK benchmark (fig. 2B) was established, then the 

base station was relocated to the temporary RTK benchmark 

and used to survey the DC resistivity profile along the eastern 

embankment.

Two DC resistivity profiles (fig. 2A) were collected along 

the western embankment to image the subsurface resistivity 

near the three transects of piezometers. DC resistivity profile 

1 trending north to south was collected so that the profile data 

could be compared to hydrogeologic and borehole geophysical data from piezometer transect 1 (LA–8L, LA–7, LA–6; 

fig. 2A) and transect 2 (LA–12L, LA–11, LA–10; fig. 2A ) 

and to image the subsurface resistivity between the toe of the 

dam and the springs. DC resistivity profile 2 trends west to 

east intersecting piezometer transects 1–3 perpendicularly 

south of piezometers LA–7, LA–11, and LA–15. DC resistivity profile 2 was collected to image the subsurface resistivity 

semi-parallel to the toe of the dam and to collect data between 

piezometer transects 1–3. DC resistivity profile 3 was collected traversing west to east along the eastern embankment 

(fig. 2B). Apparent resistivity data were inverted in the field 

to identify resistivity features similar to the features characterized along DC resistivity profiles 1 and 2. 

Two arrays, the dipole-dipole (fig. 8A) and a modification 

of the Schlumberger array (fig. 8B), the reciprocal Schlumberger array (fig. 8C), were used to collect DC resistivity data 

for each profile (Zohdy and others, 1974). The dipole-dipole 

and reciprocal Schlumberger arrays were chosen because they 

can be easily optimized to allow for the maximum number 

of potential pairs (channels) to be measured with a single 

current injection. The dipole-dipole array is good for mapping vertical structures such as dikes or cavities but relatively 

poor for mapping horizontal structures (Loke, 2004); whereas 

the Schlumberger array is superior in distinguishing lateral 

from vertical variations in resistivity (U.S. Army Corps of 

Engineers, 1995). The theorem of reciprocity states that no 

change will be observed in the measured voltage if the positions of the potential electrodes and of the current electrodes 

are interchanged; therefore the reciprocal Schlumberger array 

should have similar lateral and vertical resolution capabilities 

(Keller and Frischknecht, 1966). One disadvantage to using 

the dipole-dipole array is a decrease in signal strength for large 

values of “n” spacings (transmitter and receiver separations) 

making it more susceptible to environmental noise than the 

reciprocal Schlumberger array (Loke, 2004). 

Figure 7. Near Amistad Reservoir dam, March 2006, along direct-current resistivity profile 2: (A) real-time kinematic (RTK) Global 

Positioning System base station, and (B) RTK rover used to survey electrode locations. 

12 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
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 (Grant 

and West, 1965). To estimate the true subsurface resistivity, 

an inverse modeling program develops a model consisting of 

rectangular blocks of individual resistivities (Loke, 2004). 

The inversion program calculates the system response of that 

model to produce synthetic calculated apparent resistivity data. 

The calculated data are compared to the collected 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 model resistivity 

values and recalculating the calculated apparent resistivity; 

this alteration and recalculation of resistivities is known as 

an iteration. When the RMS difference between the calculated and measured apparent resistivity no longer improves 

appreciably between iterations (more than 1 percent between 

iterations), a solution is reached. This final model represents 

a non-unique estimate of the true distribution of subsurface 

resistivity. The inverse modeling process is described in detail 

by Loke (2004) and Advanced Geosciences, Inc. (2006). 

All apparent resistivity data were inverted using AGI 

EarthImager 2D version 2.1.2 Resistivity and Induced Polarization Inversion Software (Advanced Geosciences, Inc., 

2006). After analysis of apparent resistivities and the evaluation of both smooth and blocky (robust) inversion methods it 

was determined that the robust inverse modeling method best 

modeled the apparent resistivity data because of sharp contrast 

in the apparent resistivity data. The smooth inversion method 

gives better results where there are gradual changes in subsurface resistivity, whereas the robust method produces substantially better results where the subsurface geology consists of 

a number of regions that are almost homogeneous but with 

sharp boundaries between the different regions (Loke and others, 2003). A robust model inversion, based on the assumption 

of an exponential distribution of data errors (Burkardt, 2005; 

Advanced Geosciences, Inc., 2006), minimizes the integral of 

the absolute value of the difference between the measured and 

the calculated apparent resistivity.

Figure 8. Configuration of current (C1 and C2) and potential (P1 and P2) electrodes used to collect direct-current resistivity data for 

(A) dipole-dipole array, (B) Schlumberger array, and (C) reciprocal Schlumberger array.

A EXPLANATION

C1 – Current electrode 1

C2 – Current electrode 2

P1 – Potential electrode 1

P2 – Potential electrode 2

B

C

C1 C2 P1 P2

Dipole-dipole array

P1 C1 C2 P2

Reciprocal Schlumberger array

C1 P1 P2 C1

Schlumberger array

Methods 13
Model inversion parameters are settings that are applied 

to a model inversion to control certain criteria throughout the 

inversion process. All apparent-resistivity data were inverted 

using a maximum eight-iteration inversion. An inversion can 

be stopped prior to the eighth iteration when any of the stop 

criteria in the model inversion parameters are met. All data 

sets were first inverted without using any criteria for data 

removal. Model inversion parameters then were optimized for 

data processing and inverse modeling of the apparent resistivity data collected along the three profiles. Raw apparentresistivity data initially were filtered using the criteria for data 

removal (appendix 5) to remove poor-quality data points. Data 

also were filtered during the inversion process on the basis of 

the resistivity inversion parameters (appendix 5). Because the 

three apparent resistivity profiles used to collect both dipoledipole and reciprocal Schlumberger arrays yielded dense 

apparent-resistivity data sets, the data sets were processed as 

individual roll-alongs (subsections). By subsectioning large 

data sets and calculating model inversions for each subsection, 

data and model-convergence can be evaluated appropriately to 

address specific problem areas in a large profile. 

Borehole Drilling and Piezometer Installation

Borehole drilling was done with a Gardner-Denver model 

17W direct rotary drill rig owned and operated by the USGS. 

A combination of tri-cone air-rotary and down-hole pneumatic 

hammer methods was used to complete the boreholes. In these 

drilling methods a large compressor forces high pressure air 

through the drill pipe and through ports in the bit. The high 

pressure forces the cuttings up through the annulus between 

the drill pipe and the borehole walls, thus providing a constant 

means of cuttings removal and air circulation to cool the bit. 

When using tri-cone bits, the combination of downward pressure exerted by the drill rig and the rotating action of the steel 

or carbide-studded rollers grinds the rock. During pneumatic 

hammer drilling, the bottom of the bit acts as a jackhammer, fracturing and pulverizing the rock for removal. In both 

methods, the cuttings are either ground or pulverized to a size 

sufficient to be removed by the up-hole air velocity and collected at land surface. 

If during the drilling process, adequate ground water 

was not encountered, water from a potable source well at the 

IBWC Amistad Reservoir dam Project Office was injected into 

the boreholes to minimize the amount of dust generated and to 

assist in the cooling and lubrication of the drill bit.

A 250-millimeter tri-cone bit was used to drill the boreholes from land surface to a depth of about 1 meter. A 146-

millimeter pneumatic hammer was then used to drill a pilot 

hole from 1 to about 4.5 meters. The 250-millimeter rotary bit 

was then re-attached and used to enlarge or “ream” the pilot 

hole to a diameter that would accept the surface casing and 

Portland cement seal. After the borehole was at a depth of 

about 4.5 meters, a 5-meter length of 168-millimeter outside 

diameter steel casing with a wall thickness of 5 millimeters 

was lowered into and centered in the hole. A type I/II Portland 

cement slurry was mixed and hand-poured into the annulus to 

form a surface seal from a depth of 4.5 to 1 meter below land 

surface. The remaining annulus was back-filled to land surface 

with bentonite chips or pellets, or both, and approximately 24 

hours was allowed for the Portland cement to solidify. The 

146-millimeter pneumatic hammer was lowered through the 

surface casing and used to drill from 4.5 meters below land 

surface to the completion depth of each borehole.

The piezometers were constructed to ATSM standards 

for design and installation of monitoring wells (American 

Society for Testing and Materials, 2004d), hereinafter referred 

to as ASTM D 5092–04. The piezometers were constructed 

subsequent to drilling and geophysical logging. A 0.3-meter 

layer of 8- to 16-mesh sand pack was piped to the bottom of 

the hole prior to setting the casing and screen. The piezometers were cased with 63.5-millimeter schedule 40 PVC casing 

and screen. The PVC casing sections were threaded together 

and lowered to the top of the 0.3-meter sand pack. A sand 

pack was piped to 0.6 meter above the screened interval. Next, 

3.6 meters of bentonite pellets were placed on top of the sand 

pack. Bentonite grout was then mixed and piped to within 0.3 

meter of land surface. Bentonite chips were used to fill the 

annulus between the PVC and steel surface casings from a 

depth of 0.3 meter to land surface. 

One of the study objectives was to design and construct 

the piezometers in a similar fashion as the existing piezometers on the western embankment. Because some materials 

of the type used in the existing wells were unavailable, some 

design modifications were necessary. These modifications 

were discussed with USIBWC personnel and approved prior 

to construction. Design changes include the substitution 

of 0.040-inch (1-millimeter) (number 40) slot-size screen 

(horizontally slotted) for the 0.25-inch (6-millimeter) screen 

and 8- to 16-mesh sand pack for the 0.50-inch (13-millimeter) 

gravel pack used in the western embankment piezometers. 

According to ASTM D 5092-04, it is recommended that an 

8- to 12-mesh sand pack be used in conjunction with a number 

40 slot-size screen. The 8- to 16-mesh sand used in this project 

was factory-washed and sieved to a nominal particle size of 

0.0469 to 0.0937 inch (1 to 2 millimeters), which correspond 

to respective mesh numbers of 18 and 8. Because the sand 

available was slightly smaller than is generally recommended, 

a sediment sump was placed below the screen in each well 

to catch any fine particles that might pass through the screen 

openings.

The piezometers on the western embankment were 

constructed using 2-meter screen lengths. Screens in metric 

lengths were not readily available, and it was not possible to 

cut the screens to 2 meters because of threaded joints; therefore screens in 10-foot (3.0-meter) lengths were used. 

In addition to the screen and sand pack design modifications, the 2-meter cement plug used in the western embankment piezometers was omitted because of the likelihood of the 

cement filtering down through the lower bentonite seal and 

contaminating the sand pack prior to its solidifying.

14  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Geophysical Analysis 

To efficiently compile, store, and compare geologic and 

geophysical data, a geodatabase was created at the beginning 

of the project and used throughout the project. Data were 

added to the geodatabase as the project progressed. Borehole 

geophysical logs, both existing and newly acquired, were compiled and merged into multiple-property logs for comparison 

and analysis. These logs are contained in appendix 3 and 

are described briefly in the sections below. Many borehole 

geophysical properties were recorded and used to obtain 

information on the geologic, lithologic, and water-yielding 

properties that lead to possible explanations of differences in 

resistivity of the rocks in the formations. Geophysical differences among layers were delineated and integrated with 

surface geophysical DC resistivity in two-dimensional (2D) 

and three-dimensional (3D) visualizations. 

Each subsection of inverted DC resistivity was exported 

from AGI EarthImager 2D in an ASCII format. Subsequently, 

each subsection for a single profile was imported into a 

spreadsheet format and inputted into Oasis montaj using an 

array format that uses the distance as a reference point. An 

array format consists of multiple values for any given variable. 

In this case, there are multiple depths and resistivities for a 

given distance; therefore each depth and resistivity for a given 

distance would be placed into a new column. These data were 

imported into the geodatabase through the Oasis montaj Data 

Import Wizard, specifying the array columns.

Overlap present in the DC resistivity data from the rollalongs for profiles 2 and 3 caused multiple resistivities for a 

given depth and distance. To combine these overlapping data 

points, a nonlinear filter operation was performed along the 

array channel using a filter width of two data points. This 

nonlinear filter analyzes each individual point resistivity and 

determines the validity of the data point using an algorithm 

based on the width of data features and the relation of these 

features to a local background (Geosoft Inc., 2006; Help Topic 

NLFILT GX). The number of data points specifies the filter 

width. If the data point is determined to be noise, it is replaced 

by an estimated value on the basis of interpolation from surrounding data-point values. These filtered values were then 

gridded using an array-channel section grid. The profiles were 

gridded using a minimum-curvature gridding method with a 

0.25-meter grid-cell size.

All lithologic and geophysical data were plotted together 

on lithologic and resistivity sections for 2- and 3-D visualizations. The color scale for the DC resistivity profiles then was 

determined by matching the color zones in the DC resistivity 

profile with the lateral resistivity log for piezometer 106+00B 

(appendix 3). The color scale then was compared to the general geology throughout the profile to see if the highly resistive 

zones closely matched the zones of unweathered limestone. 

This process was repeated until a uniform color scale was 

obtained. Because of various resistivity changes between the 

dipole-dipole array and the reciprocal Schlumberger array, a 

unique color resistivity scale was determined for each. 

Analysis of Geologic Data and Geophysical Log 

Descriptions

On March 8, 10–12, and 19–22, 2006, geophysical logs 

were collected from individual wells below the Amistad 

Reservoir dam. All piezometers were completed in the Salmon 

Peak Formation except for the Brite well, which also penetrates the upper McKnight Formation. All geophysical log 

responses are discussed in relation to lithologic descriptions 

and possible anomalous features in the subsurface. All wells 

were logged using at minimum, the geophysical methods of 

natural gamma and induction conductivity, which can be collected in open holes or PVC-cased piezometers. The existing 

piezometers on the western embankment were LA–12L, LA–

11, LA–10, LA–16L, LA–15, LA–8L, and LA–7 (fig. 2A). 

The piezometers drilled by USGS and logged on the eastern 

embankment were 105+90B, 105+90A, 106+00B, 106+00A, 

106+00C, and 106+00D (fig. 2B). The Brite well also was 

logged. In addition to the previously mentioned minimum 

suite of logs, open-hole logging techniques were used on the 

eastern embankment (where applicable) including electric long 

(64N) and short (16N) normal resistivity, SP, fluid conductivity, temperature, acoustic velocity, full-waveform sonic VIL, 

ATV, and EM flowmeter, caliper, and casing collar locator. For 

various reasons, not all geophysical methods were used on all 

wells. For example, some wells did not contain a static fluid 

column over the total length of the borehole at the time of logging, which inhibits the collection of flowmeter, acoustic, and 

electric logs. The descriptions contained in this section mostly 

will pertain to geophysical variations that relate to effects on 

resistivity such as the water-yielding properties of the rocks 

and do not constitute a complete analysis of every geophysical response. All depths are in meters below land surface 

datum (LSD). The borehole geophysical logs are contained in 

appendix 3.

Western Embankment Investigation

Analysis of Hydrogeologic Data 

The geodatabase was used to create two 2D sections 

(fig. 9, at end of report) of lithologic data along DC resistivity 

profiles 1 and 2 on the western embankment. The lithologic 

sections were created by projecting lithologic data in the 

geodatabase 200 meters west or east of a line centered along 

DC resistivity profile 1 (fig. 9A). A second section was created 

by projecting all available geologic data 200 meters north and 

south of profile 2 (fig. 9B). The sections were used to identify 

the depth and extent of the alluvium, the weathered features in 

limestone, and the argillaceous fossiliferous zone along each 

DC resistivity profile.

Geologist descriptions of the borings along DC resistivity profile 1 (fig. 9A) indicate a concentration of weathered 

features in the Salmon Peak Formation extending from about 

Geophysical Analysis 15
295 meters above NAVD 88 to a few meters below land 

surface, except for boring (LA–12), which indicates a second 

zone of weathered features ranging from about 278 to 285 

meters above NAVD 88. Borings south of the toe of the dam 

(fig. 9A) are shallow and do not identify the total depth of 

weathered features in the Salmon Peak Formation. Borings 

indicate an argillaceous fossiliferous zone in the Salmon Peak 

Formation that ranges from about 280 to 290 meters above 

NAVD 88. The argillaceous fossiliferous zone was identified 

consistently in borings from Amistad Reservoir prior to dam 

construction and serves as a distinct geologic marker throughout the study area. The top of this feature was identified along 

sections of lithologic data (fig. 9A–B) to visualize its vertical 

displacement, which can be used as an indicator of faulting 

throughout the study area. The lithologic section along DC 

resistivity profile 1 shows a continuous surficial alluvial layer 

ranging from about 2 to 5 meters thick. 

The thickness of the zone of weathered features varies 

along the section ranging in thickness from about 18 to 40 

meters, and the top of the zone ranges in elevation from 332 

to 324 meters above NAVD 88. Two borings, LA–12L and 

LA–16L, indicate a secondary weathered zone; however, the 

total thickness of that zone was not identified. The alluvial 

layer along profile 2 is a continuous layer extending from a 

depth of about 320 meters above NAVD 88 to land surface and 

ranging from about 2 to 7 meters thick.

Borehole Geophysical Log Analysis

Seven piezometers along the western embankment 

(LA–12L, LA–11, LA–10, LA–16L, LA–15, LA–8L, and 

LA–7) were logged through casing using borehole geophysical 

methods in March 2006. The open boreholes were logged at 

the time of drilling in June 1995 by MxIBWC. Selected logs 

run in open boreholes in 1995 were digitized from photocopies 

for inclusion in this report (appendix 3). A detailed log analysis is provided in appendix 2 that includes discussion of the 

individual logs (appendix 3) for each piezometer starting with 

the deepest piezometer LA–12L and continuing along that 

transect to the shallowest piezometer (LA–10). A brief discussion of the geophysical properties related to the water-yielding 

properties of the formation or other information, or both, that 

could relate to resistivity follows. 

