Geophysical Delineation of the Freshwater/

Saline-Water Transition Zone in the Barton 

Springs Segment of the Edwards Aquifer, 

Travis and Hays Counties, Texas, 

September 2006

By Jason D. Payne, Wade H. Kress, Sachin D. Shah, James E. Stefanov, 

Brian A. Smith, and Brian B. Hunt 

In cooperation with the Barton Springs/Edwards Aquifer Conservation District 

Scientific Investigations Report 2007–5244

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

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Suggested citation:

Payne, J.D., Kress, W.H., Shah, S.D., Stefanov, J.E., Smith, B.A., and Hunt, B.B., 2007, Geophysical delineation of 

the freshwater/saline-water transition zone in the Barton Springs segment of the Edwards aquifer, Travis and Hays 

Counties, Texas, September 2006: U.S. Geological Survey Scientific Investigations Report 2007–5244, 21 p., 

6 appendixes.
iii

Contents

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

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

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

Previous Investigations .......................................................................................................................3

Description of the Barton Springs Segment of the Edwards Aquifer, Confining Units, 

and Study Area .......................................................................................................................5

Acknowledgments ...............................................................................................................................5

Methods and Assessment of Data .............................................................................................................6

Geodatabase Development ................................................................................................................6

Geologic Sections ................................................................................................................................6

Site Selection ........................................................................................................................................6

Time-Domain Electromagnetic Sounding ........................................................................................6

Borehole Geophysics ..........................................................................................................................9

Data Processing and Quality .............................................................................................................9

Inverse Modeling ...............................................................................................................................12

Delineation of Transition Zone Using Time-Domain Electromagnetic Soundings ...........................13

Profile 2 ................................................................................................................................................13

Profile 3 ................................................................................................................................................19

Profile 4 ................................................................................................................................................19

Profile 5 ................................................................................................................................................19

Evaluation of Findings .......................................................................................................................20

Summary and Conclusions ........................................................................................................................20

References....................................................................................................................................................21

Appendix 1—Locations and Dissolved Solids Concentrations of Existing Wells ......................... 1–1

Appendix 2—Geophysical Borehole Logs, Swinney Well ................................................................ 2–1

Appendix 3—Global Positioning System Online User Service (OPUS) Solutions ......................... 3–1

Appendix 4—Raw Time-Domain Electromagnetic Voltages and Computed Standard 

Deviations .................................................................................................................................... 4–1

Appendix 5—Graphs Showing Time-Domain Electromagnetic Resistivity From Field 

Measurements as a Function of Time and Inverse Modeling Results (Smooth 

Models) ........................................................................................................................................ 5–1

Appendix 6—Datasets of Raw and Processed Time-Domain Electromagnetic Data .................. 6–1

Figures

1–4. Maps showing:

1. Location of Barton Springs segment of the Edwards aquifer and study area, 

Travis and Hays Counties, Texas ......................................................................................2

2. Locations of time-domain electromagnetic (TDEM) sounding sites and wells, 

Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, 

Texas .....................................................................................................................................4
iv

3. Location of freshwater/saline-water transition zone, Barton Springs segment 

of the Edwards aquifer, Travis and Hays Counties, Texas ...........................................7

4. Locations of geologic sections and time-domain electromagnetic (TDEM) 

sounding profiles, Barton Springs segment of the Edwards aquifer, Travis and 

Hays Counties, Texas .........................................................................................................8

5. Geologic sections of the Barton Springs segment of the Edwards aquifer, Travis 

and Hays Counties, Texas .........................................................................................................10

7. Geologic and gamma-ray geophysical logs, test well 58–58–213, Travis County, 

Texas ............................................................................................................................................13

8–12. Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas 

(A) plan view of distribution of dissolved solids concentrations from previously 

collected data, (B) nearby geologic section interpreted from previously collected 

data and overlain by smooth inversion models of resistivity at each sounding, and 

(C) resistivity section from time-domain electromagnetic (TDEM) soundings

8. Profile 2 ...............................................................................................................................14

9. Profile 3 ...............................................................................................................................15

10. Profile 4.1 ............................................................................................................................16

11. Profile 4.2 ............................................................................................................................17

12. Profile 5 ...............................................................................................................................18

Tables

1. Location data for time-domain electromagnetic soundings used for this report, 

Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas ........3

2. Correlation between hydrogeologic and stratigraphic units associated with the 

Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas ........5

3. Root mean square (RMS) error between measured and calculated apparent 

resistivity for soundings, Barton Springs segment of the Edwards aquifer, Travis 

and Hays Counties, Texas .........................................................................................................19
v

Conversion Factors, Abbreviations, and Datums

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.02832 square meter (m2)

SI to Inch/Pound

Multiply By To obtain

Length

meter (km) 3.281 foot (ft)

Area

square meter (m2) 10.76 square foot (ft2)

Abbreviations

BSEACD, Barton Springs/Edwards Aquifer Conservation District

DS, dissolved solids

GIS, geographic information system

milligrams per liter, mg/L

ohm-meter, ohm-m

RMS, root mean square error

TDEM, time-domain electromagnetic

USGS, U.S. Geological Survey

Datums

Sounding data: Vertical coordinate information is referenced to North American Vertical Datum 

of 1988 (NAVD 88).

Geologic sections: Vertical coordinate information is referenced to National Geodetic Vertical 

Datum of 1929 (NGVD 29).

Horizontal coordinate information is referenced to North American Datum of 1927 (NAD 27) or 

1983 (NAD 83).
Blank Page
Abstract

During September 2006, the U.S. Geological Survey, in 

cooperation with the Barton Springs/Edwards Aquifer Conservation District, conducted a geophysical pilot study to determine whether time-domain electromagnetic (TDEM) sounding could be used to delineate the freshwater/saline-water 

transition zone in the Barton Springs segment of the Edwards 

aquifer in Travis and Hays Counties, Texas. There was uncertainty regarding the application of TDEM sounding for this 

purpose because of the depth of the aquifer (200–500 feet to 

the top of the aquifer) and the relatively low-resistivity clayey 

units in the upper confining unit. Twenty-five TDEM soundings were made along four 2–3-mile-long profiles in a study 

area overlying the transition zone near the Travis-Hays County 

boundary. The soundings yield measurements of subsurface 

electrical resistivity, the variations in which were correlated 

with hydrogeologic and stratigraphic units, and then with dissolved solids concentrations in the aquifer. Geonics Protem 47 

and 57 systems with 492-foot and 328-foot transmitter-loop 

sizes were used to collect the TDEM soundings. A smooth 

model (vertical delineation of calculated apparent resistivity 

that represents an estimate [non-unique] of the true resistivity) 

for each sounding site was created using an iterative software 

program for inverse modeling. The effectiveness of using 

TDEM soundings to delineate the transition zone was indicated by comparing the distribution of resistivity in the aquifer 

with the distribution of dissolved solids concentrations in the 

aquifer along the profiles. TDEM sounding data show that, in 

general, the Edwards aquifer in the study area is characterized 

by a sharp change in resistivity from west to east. The western 

part of the Edwards aquifer in the study area shows higher 

resistivity than the eastern part. The higher resistivity regions 

correspond to lower dissolved solids concentrations (freshwater), and the lower resistivity regions correspond to higher 

dissolved solids concentrations (saline water). On the basis of 

reasonably close matches between the inferred locations of the 

freshwater/saline-water transition zone in the Edwards aquifer 

in the study area from resistivities and from dissolved solids 

concentrations in three of the four profiles, TDEM sounding 

appears to be a suitable tool for delineating the transition zone. 