Piezometer LA–12L, near the toe at of the dam on the 

western embankment, extends about 50 meters below LSD 

(fig. 2A; appendix 3). The neutron log is an indicator of the 

presence of hydrogen (Keys, 1990), which absorbs neutrons 

and reduces the neutron count rate. Decreased count rates 

are shown on the neutron log at 3–6, 7.5–8.5 and 18.5 meters 

below LSD because of shale beds at those depths. The neutron 

log for piezometer LA–12L shows relatively low count rates 

from 20 meters to the bottom of the borehole compared with 

the upper part of the borehole. Compared to the other boreholes, this zone shows the lowest neutron count rate, possibly indicating the greatest water content of the all western 

embankment borehole logs. However, slightly increased count 

rates are at 27–28, 29.5–32, 37–42, and 44.5­–48.5 meters 

below LSD, zones which also are coincident with zones of 

lower natural gamma responses, indicating relatively pure 

limestone, with little shale or clay, at those depths. Possible 

zones of increased water content in the formation are at 24–27, 

32–36.5 and 41–44 meters below LSD. 

Piezometer LA–11 is about 141 meters southeast of 

piezometer LA–12L on the western embankment and extends 

about 20 meters below LSD (fig. 2A, appendix 3). The natural 

gamma log peaks from piezometer LA–11 generally correlate 

with shale and clay on the lithology log provided by MxIBWC 

(Paul Gibson, U.S. Section, International Boundary and Water 

Commission, written commun., 2006); the natural gamma 

log shows excellent correlation with the induction resistivity 

and conductivity logs and neutron log at 3.5–8 and 14.3–15.3 

meters below LSD as related to clay or shale layers. Interpretations of decreased neutron count rate and decreased natural 

gamma count rate indicate possible zones of increased water 

content in the formation at 8–9, 12–13, and 16.8–17.8 meters 

below LSD. 

Piezometer LA–10 is about 286 meters southeast of 

piezometer LA–12L on the western embankment and extends 

about 15 meters below LSD (fig. 2A; appendix 3). The natural 

gamma log from piezometer LA–10 shows two zones of 

increased count rate at 2.8–3.4 and 10.5–11 meters below LSD 

that correspond to interbedded shale beds in the lithologic 

descriptions and zones of decreased count rates on the neutron 

log. Somewhat decreased count rates also are at 3.5–4.5 and 

12.7–14 meters below LSD, zones which correspond to weathered limestone zones with decreased gamma count rates; such 

zones appear to contain higher amounts of hydrogen (Keys, 

1990) and are assumed to be water-yielding zones in the limestone. The induction resistivity log shows very little variation 

in resistivity and mostly greater than 100 ohm-meters, except 

for the interbedded shale zone at 2.5–3.5 meters below LSD 

where the resistivity decreases to less than 100 ohm-meters.

Piezometer LA–16L is about 189 meters east-northeast 

of piezometer LA–12L on the western embankment and 

extends about 47.5 meters below LSD (fig. 2A, appendix 3). 

Piezometer LA–16L logs indicate that beneath a shale layer at 

15.5–16.5 meters below LSD the neutron count rate decreased 

at 21–28 and 28.5–30.5 meters below LSD, which indicates 

increased water content in the weathered limestone. 

Piezometer LA–15 is about 225 meters south-southeast of 

piezometer LA–16L on the western embankment and extends 

about 20 meters below LSD (fig. 2A, appendix 3). The natural 

gamma log of piezometer LA–15 shows increased count rates 

at 1.75–4.5 and 11.3–12.3 meters below LSD, zones that correspond with shale layers on the lithology log (Paul Gibson, 

U.S. Section, International Boundary and Water Commission, 

written commun., 2006) and with zones of increased conductivity and decreased resistivity per those logs. The neutron log 

shows decreased count rates in the shale layers noted above. 

Below the shale layer at 11.3–12.3 meters below LSD, the 

neutron count rate decreased, indicating increased water content in the weathered limestone.

16 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Piezometer LA–8L is about 191 meters west-southwest of 

piezometer LA–12L on the western embankment and extends 

about 46.5 meters below LSD (fig. 2A, appendix 3). The natural gamma log for piezometer LA–8L shows increased count 

rates at 10–12, 14–15.5, and 37.5–40 meters below LSD. The 

zone at 10–12 meters below LSD corresponds with a shale 

layer on the lithology log (Paul Gibson, U.S. Section, International Boundary and Water Commission, written commun., 

2006). Other zones of increased natural gamma count rate do 

not correspond with a shale layer on the lithology log (Paul 

Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006) and possibly could be caused 

by a clay-filled void (at 14–15.5 meters below LSD). A zone 

of increased induction conductivity and decreased resistivity at 

5–9 meters correlates with gravel, clay, and shale, and interbedded shale layers along with a zone of decreased neutron 

count rate. Other zones of decreased neutron count rate are at 

16.5–17.5, 22.4–23.5, 24.2–25.4, 28.8, and 31 meters below 

LSD; all occur in weathered limestone (Paul Gibson, U.S. Section, International Boundary and Water Commission, written 

commun., 2006) indicating possible water-yielding zones at 

these depths.

Piezometer LA–7 is about 223 meters south-southeast of 

piezometer LA–8L on the western embankment and extends 

about 20 meters below LSD (fig. 2A, appendix 3). The natural 

gamma log shows increased count rates at 5–6, 7.7–8.6, and 

13.2–14.5 meters below LSD, zones that correspond with 

shale layers on the lithology log (Paul Gibson, U.S. Section, 

International Boundary and Water Commission, written 

commun., 2006) and with zones of increased conductivity 

and decreased resistivity per these logs. The caliper log shows 

increased borehole diameter at 3.5–6 and 16 meters below 

LSD, and large- and small-diameter responses at 16.9–17.6 

meters below LSD. These zones are probably related to karst 

openings or fractures in the weathered limestone. The zone 

at 16.9–17.6 meters below LSD is unique in that it shows an 

increase in borehole diameter above and below a 0.5-meter 

zone of the borehole that is smaller than bit size. This smallerdiameter zone possibly indicates a swelling clay layer in a 

karst opening. The neutron log count rate decreased drastically 

in this zone, also indicating clay or water-yielding media, or 

both.

Analysis of Resistivity Profiles 1 and 2

Two identifiable resistivity units are in both the dipoledipole array data and reciprocal Schlumberger array data along 

DC resistivity profile 1 (fig. 10, at end of report). Resistivity 

unit 1, a near-surface, low-resistivity unit, generally ranges in 

resistivity from less than 45 to greater than 120 ohm-meters in 

the dipole-dipole array and from less than 75 to greater than 

150 ohm-meters in the reciprocal Schlumberger array. The 

deeper, more resistive unit, resistivity unit 2, generally ranges 

in resistivity from about 120 to greater than 345 ohm-meters 

in the dipole-dipole array and from about 150 to greater than 

365 ohm-meters in the reciprocal Schlumberger array. In both 

resistivity arrays, a vertical high-low resistivity boundary 

between resistivity unit 1 and resistivity unit 2 is observed at 

a horizontal distance of about 285 meters in the profile. The 

resistivities south of this boundary are relatively low except for 

two features in the dipole-dipole array and one feature in the 

reciprocal Schlumberger array.

DC resistivity profile 2 shows a similar two-unit 

electrical structure—a continuous, near-surface low-resistivity 

unit, resistivity unit 1, ranging from about 20 to less than 10 

meters thick in the reciprocal Schlumberger array; and resistivity unit 2, a relatively high-resistivity zone that is well defined 

in the reciprocal Schlumberger array (fig. 10D), but becomes 

less so in the dipole-dipole array (fig.10C). Resistivity unit 

2 in the dipole-dipole array is less well defined (distorted) 

from the beginning of the profile to about 475 meters. This 

distortion is attributed to poor-quality data, of which about 40 

percent was removed in the inversion process. A low-resistivity feature is observed at about 250 meters along the profile in 

both arrays in about the lowermost 50 meters of the profile.

Integration of Results

The upper extent of major lithologic features identified 

in lithologic sections (fig. 9A–B) were superimposed on DC 

resistivity profiles 1 and 2 (fig. 11, at end of report) to conduct an assessment of the hydrogeologic characteristics of 

the Salmon Peak Formation along the western embankment. 

The lithologic section along DC resistivity profile 1 shows a 

concentration of weathered features extending to about 295 

meters above NAVD 88 except for boring LA–12, which 

indicates a second zone of concentrated weathered features 

ranging from about 278 to 285 meters above NAVD 88. These 

zones of weathering generally correlate with the thickness 

and extent of resistivity unit 1 (low resistivity) near the toe of 

the dam. Although the total thickness of resistivity unit 1 was 

not completely ground-truthed by the geologist descriptions, 

borings from LA–12 and LA–16 do indicate that a deeper secondary weathered zone is present; however the total depth was 

not identified. Neutron logs from LA–12 and LA–16 identify 

possible zones of increased water content in the formation 

occurring from the bottom of the boring to about 20 meters 

below LSD. Compared to the other boreholes, LA–12 shows 

the lowest neutron count rate in this zone possibly indicating 

the greatest water content of all western embankment borehole 

logs. The presence of mapped sinkholes (fig. 1) in the reservoir north of the western embankment study area and the zone 

of increased water content in LA–12 indicated by the neutron 

log leads to the conclusion that resistivity unit 1 (fig. 10A–B) 

is a preferential flow path where surface water from Amistad 

Reservoir is forced into the ground-water system (because of 

increased head from the reservoir). The increased occurrence 

of weathered limestone along this profile indicates a potential 

conduit for ground-water movement along resistivity unit 1 

identified in profile 1 (fig. 10A–B). 

A vertical high-low resistivity boundary, at about 285 

meters along profile 1 (fig. 10A–B), was identified in both 

Geophysical Analysis 17
arrays. Borings along the south end of this profile are shallow 

and do not identify the total depth of weathered features (low 

resistivity). The high resistivity side of the boundary in profile 

1 is evident in profile 2 because profile 2 intersects profile 

1 at about 257 meters. This high-resistivity zone (2) along 

profile 1 (fig. 11A–B) begins to increase in elevation from 

about 850 meters along the profile (fig. 11C) to the end of the 

profile. This increase in elevation of resistivity unit 2 at about 

850 meters correlates to the location of springs 1, 2, and 3 

(fig. 11A–B), which suggests that water moves through the 

preferential flow path (resistivity unit 1) and discharges to the 

surface through the springs because of the presence of more 

resistive, less weathered (less permeable) limestone. 

Comparison of the inverse modeling results of DC resistivity profiles 1 and 2 to the lithologic sections, together with 

interpretations of borehole geophysical neutron logs, leads 

to the conclusion that the low-resistivity unit (1) in profiles 1 

and 2 are zones of increased weathering in the Salmon Peak 

Formation which is indicative of a preferential flow path for 

ground-water movement. This electrical contrast of high and 

low resistivity was used in the evaluation of DC resistivity data 

collected in the eastern embankment study area. To provide 

an alternative and complimentary presentation of the geologist descriptions of borings, available neutron logs, and DC 

resistivity profiles 1 and 2, a 3D section map was produced 

(figs. 12–13, at end of report). 3D visualization allows a comprehensive view of selected data that were used to interpret the 

preferential flow path in the western embankment study area. 

An important consideration regarding the 3D images is that 

they are constructed from the 2D inversion results with each 

line independent of one another (McDougal and others, 2004). 

The data have been interpreted by a 2D inversion that contains 

no information of other profiles and, therefore, is not constrained by the adjacent data set. This can result in disagreement of resistivity values where the profiles cross (McDougal 

and others, 2004).

Eastern Embankment Investigation

The service road along the southern toe of the eastern 

embankment was selected for DC resistivity profile 3 (fig. 2B). 

(The service road has little traffic and a shoulder that was clear 

of brush, thus facilitating the collection of the DC resistivity data. Data collection in a westerly direction was limited 

because of the intersection of another service road where 

heavy equipment was transported for quarrying operations.) 

The starting point along the service road for DC resistivity 

profile 3 was determined by (1) examining the spatial trend 

of documented sinkholes (fig. 2B) and (2) visually analyzing 

the thickness and extent of the weathered zones in the Salmon 

Peak Formation. A line of sinkholes has formed along each 

of two zones semi-perpendicular to the eastern embankment 

(figs. 1, 2B). The first zone occurs between borings A32V 

and A23E and the second is between borings A15V and 

A15E. Analysis of the lithologic section indicated that profile 

3 should start in the first zone of sinkholes and extend east 

through the second zone of sinkholes. 

Analysis of Lithologic Data 

To assess the hydrogeologic characteristics of the Salmon 

Peak Formation along the eastern embankment, a 2D section of available lithologic data (fig. 14, at end of report) was 

developed from all data 200 meters north or south of a line 

centered between the historical borings and the service road. 

Analysis of the lithologic section indicates that the weathered 

zone is thickest along the east end of the section; thus the 

second zone of sinkholes subsequently was given priority over 

the first zone with regard to geophysical analysis for piezometer siting (see “Eastern Embankment Piezometer Placement” 

section).

Analysis of DC Resistivity Profile 3

In both the dipole-dipole array and the reciprocal 

Schlumberger array, resistivity unit 1 is separated into three 

different parts by resistivity unit 2 (fig. 15, at end of report). 

The profiles can be divided into vertical sections for detailed 

analysis and are designated in the analysis by meters along the 

profile. The first part of resistivity unit 1 is from the beginning of the profile (0) to about 280 meters (western expression) along the profile. Resistivity unit 2 starts to decline 

in elevation from the surface at about 380 meters along the 

profile (central expression). From that point, resistivity unit 

1 continues until resistivity unit 2 rises back to the surface at 

about 750 meters along the profile. The last part of resistivity 

unit 1 (eastern expression) appears again at about 990 meters 

along the profile and continues to the end of the profile. The 

base elevation of resistivity unit 1 decreases gradually from 

the beginning (west) of the profile to the end (east). At the 

beginning of the profile, the base elevation of resistivity unit 

1 is about 315 meters above NAVD 88, and at the end of the 

profile, the elevation is about 295 meters above NAVD 88. The 

last 330 meters along the profile of resistivity unit 1 is difficult 

to distinguish because of the multiple resistive features in the 

unit. These resistive features show a similarity to the features 

in profile 1 at the western embankment study site (fig. 10). In 

profile 1, a sharp vertical boundary occurs between resistivity 

unit 1 and resistivity unit 2. Resistivity unit 1 becomes appreciably thicker at that boundary. A vertical boundary between 

resistivity unit 1 and resistivity unit 2 occurs at about 990 

meters along profile 3 where resistivity unit 1 becomes thicker. 

The thickness of resistivity unit 1 from 990 meters along the 

profile to the end of the profile is more than double the average thickness of resistivity unit 1 from the beginning of the 

profile to 750 meters along the profile.

The western and eastern expressions of resistivity unit 1 

correlate with the general trend of sinkholes (fig. 2B) documented by the USIBWC in the eastern embankment study 

area. To assess the hydrogeologic characteristics of the Salmon 

18  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Peak Formation in the eastern embankment study area along 

DC resistivity profile 3, the upper extent of major lithologic 

features (fig. 14) were superimposed on DC resistivity profile 

3 (fig. 16, at end of report). The thickness and extent of weathered zones do not correlate with the thickness and extent of the 

western expression of resistivity unit 1. There is, however, a 

substantial increase in the thickness of weathered zones in the 

limestone west of DC resistivity profile 3 shown by the first 

two geologist descriptions of borings (fig. 16B).

Lithologic data show that the thickness of resistivity unit 

1, along its central and eastern expressions, corresponds generally with the extent and thickness of weathered zones in the 

limestone (fig. 16); however, there is an increase in the thickness of alluvial deposits (fig. 14) along the central expression 

of resistivity unit 1. Because alluvium tends to be less resistive 

than limestone (Loke, 2004), it is possible that the decrease in 

resistivity along the central expression of resistivity unit 1 is 

predominantly influenced by the increased thickness of alluvium more so than the underlying weathered limestone.

Because of the lack of lithologic correlation along the 

western expression and the increased thickness of alluvium 

along the central expression, these features were not chosen 

for further exploration. The zone of concentrated weathered 

features on the eastern end of DC resistivity profile 3 is thickest and extends to a greater depth along the eastern end of the 

section. The correlation of surface geophysical DC resistivity, 

historical lithologic data, and general trend of documented 

sinkholes along the eastern end of profile 3 were used to justify further exploration (drilling of piezometers) in the eastern 

expression of resistivity unit 1.

Borehole Geophysical Log Analysis

Geophysical logging occurred immediately after drilling was completed, before casing was set on all but one well 

logged on the eastern embankment. A detailed log analysis 

provided in appendix 2 includes discussion of the individual 

logs (appendix 3) for each piezometer from the deepest 

piezometers (105+90B and 106+00B) to the shallowest 

piezometer (105+90A). A brief discussion of the geophysical properties related to the water-yielding properties of the 

formation or other information, or both, that could relate to 

resistivity follows. 

Piezometer 105+90B is the deepest of the 105+90 pair 

of piezometers, extending to about 36 meters below LSD 

(fig. 2B, appendix 3). Several zones of enlarged borehole 

diameter are evident on the caliper log, probably related to 

karstification or fracture openings in the limestone. These 

zones are centered at 6, 8, 12.2, 22.5, 24.5, 27.3, 27.8, and 

30.5 meters below LSD. The natural gamma log shows 

increased count rates in all of these enlarged zones. Increased 

count rates probably are caused by clay commonly contained 

in karst openings. Visible in the borehole televiewer image 

are some indications of open dissolution voids and bedding 

planes or low-angle fractures, many of which correspond with 

caliper enlargements. Some easily identifiable voids occur 

at 7.5–8.5, 12.2, 14.2–14.5, 15, 22.5, 24.5, 25, 27–28.7, and 

30.3–30.8 meters below LSD. The fluid conductivity curve 

indicates a slightly lower-conductivity (fresher) fluid at 

the bottom of the hole, which probably is aquifer water 

entering the hole at the bottom and displacing slightly higherconductivity fluid that was left in the hole from drilling. 

The temperature curve has a trend similar to that of the fluid 

conductivity and shows a slightly lower temperature near the 

bottom of the borehole. 

Piezometer 105+90A extends to about 10.7 meters below 

LSD and is a shallow companion to piezometer 105+90B, 

which is 3.5 meters southeast (fig. 2B, appendix 3). The caliper log shows zones similar to those of 105+90B, increased 

borehole diameter at 6 and 8 meters below LSD, which indicates possible laterally continuous openings at those depths 

between piezometers 105+90A and 105+90B. The coincidence 

of natural gamma peaks and caliper anomalies indicates clay 

is in the karst openings as was indicated in 105+90B. An 

anomaly of decreased fluid conductivity at about 9 meters 

below LSD could indicate relatively fresh inflow into the borehole from the aquifer. 

Piezometer 106+00B is the deepest of the 106+00 

pair of piezometers and the deepest of the USGS-drilled 

wells, extending to about 48 meters below LSD (fig. 2B, 

appendix 3). The zone at 16.5–21 meters below LSD shows 

a smaller decrease in resistivity (but shows a similar trend) 

compared to that for piezometer 105+90B, which is 3 meters 

from piezometer 106+00B. The geologist description of this 

zone in piezometer 106+00B does not note limey shale as 

does the description of the less resistive zone in piezometer 

105+90B but does note hard clay layers and a decrease in 

clay content with depth. A few zones of enlarged borehole 

diameter evident on the caliper log probably are related to 

dissolution or fracture openings in the limestone. Two large 

openings occur at 6.5–8.5 meters below LSD (referred to as 

the upper void) and 10.5–12 meters below LSD (referred to 

as the lower void) with diameters of 30 centimeters in the 

upper void and greater than 45 centimeters in the lower void. 

At the time of logging, the water level in the borehole was 

about 8.4 meters below LSD, which corresponds to the bottom 

of the upper void. This indicates that water could be exiting the borehole at this point. The natural gamma log shows 

increased count rates on both of these borehole enlargements. 

Increased counts probably are caused by the presence of clay 

that commonly is contained in karst openings. Elsewhere in 

piezometer 106+00B, natural gamma peaks coincide with 

small enlargements at 14.8, 24.3, 30.7, 34, and 37.5 meters 

below LSD, which probably are associated with dissolutionenhanced bedding-plane- or fracture-related openings. The 

ATV image and acoustic delta-T fluctuations (or cycle-skips) 

also indicate fracture or bedding-plane-related anomalies at 

these depths. 

A computer generated virtual core analysis of piezometer 106+00B using the caliper and ATV data of zones not 

showing natural gamma peaks indicates several additional 

zones of fractures or dissolution openings at 15–16, 19.7, 

Geophysical Analysis 19
21–21.5, 22.5, 27.25, 34.5–35.5, 38–39, and 45.5–48 meters 

below LSD. The horizontal features intersecting the borehole 

at 22.5, 24.3, and 30.6 meters below LSD appear to produce 

discontinuities for acoustic wave-energy propagation along the 

borehole, creating a diagonal effect on the full-waveform sonic 

VIL near these depths.

Piezometer 106+00A extends about 26 meters below 

LSD and is a shallow companion to piezometer 106+00B, 

which is 3.5 meters southeast (fig. 2B, appendix 3). Resistivity logs, including induction, normal, and lateral logs, all 

show some variation in resistivity, especially the zones at 

10–12.5 meters below LSD (similar to 106+00B) and 17–18 

meters below LSD. The 64N resistivity log, which measures 

deeper into the formation (with less resolution) than the 16N 

decreases only slightly at 17–18 meters and actually reads 

higher than the 16N and lateral logs at the 10–12.5-meter 

zone, which indicates that the 16N and lateral logs probably 

are being influenced by borehole fluid in the area of the formation closest to the borehole wall; this also was the case in 

piezometer 106+00B. Both zones show erratic acoustic delta-T 

readings (cycle-skips) because of irregular borehole diameter, 

or fractures, or both. The horizontal features intersecting the 

borehole at 12, 15, 17.5, 20, 22.5, and 24.5 meters below LSD 

appear to produce discontinuities for acoustic wave-energy 

propagation along the borehole, creating a diagonal effect 

on the full-waveform sonic VIL near these depths. At about 

12 meters below LSD, the caliper indicates a 10-centimeter 

diameter increase, and the ATV image indicates a large void; 

a similar feature is at 17.5 meters below LSD. Both features 

appear to be karst openings that are transmitting water. The 

EM flowmeter shows upward flow that originates below 24 

meters below LSD and decreases at about 16 meters below 

LSD. The water level continuously declined as the logging 

was in progress and became stable at about 12 meters below 

surface, at the same depth as the upper karst opening. 

Piezometer 106+00C is about 24 meters below LSD and 

is the third piezometer in the 106+00 transect. The piezometer 

was placed 137 meters south of piezometers 106+00A and 

106+00B, which are near the toe of the dam (fig. 2B, appendix 

3). The geophysical logs for piezometer 106+00C indicate 

a decrease in induction resistivity and an increase in natural 

gamma count rate at 10–12.5 meters below LSD, probably in 

response to the large borehole diameter that is likely the result 

of a dissolution cavity. The cavity appears to be about 2 meters 

high on the basis of the caliper log response. The increase 

in natural gamma count rate at 10.5–12.5 meters probably is 

caused by the abundance of clay in the cavity, as described 

in the geologist description. This cavity also was the likely 

exit point of some water added to the borehole. The ATV 

shows an excellent image of the trace of the bottom of this 

cavity at 12.5 meters below LSD. Similar sets of responses 

occur at 13–13.5 meters below LSD at a much smaller scale. 

At 13.5–16 meters below LSD the increase in resistivity, as 

shown on the 16N, 64N, and lateral logs, probably is the result 

of unsaturated, dense limestone (as given in the geologist 

description). Immediately below the relatively high-resistivity 

zone at 16–16.4 meters below LSD, a natural gamma count 

rate increase appears to correspond to a dark trace on the ATV 

that is probably a horizontal opening containing clay. This 

horizontal feature intersecting the borehole appears to produce 

discontinuities for acoustic wave-energy propagation along the 

borehole, creating a diagonal effect on the full-waveform sonic 

VIL at 15.7–16.4 meters below LSD. 

Piezometer 106+00D is about 21.6 meters below LSD 

and is the fourth piezometer in the 106+00 transect. The 

piezometer was placed 275 meters south of piezometers 

106+00A and 106+00B, which are near the toe of the dam 

(fig. 2B; appendix 3). Resistivity logs, including induction, 

normal, and lateral logs, all show some variation in resistivity, with a decrease (in induction resistivity) at 10–12 meters 

below LSD and an increase (in 16N, 64N, and lateral log 

resistivity) at 15–17 meters below LSD. The log responses 

from this piezometer are very similar to those of piezometer 

106+00C. The zones mentioned above correlate with similar 

responses in piezometer 106+00C on the induction resistivity, 

16N/64N/lateral log resistivity, natural gamma, caliper, ATV 

image, and VIL.

The Brite well is located 662 meters southwest of 

piezometer 106+00A and extends about 133 meters below 

LSD (fig. 2B, appendix 3). On March 11, 2006, at the time of 

logging, the well flowed at about 2.8 liters per second from 

the casing at land surface. The well log indicates an interval of 

variable and irregularly increased borehole diameter, probably associated with karst- or fracture-related openings. This 

interval occurs in the McKnight Formation below 119 meters 

below LSD. Consequently, most of the flow that is moving 

up the borehole originates from the McKnight Formation as 

evident in the ambient EM flowmeter measurements, increasing from about 0.2 liter per second (upward) at 126 and 128 

meters below LSD to greater than 3 liters per second (upward) 

at 121.5 meters below LSD. Also, the fluid conductivity shows 

greater variation of fluid conductivity in the McKnight Formation than in the Salmon Peak Formation; fluid conductivity 

ranges from about 1,160 to about 1,190 microseimens per 

centimeter, possibly indicating a mixing of various sources of 

water in the borehole.

Integration of Results

Comprehensive evaluation of surface geophysical DC 

resistivity data, borehole geophysical data, and lithologic 

data were used to establish a relation between low-resistivity 

features identified in the surface geophysical resistivity data 

and zones of increased weathering in the Salmon Peak Formation at the western embankment study site. By evaluating the 

general trend of sinkholes and sections of lithologic data using 

a geodatabase, an area was selected for DC resistivity data 

collection along the eastern embankment. The results of the 

DC resistivity survey were used to identify three expressions 

of resistivity unit 1 (low-resistivity unit) showing similar 

characteristics to those identified at the western embankment 

study site. The eastern expression of resistivity unit 1 was 

20 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
selected for further exploration and piezometric head monitoring. The piezometer 106+00 transect intersects profile 3 

at about 1,140 meters along the profile. Although borehole 

geophysical logs provide highly detailed information proximal 

to the boring, and surface geophysical data provide continuous 

big-picture information, the results of the dipole-dipole array 

and reciprocal Schlumberger array data collected along profile 

3 have been superimposed on the borehole geophysical logs 

for piezometers 105+90B, 105+90A, 106+00B, and 106+00A 

for comparison. Combining the two data sets allows the 

geophysical analyst to identify general log characteristics 

to better understand the hydrogeologic characteristic most 

controlling the changes in resistivity shown in the surface 

geophysical DC resistivity data.

Included in the piezometer 105+90B log is a 6-meter 

section of surface DC resistivity for the dipole-dipole array 

and for the reciprocal Schlumberger array. The surface DC 

resistivity sections are superimposed on the resistivity logs and 

indicate three discernible resistivity units. These three resistivity units can be observed in the dipole-dipole array and in the 

borehole resistivity logs and correlate with resistivity units 

2, 1, and 2, respectively, identified in the western embankment study area (appendix 3). These three resistivity units 

are observed in their entirety in the reciprocal Schlumberger 

array for this piezometer even though there appears to be only 

a slight increase in resistivity near the bottom of the well. In 

the dipole-dipole array, the uppermost expression of resistivity 

unit 2 decreases in resistivity at about 7.5 meters below LSD; 

in the reciprocal Schlumberger array, the decrease in resistivity begins at about 6.5 meters below LSD. This decrease in 

resistivity is the expression of the upper extent of resistivity 

unit 1 and continues downward to the base of resistivity unit 1 

at 34 meters below LSD where resistivity unit 2 appears again. 

The expression of resistivity unit 2 at 34 meters below LSD is 

more evident in the dipole-dipole array as a sharp resistivity 

increase but appears only as a slight increase in resistivity as 

shown in the reciprocal Schumberger array. The delineation of 

resistivity unit 1 is at 7.5–34 meters below LSD in the dipoledipole array and at 6.5–34 meters below LSD in the reciprocal 

Schlumberger array. The lowest DC resistivity values in resistivity unit 1 appear at about 18–22 meters below LSD in the 

dipole-dipole array and at about 15–23.5 meters below LSD in 

the reciprocal Schlumberger array. These lowest values of DC 

resistivity correspond with the zone of lowest resistivity on the 

normal and lateral resistivity logs at 17–22 meters below LSD. 

Included in the piezometer 105+90A log is a 6-meter section of surface DC resistivity for the dipole-dipole array and 

for the reciprocal Schlumberger array. Both arrays indicate a 

single resistive layer (resistivity unit 2) near this piezometer.

The surface DC resistivity sections superimposed on the 

piezometer 106+00B resistivity logs indicate three discernible resistivity units. These three resistivity units, observed 

in the dipole-dipole array and in the borehole resistivity logs, 

correlate with resistivity units 2, 1, and 2, respectively, identified in the western embankment study area (appendix 3). 

The dipole-dipole and reciprocal Schlumberger arrays for the 

surface DC resistivity indicate three resistivity units near this 

piezometer. The three resistivity units consist of resistivity unit 

2 at the top and bottom of the well and a conductive resistivity 

unit 1 in the middle. In both arrays, the uppermost expression 

of resistivity unit 2 decreases in resistivity at about 11 meters 

below LSD and continues downward to the base of resistivity 

unit 1 at 35 meters below LSD, where resistivity unit 2 appears 

again. In the dipole-dipole array, this decrease in resistivity is 

the expression of the upper extent of resistivity unit 1 and continues to about 35 meters below LSD, becoming very resistive 

at about 46 meters below LSD. The expression of resistivity 

unit 2 at 35 meters below LSD is more evident in the dipoledipole array as a sharp resistivity increase but appears only as 

a slight increase in resistivity in the reciprocal Schumberger 

array. The delineation of resistivity unit 1 is at 11–35 meters 

below LSD in the dipole-dipole array and at 11–38 meters 

below LSD in the reciprocal Schlumberger array. The lowest DC resistivity values in resistivity unit 1 appear at about 

18–23.5 meters below LSD in the dipole-dipole array and at 

about 15–23.5 meters below LSD in the reciprocal Schlumberger array. These lowest values of DC resistivity correspond 

with the zone of lowest resistivity on the normal and lateral 

resistivity logs at 17–23 meters below LSD, except for the 

lower void at 10.5–12 meters below LSD. 

Included in the piezometer 106+00A log is a 6-meter section of the surface DC resistivity profile for the dipole-dipole 

array and for the reciprocal Schlumberger array. Both arrays 

indicate two resistivity units near this piezometer. These two 

resistivity units consist of resistivity unit 2 directly above a 

more conductive resistivity unit 1. In the dipole-dipole array, 

the uppermost expression of resistivity unit 2 decreases in 

resistivity at about 12 meters below LSD and contains a highly 

resistive zone at about 4–7 meters below LSD. The uppermost 

expression of resistivity unit 2 in the reciprocal Schlumberger 

array decreases in resistivity at about 12.5 meters below LSD. 

The lowest DC resistivity values in resistivity unit 1 appear at 

about 21.5–24 meters below LSD in the dipole-dipole array 

and from about 21 meters below LSD to below the bottom of 

the piezometer in the reciprocal Schlumberger array. These 

lowest values of DC resistivity do not correspond well to the 

lowest values on the normal resistivity logs nor do the highest values of DC resistivity. A possible reason for this lack of 

agreement is that water was added to the borehole just prior 

to logging, thus the rock and clay surrounding the borehole 

probably were not saturated at the time of logging. The water 

level was declining as water exited the borehole during logging at the void at 12 meters below LSD and then became 

stable. Another explanation might be that the karst system is 

nonhomogeneous or that the DC resistivity data were collected 

20–30 meters south of the piezometer.

The surface DC resistivity sections superimposed on 

the resistivity logs for piezometers 105+90B and 106+00B 

indicate three discernible resistivity units that can be observed 

in the dipole-dipole array and in the borehole resistivity logs. 

These three resistivity units correlate with resistivity units 2, 

1, and 2, respectively, identified in the western embankment 

Geophysical Analysis 21
Table 3. Locations and construction information for observation wells near Amistad Reservoir dam, Val Verde County, Texas, and Coahuila, Mexico. 

[LSD, land surface datum; --, not applicable]

Embankment Piezometer identifier Latitude Longitude

Altitude

of LSD 

(meters 

above

NGVD 88)

Altitude of

top of casing 

(meters 

above

NGVD 88)

Altitude of top

of screened

interval

(meters above

NGVD 88)

Distance

from LSD

to top of

screened

interval

(meters)

Altitude of

bottom of

screened

interval

(meters above

NGVD 88)

Altitude of

bottom of

completed

well

(meters above

NGVD 88)

Length of

screened

interval

(meters)

Total

drilled

depth

below LSD

(meters)

Western LA–12L 29 26.928 101 04.758 333.344 -- 289.34 44 287.34 283.34 2 50Western LA–11 29 26.859 101 04.721 331.143 -- 315.14 16 313.14 311.14 2 20Western LA–10 29 26.786 101 04.686 328.942 -- 316.94 12 314.94 313.94 2 15Western LA–16L 29 26.963 101 04.647 334.734 -- 296.48 38.25 293.73 287.18 2 47.5Western LA–15 29 26.877 101 04.632 331.886 -- 317.88 14 315.88 311.88 2 20Western LA–8L 29 26.877 101 04.861 334.355 -- 294.35 40 292.35 284.35 2 46.5Western LA–7 29 26.816 101 04.808 332.296 -- 317.79 14.5 315.79 312.29 2 20Eastern 105+90B 29 27.160 101 02.363 1335.149 334.13 304.91 30.23 301.86 299.14 3.05 36Eastern 105+90A 29 27.160 101 02.363 1335.149 334.80 328.16 6.98 325.14 324.34 3.02 10.7Eastern 106+00B 29 27.160 101 02.363 1335.149 335.36 290.89 44.25 287.84 286.84 3.05 48.2Eastern 106+00A 29 27.167 101 02.365 1335.149 335.25 314.26 20.88 311.21 309.14 3.05 26Eastern 106+00C 29 27.096 101 02.325 2334.573 335.35 318.25 16.32 315.20 310.57 3.05 24Eastern 106+00D 29 27.037 101 02.288 2333.875 335.36 319.61 14.26 316.56 311.87 3.05 21.6Eastern Brite well 29 26.810 101 02.387 324 -- 314 10 194 276.45 120 133

1 Same altitude used for 105+90B, 105+90A, 106+00B, and 106+00A (Paul Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006).2 From Paul Gibson (U.S. Section, International Boundary and Water Commission, written commun., 2006).