Introduction

The Barton Springs segment of the Edwards aquifer 

in Travis and Hays Counties, Texas (fig. 1) is an important 

resource for municipal, industrial, domestic, recreational, and 

ecological water supply. The concentration of dissolved solids 

(DS), or salinity, in water in the aquifer increases from west to 

east, resulting in a zone of transition from freshwater (DS concentration less than 1,000 milligrams per liter [mg/L]) to saline 

water. The freshwater/saline-water interface thus is defined as 

the 1,000-mg/L DS concentration threshold. 

During September 2006, the U.S. Geological Survey 

(USGS), in cooperation with the Barton Springs/Edwards 

Aquifer Conservation District (BSEACD), conducted a 

geophysical pilot study to determine whether time-domain 

electromagnetic (TDEM) sounding could be used to delineate the freshwater/saline-water transition zone in the Barton 

Springs segment of the Edwards aquifer. TDEM sounding 

(Fitterman and Labson, 2005) is one of a number of surfacegeophysical methods used to characterize subsurface geologic 

and hydrogeologic properties. TDEM sounding can be used 

to detect variations in electrical resistivity of the subsurface 

that can be related to variations in the physical and chemical 

properties of soil, rock, and pore fluids. This application of 

TDEM sounding also was a test to indicate whether highquality data could be obtained from the required depths 

(200–500 feet [ft] to the top of the aquifer plus a substantial 

Geophysical Delineation of the Freshwater/

Saline-Water Transition Zone in the Barton 

Springs Segment of the Edwards Aquifer, 

Travis and Hays Counties, Texas, 

September 2006 

By Jason D. Payne1, Wade H. Kress1, Sachin D. Shah1, James E. Stefanov1, Brian A. Smith2, and Brian B. Hunt2

1

 U.S. Geological Survey.

2

 Barton Springs/Edwards Aquifer Conservation District.
TRAVIS COUNTY

HAYS COUNTY

TRINITY AQUIFER

5 KILOMETERS

0 5 MILES

0

Edwards aquifer recharge zone

Edwards aquifer confined zone

EXPLANATION

Base modified from Texas Natural Resources Information

System (TNRIS) digital data, 2005, 1:24,000

Albers equal area conic projection, NAD27

Barton Springs segment of the

Edwards aquifer modified from

Slagle and others, 1986

Barton

 Springs

Study area

Travis County

Hays County

TEXAS

Williamson

Creek

Slaughter

Creek

Creek

Bear

Creek

Onion

LAKE

 TRAVIS

COLORADO RIVER

Shoal

Creek

TOWN LAKE

Walnut

Creek

Bull

Creek

Creek

Barton

Little Bear

Creek

LAKE

 AUSTIN

Creek

Garlic

IH-35

Saline

zone

boundary

Ground-water

divide

AUSTIN

Buda

97o45' 97o

52'30"

30o22'30"

98o

30o07'30"

30o15'

30o00'

Figure 1. Location of Barton Springs segment of the Edwards aquifer and study area, Travis and Hays Counties, Texas. 

   Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
thickness of the aquifer); and whether relatively low-resistivity 

(relatively high-conductivity) clayey units that compose the 

upper confining unit of the aquifer would adversely affect the 

ability of TDEM to detect relatively high-conductivity saline 

water that indicates the transition zone. The pilot application 

of TDEM sounding also was an opportunity to test whether 

urban features would adversely affect the sounding data. 

Power lines associated with certain frequencies, nearby radio 

and radar transmitters, metallic structures, and buried metal 

objects can cause noise in TDEM measurements (Lucius and 

others, 2007).

Twenty-eight sites were selected for TDEM sounding 

across four profiles (one of which comprises two alternative 

lines) in a study area overlying the transition zone of the aquifer in Travis and Hays Counties (fig. 2). The TDEM profiles 

were about 2–3 miles (mi) long. Because of land-access difficulties at two sites and equipment malfunction at another, data 

from three of the original 28 sites were not collected. Twentyfive TDEM soundings (measurements) (table 1) were used for 

this report. 

Purpose and Scope

This report documents a pilot study in the Barton Springs 

segment of the Edwards aquifer done in September 2006 to 

determine whether TDEM sounding can be used to delineate 

the freshwater/saline-water transition zone in the aquifer. 

There was uncertainty regarding the application of TDEM for 

this purpose because of the depth of the aquifer and the relatively low-resistivity clayey units in the upper confining unit. 

The report describes the results of 25 TDEM soundings along 

four profiles across the transition zone. Data were collected 

during September 2006. The soundings yield measurements 

of subsurface electrical resistivity, the variations in which are 

correlated with hydrogeologic and stratigraphic units, and 

then with DS concentrations in the aquifer. The report briefly 

describes the aquifer, upper confining unit, and study area; 

and briefly describes TDEM theory and its application to 

characterize properties of the subsurface. The interpretation 

of subsurface resistivity, aided by selected borehole geophysical data, is discussed. The data for each profile are shown as 

two-dimensional subsurface sections of resistivity generated 

from one-dimensional inverse modeling results and compared 

graphically with an associated geologic section and plan view 

of DS concentrations along the profile. 

Previous Investigations

Previous investigations that describe the regional setting, 

hydrogeologic characteristics, and electrical stratigraphy of 

the area were used as guidance for this study. Smith and others 

(2003) conducted a helicopter electromagnetic survey across 

the freshwater/saline-water transition zone in the Seco Creek 

area in Medina County about 60 mi southwest of the study 

area. Data from that report were used to identify apparent 

resistivity ranges for each of the stratigraphic units in the study 

area. 

Baker and others (1986) describes the regional geologic 

setting and hydrogeologic characteristics of the Edwards 

aquifer in the Austin area. That report was used to locate 

the freshwater/saline-water transition zone and to determine 

required depths of investigation at different points across each 

profile. Maclay (1995) presents a comprehensive study of 

the geology and hydrogeology of the Edwards aquifer in the 

San Antonio area that includes the Barton Springs segment. 

Small and others (1996) conducted a study to characterize the 

geologic framework and hydrogeologic characteristics of the 

Edwards aquifer in northeastern Hays and southeastern Travis 

Counties. Both reports provided information on the geologic 

characteristics of the study area. 

Table 1. Location data for time-domain electromagnetic 

soundings used for this report, Barton Springs segment of the 

Edwards aquifer, Travis and Hays Counties, Texas. 