22 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
study area. The delineation of resistivity units 1 and 2 in the 

DC resistivity profiles generally corresponds with low and 

high resistivity zones, respectively, on the normal and lateral 

resistivity logs collected in the nearby piezometers at the time 

of installation. The surface DC resistivity sections superimposed on the resistivity logs for piezometer 106+00A indicate 

two discernible resistivity units that can be observed in the 

dipole-dipole array and that correlate with resistivity units 2 

and 1, respectively; however, these units are not discernible 

on the resistivity logs for piezometer 106+00A. Piezometer 

105+00A was not deep enough to observe both resistivity 

units. 

A geologic characteristic that generally controls changes 

in resistivity is clay that commonly occurs in karst openings. 

This clay often is removed as variable lake levels force water 

through the karst openings at varying pressure heads. 

Eastern Embankment Piezometer 

Placement

The piezometer monitoring network along the eastern 

embankment was designed to function similarly to the western 

embankment network. The spatial location of the piezometers 

and the screened intervals were selected using the various 2D- and 3D-visualizations of lithologic data, borehole 

geophysical logs, and the inversion results of DC resistivity 

data to best match the locations of the screened interval of the 

western embankment piezometers and best satisfy the recommendations of Joint Report of the Technical Advisors of the 

International Boundary and Water Commission Regarding the 

Geotechnical, Electrical, Mechanical & Structural Safety of 

Amistad dam (Ken Breiton, U.S. Section, International Boundary and Water Commission, written commun, 2005). 

Three piezometer nests (LA–8, LA–12, and LA–16) 

on the western embankment each consist of three separate 

piezometers open to zones that reflect deep, medium, and 

shallow hydraulic heads. These nests are at the toe of the dam 

and mark the northernmost piezometers in the three western 

embankment transects (fig. 2A). The piezometer transect constructed on the eastern embankment was designed to match the 

LA–12/LA–11/LA–10 transect on the western embankment in 

regard to hydrogeologic setting, lithologic zones, and similar 

elevations. 

The horizontal location of the eastern embankment 

piezometers was determined by comparing the DC resistivity 

profiles for the two embankments (figs. 10–16). The locations 

of the screened intervals were duplicated by constructing the 

intervals at similar altitudes in some piezometers and similar 

lithologies in others (using core data and geophysical logs). 

Five piezometers were constructed on the eastern embankment 

to match the piezometers in that transect with the following 

piezometer numbers and screened interval rationale:

106+00B—similar screen elevation as LA–12L

105+90B—similar screen elevation as LA–12M

106+00A—same lithologic unit as LA–12L

105+90A—same lithologic unit as LA–12M

106+00C—same lithologic unit as LA–11

106+00D—similar elevation as LA–10 (because same lithologic unit is not present) 

Piezometer LA–12M is a nearby shallow piezometer (depth 

of 34 meters and screened interval of 30 to 32 meters below 

LSD) paired with the deep piezometer LA–12L. Piezometer 

LA–12M was not logged for the reconnaissance study, but 

lithologies determined for piezometer LA–12L are considered 

similar to those in piezometer LA–12M. 

Both USGS and USIBWC personnel agreed upon the 

horizontal and vertical placement of the eastern embankment 

piezometers. Table 3 shows the locations and construction 

information of the piezometers and wells used in the study. 

Schematic drawings of piezometer construction detail are 

included on the geophysical logs in appendix 3. 

The results of the eastern embankment DC resistivity 

survey were used to identify three expressions of low-resistivity (resistivity unit 1) units showing characteristics similar to 

those identified and characterized as weathered limestone at 

the western embankment study site. This information was used 

to select vertical placement of well screens in the new piezometers (table 3).

Summary

Amistad Reservoir dam, which impounds Amistad 

Reservoir on the Rio Grande in Val Verde County, Texas, and 

Coahuila, Mexico, was constructed in the late 1960s for flood 

control and water conservation storage for the benefit of the 

United States and Mexico. Impoundment of water began May 

31, 1968. The dam is operated and maintained jointly by the 

U.S. and Mexico Sections of the International Boundary Water 

Commission (IBWC). Since 1992, numerous sinkholes have 

developed northwest of Amistad Reservoir dam. Increases in 

the discharge of springs south of the dam on the western side 

of the Rio Grande, in Coahuila, Mexico, have been documented. In 1995 the Mexico Section of the IBWC completed 

a study of the western embankment (Coahuila, Mexico) of the 

dam that included surface geophysics, borehole geophysics, 

and installation of piezometers to learn more about subsurface 

conditions. Piezometers were installed in three lines perpendicular to the centerline of the western embankment, with each 

line containing four piezometers. After the 1995 study, the 

centerline of a part of the western embankment was regrouted. 

As part of a 5-year safety inspection in 2005, technical advisors recommended that one line of piezometers be installed on 

the eastern embankment (Val Verde County, Tex.) of the dam 

at identical land-surface altitudes and with depths similar to 

those on the western embankment. The purpose of the piezometers is to compare water levels (potentiometric head) on both 

Summary 23
the western and eastern embankments of Amistad Reservoir 

dam.

To provide technical assistance for the horizontal and 

vertical placement of piezometers on the eastern embankment 

of Amistad Reservoir dam, the U.S. Geological Survey, in 

cooperation with the U.S. Section of the IBWC, conducted a 

study along the western and eastern embankments of Amistad 

Reservoir dam. The study involved an integrated approach 

consisting of historical data review, geodatabase development, 

and borehole and surface geophysical investigations. To 

efficiently compile, store, and compare geologic and geophysical data, a geodatabase was created at the beginning of the 

project and used throughout the project. Data were added to 

the geodatabase as the project progressed. Borehole geophysical logs, both existing and newly acquired, were compiled 

and merged into multiple-property logs for comparison and 

analysis. The results of this approach were analyzed using the 

geodatabase to optimally locate piezometers along one line 

perpendicular to the centerline of the eastern embankment. 

This report presents the geophysical analysis from a reconnaissance-level study conducted on the eastern and western 

embankments of Amistad Reservoir dam on the Rio Grande in 

March 2006 to characterize the near-surface hydrogeology in 

the Salmon Peak Formation to facilitate optimum placement of 

piezometers along the eastern embankment. 

In the western embankment investigation, geologic 

and geophysical characteristics that indicate relatively large 

water-yielding properties of the Salmon Peak Formation were 

identified. The direct-current (DC) resistivity method was 

selected as the surface geophysical reconnaissance technique 

to correlate relatively large water-yielding properties of the 

Salmon Peak Formation, identified from analysis of borehole 

geophysical logs, with variations in subsurface resistivity. The 

reciprocal Schlumberger array and the dipole-dipole array 

were selected as the most applicable DC resistivity arrays. 

Two identifiable resistivity units are present in both the dipoledipole array data and reciprocal Schlumberger array data along 

DC resistivity profiles on both embankments. These resistivity 

units were first identified in profile 1 on the western embankment. Resistivity unit 1, a near-surface, low-resistivity unit, 

generally ranges in resistivity from less than 75 to greater 

than 150 ohm-meters in the reciprocal Schlumberger array 

and from less than 45 to greater than 120 ohm-meters in the 

dipole-dipole array. The deeper, more resistive unit, resistivity 

unit 2, generally ranges in resistivity from about 150 to greater 

than 365 ohm-meters in the reciprocal Schlumberger array and 

from about 120 to greater than 345 ohm-meters in the dipoledipole array. In both resistivity arrays, a vertical high-low 

resistivity boundary between resistivity unit 1 and resistivity 

unit 2 is observed. The presence of mapped sinkholes in the 

reservoir north of the western embankment study area and the 

zone of increased water content, as indicated by the lowest 

neutron log count rates in piezometer LA–12, leads to the 

conclusion that resistivity unit 1 is a preferential flow path 

where surface water from Amistad Reservoir is forced into 

the ground-water system (because of increased head from the 

reservoir). The increased occurrence of weathered limestone 

along this profile also indicates a potential conduit for groundwater movement along resistivity unit 1 identified in profile 1. 

Borehole resistivity logs were collected to relate geophysical 

variations in existing gamma, caliper, and neutron logs, identified as changes in water-yielding properties, to changes in the 

electrical stratigraphy of the subsurface along the DC resistivity profiles as resistivity units 1 and 2.

Resistivity units 1 and 2, as identified on the western embankment, were identified as present on the eastern 

embankment by geophysical correlation. On the eastern 

embankment in both the dipole-dipole array and the reciprocal 

Schlumberger array, resistivity unit 1 is separated into three 

different parts by resistivity unit 2 in profile 3. These resistive features show a similarity to the features in profile 1 on 

the western embankment. In western embankment profile 1, a 

sharp vertical boundary occurs between resistivity unit 1 and 

resistivity unit 2; resistivity unit 1 becomes appreciably thicker 

at that boundary. In eastern embankment profile 3, a vertical 

boundary also occurs between resistivity unit 1 and resistivity 

unit 2, and resistivity unit 1 becomes thicker at that boundary. 

In the eastern embankment investigation, trends in the 

spatial distribution of sinkholes were identified by analyzing historical data from the IBWC and the occurrence of 

weathered zones identified in geologic descriptions of cores 

collected by the IBWC along the eastern embankment. The 

western and eastern expressions of resistivity unit 1 correlate 

with the general trend of sinkholes documented by IBWC in 

the eastern embankment study area. To assess the hydrogeologic characteristics of the Salmon Peak Formation in the eastern embankment study area along DC resistivity profile 3, the 

upper extent of major lithologic features were superimposed 

on DC resistivity profile 3. The thickness and extent of weathered zones do not correlate with the thickness and extent of the 

western expression of resistivity unit 1. There is, however, a 

substantial increase in the thickness of weathered zones in the 

limestone west of DC resistivity profile 3 shown by geologist 

descriptions of borings.

Lithologic data show that the thickness of resistivity unit 1, along its central and eastern expressions on the 

eastern embankment, corresponds generally with the extent 

and thickness of weathered zones in the limestone; however, 

the thickness of alluvial deposits increases along the central 

expression of resistivity unit 1. Because alluvium tends to be 

less resistive than limestone, it is possible that the decrease 

in resistivity along the central expression of resistivity unit 

1 is predominantly influenced by the increased thickness of 

alluvium more so than the underlying weathered limestone. 

Because of the lack of lithologic correlation along the western 

expression and the increased thickness of alluvium along the 

central expression, these features were not chosen for further 

exploration. The zone of concentrated weathered features 

on the eastern end of DC resistivity profile 3 is thickest and 

extends to a greater depth along the eastern end of the section. 

The correlation of surface geophysical DC resistivity, historical lithologic data, and general trend of documented sinkholes 

24  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
along the eastern end of profile 3 were used to justify further 

exploration (drilling of piezometers) in the eastern expression 

of resistivity unit 1.

The piezometer monitoring network along the eastern 

embankment was designed to function similarly to the western 

embankment network. The spatial location of the piezometers 

and the screened intervals were selected using the various 

two- and three-dimensional visualizations of lithologic data, 

borehole geophysical logs, and inversion results of DC resistivity data to best match the locations of the screened intervals 

of the western embankment piezometers and to best satisfy 

the recommendations of Joint Report of the Technical Advisors of the International Boundary and Water Commission. Six 

piezometers were installed on the eastern embankment 

and logged using borehole geophysical techniques including 

electric long (64-inch) and short (16-inch) normal resistivity, spontaneous potential, fluid conductivity, temperature, 

acoustic velocity, full-waveform sonic, acoustic televiewer, 

electromagnetic flowmeter, caliper, and casing collar locator. 

Borehole geophysical logs provide highly detailed information 

proximal to the boring, and surface geophysical data provide 

continuous big-picture information. Combining the borehole 

and surface geophysical results allows the geophysical analyst 

to identify general log characteristics to better understand the 

hydrogeologic characteristic most controlling the changes in 

resistivity shown in the surface geophysical DC resistivity 

data. 

The surface DC resistivity sections superimposed on 

the resistivity logs for piezometers 105+90B and 106+00B 

indicate three discernible resistivity units in the dipole-dipole 

array and in the borehole resistivity logs. These three resistivity units correlate with resistivity units 2, 1, and 2, respectively, as identified in the western embankment study area. 

Resistivity units 1 and 2 in the DC resistivity profiles generally 

correspond with low- and high-resistivity zones, respectively, 

on the normal and lateral resistivity logs collected in the 

nearby piezometers at the time of installation. The surface DC 

resistivity sections superimposed on the resistivity logs for 

piezometer 106+00A indicate two discernible resistivity units 

that can be observed in the dipole-dipole array and that correlate with resistivity units 2 and 1, respectively; however, these 

units are not discernible on the resistivity logs for piezometer 

106+00A. 

References Cited

Advanced Geosciences, Inc., 2006, Instruction manual for 

EarthImager 2D version 2.1.0—Resistivity and IP inversion 

software: Austin, Tex., accessed November 1, 2006, at 

http://www.agiusa.com/files/pub/softwareproducts.shtml

American Society for Testing and Materials, 1999, Standard 

guide for using the direct current resistivity method for 

subsurface investigation: ASTM D 6431–99, 14 p.

American Society for Testing and Materials, 2001, Standard 

guide for conducting borehole geophysical logging—

electromagnetic induction: ASTM standard D 6726–01, 7 p.

American Society for Testing and Materials, 2004a, Standard 

guide for conducting borehole geophysical logging—

Gamma: ASTM D 6274–98, 11 p.

American Society for Testing and Materials, 2004b, Standard 

guide for conducting borehole geophysical logging—

Mechanical caliper: ASTM D 6167–97, 6 p.

American Society for Testing and Materials, 2004c, Standard 

guide for use of the metal detection method for subsurface 

investigation: ASTM D 7046–04, 7 p.

American Society for Testing and Materials, 2004d, Standard 

practice for design and installation of ground water monitoring wells: ASTM D 5092–04, 16 p.

Biot, M.A., 1952, Propagation of elastic waves in a cylindrical 

bore containing a fluid: Journal of Applied Geophysics, 

v. 23, p. 997–1,005.

Boyd, T.M., Introduction to geophysical exploration, DC 

resistivity notes—Resistivity guess, accessed November 3, 

2006, at http://galitzin.mines.edu/INTROGP/

notes_template.jsp?url=RES%2FNOTES%2F1estres.

html&page=DC%20Resistivity%3A%20Notes%3A%20Resi

stivity%20Guess

Burkardt, John, 2005, MATH2070, numerical methods in scientific computing I—LAB #7, accessed January 24, 2007, 

at http://www.scs.fsu.edu/~burkardt/math2070/lab_07.html

Century Geophysical Corp., 2006, accessed August 24, 2006, 

at http:// www.century-geo.com

Dunham, R.J., 1962, Classification of carbonate rocks according to depositional texture, in Classification of Carbonate 

Rocks Symposium: American Association of Petroleum 

Geologists Memoir 1, p. 108–121. 

Fetter, C.W., 1994, Applied hydrogeology (3d ed.): PrenticeHall, Inc., 691 p.

Ford, D.C., and Ewers, R.O., 1978, The development of limestone cave systems in the dimensions of length and depth: 

The Canadian Journal of Earth Sciences, v. 15, p. 1,783–

1,798.

Geonics Limited, 1992, Geonics EM39 borehole conductivity logger operating manual: Mississauga, Ontario, Canada, 

Geonics Limited. 

Geosoft Inc., 2006, Mapping and processing system help topics: Oasis montaj MAPS version 6.3.

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

applied geophysics—Part 3, Electrical conduction and 

electromagnetic induction methods: New York, McGrawHill, p. 384–572.

References Cited 25
Greenwood, Robert, 1956, Submarine volcanic mudflows and 

limestone dikes in the Grayson Formation (Cretaceous) of 

central Texas: Gulf Coast Association of Geological Societies Transactions, v. 6, p. 167–177.

Hearst, J.R., Nelson, P.H., and Paillet, F.L., 2000, Well logging 

for physical properties—A handbook for geophysicists, 

geologists, and engineers (2d ed.): John Wiley, 483 p.

Humphreys, C.H., 1984, Stratigraphy of the Lower Cretaceous (Albian) Salmon Peak Formation of the Maverick 

Basin, south Texas, in Miller, B.C., Smith, C.I., Rose, P.R., 

Humphreys, C.H., and Webster, R.E., eds., Stratigraphy and 

structure of the Maverick Basin and Devils River Trend, 

Lower Cretaceous, southwest Texas: San Antonio Geological Society Guidebook, p. 34–59.

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

Pergamon Press, Inc., 519 p.

Keys, W.S., 1990, Borehole geophysics applied to groundwater investigations: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 2, chap. E2, 150 p.

Keys, W.S., 1997, A practical guide to borehole geophysics 

in environmental investigations: CRC-Lewis Publishers, 

176 p. 

Lapham, W.W., Wilde, F.D., and Koterba, M.T., 1997, Guidelines and standard procedures for studies of ground-water 

quality—Selection and installation of wells, and supporting 

documentation: U.S. Geological Survey Water-Resources 

Investigations Report 96–4233, 110 p.

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

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

http://www.geoelectrical.com

Loke, M.H., Acworth, Ian, and Dahlin, Torleif, 2003, A 

comparison of smooth and blocky inversion methods in 2D 

electrical imaging surveys: Exploration Geophysics, v. 34, 

p. 182–187.

Lozo, F.E., Jr., and Smith, C.I., 1964, Revision of Comanche 

Cretaceous stratigraphic nomenclature, southern Edwards 

Plateau, southwest Texas: Gulf Coast Association of Geological Societies Transactions, v. 14, p. 285–306.

McDougal, R.R., Abraham, J.D., and Bisdorf, R.J., 2004, 

Results of electrical resistivity data collected near the town 

of Guernsey, Platte County, Wyoming: U.S. Geological 

Survey Open-File Report 2004–1095, 7 p.

National Oceanic and Atmospheric Administration, 2006, 

Online Positioning User Service, accessed November 3, 

2006, at http://www.ngs.noaa.gov/OPUS/. 

Paillet, F.L., 1991, Qualitative and quantitative interpretation 

of fracture permeability using acoustic full-waveform logs: 

The Log Analyst, v. 32, no. 3, p. 256–270.

Palmer, A.N., 1984, Geomorphic interpretation of karst features, in LaFleur, R.G., ed., Groundwater as a geomorphic 

agent: London, Allen & Unwin, p. 173–209.

Raymer, L.L., Hunt, E.R., and Gardner, J.S., 1980, An 

improved sonic transit time to porosity transform, in Transactions, Society of Professional Well Log Analysts 21st 

Annual Logging Symposium: Houston, Tex., 12 p.

Reeves, R.D., and Small, T.A., 1973, Ground-water resources 

of Val Verde County, Texas: Texas Water Development 

Board Report 172, 144 p.

Shah, S.D., and Quigley, S.M., 2005, Geodatabase of environmental information for Air Force Plant 4 and Naval Air Station-Joint Reserve Base Carswell Field, Fort Worth, Texas, 

1990–2004: U.S. Geological Survey Open-File Report 

2005–1041, 1 CD. 

Sharma, P.V., 1997, Environmental and engineering geophysics: Cambridge, United Kingdom, Cambridge University 

Press.

Sumner, J.S., 1976, Principles of induced polarization for 

geophysical exploration: Elsevier, 277 p.

Texas Water Development Board, 1971, Engineering data on 

dams and reservoirs in Texas—Part III: Texas Water Development Board Report 126. 

U.S. Army Corps of Engineers, 1995, Geophysical exploration 

for engineering and environmental investigations: Engineering Manual 1110-1-1802, chap. 4, 57 p.

Wylie, M.R.J., 1963, The fundamentals of well log interpretation: New York, Academic Press, 238 p.

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.

26 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
240

250260

270

280290300310320330340

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

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

Spr ni g 1 Spr ni g 2

Spr ni g 3

200 m

200 m

S N

A78E A7 W8

A79E A7 W9

A80E A8 W0

Profi el 1

P

rofi

l

e

2

LA 6-

LA 7-

LA 8- L

LA 1- 0 LA 1- 1

LA 1- 2L

LA 1- 4

LA 1- 5

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

Selected core hole and identifier

Piezometer and identifier—Underlinedidentifier indicates logged well

Limestone

Zone of weathered features in limestone

Alluvium

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone Direct-current resistivity profile

Section trace line

Spring and identifier

EXPLANATION

Figures

Figure 9A. Section showing available lithologic data near direct-current resistivity profile 1.

Figures 27
240

250260

270

280290300310320330340

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

Sinkhole and identifier

Spr ni g 2

Spr ni g 3

GG15

GG17

200 m200 m

W

E

A70E A7 W0

A72E A7 W2

A74E A7 W4

A76E A7 W6

A80E A8 W0

LA 6-

LA 7-

LA 8- L

LA 1- 0

LA 1- 1

LA 1- 2L

LA 1- 4

LA 1- 5

LA 1- 6L

Profile 1

Profile 2

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

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

Selected core hole and identifier

Piezometer and identifier

—Underlined

identifier indicates logged well

Limestone

Zone of weathered features in limestone

Alluvium

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone Direct-current resistivity profile

Section trace line

Spring and identifier

EXPLANATIO

N

Figure 9B. Section showing available lithologic data near direct-current resistivity profile 2.

28  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Figure 10. Robust inverse model results of direct-current resistivity profile 1 (A) reciprocal Schlumberger array and (B) dipole-dipole 

array; and profile 2 (C) reciprocal Schlumberger array (D) and dipole-dipole array. 

A

B

0 100

meters

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

S N

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

-50 0 50 100 150 200 250 300 350 400 450 500

340

320

300

280

260

Resistivity unit 1

Resistivity unit 2

0 100

meters

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

S N

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

-50 0 50 100 150 200 250 300 350 400 450 500

340

320

300

280

260

Resistivity unit 1

Resistivity unit 2

0 100

METERS

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

S N

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

-50 0 50 100 150 200 250 300 350 400 450 500

340

320

300

280

260

Resistivity unit 1

Resistivity unit 2

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

S N

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

-50 0 50 100 150 200 250 300 350 400 450 500

340

320

300

280

260

Resistivity unit 1

Resistivity unit 2

EXPLANATION

EXPLANATION

0 100

METERS

Trace of section shown on figure 9A

Trace of section shown on figure 9A

Figures 29
Figure 10. Continued. 

C

D

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

Resistivity unit 1

Resistivity unit 2

W E

45 345

-50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

340

320

300

280

260

100 0 100

metres

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

Resistivity unit 1

Resistivity unit 2

W E

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

-50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

340

320

300

280

260

-100 0 100

METERS

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

Resistivity unit 1

Resistivity unit 2

W E

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

-50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

340

320

300

280

260

, 

Resistivity unit 1

Resistivity unit 2

less than

129 270

greater than

RESISTIVITY,

IN OHM-METERS

EXPLANATION

EXPLANATION

Trace of section shown on figure 9B

Trace of section shown on figure 9B

-100 0 100

METERS

30 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Spr ni g 1 Spr ni g 2

Spr ni g 3

200 m

200 m

S N

A78E A7 W8

A79E A7 W9

A80E A8 W0

Profi el 1

P

rofi

l

e

2

LA 6-

LA 7-

LA 8- L

LA 1- 0 LA 1- 1

LA 1- 2L

LA 1- 4

LA 1- 5

240250260

270

280290300310320330340

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

less than greater than

45 129 270 345

RESISTIVITY,

IN OHM-METERS

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

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

Direct-current resistivity profile

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone

Section trace line

EXPLANATION

Selected core hole and identifier

Piezometer and identifier—Underlinedidentifier indicates logged well

Spring and identifier

Figure 11A. Section showing lithologic tops from core holes and piezometers superimposed on direct-current resistivity profile 1 dipole-dipole array.

Figures  31
Spr ni g 1

Spr ni g 2

Spr ni g 3

200 m

200 m

S

N

A78E A7 W8

A79E A7 W9

A80E A8 W0

Profi el 1

Profile 2

LA 6-

LA 7-

LA 8- L

LA 1- 0

LA 1- 1

LA 1- 2L

LA 1- 4

LA 1- 5

240250260

270

280290300310320330340

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

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

Direct-current resistivity profile

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone

Section trace line

EXPLANATIO

N

Selected core hole and identifier

Piezometer and identifier

—Underlined

identifier indicates logged well

Spring and identifier

Figure 11B. Section showing lithologic tops from core holes and piezometers superimposed on direct-current resistivity profile 1 reciprocal Schlumberger array.

32 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Spr ni g 2 Spr ni g 3

GG15

GG17

200 m200 m

W E A70E A7 W0

A72E A7 W2 A74E A7 W4 A76E A7 W6 A80E A8 W0

LA 6-

LA 7-

LA 8- L

LA 1- 0

LA 1- 1

LA 1- 2L

LA 1- 4

LA 1- 5

LA 1- 6L

Profile 1

Profile 2

240250260

270

280290300310320330340

300

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

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

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

Direct-current resistivity profile

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone

Section trace line

EXPLANATION

Selected core hole and identifier

Piezometer and identifier—Underlinedidentifier indicates logged well

Spring and identifier

Sinkhole and identifier

Figure 11C. Section showing lithologic tops from core holes and piezometers superimposed on direct-current resistivity profile 2 dipole-dipole array.

Figures  33
Spr ni g 2

Spr ni g 3

GG15

GG17

200 m200 m

W

E

A70E A7 W0

A72E A7 W2

A74E A7 W4

A76E A7 W6

A80E A8 W0

LA 6-

LA 7-

LA 8- L

LA 1- 0

LA 1- 1

LA 1- 2L

LA 1- 4

LA 1- 5

LA 1- 6L

Profile 1

Profile 2

240250260

270

280290300310320330340

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

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

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

Direct-current resistivity profile

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone

Section trace line

EXPLANATIO

N

Selected core hole and identifier

Piezometer and identifier

—Underlined

identifier indicates logged well

Spring and identifier

Sinkhole and identifier

Figure 11D. Section showing lithologic tops from core holes and piezometers superimposed on direct-current resistivity profile 2 reciprocal Schlumberger array.

34  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Centerline of dam

A

LA-16L

LA-15

LA-14

LA-12L

LA-8L

LA-7

LA-6

LA-11

LA-10

A78E A78W

A79E A79W

A80E A80W

Spring 3

Spring 2 Spring 1

EXPLANATION

Limestone

Weathered features located within limestone

Alluvium

Directional core

 and identifier

 Neutron log

and identifier

LA-16L

A78W

Direction of increasing

 counts per second

Spring

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

Figure 12. Three-dimensional image of merged dipole-dipole direct-current resistivity profile at western embankment study site, 

looking (A) southeast and (B) southwest. 

Figures  35
EXPLANATION

Limestone

Weathered features located within limestone

Alluvium

 Neutron log

and identifier

LA-16L

A78W

Direction of increasing

 counts per second

Spring

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

Centerline of dam

B

LA-16L

LA-15

LA-14

LA-12L

LA-8L

LA-7 LA-6

LA-11

LA-10

A78E

A78W

A79E

A79W

A80E A80W

Spring 3

Spring 2

Spring 1

EXPLANATION

Directional core

 and identifier

Figure 12. Continued. 

36 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Centerline of dam

A

LA-16L

LA-15

LA-14

LA-12L

LA-8L

LA-7

LA-6

LA-11

LA-10

A78E A78W

A79E A79W

A80E A80W

Spring 3

Spring 2

Spring 1

EXPLANATION

Limestone

Weathered features located within limestone

Alluvium

Directional core

 and identifier

 Neutron log

and identifier

LA-16L

A78W

Direction of increasing

 counts per second

Spring

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

Figure 13. Three-dimensional image of merged reciprocal Schlumberger direct-current resistivity profile at western embankment 

study site, looking (A) southeast and (B) southwest. 

Figures  37
Limestone

Weathered features located within limestone

Alluvium

Directional core

 and identifier

 Neutron log

and identifier

LA-16L

A78W

Direction of increasing

 counts per second

Spring

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

EXPLANATION

Centerline of dam

B

LA-16L

LA-15

LA-14

LA-12L

LA-8L

LA-7 LA-6

LA-11

LA-10

A78E

A78W

A79E A79W

A80E A80W

Spring 3

Spring 2

Spring 1

Figure 13. Continued. 

38  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
BB 4- 5

BB 1- 4

BB 1- 6

BB 1- 7

BB 2- 2

BB 2- 4

6 4- 7

le 3

A120V A120D

A125D A25E A2 W5 A26V A27E A2 W7 A2 V A29E A2 W9 A30V A31E A3 W1 V

BB 2- 3

BB 2- 5 BB4 BB

BB 4- 8

200 m200 m

W E

Profi

A121V

A121D A123D

A126D

A12V

A12E A1 W2

A136D A13V A13E A1 W3 A14VA14E A1 W4 A15VA15E A1 W5 A16VA16E A1 W6 A17E A1 W7 A19E A1 W9 A21E A2 W1A23E A2 W3 A32 106 00

340330320310300290280270260250240

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

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

General trend of sinkholes

Piezometer and identifier—

Selected core hole and identifier

Sinkhole and identifier

Underlined 

identifier indicates logged well

Alluvium

Zone of weathered features in limestone

Limestone

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone Direct-current resistivity profile

Section trace line

EXPLANATION

Figure 14. Section showing available lithologic data near direct-current resistivity profile 3. 

Figures  39
Figure 15. Robust inverse model results of direct-current resistivity profile 3 (A) reciprocal Schlumberger array and (B) dipole-dipole array. 

A

-100 0 100

METERS

DISTANCE, IN METERS

R S T E M E IN N , IO E V A T E L

8 8 V D N A E O V A B

Resistivity unit 1

Resistivity unit 1

Resistivity unit 1

Resistivity unit 2

W

E

less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1,000 1,050 1,100 1,150 1,200 1,250 1,300 1,350 -50 1,400

340320300280260

EXPLANATION

Trace of section shown on figure 14

B

DISTANCE, IN METERS

R S T E M E IN N , IO E V A T E L

8 8 V D N A E O V A B

Resistivity unit 1

Resistivity unit 1

Resistivity unit 1

Resistivity unit 2

W

E

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

340320300280260

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1,000 1,050 1,100 1,150 1,200 1,250 1,300 1,350 -50 1,400

EXPLANATION

Trace of section shown on figure 14

-100 0 100

METERS

40 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
BB 4- 5

BB 1- 4

BB 1- 6

BB 1- 7

BB 2- 2

BB 2- 4

BB 2- 5 BB46 BB 4- 7

BB 4- 8

200 m200 m

W E

Profile 3

A120V A120D

A121V

A121D A123D A125D

A126D

A12V

A12E A1 W2

A136D A13V A13E A1 W3 A14VA14E A1 W4 A15VA15E A1 W5 A16VA16E A1 W6 A17E A1 W7 A19E A1 W9 A21E A2 W1A23E A2 W3 A25E A2 W5 A26V A27E A2 W7 A28V A29E A2 W9 A30V A31E A3 W1 A32V 106+00B BB 2- 3

less than

45 129 270

greater than

345

RESISTIVITY,

IN OHM-METERS

240250260270280290300310320330340

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

DISTANCE, IN METERS

RS TE ME IN N, IO AT ELEV

88 VD NA E OV AB

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

General trend of sinkholes

Sinkhole and identifier

Selected core hole and identifier

Piezometer and identifier—Underlinedidentifier indicates logged well

Limestone

Zone of weathered features inlimestone

Alluvium

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone Direct-current resistivity profile

Section trace line

EXPLANATION

Figure 16A. Section showing lithologic tops from core holes and piezometers superimposed on direct-current resistivity profile 3 dipole-dipole array.

Figures  41
less than

75 166 256

greater than

365

RESISTIVITY,

IN OHM-METERS

BB 4- 5

BB 1- 4

BB 1- 6

BB 1- 7

BB 2- 2

BB 2- 4

BB 2- 5

BB46

BB 4- 7

BB 4-

200 m200 m

W

E Profile 3

A120V A120D

A121V

A121D A123D

A125D

A126D

A12V

A12E

A1 W2

A136D

A13V

A13E A1 W3

A14V

A14E

A1 W4

A15V

A15E

A1 W5

A16V

A16E A1 W6

A17E

A1 W7

A19E A1 W9

A21E

A2 W1

A23E

A2 W3

A25E A2 W5

A26V

A27E A2 W7

A2 V

A29E

A2 W9

A30V

A31E A3 W1

A32V

106 00

340330320310300290280270260250240

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

DISTANCE, IN METERS

ELEVATION, IN METERS

ABOVE NAVD 88

BB 2- 3

Base from U.S. Department of Agriculture National Agricultural Imagery Digital Ortho Photo ImageUniversal Transverse Mercator projection, Zone 14North American Datum of 1983

General trend of sinkholes

Piezometer and identifier—

Selected core hole and identifier

Sinkhole and identifier

Underlined 

identifier indicates logged well

Alluvium

Zone of weathered features in limestone

Limestone

Top of zone of weathered features in limestone

Top of limestone

Argillaceous fossiliferous zone Direct-current resistivity profile

Section trace line

EXPLANATION

Figure 16B. Section showing lithologic tops from core holes and piezometers superimposed on direct-current resistivity profile 3 reciprocal Schlumberger array.

42 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Appendix 1

Appendix 1—Detailed Descriptions of Borehole Geophysical 

Methods
Blank Page
Electromagnetic Induction Logs

Electromagnetic (EM) induction probes measure conductivity in air- or water-filled holes and perform well in open holes 

or PVC cased holes. The measurement of conductivity commonly is reciprocated to provide logs with curves of both resistivity 

and conductivity (Keys, 1997), as are the logs contained in this report. Conductivity is affected by the salinity of borehole and 

formation fluids and the type of lithology encountered. Generally, pure carbonates, sands, and gravels have lower conductivity 

(thus higher resistivity) than clays or shales (Keys, 1997). A Geonics EM39 induction conductivity probe was used in all wells. 

The EM39 was calibrated twice daily using manufacturer’s recommended procedures (Geonics Limited, 1992) at temperatures 

within the range expected in the boreholes. To attain a stable temperature, the probe was hung in a well for 20–30 minutes prior 

to calibration. During a two-point calibration process, the probe was calibrated (1) to a zero-conductivity environment and (2) to 

a calibration coil of known conductivity with the bottom of the probe at least 3 meters above land surface. The calibration also 

was checked periodically between calibrations. The EM induction conductivity measurements (commonly sensitive to metallic 

conductive objects) were affected at depths corresponding with metal objects such as centralizers and stainless steel screens. 