Profile

Site

identifier

(fig. 2)

Easting

(feet)

Northing

(feet)

Altitude

(feet above 

NAVD 88) 

2 200 2,013,702 10,936,180 634.20

2 240 2,018,195 10,931,173 740.71

2 245 2,019,691 10,930,398 706.88

2 247 2,020,355 10,930,018 706.92

2 250 2,021,358 10,930,533 681.51

2 260 2,023,146 10,930,023 665.34

3 320 2,010,614 10,921,904 707.95

3 330 2,013,558 10,918,985 754.75

3 340 2,015,854 10,919,689 744.20

3 350 2,018,280 10,920,862 722.11

3 360 2,021,176 10,921,365 751.63

3 370 2,023,325 10,922,600 752.60

4.1 410 2,004,443 10,909,101 742.70

4.1, 4.2 420 2,012,228 10,908,088 693.72

4.1, 4.2 440 2,006,948 10,910,110 736.82

4.1, 4.2 450 2,008,527 10,909,042 706.76

4.1, 4.2 460 2,010,612 10,909,099 692.00

4.2 470 2,005,533 10,912,378 739.23

5 520 2,001,415 10,902,646 711.76

5 522 2,001,850 10,903,909 727.65

5 524 2,003,092 10,904,189 757.70

5 530 2,005,478 10,905,299 742.70

5 540 2,006,559 10,904,444 715.35

5 550 2,008,722 10,904,029 696.57

5 560 2,009,319 10,904,949 727.43

Introduction 
Figure 2. Locations of time-domain electromagnetic (TDEM) sounding sites and wells, Barton Springs segment of the Edwards aquifer, 

Travis and Hays Counties, Texas. 

IH-35

Profile 2

Profile 3

Profile 4.1

Profile 4.2

Profile 5

58–58–213 (Flores, 1990)

58–58–428 (Swinney well)

200

210 240

245 250 260

320

330 340

350 360

370

410

420

440

450 460

470

520

522 524

530

540 550

560

247

310

430 TRAVIS COUNTY

HAYS COUNTY

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

Universal Transverse Mercator Projection, Zone 14 North

North American Datum 1983 (NAD 83)

1,990,000 2,000,000 2,010,000 2,020,000 2,030,000 2,040,000

EASTING, IN FEET

10,900,000 10,910,000 10,920,000 10,930,000 10,940,000

NORTHING, IN FEET

EXPLANATION

TDEM sounding profile

TDEM sounding site and identifier

TDEM sounding site and identifier—Data not collected for this report

Well used for dissolved solids grid

550

Edwards aquifer recharge zone

Edwards aquifer confined freshwater zone

210

0 5,000 10,000 15,000 FEET

Freshwater/saline-water

interface (Slagle and 

others, 1986)

 Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
Description of the Barton Springs Segment of 

the Edwards Aquifer, Confining Units, and Study 

Area

The Barton Springs segment of the Edwards aquifer 

comprises carbonate rocks, primarily faulted and fractured 

limestone. The aquifer is bounded on the north by the Colorado River; on the south by a ground-water divide; on the 

west by the Trinity aquifer; and on the east by the saline 

zone (fig. 1). The aquifer crops out and is unconfined on the 

western side (recharge zone) and dips to the east and becomes 

more deeply buried and confined with distance toward the 

saline zone. 

The rocks of the aquifer consist of the Edwards Group 

and the Georgetown Formation of Lower Cretaceous age 

(Maclay, 1995; Small and others, 1996) (table 2). Where 

confined above, the upper confining unit comprises (from 

oldest to youngest) the Del Rio Clay, Buda Limestone, Eagle 

Ford Group, Austin Group, Taylor Group, and Navarro Group 

(Barker and Ardis, 1996, plate 1) of Upper Cretaceous age. 

The lower confining unit consists of the generally less permeable (than the Edwards Group) Glen Rose Limestone of the 

Trinity aquifer. 

Recharge to the aquifer occurs primarily by downward 

leakage through the beds of major streams that cross the 

recharge zone from west to east (Slade and others, 1986). 

Barton Springs on Barton Creek near the Colorado River 

(fig. 1) is the primary natural discharge point of the aquifer. 

Solution-enhanced karst features (fractures, faults, sinkholes) 

enhance the transmissivity of the aquifer, probably more so in 

the freshwater zone than in the saline zone. Rocks of the freshwater zone are thought to be relatively more transmissive than 

rocks of the saline zone (Flores, 1990; Maclay, 1995). 

Water in the freshwater zone of the aquifer generally is a 

calcium-bicarbonate to calcium-magnesium-bicarbonate type 

that contains less than 500 mg/L DS concentration (Senger 

and Kreitler, 1984) (although for this report, freshwater is 

defined as water containing less than 1,000 mg/L DS concentration). Sodium and chloride are the dominant ions of water 

in the saline zone, which has DS concentration greater than 

1,000 mg/L.

The study area overlies the transition zone where the 

northwest-to-southeast Travis County-Hays County line passes 

through near Buda (fig. 1). Land cover east of Interstate Highway 35 (IH–35) primarily is rural open ranchland, and land 

cover west of IH–35 largely is commercial and urban. Depth 

from land surface to the top of the aquifer in the study area 

ranges from negligible to about 800 ft.

Acknowledgments

The authors acknowledge and extend appreciation to Joe 

Beery and Guy Rials, BSEACD, for their assistance during 

data collection. Thanks also are extended to all landowners 

Table 2. Correlation between hydrogeologic and stratigraphic units associated with the Barton Springs 

segment of the Edwards aquifer, Travis and Hays Counties, Texas.

[Stratigraphic units modified from Barker and Ardis (1996, pl. 1); Small and others (1996, table 1)]

Series Hydrogeologic unit Stratigraphic unit

Upper Cretaceous Upper confining unit

Navarro Group

Taylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Lower Cretaceous

Edwards aquifer, Barton 

Springs segment

Georgetown Formation

Edwards 

Group

Person Formation

Cyclic and marine 

members, undivided

Leached and collapsed 

members, undivided

Regional dense member

Kainer Formation

Grainstone member

Kirschberg evaporite 

member

Dolomite member

Basal nodular member

Lower confining unit 

(Trinity aquifer) Glen Rose Limestone

Introduction 
for permission to access to their properties; especially to 

Kathleen Adkins and Ken Rutlidge, Adkins Ranch; Don 

Killibrew, Texas Lehigh Cement Co.; Steve Bartlett, Sunfield 

Municipal Utility District No. 2; and Donald and Douglas 

Dacy. 

Methods and Assessment of Data

Geodatabase Development

Geodatabases are spatial extensions of tabular data that 

allow users to correlate numerical data with physical and 

spatial components. With geodatabases, geographic data can 

be manipulated to represent the real world using a geographic 

information system (GIS) to produce maps, interactive queries, 

and various types of spatial analyses in one integrated environment. For this study, Oasis montaj (Geosoft, Inc., 2006) was 

the software used to compile geologic, lithologic, geochemical, and TDEM geophysical data. The TDEM data imported 

into Oasis montaj are “smooth” models (vertical delineations 

of estimated true resistivity) generated for each sounding. The 

software typically is used to create two- and three-dimensional 

sections of subsurface electrical stratigraphy. Two-dimensional 

sections were created for this report.