Natural Gamma Logs

Natural gamma logs provide a record of gamma radiation detected at depth in a borehole. Fine-grained sediments that contain abundant clay tend to be more radioactive than quartz-grain sandstones or carbonates (Keys, 1997). The natural gamma log 

was run in conjunction with the fluid resistivity log and was recorded in natural gamma counts per second simultaneously as the 

induction log was recorded in both cased and open boreholes. A Mount Sopris 2PGA-1000 natural gamma probe with a sodium 

iodide detector was used. The natural gamma probe is calibrated at the factory and does not require calibration in the field. The 

natural gamma and induction logs collectively can be useful to identify lithologies and contact depths of the strata penetrated in 

the borehole. Natural gamma count rates, which commonly will increase near clay and shale, could be slightly increased adjacent to any bentonite seals in the wells.

Electric Logs

Electric logs use a series of electrodes mounted on the downhole probe and a surface electrode in the ground to measure 

potential (or voltage) that varies with the electrical properties of fluids and rock materials. Electric logs require an uncased, 

fluid-filled hole to allow the current to flow into the formation. 

 Normal resistivity methods actually measure apparent resistivity in ohm-meters using two current electrodes at each end of 

the tool and two potential electrodes at two different spacings, 16 inches (short normal) and 64 inches (long normal). A constant 

current is applied to the electrodes by the surface instrumentation regardless of the resistance of the formation. The current goes 

through the borehole fluid and into the formation, and the voltage is measured by the potential electrodes at the two spacings. 

Using Ohm’s law, resistance is computed as resistance (ohms) equals the voltage measured by the potential electrodes divided by 

the constant current (amperes). A spatial component is introduced with the electrode spacing and ohm-meters are the resulting 

unit. About 50 percent of the signal is measured from a radius of investigation that is about twice the distance from the borehole 

as the electrode spacing. Consequently, the long normal measures a larger volume of the subsurface than the short normal. When 

estimating thickness of a lithologic unit, the normal resistivity-curve deflections are subject to error where the thickness of the 

unit is less than the distance of the 16- or 64-inch spacing for the short normal and long normal, respectively. The normal resistivity log curves are usually recorded on a logarithmic scale to minimize the high-resistivity values going off-scale yet providing detail at low-resistivity values. Generally, carbonate rocks have a higher resistivity than sandstone, and sandstone generally 

has higher resistivity than clay or shale. Therefore the normal resistivity logs are useful for determining and correlating various 

lithologies but also are affected by the resistivity of the fluids in the borehole and formation. The normal resistivity probe can be 

calibrated in the field using a special resistor box with various resistor switch positions and cables that connect to the electrodes. 

A detailed explanation of the theory of normal resistivity logging is in Keys (1997). A Century model 8044 multiparameter 

E-log probe was used to measure the normal resistivity and was calibrated prior to logging.

The lateral resistivity is measured with four electrodes as is the normal resistivity, but the electrodes are in a different configuration to focus the current flow away from the borehole into the rocks that surround the (Keys, 1990) borehole. The lateral 

resistivity is in a 48-inch-spacing configuration that actually measures a greater distance from the borehole than the 64-inch 

normal resistivity. The sheetlike current pattern of the lateral log increases the resolution and decreases the effect of adjacent 

beds in comparison with the normal resistivity logs (Keys, 1990). A Century model 8044 multiparameter E-log probe was used 

to measure the lateral resistivity and was calibrated in conjunction with the normal resistivity. 

The spontaneous potential (SP) is one of the oldest logging techniques and uses a very simple method of measuring the 

potentials produced by various salinity conditions (Keys, 1990). SP is a function of the chemistry of fluids in the borehole 

Appendix 1  45
and adjacent rocks, the temperature, and the clay present and is not related directly to porosity and permeability (Keys, 1997). 

Electrochemical effects are the main source of natural spontaneous potentials in most boreholes and result from the migration of 

ions from concentrated to more dilute solutions (Keys, 1997). Clay or shale decreases the mobility of the ions. The SP log curve 

is scaled in millivolts of potential and deflects in a positive or negative direction as the salinity of the formation fluid or borehole 

changes vertically. When the borehole fluid column is fresher than the formation fluid, current flow is such that the SP curve 

deflects in a negative direction; when the borehole fluid column is more saline than the formation fluid, the current flow and 

curve deflection will be reversed (Keys, 1997). Rapid oscillations in a SP curve that is normally smooth are caused by streaming potentials that are commonly a result of water moving in or out of the borehole through a permeable media (Keys, 1997). 

SP logs are susceptible to extraneous effects from stray electrical currents and equipment problems. SP logs that are periodically rerun on the same well will most likely show a difference in log response because any changes of water contained in the 

borehole (drilling fluid or aquifer water), or its depth of invasion into the formation, can cause the log to change (Keys, 1997). A 

Century model 8044 multiparameter E-log probe was used to measure the SP values and was calibrated in conjunction with the 

normal resistivity. 

Caliper Logs

Caliper logs provide a measurement of the diameter of the borehole and are useful in determining changes in borehole 

diameter that can be related to drilling techniques, cavernous formations, lithology, and well construction. The Century model 

no. 7074 three-arm caliper probe used in this study records an average diameter measured by the three arms. The caliper is calibrated by performing a two-point calibration on short sections of pipe where diameters are larger and smaller than the borehole 

sizes that are expected to be encountered.

Fluid Resistivity Logs

Fluid resistivity logs provide a record of the capacity of the borehole fluid to conduct electrical current (Keys, 1990). 

Changes in fluid resistivity are measured by ring electrodes inside a housing that allows borehole fluid to flow through it. Best 

fluid resistivity logging results are achieved when logging downward into boreholes containing ambient water that has had sufficient time to stabilize. Ideally, fluid resistivity logs are run as the first logging run to record the ambient conditions before other 

probes have passed through the borehole and vertically mixed the borehole fluid. Curve deflections on the fluid resistivity log 

can indicate horizontal or vertical flow, stratification of borehole fluid, or screened intervals in cased wells. The fluid resistivity values also can be used in calculations with other logs. The reciprocal of fluid resistivity, fluid conductivity, is shown on the 

logs in this study for comparison to specific conductance values collected at springs. The fluid conductivity values contained in 

the logs for this study are the values recorded at the ambient borehole temperature and are not corrected to a standard temperature. A Century model 8044 multiparameter E-log probe was used to log fluid resistivity on uncased (open) boreholes, and the 

Century model 9042 probe was used to measure fluid resistivity in cased wells. Calibration of the fluid resistivity logging probes 

was done with solutions of known conductivity in a two-point calibration. 

Acoustic Velocity Logs

Acoustic velocity logs record the travel time of pulsed acoustic waves from a transmitter to one or more receivers. The 

acoustic pulse travels through the fluid in the well and through the surrounding rock at a velocity that is related to the lithology 

and porosity of the rocks (Keys, 1997). Consequently, the well must be filled with mud or water to transmit the acoustic energy 

to the borehole wall. The acoustic velocity probe must be centered with bowspring or rubber centralizers to keep the travel path 

of the acoustic energy at a consistent length. The Century model 9320 acoustic probe uses a single transmitter and dual receiver 

system for recording the travel times through the formation. The receivers are spaced at 0.6 and 0.9 meter from the transmitter; 

therefore, a 0.3-meter calculation can be made to measure this interval transit time (delta-T) in microseconds per meter (or foot). 

The acoustic wave train received by the two receivers can be divided into several components; the most important are the 

compressional (P) waves and shear (S) waves. P waves have particle movement in the direction of propagation and have a lower 

amplitude and higher velocity than S waves. S waves have particle movement perpendicular to the direction of propagation 

(Keys, 1997). S waves have a velocity about one-half of that of P waves (Keys, 1997). Generally, acoustic velocity logs record 

the travel time of the P waves, or the first arrival of amplitude. The Century acoustic logging system contains a full analog-todigital conversion of the receiver signals. The processor then scans the sampled data for the first negative arrival below a preset 

threshold for each receiver and calculates the travel time in microseconds per meter (or foot) (Century Geophysical Corp., 2006). 

Acoustic logging data are subject to many sources of error such as large or irregular borehole diameter, poor centralization, 

46 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
cycle skipping, and unsaturated or highly unconsolidated formation material. In cycle skipping, the waveform first arrival at the 

far receiver might be too weak to be detected, and the acquisition program triggers on a later cycle in the waveform. When this 

happens, the recorded travel time is greater by the amount of time between the first peak on the far waveform and the peak that 

is actually detected.

For rocks with uniformly distributed, intergranular-pore spaces, porosity can be derived from the Wylie time-average equation (Wylie, 1963) or the Raymer-Hunt Formula (Raymer and others, 1980). This is usually very difficult in fractured carbonate 

rock.

Full-Waveform Logs

Full-waveform logs, which are sometimes collected simultaneously with acoustic velocity logs, show the waveform of 

the received acoustic signal. The most common presentation of the full-waveform log is the variable intensity log (VIL). On 

a VIL, the intensity of the trace is modulated with the acoustic signal so that a zero-amplitude signal shows a gray area on the 

log; a very strong negative signal shows a white area on the log, and a very strong positive signal shows a darker area on the log 

(Hearst and others, 2000). A common application of a VIL is a qualitative indicator of rock properties in the vicinity of the borehole (Hearst and others, 2000). In unfractured rock, this display represents the waveforms as a series of parallel wavefronts. The 

wavefronts become less distinctive for zones where seismic velocities vary. Nonvertical fractures intersecting the borehole produce large discontinuities for wave-energy propagation along the borehole (Paillet, 1991). Studies indicate that a relation exists 

between fracture permeability and attenuation of the part of the waveform with a velocity about equal to that of the fluid in the 

borehole (Paillet, 1991). Waveforms display several “modes” or propagation patterns; however the attenuation and permeability 

correlation primarily applies to the tube-wave mode (Paillet, 1991). The tube-wave is a guided-wave mode associated with the 

same interface mode of propagation as the Stoneley wave known to exist between a fluid and an elastic solid (Biot, 1952; Paillet, 

1991). 

Acoustic Televiewer

The acoustic televiewer (ATV) is an oriented logging device that can provide a high-resolution, 360-degree image or “cylindrical picture” of the circumference of the borehole that can be used to evaluate secondary porosity features such as fractures 

and solution openings. The ATV image shows the borehole-fracture intersection by scattering acoustic energy such that the 

location of a fracture or other opening appears as a dark line on the ATV log display (Paillet, 1991). The vertical interval over 

which the fracture intersects the borehole is used to determine the orientation of a fracture, and the width of the fracture image in 

the ATV image is a qualitative indicator of fracture aperture (Paillet, 1991). Furthermore, the ATV image also can be used to calculate the strike and dip of planar features such as fractures and bedding planes (Keys, 1997); however, the depth of the fracture 

into the well bore cannot be determined using this method.

Electromagnetic Flowmeter

The EM flowmeter measures the vertical flow rate and direction in a borehole using the principal of Faraday’s Law of EM 

Induction (Century Geophysical Corp., 2006). The EM flowmeter probe consists of an electromagnet and two electrodes 180 

degrees apart and oriented 90 degrees to the magnetic field inside a hollow cylinder or tube. The voltage induced by a conductor moving at right angles through the magnetic field is directly proportional to the velocity of the conductor (water) through the 

field (Century Geophysical Corp., 2006). Generally, when using the tool to measure low-velocity flow, rubber diverters direct the 

water flow through the tube, which is open at both ends, instead of around the tool. Because the diameter of the tube and voltage 

response is calibrated, the volume of flow is instantaneously recorded. The direction of water flow is determined by the polarity 

of the response; upward flow is positive and downward flow is negative.

Appendix 1  47
Blank Page
Appendix 2—Detailed Descriptions of Geophysical Logs
Blank Page
Western Embankment Piezometers

Seven piezometers along the western embankment (LA–12L, LA–11, LA–10, LA–16L, LA–15, LA–8L, and LA–7) were 

logged through casing using borehole geophysical methods in March 2006. The open boreholes were logged at the time of 

drilling in 1995 by the Mexico Section, International Boundary and Water Commission (MxIBWC). Selected logs run in open 

boreholes in 1995 were digitized and are presented with 2006 logs (appendix 3) as denoted in log headers of western embankment piezometers only.

Piezometer LA–12L

Piezometer LA–12L is near the toe at of the dam on the western embankment (fig. 2A) and extends about 50 meters below 

land surface datum (LSD). The well is constructed of polyvinyl chloride (PVC) casing to a depth of 43 meters and is screened 

from about 44 to 50 meters below LSD. The log data collected on March 8, 2006, by the U.S. Geological Survey (USGS) 

include natural gamma, induction resistivity and conductivity, and fluid conductivity. Open borehole geophysical log data were 

collected in 1995 at the time of drilling by MxIBWC and include caliper, natural gamma, gamma-gamma (density), and neutron. 

The geophysical log plots for this well are in appendix 3 and include logs run in 2006 and 1995 and a lithologic description 

translated from the MxIBWC drilling logs (Paul Gibson, U.S. Section, International Boundary and Water Commission, written 

commun., 2006). 

The natural gamma logs collected at the time of drilling in 1995 and the natural gamma log recorded through casing during this study are considerably different, probably because of bentonite or other clay material used in the well completion and 

grouting. The 2006 natural gamma shows a greater count rate than the log run in open hole in 1995 from near-surface to about 

42.5 meters below LSD. However, the two natural gamma logs do show a correlation of clay or shale layers near 8, 12, 16, 21, 

24.5-25, and 32 meters below LSD. The presence of clay material in the grout could have affected the induction conductivity 

and resistivity. Another log artifact derived from well construction is the slight deflection of induction conductivity/resistivity 

and fluid conductivity curves about every 3 meters throughout the well. This response likely is caused by conductive material 

(possibly pipe-thread compound) used in the joints of casing sections. The induction resistivity and conductivity logs show about 

150 ohm-meters, except for the previously mentioned casing joints and shale zones at about 16 and 4–9 meters below LSD, 

which show slightly lower resistivity/higher conductivity measurements. The lack of deflection of resistivity and conductivity 

measurements deeper than 9 meters below LSD possibly could be caused by grout in the annular space shielding the formation. 

The neutron log is an indicator of the presence of hydrogen (Keys, 1990), which absorbs neutrons and reduces the neutron count rate. The neutron log for LA–12L was run in open-hole conditions in 1995. Increased count rates are shown on the 

neutron log at 3–6, 7.5–8.5, and 18.5 meters below LSD because of shale beds at those depths. The neutron log shows relatively 

low count rates from 20 meters to the bottom of the borehole compared with the upper part of the borehole. However, slightly 

increased count rates are at 27–28, 29.5–32, 37–42, and 44.5–48.5 meters below LSD, which also is coincident with lower natural gamma responses, indicating a more pure limestone lithology at those depths. Possible zones of increased water content in 

the formation are at 24–27, 32–36.5, and 41–44 meters below LSD. 

The fluid conductivity log (appendix 3) indicates a sharp increase in conductivity at 44.1 meters below LSD, indicating the 

screened interval from that depth to the deepest point reached during the 2006 logging at about 45.1 meters below LSD. The 

difference between the 2006 logged depth of 45.1 meters below LSD and the well completion record depth of 47 meters below 

LSD could indicate about 3 meters of fill in the bottom of the well.

Piezometer LA–11

Piezometer LA–11 is about 141 meters southeast of piezometer LA–12L on the western embankment (fig. 2A) and extends 

about 20 meters below LSD. The well is constructed of PVC casing to a depth of 16.25 meters and is screened at about 16.25–18 

meters below LSD. The log data collected on March 8, 2006, by USGS include natural gamma, induction resistivity and conductivity, and fluid conductivity. Open borehole geophysical log data were collected in 1995 at the time of drilling by MxIBWC and 

include caliper, natural gamma, gamma-gamma (density), and neutron. The geophysical log plots for this well are in appendix 3 

and include logs run in 2006 and 1995 and a lithologic description translated from the MxIBWC drilling logs (Paul Gibson, U.S. 

Section, International Boundary and Water Commission, written commun., 2006). 

Natural gamma log peaks generally correlate with shale and clay on the lithology log provided by MxIBWC (Paul 

Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006) and correlate well with the 

induction resistivity and conductivity logs and neutron log at 3.5–8 and 14.3–15.3 meters below LSD. Decreased neutron count 

rate and decreased natural gamma count rate at 8–9, 12–13, and 16.8–17.8 meters below LSD are interpreted as possible zones 

of increased water content in the formation. 

Appendix 2  51
The fluid conductivity log indicates a sharp increase in conductivity at 16.3 meters below LSD, indicating the screened 

interval from that depth to the deepest point reached during the 2006 logging at about 18.5 meters below LSD. 

Piezometer LA–10

Piezometer LA–10 is about 286 meters southeast of piezometer LA–12L on the western embankment (fig. 2A) and extends 

about 15 meters below LSD. The well is constructed of PVC casing to a depth of 12 meters and is screened at 12–14 meters 

below LSD. The log data collected by USGS include natural gamma and induction resistivity and conductivity. Open borehole log data were collected at the time of drilling in 1995 by MxIBWC and include caliper, natural gamma, gamma-gamma 

(density), and neutron. The geophysical log plots for this well are in appendix 3 and include logs run in 2006 and 1995 and a 

lithologic description translated from the MxIBWC drilling logs (Paul Gibson, U.S. Section, International Boundary and Water 

Commission, written commun., 2006).