A site map comprising roads, urban areas, and all 25 

sounding sites was imported into the geodatabase. Additional 

data were imported with information to facilitate analysis 

of the collected TDEM data. A table of DS concentrations 

obtained from wells in the study area (appendix 1) were used 

to generate an areal grid of concentrations from which the 

approximate lateral location of the freshwater/saline-water 

transition zone could be identified (fig. 3). The grid was used 

as a reconnaissance guide to locate sounding sites across the 

transition zone. Geologic (stratigraphic unit) and hydrogeologic (aquifer and confining unit) contact data from geologic 

sections constructed from existing data along each TDEM 

profile also were input to the geodatabase to associate with 

TDEM measurements.

Geologic Sections 

Geologic sections (figs. 4, 5) were constructed so that 

TDEM resistivity data could be associated with hydrogeologic and stratigraphic units. Subsurface geologic data in the 

BSCEAD database (Barton Springs/Edwards Aquifer Conservation District, written commun., 2006), primarily from geophysical and drillers’ logs, test holes, and water wells, were 

used to construct geologic sections. Geologic control points 

are sparse east of IH–35; therefore there is less confidence in 

the sections in the easternmost part of the study area. In a few 

cases, the depth of the casing was interpreted to be the top of 

the aquifer, as most drillers install casing through the incompetent Del Rio Clay, completing the casing in the upper part 

of the Georgetown Formation. Within the Edwards aquifer and 

shown in the sections of figure 5 is the regional dense member, 

a poorly permeable, dense, carbonate mudstone that is recognizable in test hole cores by its lithology and on geophysical 

logs by distinct shifts in log traces (Maclay, 1995). 

Site Selection

Twenty-five sounding sites were selected to provide a 

uniform distribution of data to determine variations in electrical properties across the study area. A grid was generated 

from existing water-quality data-collection sites using kriging 

(Geosoft Inc., 2006) to aid in selecting sounding sites where 

DS concentrations transition from freshwater to saline water 

in the study area (fig. 3). Kriging interpolates values (DS 

concentrations in this application) from measured values at 

known locations (appendix 1). TDEM sounding sites were 

chosen to avoid urban features and where land was accessible. 

Urban features hinder data collection because of electromagnetic noise from sources such as power lines and underground 

pipelines. Initial sites along each profile were selected and 

sounding data collected. After preliminary data processing, 

additional sounding sites were sought to fill data gaps; however, in some cases desired sites could not be accessed because 

landowner permission could not be obtained.

Time-Domain Electromagnetic Sounding

TDEM sounding can be used to identify variations in the 

electrical resistivity of the subsurface, which can be linked 

to changes in the physical and chemical properties of soil, 

rock, and pore fluids. The resistivity of soils and rocks are 

controlled by mineralogy, clay content, water content, salinity, metallic minerals, and porosity. Changes in the resistivity 

of soils and rocks, either vertically or horizontally, produce 

variations in the electromagnetic signature measured by 

geophysical tools. Changes in the resistivity can be correlated 

to variations in the composition and physical properties of 

the subsurface geology, to the level that differences in lithology or rock type are accompanied by variations in resistivity 

(U.S. Army Corps of Engineers, 1995). However, to efficiently 

distinguish these differences, the geologic characteristic to be 

defined must have properties considerably different from background conditions (American Society for Testing and Materials, 1999). Typically, clay and shale are less resistive than sand 

and gravel, which in some cases can provide contrasts that 

could be noticeable with TDEM methods (U.S. Army Corps of 

Engineers, 1995).

Electromagnetic measurements are made by transmitting an alternating current into a square loop of insulated wire 

deployed on the land surface. The current consists of equal 

periods of time-on and time-off base frequencies that ranged 

in this application from 285 to 3 hertz (Hz), which produces an 

electromagnetic field near the loop. Termination of the current 

   Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
Figure 3. Location of freshwater/saline-water transition zone, Barton Springs segment of the Edwards aquifer, Travis and Hays 

Counties, Texas. 

Freshwater/saline-water

interface (Slagle and 

others, 1986)

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

Universal Transverse Mercator Projection, Zone 14 North

North American Datum 1983 (NAD 83)

0 5,000 10,000 15,000 FEET

IH-35

Profile 2

Profile 3

Profile 4.1

Profile 4.2

Profile 5

TRAVIS COUNTY

HAYS COUNTY

200

240

245 250 260

320

330 340

350 360

370

410

420

440

450 460

470

520

522 524

530

540 550

560

247

Dissolved solids concentration, in milligrams per liter

46 325 641 925 1,202 1,583 2,081

Time-domain electromagnetic (TDEM) sounding profiles

TDEM sounding site and identifier

Well used for dissolved solids concentration grid

550

EXPLANATION

EASTING, IN FEET

1,990,000 2,000,000 2,010,000 2,020,000 2,030,000 2,040,000

NORTHING, IN FEET

10,900,000 10,910,000 10,920,000 10,930,000 10,940,000

Methods and Assessment of Data 
IH-35

Profile 2

Profile 3

Profile 4.1

Profile 4.2

Profile 5

TRAVIS COUNTY

HAYS COUNTY

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

Universal Transverse Mercator Projection, Zone 14 North

North American Datum 1983 (NAD 83)

1,990,000 2,000,000 2,010,000 2,020,000 2,030,000 2,040,000

EASTING, IN FEET

10,900,000 10,910,000 10,920,000 10,930,000 10,940,000

NORTHING, IN FEET

0 5,000 10,000 15,000 FEET

A

A'

B

B'

C

C'

D

D'

58-58-702

58-50-846

58-50-820

58-50-851

58-58-202

58-50-840

58-58-213

58-58-2MB

58-58-101

58-58-412

58-58-211

58-58-503

58-58-410 58-58-505

58-58-406

58-58-502

58-58-417

58-57-9N1 58-58-704

58-58-7MU

58-58-4WM

58-58-411

58-58-4BS

58-58-801

58-58-213

EXPLANATION

Edwards aquifer recharge zone

Edwards aquifer confined freshwater zone

Section trace (see fig. 5)

TDEM sounding profile

Well and State number—Provided geologic data for section 

(Barton Springs/Edwards Aquifer Conservation District geologic database)

D D’

Freshwater/saline-water

interface (Slagle and 

others, 1986)

Figure 4. Locations of geologic sections and time-domain electromagnetic (TDEM) sounding profiles, Barton Springs segment of the 

Edwards aquifer, Travis and Hays Counties, Texas. 

   Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
flow is not instantaneous, but occurs over a few microseconds, 

known as the ramp time, during which the magnetic field is 

time-variant. The time-variant nature of the induced electromagnetic field creates a secondary electromagnetic field in the 

ground underneath the loop in accordance with Faraday’s Law 

(Halliday and Resnick, 1974). This resultant field instantly 

begins to decay, in the process generating additional eddy currents that spread downward and outward into the subsurface 

like a succession of smoke rings (North Carolina Division of 

Water Resources, 2006). Measurements of the eddy currents 

are made throughout the time-off period by a receiver positioned in the center of the transmitter loop. Depth of exploration depends on the time interval after shutoff of the current, 

because at later times, the receiver is measuring eddy currents 

at increasingly greater depths. The magnitude of the eddy 

current voltage at specific times and depths is determined by 

the overall conductivity of subsurface rock units and fluids 

(Stewart and Gay, 1986). From this voltage measurement, the 

apparent resistivity can be calculated.

Transmitter loop, or antenna, selection was critical 

because of relatively large depths to and into the Edwards 

aquifer in the eastern part of the study area. Data from squareshaped transmitter loops of three sizes, 164 by 164 ft (50 

meter [m]), 328 by 328 ft (100 m), and 492 by 492 ft (150 m), 

were examined for data quality at depth. A larger loop size 

increases the depth of exploration but increases resistance 

of the loop wire, which therefore decreases the current. For 

example, a 492-ft (150-m) loop transmits only 8 amps because 

of the added resistance of the wire, whereas a 328-ft (100-m) 

loop will transmit between 12 and 14 amps. Reduced current 

in the loop decreases the electromagnetic field strength, which 

decreases the reliability of data from relatively deep zones. 

It was determined that the 164-ft (50-m) loop was not large 

enough to generate reliable data at the depths needed for most 

of the survey. The judgment was made not to use sounding 

data from depths greater than 656 ft (200 m) for interpretations because of the probable lack of reliability of data from 

those depths. 

Geonics Protem 47 (fig. 6A) and 57 (fig. 6B) systems 

with 492-ft (150-m) and 328-ft (100-m) transmitter-loop sizes 

were used to collect the TDEM soundings. The Protem 47 and 

57 use a multi-turn coil to measure electromagnetic fields in 

the center of the transmitter loop. The Protem 47 receiver coil 

has an effective area of 338 square feet (31.4 square meters), 

and the Protem 57 receiver has an effective area of 1,076 

square feet (100 square meters). The Protem 47 produced currents of 2.2–2.7 amps (depending on the size of the loop used), 

and the Protem 57 produced currents of 8–21 amps. The Protem 47 and 57 each have preset frequencies or repetition rates. 

The Protem 47 uses repetition rates of 285 Hz (ultra-high), 75 

Hz (very high), and 30 Hz (high) (Geonics Limited, 2006a). 

The Protem 57 uses slower repetition rates of 30 Hz (high), 

7.5 Hz (medium), and 3 Hz (low) (Geonics Limited, 2006b). 

At each sounding an integration time of 60 seconds was used 

to measure five different datasets or duty cycles. The five duty 

cycles are individual datasets that are later averaged before 

inverse modeling. Averaging is done to ensure data quality 

and repeatability. The 60-second integration time was selected 

after comparison with data collected with 30-second integration times. The larger integration time increased late-time data 

quality, which increased data quality at depth.

A Trimble 5800 dual frequency kinematic global positioning system (GPS) unit was set up at each sounding to 

obtain the precise location of the transmitter. Each sounding 

was configured from west to east using a compass, with the 

transmitter at the west corner, so that the receiver location in 

the center of the loop could be computed using the Pythagorean theorem. From the position of the transmitter, the receiver 

position was computed as one-half the diagonal distance of the 

square loop due east. 

Borehole Geophysics

Borehole geophysical methods typically are used to measure and analyze the physical properties of the subsurface in 

wells. Geophysical probes that measure various properties are 

lowered into the borehole to collect data that can be displayed 

graphically with depth as a geophysical log. Typically, several 

different logs are collected because more can be observed in 

the analysis of a suite of logs than by analyzing single logs 

alone. Borehole logs offer a means to accurately estimate the 

thickness and physical properties of the diverse materials penetrated by a well (Keys, 1990). 

The top of the Edwards aquifer is identified on a borehole 

geophysical log from a previous study in the area (Flores, 

1990) (fig. 2) by the increase in gamma-ray activity across 

the Georgetown Formation/Del Rio Clay contact (fig. 7), 

likely caused by higher clay content in the Del Rio Clay. 

Also prominent on the geophysical log above the Del Rio Clay 

are increases in gamma-ray activity that Flores (1990) interpreted as marking the Eagle Ford Group and the Navarro and 

Taylor Groups (undifferentiated). These units also likely have 

relatively high clay content. It is these clayey units, which are 

relatively conductive like saline water, that were of concern at 

the outset of the study regarding their potential to adversely 

affect TDEM measurements indicating saline water in the 

aquifer. 

Geophysical borehole logs were collected at State well 

58–58–428 (Swinney well) in Hays County (fig. 2; appendix 

2) for comparison with the Flores (1990) log and to better 

determine the electrical stratigraphy of the site. Gamma, resistivity, temperature, and specific conductance logs were collected. The gamma log indicates variations in lithology similar 

to those of the Flores (1990) gamma log. The same relatively 

clayey units are apparent on both logs. 

Data Processing and Quality

The dual frequency kinematic GPS data were processed through the Online Positioning User Service (OPUS) 

(National Geodetic Survey, 2006) 1 week after collection to 

Methods and Assessment of Data 
Figure 5. Geologic sections of the Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas. 

Regional dense member See figure 4 for locations of sections

A A’

B B’

Geology from Barton Springs/Edwards Aquifer Conservation District geologic database

0

Vertical scale greatly exaggerated

4,000 8,000 FEET

EXPLANATION

Faults shown as vertical because of verticalexaggeration—Dashed where inferred

1 58–58–10 Well geologic control point and State well

number—Type of geologic control indicatedat bottom (GR, gamma ray; DL, driller’s log)

Taylor and Navarro Groups (undivided)Austin GroupEagle Ford GroupBuda LimestoneDel Rio ClayGeorgetown FormationEdwards Group

GR

GR

GR

GR

GR

GR

GR

Travis County

6 58–50–84

0 58–50–82

1 58–50–85

2 58–58–20

0 58–50–84

3 58-58-21

B 58-58-2M

1 58–58–10

2 58–58–41

1 58–58–21

3 58–58–50

58–58–2MB

58–58–505

Hays County Travis County

Geologic section along time-domain electromagnetic (TDEM) sounding profile 2 Geologic section along TDEM sounding profile 3

NORTHWEST SOUTHEAST WEST-SOUTHWEST EAST-NORTHEAST

IH-35

5 IH-3 650

550

450

350

250

150

50

-100

-150

-250

ALTITUDE, IN FEET ABOVE NGVD 29

650

550

450

350250150

50

-100-150

-250

650

550

450350

250

150

50

-100

-150

-250

650550450350250150

50

-100-150-250

Upper confining unit Edwards aquifer

10 Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
Figure 5. Continued. 