The natural gamma log (2006) shows two zones of increased counts per second at 2.8–3.4 and 10.5–11 meters below LSD 

that correspond to shale beds in the lithologic descriptions and zones of decreased count rates on the neutron log. Neutron count 

rates also are decreased somewhat at 3.5–4.5 and 12.7–14 meters below LSD, intervals that correspond to weathered limestone 

zones. These intervals of decreased neutron count rates that likely contain higher amounts of hydrogen (Keys, 1990) also show 

decreased gamma count rates and are assumed to be water-bearing zones in the limestone. There is little resistivity variation 

on the induction resistivity log; most of the log shows greater than 100 ohm-meters except for the interbedded shale zone at 

2.5–3.5 meters below LSD, where the resistivity decreases to less than 100 ohm-meters. The 2006 induction log also shows a 

slight deflection of induction conductivity/resistivity about every 3 meters throughout the well, which probably is an artifact of 

well construction. This response is similar to that in LA–12L and likely is caused by conductive material (possibly pipe-thread 

compound) used in the joints of casing sections. The 1995 caliper shows a constant diameter of about 6 centimeters in the initial 

borehole. The well currently (2006) has a screened interval at a depth of about 12–14 meters, as shown by the fluid conductivity 

log. The logging tools reached the deepest point at about 14 meters below LSD during the 2006 logging. 

Piezometer LA–16L

Piezometer LA–16L is about 189 meters east-northeast of piezometer LA–12L on the western embankment (fig. 2A) and 

extends about 47.5 meters below LSD. The well is constructed of PVC casing to a depth of 38.25 meters and is screened at 

38.25–40.25 meters below LSD. The log data collected on March 9, 2006, by USGS include natural gamma, induction resistivity 

and conductivity, and fluid conductivity. Open borehole log data were collected at the time of drilling in 1995 by MxIBWC and 

include caliper, natural gamma, gamma-gamma (density), and neutron. The geophysical log plots for this well are in appendix 

3 and include logs collected in 2006 and 1995 and a lithologic description from the MxIBWC drilling logs (Paul Gibson, U.S. 

Section, International Boundary and Water Commission, written commun., 2006).

The natural gamma logs collected at the time of drilling in 1995 and the natural gamma log recorded through casing during 

this study are considerably different, partly because of bentonite or other clay material used in the well completion and grouting. 

The open borehole (1995) natural gamma shows an overall lower count rate (less than 20 counts per second) than all of the other 

open hole natural gamma logs, the explanation for which is unknown but is likely caused by an equipment malfunction. However, both natural gamma logs show a correlation with clay and shale layers per the lithologic description (Paul Gibson, U.S. 

Section, International Boundary and Water Commission, written commun., 2006) from near-surface to 7.5 and about 16 meters 

below LSD. The presence of bentonite in the grout could have affected the induction conductivity and resistivity logs. Another 

log artifact of well construction is the slight deflection of induction conductivity and resistivity and fluid conductivity curves 

about every 3 meters throughout the well. This response likely is caused by conductive material (possibly pipe-thread compound) used in the joints of casing sections. The clay and shale layers at near-surface to 7.5 and about 16 meters below LSD also 

correlate with zones of decreased count rates on the neutron log. However, the shale layer described in the lithologic description 

(Paul Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006) at 7.5–11.5 meters below 

LSD corresponds with a very high neutron count rate (greater than 2,000 counts per second) and a low natural gamma count 

rate (in both the 1995 and 2006 logs), which are uncharacteristic of a shale lithology; but the induction conductivity log shows 

an increase in conductivity (and the resistivity log shows a decrease in resistivity), which indicates more clay or shale present. 

Below the shale layer at 15.5–16.5 meters below LSD, the neutron count rate is decreased at 21–28 and 28.5–30.5 meters below 

LSD, which reflects increased water content in the weathered limestone.

The fluid conductivity log indicates a sharp increase in conductivity at 38.25 meters below LSD, indicating the screened 

interval from that depth to the deepest point reached during the 2006 logging at about 40 meters below LSD. 

52 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Piezometer LA–15

Piezometer LA–15 is about 225 meters south-southeast of piezometer LA–16L on the western embankment (fig. 2A) 

and extends about 20 meters below LSD. The well construction data contained in the drilling records indicate that LA–16L is 

constructed of PVC casing to a depth of 14 meters and is screened at 14–16 meters below LSD (appendix 3). The log data collected on March 10, 2006, by USGS include natural gamma, induction resistivity and conductivity, and fluid conductivity. Open 

borehole log data were collected at the time of drilling in 1995 by MxIBWC and include caliper, natural gamma, gamma-gamma 

(density), and neutron. The geophysical log plots for this well are in appendix 3 and include logs collected in 2006 and 1995 and 

a lithologic description translated from the MxIBWC drilling logs (Paul Gibson, U.S. Section, International Boundary and Water 

Commission, written commun., 2006). 

The natural gamma log run in 2006 shows increased count rate at 1.75–4.5 and 11.3–12.3 meters below LSD, which correspond with shale layers on the lithology log (Paul Gibson, U.S. Section, International Boundary and Water Commission, written 

commun., 2006) along with increased conductivity and decreased resistivity at these depths. The increased natural gamma count 

rate extends from the base of the shale layer at about 12.3 meters below LSD into the top of weathered limestone above the 

screened interval at 13.2 meters below LSD. The neutron log (1995) shows decreased count rates in the shale layers. Below the 

shale layer at 11.3–12.3 meters below LSD, the neutron count rate decreases at 13.5–14.7 and 16–19 meters below LSD, indicating increased water content in the weathered limestone at these depths.

The fluid conductivity log indicates a sharp increase in conductivity at 14.8 meters below LSD, indicating the screened 

interval from that depth to the deepest point reached during the 2006 logging at about 16.5 meters below LSD. The screen interval indicated by the fluid conductivity log is about 0.8 meter deeper than the well completion information indicates.

Piezometer LA–8L

Piezometer LA–8L is about 191 meters west-southwest of piezometer LA–12L on the western embankment (fig. 2A) and 

extends about 46.5 meters below LSD. The well is constructed of PVC casing to a depth of 40 meters and is screened at 40–42 

meters below LSD. The log data collected on March 9, 2006, by USGS include natural gamma, induction resistivity and conductivity. Open borehole log data were collected at the time of drilling in 1995 by MxIBWC and include caliper, natural gamma, 

gamma-gamma (density), and neutron. The geophysical log plots for this well are in appendix 3 and include logs collected in 

2006 and 1995 and a lithologic description from the MxIBWC drilling logs (Paul Gibson, U.S. Section, International Boundary 

and Water Commission, written commun., 2006).

The natural gamma log run in 2006 shows increased count rate at 10–12, 14–15.5, and 37.5–40 meters below LSD. The 

zone at 10–12 meters below LSD corresponds with a shale layer on the lithology log (Paul Gibson, U.S. Section, International 

Boundary and Water Commission, written commun., 2006). Other zones of natural gamma count rate increase do not correspond with shale layers on the lithology log (Paul Gibson, U.S. Section, International Boundary and Water Commission, 

written commun., 2006) and possibly could be caused by a clay-filled void (at 14–15.5 meters below LSD) and a bentonite seal 

(at 37.5–40 meters below LSD). The induction conductivity and resistivity logs both show erratic measurements at 14.2–15.1 

meters below LSD, which probably are caused by some metal in the borehole or well construction. Another log artifact of well 

construction is the slight deflection of induction conductivity and resistivity logs about every 3 meters throughout the well. This 

response likely is caused by conductive material (possibly pipe-thread compound) used in the joints of casing sections. A zone 

of increased induction conductivity and decreased resistivity at 5–9 meters below LSD correlates with gravel, clay, and shale and 

with interbedded shale layers along with a zone of decreased neutron count rate. Other zones of decreased neutron count rate 

appear at 16.5–17.5, 22.4–23.5, 24.2–25.4, 28.8, and 31 meters below LSD and all occur in weathered limestone per the lithologic description (Paul Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006), which 

indicates possible water-bearing zones at these depths.

Piezometer LA–7

Piezometer LA–7 is about 223 meters south-southeast of piezometer LA–8L on the western embankment (fig. 2A) and 

extends about 20 meters below LSD. The well is constructed of PVC casing to a depth of 14.5 meters and is screened at 14.5–

16.5 meters below LSD. The log data collected on March 10, 2006, by USGS include natural gamma, induction resistivity and 

conductivity, and fluid conductivity. Open borehole log data were collected at the time of drilling in 1995 by MxIBWC and 

include caliper, natural gamma, gamma-gamma (density), and neutron. The geophysical log plots for this well are contained in 

appendix 3 and include logs collected in 2006 and 1995 and a lithologic description translated from the MxIBWC drilling logs 

(Paul Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006).

Appendix 2  53
The natural gamma log run in 2006 shows increased count rate at 5–6, 7.7–8.6, and 13.2–14.5 meters below LSD, all of 

which correspond with shale layers on the lithology log (Paul Gibson, U.S. Section, International Boundary and Water Commission, written commun., 2006) and with increased conductivity and decreased resistivity at these intervals. The natural gamma log 

at 13.2–14.5 meters below LSD also is affected by the bentonite seal at that interval. 

The caliper log shows increased borehole diameter at 3.5–6 and 16 meters below LSD and large and small diameter 

responses at 16.9–17.6 meters below LSD. These zones probably are related to karst openings or fractures in the weathered 

limestone. The zone at 16.9–17.6 meters below LSD is unique in that it shows an increase in borehole diameter above and below 

a 0.5-meter zone of borehole that is smaller than bit size. This smaller-diameter zone possibly indicates a swelling clay layer 

within a karst opening. The neutron-log count rate is drastically decreased in this zone, which also indicates clay or waterbearing media, or both.

Eastern Embankment Piezometers

Geophysical logging of the eastern embankment piezometers occurred in March 2006 immediately after drilling was completed and before casing was set. One additional well on a nearby ranch was logged for a better understanding of the underlying 

hydrostratigraphy.

Piezometer 105+90B

Piezometer 105+90B is the deepest of the 105+90 pair of piezometers (fig. 2B), extending about 36 meters below LSD. The 

borehole geophysics data collected in this well (appendix 3) include natural gamma, induction resistivity, 64N and 16N normal 

resistivity, lateral log, SP, caliper, casing collar locator (CCL), fluid conductivity, temperature, acoustic televiewer (ATV), and 

EM flowmeter measurements. Included in the well log is a 6-meter section of surface direct current (DC) resistivity for both 

the dipole-dipole array and reciprocal Schlumberger array. The CCL indicates that the bottom of the steel surface casing is at 

3.75 m. Resistivity logs, including induction, normal, and lateral logs all show little variation in resistivity except for the zone 

at 16.5–21.5 meters below LSD. That zone is described as limestone and limey shale, which generally is less resistive than pure 

limestone. However, the natural gamma does not indicate an abundance of shale (no increased count rate) in this zone. Several 

zones of enlarged borehole diameter are evident on the caliper log, probably related to karstification or fracture openings in the 

limestone. These zones are centered at 6, 8, 12.2, 22.5, 24.5, 27.3, 27.8, and 30.5 meters below LSD. The natural gamma shows 

increased counts per second on all of these caliper enlargements. Increased counts probably are caused by the clay commonly 

contained in karst openings. 

The ATV indicated several different rock units that are easily distinguished by texture. Many of these textural units correspond to the described lithologic units. Also visible in the borehole televiewer are some indications of open dissolution voids 

and bedding planes or low-angle fractures, many of which correspond with caliper enlargements. Some easily identifiable voids 

occur at 7.5–8.5, 12.2, 14.2–14.5, 15, 22.5, 24.5, 25, 27–28.7, 30.3–30.8 meters below LSD. 

The EM flowmeter measurements indicate low upward (+) flow of about 0.006 liter per second (0.1 gallon per minute) over 

nearly the entire length of the borehole. Because the geophysical logging occurred shortly after the hole was drilled, the water 

level was still rising slightly during the EM flowmeter logging; however the largest increase in flow occurs at 35–33 meters 

below LSD (near bottom of borehole), where measurements indicate the beginning of upward flow at about 0.006 liter per 

second (0.1 gallon per minute). Because this flow rate appeared steady above the 35–33-meter interval, the conclusion is that the 

interval is contributing the flow into the borehole. The fluid conductivity curve indicates a slightly lower conductivity (fresher) 

fluid at the bottom of the hole, which probably is aquifer water entering the hole at the bottom and displacing slightly higherconductivity fluid left in the hole from drilling. The temperature curve has a trend similar to that of fluid conductivity, showing a 

slightly lower temperature near the bottom of the borehole. 

The 6-meter surface DC resistivity section superimposed on the resistivity logs indicates three discernible layers that can 

be observed in the dipole-dipole array. The resistivity characteristics of these layers are described in the “Eastern Embankment 

Investigation/Integration of Results” section of the report.

Piezometer 105+90A

Piezometer 105+90A is about 10.7 meters below LSD and is a shallow companion to piezometer 105+90B, which is 3.5 

meters southeast (fig. 2B) . The borehole was filled with fluid from the drill rig water tank to facilitate logging; however the 

water level could not be maintained so some logs were not run. The borehole geophysics data collected in this well include 

natural gamma, induction resistivity, caliper, CCL, fluid conductivity, and temperature (appendix 3). Included in the well log is a 

6-meter section of surface DC resistivity for both the dipole-dipole array and reciprocal Schlumberger array. The CCL indicates 

54  Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
the bottom of steel surface casing at about 4 meters below LSD. The natural gamma and induction resistivity logs closely resemble those of piezometer 105+90B, showing similar natural gamma peaks at 3.3, 6, and 7.9 meters below LSD and similar induction resistivity peaks about 7–7.25 meters below LSD. The caliper log shows zones similar to those of 105+90B—increased 

borehole diameter at 6 and 8 meters below LSD, which indicates possible continuous openings at those depths between piezometers 105+90A and 105+90B. The coincidence of natural gamma peaks and caliper anomalies in the two wells indicates clay is 

present in the karst openings of both. The fluid conductivity log indicates increased conductivity of the borehole fluid compared 

to that of piezometer 105+90B; this likely is because the source of the borehole fluid in piezometer 105+90B was the drill rig 

water tank, which was filled from a local well. However, a decrease in fluid conductivity was noted at about 9 meters below 

LSD, which could indicate fresher inflow into the borehole from the aquifer. Both the dipole-dipole array and the reciprocal Schlumberger array for the surface DC resistivity show a single resistive layer near this well.

Piezometer 106+00B

Piezometer 106+00B is the deepest of the 106+00 pair of piezometers and the deepest of the USGS-drilled wells, extending about 48 meters below LSD (fig. 2B). The borehole geophysics data collected in this well include natural gamma, induction resistivity, 64N and 16N normal resistivity, lateral, SP, caliper, CCL, fluid conductivity, temperature, acoustic velocity, 

full-waveform/variable intensity log (VIL), ATV, and EM flowmeter measurements (appendix 3). Included in the well log is a 

6-meter section of surface DC resistivity for both the dipole-dipole array and reciprocal Schlumberger array. The CCL indicates 

that the bottom of the steel surface casing is at 4.5 meters below LSD. 

Resistivity logs, including induction, normal, and lateral, all show little variation in resistivity except for the zones at 16.5–

21 meters below LSD (similar to variation in logs of piezometer 105+90B) and at 10–12.5 meters below LSD, which show slight 

decreases in resistivity. The 64N resistivity log curve shows greater resistivity than the 16N and lateral log at the 10–12.5-meter 

zone, which indicates that the 16N and lateral log probably are being influenced by borehole fluid in the area of the formation 

closest to the borehole wall. The zone at 16.5–21 meters below LSD shows a smaller decrease in resistivity than the similarly 

located resistivity decrease in piezometer 105+90B, which is 3 meters from piezometer 106+00B. The lithologic description of 

this zone in well 106+00B does not note limey shale as does the description of the less resistant zone in piezometer 105+90B, 

but does note hard clay layers and a decrease in clay content with depth. The natural gamma does not indicate an abundance of 

shale or clay (no increased count rate) at 16.5–21 meters below LSD on either log. 

A few zones of enlarged borehole diameter are evident on the caliper log, probably related to dissolution or fracture openings in the limestone. These zones are described in the “Eastern Embankment Investigation/Borehole Geophysical Log Analysis” section of the report. 

The ambient EM flowmeter measurements indicate a consistent upward flow of about 0.006 liter per second (0.1 gallon 

per minute) from the bottom of the borehole to about 12.5 meters below LSD, where it decreases slightly. The presence of clay 

could be restricting flow through some of the openings. 

Computer-generated virtual core analysis using the caliper and ATV data of zones not showing natural gamma peaks indicates several additional zones of fractures and or dissolution openings at 15–16, 19.7, 21–21.5, 22.5, 27.25, 34.5–35.5, 38–39, 

and 45.5–48 meters below LSD. The horizontal features intersecting the borehole at 22.5, 24.3, and 30.6 appear to produce discontinuities for acoustic wave-energy propagation along the borehole, creating a diagonal effect on the acoustic full-waveform 

VIL near these depths.

The surface DC resistivity sections superimposed on the piezometer 106+00B resistivity logs indicate three discernible 

layers that can be observed in the dipole-dipole array. The resistivity characteristics of these layers are described in the “Eastern 

Embankment Investigation/Integration of Results” section of the report. 

Piezometer 106+00A

Piezometer 106+00A is about 26 meters below LSD and is a shallow companion to piezometer 106+00B, which is 3.5 

meters southeast (fig. 2B). The borehole geophysical data collected in this well include natural gamma, induction resistivity, 64N 

and 16N normal resistivity, lateral log, SP, caliper, CCL, temperature, acoustic velocity, full waveform/VIL, ATV, and EM flowmeter measurements (appendix 3). Fluid conductivity was not collected on this borehole because of tool malfunction. Included 

in the well log is a 6 meter section of surface DC resistivity for both the dipole-dipole array and reciprocal Schlumberger array. 