C C’ D D’

Geology from Barton Springs/Edwards Aquifer Conservation District geologic database

0

Vertical scale greatly exaggerated

4,000 8,000 FEET

See figure 4 for locations of sections

Geologic section along TDEM sounding profile 4 Geologic section along TDEM sounding profile 5

Regional dense member

EXPLANATION

Faults shown as vertical because of verticalexaggeration—Dashed where inferred

1 58–58–10 Well geologic control point and State well

number

Taylor and Navarro Groups (undivided)Austin GroupEagle Ford GroupBuda LimestoneDel Rio ClayGeorgetown FormationEdwards Group

GR

WEST-NORTHWEST EAST-SOUTHEAST SOUTHWEST NORTHEAST

IH-35

IH-35

650550

450

350

250

150

50

-100

-150

-250

ALTITUDE, IN FEET ABOVE NGVD 29

650

550

450

350250150

50

-100-150

-250

650

550

450350

250

150

50

-100

-150

-250

650550450350250150

50

-100-150-250

750 750 750 750

6 58-58-40

M 58-58-4W0 58-58-41

58-58-4BS

1 58-58-41 1 58-58-80 2 58-58-50

1 58-57-9N 7 58-58-414 58-58-70 U 58-58-7M2 58-58-70 1 58-58-80

Upper confining unit Edwards aquifer

Hays County Hays County

Methods and Assessment of Data 11
ensure accuracy of sounding locations. There are three different levels of OPUS solutions: ultra-rapid, rapid, and precise. 

Ultra-rapid solutions are collected in near real-time and are 

processed using a predicted satellite orbit. Rapid solutions are 

processed within 1 day of collection and use a partially predicted orbit; and precise solutions are calculated after 3 days 

and use the exact orbit. OPUS suggests collecting GPS data 

for a minimum of 2 hours for best results. For this survey, GPS 

data were collected for about 30 minutes at each site; highly 

accurate locations were not critical for this survey. The GPS 

solutions are in appendix 3.

For each sounding, the voltage data were averaged and 

evaluated statistically. The raw field voltage data were checked 

first for uncertainty by computing the standard deviation of the 

data. In some cases, the voltages were negative at the late-time 

gates, so the absolute values for those voltages were used to 

evaluate the data statistically. The negative data then were 

deleted before any other processing was done. The raw voltage 

data were averaged over the five duty cycles for each gate for 

each frequency using TEM2IX1D (Interpex Limited, 1996). 

TEM2IX1D is a program used to analyze and average the duty 

cycles for each sounding. Voltages with standard deviation 

greater than 3 percent were deleted before modeling, which 

eliminated data from late-time gates that yielded the lowest 

signal-to-noise ratio. The computed standard deviations of the 

voltages are in appendix 4. 

Inverse Modeling

Apparent resistivity, as calculated from field measurements of voltage, is the resistivity of an equivalent electrically homogeneous and isotropic subsurface (Grant and West, 

1965). Inverse modeling is the process of creating an estimate 

of the true distribution of subsurface resistivity (which reflects 

heterogeneous, anisotropic rocks) from the measured apparent resistivity obtained from TDEM soundings (which reflects 

homogeneous, isotropic rocks). IX1D, a program developed 

by Interpex Limited (1996), was used for inverse modeling. 

The measured apparent resistivity data were graphed as a function of time on a linear scale (appendix 5). Data points that 

deviated severely (a judgment decision) from the curve were 

deleted before inverse modeling. A smooth model consisting 

of 100 layers with a minimum depth of 3.281 ft (1 m), a maximum depth of 2,296 ft (700 m), and a starting resistivity of 10 

ohm-meters (ohm-m) was used to approximate the measured 

resistivity points. A smooth model is a vertical delineation of 

calculated apparent resistivity that represents a (non-unique) 

Figure 6. Receiver coil for Geonics (A) Protem 47 and (B) Protem 57. 

(A) (B)

12 Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
 Navarro and 

 Taylor 

 Groups 

undifferentiated

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown

 Limestone

Edwards Group

Modified from Flores, 1990, appendix 2

0 50

Counts per second

Gamma Ray Depth Geologic

(Feet)

EDWARDS AQUIFER

0

100

200

300

400

500

600

700

800

900

1000

estimate of the true resistivity. Ridge regression (Interpex 

Limited, 1996) was used by the inversion software in a series 

of iterations to create a smooth model for each sounding site. 

Iterations were continued until the root mean square (RMS) 

error between measured and calculated apparent resistivity 

was less than 0.1 percent between iterations. Graphs of the 

smooth models for the 25 sounding sites are in appendix 5. 

The entire datasets of raw and processed TDEM data are in 

appendix 6. 

RMS errors between measured and calculated apparent 

resistivity for the soundings range from 1.71 to 8.72 percent 

(table 3). After each sounding was inverted in IX1D, the onedimensional smooth model for the sounding was imported into 

the geodatabase to compare with models from other soundings along the same profile. The TDEM smooth models were 

superimposed on the geologic sections. 

Delineation of Transition Zone Using 

Time-Domain Electromagnetic 

Soundings

The effectiveness of using TDEM soundings to delineate 

the transition zone is indicated by comparing the distribution of resistivity in the aquifer with the distribution of DS 

concentrations in the aquifer along profiles 2–5, as shown in 

figures 8–12. (Profile 1 was abandoned because of a lack of 

landowner permissions to access the necessary sites.) TDEM 

sounding data show that, in general, the Edwards aquifer in 

the study area is characterized by a sharp change in resistivity 

from west to east. The western part of the Edwards aquifer in 

the study area shows higher resistivity than the eastern part. 

The higher resistivity regions correspond to lower DS concentrations (freshwater), and the lower resistivity regions correspond to higher DS concentrations (saline water). Resistivities from soundings along each profile relative to known DS 

concentrations are discussed below.

Profile 2 

Profile 2 (fig. 3) comprises six TDEM soundings (fig. 8). 

Sounding 200 on the far west end of the profile depicts a 

continuous vertical decrease in resistivity (fig. 8B, C) that is 

interpreted as indication of an increase in DS concentration 

with depth. More-saline water apparently occurs in the lower 

part of the aquifer at the site. Sounding 240 also indicates a 

vertical decrease in resistivity (fresher water overlying moresaline water), but the decrease is attenuated compared to that 

of sounding 200. Soundings 245, 247, 250, and 260 generally 

show decreasing resistivity with distance to the east, which is 

interpreted as increasing salinity in the aquifer with distance to 

the east. Comparison of the lateral location of the interpreted 

freshwater/saline-water interface based on resistivity with the 

location based on DS concentrations (fig. 8A) shows general 

agreement (approximately between soundings 240 and 245). 

Figure 7. Geologic and gamma-ray geophysical logs, test well 

58–58–213, Travis County, Texas. 

Delineation of Transition Zone Using Time-Domain Electromagnetic Soundings 13
Figure 8. Profile 2, Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas (A) plan view of distribution of 

dissolved solids concentrations from previously collected data, (B) nearby geologic section interpreted from previously collected data 

and overlain by smooth inversion models of resistivity at each sounding, and (C) resistivity section from time-domain electromagnetic 

(TDEM) soundings. 

14  Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer

500 ft

200

500 ft

240

500 ft

245

500 ft

247

500 ft

250

500 ft

260

ALLTIT

U

DE, IN FEET

ABO

VE N

G

V

D 29

A

B

-100

0

100

200

300

400

500

600

700

050100150200250

050100

050050100

050

050

EXPLANATION

Taylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown Formation

Edwards Group

Regional dense member TDEM apparent resistivity,

in ohm-meters Edwards aquifer

Upper confining unit

TDEM sounding

and identifier

1,000 0 1,000 FEET

-100

0

100

200

300

400

500

600

700

Top of Edwards aquifer

Approximate land-surface elevation

ALTIT

U

DE, IN FEET

ABO

VE N

G

V

D 29

200

240

245

247

250

260

C

6 20 43 66 89 113 145

Resistivity, in ohm-meters

200

0 46 325 641 925 1,202 1,583 2,081

Dissolved solids concentration,

in milligrams per liter

200

240 245

250

260

247
Figure 9. Profile 3, Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas (A) plan view of distribution of 

dissolved solids concentrations from previously collected data, (B) nearby geologic section interpreted from previously collected data 

and overlain by smooth inversion models of resistivity at each sounding, and (C) resistivity section from time-domain electromagnetic 

(TDEM) soundings. 

0 46 325 641 925 1,202 1,583 2,081

Dissolved solids concentration,

in milligrams per liter

EXPLANATION

Taylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown Formation

Edwards Group

Regional dense member TDEM apparent resistivity,

in ohm-meters Edwards aquifer

Upper confining unit

TDEM sounding

and identifier

1,000 0 1,000 2,000 FEET

-400

-200

0

200

400

600

800

500 ft

1000

320

500 ft

1000

330

500 ft

1000

340

500 ft

1000

350

500 ft

1000

360

500 ft

1000

370

A

B

ALTIT

U

D

E, IN FE

E

T

A

B

O

V

E

N

G

V

D 29

050100150200250300

050

050100

050

050

050

320

330 340

350

360

370

320330

340

350

360

370

-400

-200

0

200

400

600

800

Approximate land-surface elevation

Top of Edwards aquifer

C

ALTIT

U

D

E, IN FE

E

T

A

B

O

V

E

N

G

V

D 29

6 20 43 66 89 113 145

Resistivity, in ohm-meters

320

Delineation of Transition Zone Using Time-Domain Electromagnetic Soundings 15
Figure 10. Profile 4.1, Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas (A) plan view of distribution of 

dissolved solids concentrations from previously collected data, (B) nearby geologic section interpreted from previously collected data 

and overlain by smooth inversion models of resistivity at each sounding, and (C) resistivity section from time-domain electromagnetic 

(TDEM) soundings. 

58-58-407

410

420

440

450 460

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

Universal Transverse Mercator Projection, Zone 14 North.

North American Datum, (NAD 83).

0 46 325 641 925 1,202 1,583 2,081

Dissolved solids concentration,

in milligrams per liter

ALTIT

U

D

E, IN FE

E

T

A

B

O

V

E

N

G

V

D 29

EXPLANATION

Taylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown Formation

Edwards Group

Regional dense member TDEM apparent resistivity,

in ohm-meters Edwards aquifer

Upper confining unit

TDEM sounding

and identifier

1,000 0 1,000 2,000 FEET

500 ft

410

500 ft

420

500 ft

440

500 ft

450

500 ft

460

0

100

200

300

400

500

600

700

A

B

020406080100 120

020

020406080100

02040

02040

ALTIT

U

D

E, IN FE

E

T

A

B

O

V

E

N

G

V

D 29

410

440

450

460

420

0

100

200

300

400

500

600

700 Approximate land-surface elevation

Top of Edwards aquifer

C

6 20 43 66 89 113 145

Resistivity, in ohm-meters

410

16 Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
Figure 11. Profile 4.2, Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas (A) plan view of distribution of 

dissolved solids concentrations from previously collected data, (B) nearby geologic section interpreted from previously collected data 

and overlain by smooth inversion models of resistivity at each sounding, and (C) resistivity section from time-domain electromagnetic 

(TDEM) soundings. 

420

440 450

460

470

500 ft

420

500 ft

440

500 ft

450

500 ft

460

500 ft

470

01020

0102030405060708090100

02040

02040

0204060

0 46 325 641 925 1,202 1,583 2,081

Dissolved solids concentration,

in milligrams per liter

0

100

200

300

400

500

600

700

ALTIT

U

DE, IN FEET

ABO

VE N

G

V

D 29

A

B

0

470

440

450

460

420

100

200

300

400

500

600

700 Approximate land-surface elevation

Top of Edwards aquifer

ALTIT

U

DE, IN FEET

ABO

VE N

G

V

D 29

C

6 20 43 66 89 113 145

Resistivity, in ohm-meters

EXPLANATION

Taylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown Formation

Edwards Group

Regional dense member TDEM apparent resistivity,

in ohm-meters Edwards aquifer

Upper confining unit

TDEM sounding

and identifier

1,000 0 1,000 2,000 FEET

420

Delineation of Transition Zone Using Time-Domain Electromagnetic Soundings 17
Figure 12. Profile 5, Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas (A) plan view of distribution of 

dissolved solids concentrations from previously collected data, (B) nearby geologic section interpreted from previously collected data 

and overlain by smooth inversion models of resistivity at each sounding, and (C) resistivity section from time-domain electromagnetic 

(TDEM) soundings. 

58-58-701

520

522 524

530

540

550

560

0 46 325 641 925 1,202 1,583 2,081

Dissolved solids concentration,

in milligrams per liter

500 ft

520

500 ft

522

500 ft

524

500 ft

530

500 ft

540

500 ft

550

500 ft

560

ALTIT

U

D

E, IN FE

E

T

A

B

O

V

E

N

G

V

D 29

A

B

0102030405060708090

0102030405060

010203040

02040

0102030405060

0102030405060708090

01020304050

600

700

500

400

300

200

100

0

0

100

200

300

400

500

600

700

Approximate land-surface elevation

ALTIT

U

D

E, IN FE

E

T

A

B

O

V

E

N

G

V

D 29

C

Top of Edwards aquifer

6 20 43 66 89 113 145

Resistivity, in ohm-meters

520

522

524

530540550

560

EXPLANATION

Taylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown Formation

Edwards Group

Regional dense member TDEM apparent resistivity,

in ohm-meters Edwards aquifer

Upper confining unit

TDEM sounding

and identifier

1,000 0 1,000 2,000 FEET

520

Possible fault with relative 

movement indicated.

18 Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
Profile 3

Profile 3 (fig. 3) comprises six TDEM soundings (fig. 

9), only four of which penetrate the upper part of the aquifer 

within the estimated range of reliability (656 ft [200 m]). 

Sounding 320 on the far west end of the profile shows very 

low resistivity (about 4 ohm-m [appendix 5, fig. 5.7) in the 

Edwards aquifer (fig. 9B, C) that would indicate saline water. 