The CCL indicates that the bottom of the steel surface casing is at 4.5 m. 

Resistivity logs, including induction, normal, and lateral, all show some variation in resistivity. The variations, and other 

characteristics of logs that provide subsurface information, are described in the “Eastern Embankment Investigation/Borehole 

Geophysical Log Analysis” section of the report.

Appendix 2  55
Both the dipole-dipole array and the reciprocal Schlumberger array for the surface DC resistivity indicate a two-layer model 

near this well. The resistivity characteristics of the layers are described in the “Eastern Embankment Investigation/Integration of 

Results” section of the report.

Piezometer 106+00C

Piezometer 106+00C is about 24 meters below LSD and is the third piezometer in the 106+00 transect. The well was 

installed 137 meters south of piezometers 106+00A and 106+00B, which are near the toe of the dam (fig. 2B). The log data 

include natural gamma, induction resistivity, 64N and 16N normal resistivity, lateral, SP, caliper, CCL, temperature, acoustic 

velocity, full-waveform/VIL, and ATV (appendix 3). Fluid conductivity was not collected on this borehole because of tool malfunction. The CCL indicates that the bottom of the steel surface casing is at 4.5 meters below LSD. This borehole was mostly 

dry immediately after drilling, so water was added to the borehole to facilitate logging. With water added, the borehole would 

only sustain a water level about 11.5 meters below LSD and would not fill above this level. Resistivity logs, including induction, 

normal and lateral logs all show some variation in resistivity, the most prominent of which was a decrease (in induction resistivity) at 10–13.5 meters below LSD and an increase (in 16N, 64N, and lateral log resistivity) at 13.5–16 meters below LSD. 

At 10–12.5 meters below LSD, a decrease in induction resistivity and increase in natural gamma counts per second probably are associated with the large borehole diameter that likely is the result of a dissolution cavity that appears to be about 2 

meters high, per the caliper log response. The increase in natural gamma counts per second from 10.5 to 12.5 meters below 

LSD probably is caused by the abundance of clay in the cavity, as noted in the lithologic description. This cavity also was the 

likely exit point of some water added to the borehole, as previously noted. The ATV shows an excellent image of the trace of the 

bottom of this cavity at 12.5 meters below LSD. A similar set of responses, although at a much smaller scale, occurs at 13–13.5 

meters below LSD.

At 13.5–16 meters below LSD, the increases in resistivity, as shown on the 16N, 64N, and lateral logs probably are the 

result of unsaturated dense, limestone (as noted in the lithologic description). Immediately below the higher-resistivity zone at 

16–16.4 meters below LSD, a natural gamma count increase appears to correspond to a dark trace on the ATV that probably is 

a horizontal opening that contains clay. This horizontal feature intersecting the borehole appears to produce discontinuities for 

acoustic wave-energy propagation along the borehole, creating a diagonal effect on the acoustic full-waveform VIL at 15.7–16.4 

meters below LSD. 

At 16.5–21 meters below LSD, the ATV image shows a vertical striped effect that possibly is the result of hammer-bit 

drilling impacts that caused some rugosity of the borehole wall. This borehole rugosity is a possible cause of the erratic delta-T 

fluctuations for acoustic velocity in this interval that likely are erroneous responses.

Piezometer 106+00D

Piezometer 106+00D is about 21.6 meters below LSD and is the fourth piezometer in the 106+00 transect. The well was 

installed 275 meters south of piezometers 106+00A and 106+00B, which are near the toe of the dam (fig. 2B). The log data 

include natural gamma, induction resistivity, 64N and 16N normal resistivity, lateral, SP, caliper, CCL, temperature, full-waveform/VIL, and ATV (appendix 3). Fluid conductivity and acoustic velocity were not collected on this borehole because of tool 

malfunction. The CCL indicates that the bottom of the steel surface casing is at 4.5 meters below LSD. This borehole was 

mostly dry immediately after drilling, so water was added to the borehole to facilitate logging. With water added, the borehole 

would only sustain a water level about 13.5 meters below LSD and would not fill above this level. Resistivity logs, including 

induction, normal, and lateral, all show some variation in resistivity, the most prominent of which was a decrease (in induction 

resistivity) at 10–12 meters below LSD and an increase (in 16N, 64N, and lateral log resistivity) at 15–17 meters below LSD. 

The log responses from this well are very similar to those of piezometer 106+00C. The zones noted above correlate with similar 

zones and responses in piezometer 106+00C on the induction resistivity, 16N/64N/lateral log resistivity, natural gamma, caliper, 

ATV image, and VIL.

Brite Well

The Brite well is 662 meters southwest of piezometer 106+00A and extends about 133 meters below LSD (fig. 2B). The 

well is cased with steel casing to a depth of 9 meters and is open to the formation below that depth. The log data collected on 

March 11, 2006, by USGS include natural gamma, induction resistivity, 16N/64N/lateral log resistivity, SP, caliper, acoustic 

travel time (delta-T), fluid conductivity, and ambient EM flowmeter measurements (appendix 3). The Brite well was drilled 

into the Salmon Peak and McKnight Formations in 1965 as a water well for stock use. IBWC water-level data indicate a water 

level of about 44 meters below LSD in July 1968, which rose to 2.71 meters below LSD in November 1969 in response to the 

56 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
reservoir filling. At the time of logging on March 11, 2006, the well flowed at about 2.8 liters per second from the casing at land 

surface. 

The natural gamma log indicates that most of the borehole is drilled through rocks containing very little shale or clay (low 

count rate) with the exception of scattered, thin shale layers at the following depths (in m) below LSD showing greater count 

rates: 5.8–6.6 (probably clay), 10.3, 25.7, 28.7, 32.3, 35, 36.2, 40–41, 44.8, 50.4, 51.6, 53.7, 56, 57.5, 69.9–76.5 (interbedded 

shale), 79, 80.5, 82.7, 85.7, 92.7, 94–94.5, 101–104, 106, and 113. The induction resistivity and 16N/64N/lateral log resistivity 

logs show slight decreases in resistivity associated with most of these shale layers. The upper McKnight Formation generally 

contains anhydrites, fecal pelleted packstone and organic-rich, laminated mudstone with thin layers of oyster shells, all of which 

can increase the natural gamma count rate. Increased count rates on the natural gamma occurred at depths below the top of the 

McKnight at 118.5 meters below LSD continuing to the bottom of the hole at 134 meters below LSD. The acoustic travel time 

(delta-T) was post-processed to reduce cycle skips and mostly varies between 147 and 197 microseconds per meter (45 and 60 

microseconds per foot), which is consistent with limestone of low porosity. The acoustic velocity tool could not be lowered into 

the borehole past 109 meters below LSD because of an obstruction.

The caliper log shows casing to be about 17 centimeters in diameter and the bottom of casing at 9 meters below LSD. 

Below the bottom of casing in the Salmon Peak Formation, the diameter generally is representative of bit size, which appears to 

be about 22 centimeters. An interval of variable and irregularly increased borehole diameter, which probably is associated with 

karst evaporite dissolution or fracture-related openings, occurs in the McKnight Formation below 119 meters below LSD. Consequently, most of the flow that is moving up the borehole originates from the McKnight Formation as evident on the ambient 

EM flowmeter measurements, which increase from about 0.22 liter per second (upward) at 126 and 128 meters below LSD to 

greater than 3 liters per second (48 gallons per minute) (upward) at 121.5 meters below LSD. Also, the fluid conductivity shows 

a greater variability in salinity in the McKnight Formation with fluid conductivity ranging from about 1,160 to 1,190 microseimens per centimeter, possibly indicating a mixing of various sources of water in the borehole.

Appendix 2  57
Blank Page
Appendix 4 Appendix 3—Borehole Geophysical Logs
Blank Page
Appendix 3 contains borehole geophysical logs collected March 8, 9, 11, 19, and 22, 2006, in Val Verde County, Tex., and 

Coahuila, Mexico. The logs collected in Coahuilla also contain some log data collected in 1995 (as marked on logs).

The log data identifiers are located at the top and bottom of each log as follows:

GAM(NAT)—natural gamma

SP—spontaneous potential

BHC-DELT—acoustic travel time or delta-T

DT—acoustic travel time or delta-T

CALIPER—borehole diameter

CCL—casing collar locator

FLUID COND—fluid conductivity at borehole temperature

TEMP—borehole temperature

Ambient flow—vertical flow in the borehole (+, upward; -, downward)

RES(16N) —16-inch normal resistivity (short normal)

RES(64N)—64-inch normal resistivity (long normal)

LATERAL—lateral resistivity

I. Res.—induction resistivity

AMP(F) (MV)—variability intensity log of acoustic amplitude of the far detector (in millivolts)

Acoustic Televiewer—acoustic televiewer (ATV) image

D-C Resistivity, dipole-dipole—surface geophysical DC resistivity (dipole-dipole array)

D-C Resistivity, rec. schl.—surface geophysical DC resistivity (reciprocal Schlumberger array)

NOTE: All borehole geophysical logs available at http://pubs.usgs.gov/sir/2007/5143/5143-app.html

Appendix 3  61
Blank Page
Appendix 4—Online Positioning User System (OPUS) Solutions 

of Global Positioning System Data
Blank Page
4–1. OPUS Base 1

FILE: 18200700.dat 000446317

 NGS OPUS SOLUTION REPORT

 ========================

 USER: U.S. Geological Survey DATE: March 13, 2006

RINEX FILE: 1820070u.06o TIME: 02:46:57 UTC

 SOFTWARE: page5 0601.10 master30.pl START: 2006/03/11 20:13:00

EPHEMERIS: igr13656.eph [rapid] STOP: 2006/03/11 23:26:30

 NAV FILE: brdc0700.06n OBS USED: 8000 / 8164 : 98%

 ANT NAME: TRM5800 NONE # FIXED AMB: 42 / 42 : 100%

ARP HEIGHT: 2.250 OVERALL RMS: 0.016(m)

REF FRAME: ITRF00(EPOCH:2004.0000) ITRF00 (EPOCH:2006.1915)

 X: -1068140.677(m) 0.016(m) -1068140.707(m) 0.016(m)

 Y: -5455267.761(m) 0.033(m) -5455267.763(m) 0.033(m)

 Z: 3117407.122(m) 0.012(m) 3117407.110(m) 0.012(m)

 LAT: 29 26 53.46040 0.009(m) 29 26 53.45994 0.009(m)

 E LON: 258 55 17.93172 0.010(m) 258 55 17.93064 0.010(m)

 W LON: 101 4 42.06828 0.010(m) 101 4 42.06936 0.010(m)

 EL HGT: 310.363(m) 0.035(m) 310.364(m) 0.035(m)

 UTM COORDINATES

 UTM (Zone 14)

Northing (Y) [meters] 3259440.074

Easting (X) [meters] 298425.036

Convergence [degrees] -1.02213848

Point Scale 1.00010139

Combined Factor 0.99962747

 BASE STATIONS USED

PID DESIGNATION LATITUDE LONGITUDE DISTANCE(m)

DH3844 TXSN SANDERSON CORS ARP N300909.264 W1022434.121 150536.6

DG6566 TXDR DEL RIO CORS ARP N292151.860 W1005358.200 19686.2

DH3842 TXFR FREDERICKSBURG CORS ARP N301445.496 W0985048.428 233068.3

This position and the above vector components were computed without any

knowledge by the Instituto de Estadistica, Geografia e Informatica (INEGI)

and the National Geodetic Survey (NGS) regarding the equipment or field

operating procedures used.

Appendix 4  65
4–2. OPUS Base 2

FILE: 59980701.dat 000446316

 NGS OPUS SOLUTION REPORT

 ========================

 USER: U.S. Geological Survey DATE: March 13, 2006

RINEX FILE: 5998070t.06o TIME: 02:46:41 UTC

 SOFTWARE: page5 0601.10 master28.pl START: 2006/03/11 19:52:00

EPHEMERIS: igr13656.eph [rapid] STOP: 2006/03/11 23:19:30

 NAV FILE: brdc0700.06n OBS USED: 8625 / 8733 : 99%

 ANT NAME: TRM5800 NONE # FIXED AMB: 36 / 36 : 100%

ARP HEIGHT: 1.669 OVERALL RMS: 0.016(m)

REF FRAME: ITRF00(EPOCH:2004.0000) ITRF00 (EPOCH:2006.1915)

 X: -1068306.719(m) 0.014(m) -1068306.749(m) 0.014(m)

 Y: -5455307.685(m) 0.029(m) -5455307.687(m) 0.029(m)

 Z: 3117277.762(m) 0.008(m) 3117277.750(m) 0.008(m)

 LAT: 29 26 48.66697 0.010(m) 29 26 48.66651 0.010(m)

 E LON: 258 55 12.17016 0.008(m) 258 55 12.16909 0.008(m)

 W LON: 101 4 47.82984 0.008(m) 101 4 47.83091 0.008(m)

 EL HGT: 308.670(m) 0.030(m) 308.671(m) 0.030(m)

 UTM COORDINATES

 UTM (Zone 14)

Northing (Y) [meters] 3259295.265

Easting (X) [meters] 298267.143

Convergence [degrees] -1.02288401

Point Scale 1.00010218

Combined Factor 0.99958509

 BASE STATIONS USED

PID DESIGNATION LATITUDE LONGITUDE DISTANCE(m)

DH3844 TXSN SANDERSON CORS ARP N300909.264 W1022434.121 150481.7

DG6566 TXDR DEL RIO CORS ARP N292151.860 W1005358.200 19754.7

DH3842 TXFR FREDERICKSBURG CORS ARP N301445.496 W0985048.428 233268.7

This position and the above vector components were computed without any

knowledge by the Instituto de Estadistica, Geografia e Informatica (INEGI)

and the National Geodetic Survey (NGS) regarding the equipment or field

operating procedures used.

66 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico
Appendix 5—Parameters Used for Inversion Process of 

Direct-Current Resistivity Data
Blank Page
[Startup]

Xpos=-4

Ypos=-4

Width=1288

Height=1002

State=2

FontName=Times New Roman

FontSize=10

LastProjDir=S:\Projects\CG600_Amistad\Data\DCRes_Geophysical_Data\Processed_Data\EarthImager_110106\Line1\rSch

LastLineDir=S:\Projects\CG600_Amistad\Data\DCRes_Geophysical_Data\Processed_Data\EarthImager_110106\Line1\rSch\L1_rS_r0

LastTrialDir=S:\Projects\CG600_Amistad\Data\DCRes_Geophysical_Data\Processed_Data\EarthImager_110106\Line1\rSch\L1_rS_r0\trial1

LastDataFile=S:\Projects\CG600_Amistad\Data\DCRes_Geophysical_Data\Processed_Data\EarthImager_110106\Line1\rSch\L1_rS_r0.dat

MetersFeet=Meters

JobCodeK=Job Code

ProjSiteK=Project Site

ApprovedByK=Approved By

SurveyDateK=Survey Date

InstrumentK=Instrument

InvSoftwareK=Software

DataFileK=Data File

EmployerName=U.S. Geological Survey

JobCode=CG602

ProjSite=Amistad

ApprovedBy=Wade H. Kress

Instrument=Syscal Pro

IPUnit=0

[Initial]

MinVoltage=1

MinVoverI=1E-5

MinAppRes=1

MaxAppRes=100000

MaxRepeatErr=3

MaxRecipErr=5

RemoveNegERT=0

KeepAllData=1

ExcludeEdges=0

InvMethod=2

VerticalAxis=0

YAxis=0

MinElecSepX=0.01

MinElecSepZ=0.01

ContourLines=0

BlnAspectRatio=1

[Forward]

ForwModMeth=1

ForwSolver=0

BCType=1

ForwAccuracy=1

Anisotropy=1

ElemDivision=2

ThickFactor=1.1

DepthFactor=1.1

ForwCGIter=100

ForwCGResid=1E-6

[ResInv]

Appendix 5  69
MaxNumInvIter=10

MaxRMSRes=1

MinErrReduction=5

StopOnMaxIter=1

StopOnMaxRMS=1

StopOnMinErrDiff=1

DataReweight=0

UseRecipErr=0

StopOnL2Norm=1

Lagrange=10

RoughConditioner=0.2

ResStartID=0

StartRes=97.21

MinResis=1

MaxResis=100000

ParameterWidth=1

ParameterHeight=1

HVRoughRatio=0.5

ResNoisePC=3

MaxNumIterInvCG=10

DampFactorRes=10

EpsilonD=1

EpsilonM=1

QuasiNewton=20

[IPInv]

IPInvMethod=0

MaxNumInvIterIP=8

MaxRMSIP=3

MinErrReductionIP=5

StopOnMaxIterIP=1

StopOnMaxRMSIP=1

StopOnMinErrDiffIP=0

DataReweightIP=1

StopOnL2NormIP=1

MinIP=-0.2

MaxIP=0.5

IPNoisePC=3

IPNoiseMin=-0.2

IPNoiseMax=0.5

MinIPCorr=0.5

RemoveNegIP=0

Positivity=0

LagrangeIP=10

DampFactorIP=10

IPWinStart=1

IPWinEnd=6

IPStartID=1

StartIP=0.01

[Terrain]

MeshXformMethod=0

Prepared by the USGS Lafayette Publishing Service Center.

Information regarding water resources in Texas is available at 

http: //tx.usgs.gov/

70 Geophysical Analysis of the Salmon Peak Formation, Val Verde County, Texas, and Coahuila, Mexico