This response might be influenced by noise caused by urban 

features, as the sounding was collected about 330 ft (100 m) 

west of a large commercial building. The sounding was collected there because it was the only area available on what was 

thought to be the west side of the transition zone. Adjacent 

sounding 330 also indicates low resistivity (about 15 ohm-m 

[appendix 5, fig. 5.8]) that is in a range indicative of saline 

water. Similarly, soundings 340 and 350 indicate lower resistivity (appendix 5, figs. 5.9, 5.10) indicative of saline water in 

the upper part of the aquifer. The top of the Edwards aquifer 

at soundings 360 and 370 is deeper than 656 ft, thus the depth 

of the aquifer there exceeds the capability of the soundings 

to yield reliable data. The four soundings of profile 3 that 

penetrate the top of the aquifer indicate saline water, which 

in turn indicates that the transition zone is west of the westernmost sounding (320). The lateral location of the interface 

along the profile based on DS concentrations (fig. 9A) appears 

to be between soundings 320 and 330. The lack of agreement 

between the location of the transition zone based on resistivity and the location based on DS concentrations cannot be 

explained with available data.

Profile 4

Profile 4 comprises two alternate profile lines designated profiles 4.1 and 4.2 (fig. 3). Each of these contains five 

soundings (figs. 10, 11). The two profiles are the same except 

that the westernmost sounding is 410 in profile 4.1 and 470 in 

profile 4.2. Two profiles were done because of the relatively 

large north-south distance between soundings 410 and 470, 

necessitated by lack of permission to access property. The 

profiles differ substantially in orientation relative to points of 

the compass. It is evident from both profiles that resistivity of 

the Edwards aquifer decreases from west to east. In profile 

4.1 (fig. 10B, C), sounding 410 shows relatively high resistivity in the aquifer (about 30–100 ohm-m) (appendix 5, 

fig. 5.13) indicative of freshwater. Soundings 440 and 450 

show progressively decreasing resistivity toward the east, 

and the transition to saline water appears to occur near sounding 440. Low resistivity at soundings 460 and 420 on the 

east end of the profile are interpreted as indicative of saline 

water. 

In profile 4.2 (fig. 11) in which sounding 470 replaces 

sounding 410, the expected indication of freshwater at sounding 470 (based on relatively high resistivity at sounding 

410) is missing; resistivity at sounding 470 is low and in a 

range indicative of saline water. This might be because of 

electromagnetic noise in the area—the west end of the 

profile was in a relatively urbanized part of the city of Buda. 

Although the ranges of resistivity in resistivity section 4.2 

(fig. 11C) are somewhat different from those in resistivity 

section 4.1 (fig. 10C), the pattern of resistivity and the inferred 

location of the freshwater/saline-water interface (around 

sounding 440) are essentially the same in both sections, as 

would be expected. The location of the interface along the two 

profiles based on DS concentrations (figs. 10A, 11A) appears 

to be farther east, in the area of soundings 450 and 460, but 

not substantially different from that indicated by the sounding 

data.

Profile 5

 Profile 5 (fig. 3) comprises seven TDEM soundings 

(fig. 12). Soundings 520 and 522, at the far western end of 

the profile, show resistivities indicative of freshwater. Moving eastward, resistivities indicate the probable location of 

the interface between soundings 524 and 530 (fig. 12C). At 

sounding 540, the resistivity clearly indicates saline water. 

Continuing eastward, however, relatively higher resistivities 

from soundings 550 and 560 indicate that the water is fresher 

Table 3. Root mean square (RMS) error between measured and calculated apparent resistivity for soundings, Barton Springs segment 

of the Edwards aquifer, Travis and Hays Counties, Texas. 

Profile 2 Profile 3 Profiles 4.1, 4.2 Profile 5

Sounding RMS error

(percent) Sounding RMS error(percent) Sounding RMS error(percent) Sounding RMS error(percent)

200 3.88 320 3.63 410 4.25 520 3.48

240 5.47 330 2.64 420 1.88 522 2.53

245 5.4 340 2.49 440 1.71 524 2.66

247 2.27 350 2.55 450 2.73 530 1.78

250 2.79 360 3.76 460 3.91 540 6.04

260 8.55 370 3.22 470 4.33 550 7.65

560 8.72

Delineation of Transition Zone Using Time-Domain Electromagnetic Soundings 19
at those sites than the water immediately to the west, an 

anomaly relative to what would be expected. The location 

of the interface in profile 5 based on DS concentrations (fig. 

12A) appears to be at about sounding 524, close to the location 

indicated by the resistivity data. The anomalous zone of relatively high resistivity toward the eastern end of profile 5 could 

not be confirmed by DS concentrations because of sparse DS 

data in that area; however, the presence of a possible fault 

could have a local effect on water quality.

Evaluation of Findings

On the basis of reasonably close matches between the 

inferred locations of the freshwater/saline-water transition 

zone in the Edwards aquifer in the study area from resistivities and from DS concentrations in three of the four profiles, 

TDEM sounding appears to be a suitable tool for delineating 

the transition zone. The fact that the Edwards aquifer lies 

about 200–500 ft below land surface in the area of the transition zone did not appear to adversely affect TDEM measurements. The ability to identify the transition zone from TDEM 

measurements was not hindered by the presence of relatively 

low-resistivity/high-conductivity lithologic units (Del Rio 

Clay, Eagle Ford Group, and Navarro and Taylor Groups 

[undifferentiated]) in the upper confining unit of the aquifer. 

Few sounding sites were near enough to urban features for 

the data to potentially have been affected. However, urban 

features might have adversely affected data from two of the 25 

soundings (soundings 320 and 470). Whether urban features 

adversely affected the sounding data could not be determined. 

Judicious site selection to avoid urban features to the extent 

possible is necessary in future TDEM applications in the Barton Springs segment of the Edwards aquifer.

Summary and Conclusions

During September 2006, the USGS, in cooperation with 

the Barton Springs/Edwards Aquifer Conservation District, 

conducted a geophysical pilot study to determine whether 

time-domain electromagnetic (TDEM) sounding could be used 

to delineate the freshwater/saline-water transition zone in the 

Barton Springs segment of the Edwards aquifer in Travis and 

Hays Counties, Texas. This application of TDEM sounding 

also was a test to indicate whether high-quality data could be 

obtained from the required depths (200–500 ft to the top of 

the aquifer plus a substantial thickness of the aquifer); and 

whether relatively low-resistivity clayey units that compose 

the upper confining unit of the aquifer would adversely affect 

investigators’ ability to identify the transition zone from 

TDEM measurements. The pilot application of TDEM sounding also was an opportunity to test whether urban features 

would adversely affect the sounding data. Twenty-five TDEM 

soundings were made along four 2–3-mi-long profiles in a 

study area overlying the transition zone. The soundings yield 

measurements of subsurface electrical resistivity, the variations 

in which were correlated with hydrogeologic and stratigraphic 

units, and then with dissolved solids (DS) concentrations in 

the aquifer. 

A geodatabase consisting of geologic, lithologic, geochemical, and TDEM data was developed. The geodatabase 

aided in the selection of sounding sites and was used to create 

two-dimensional sections of estimated true resistivity (electrical stratigraphy). 

Geonics Protem 47 and 57 systems with 492-ft (150-m) 

and 328-ft (100-m) transmitter-loop sizes were used to collect 

the TDEM soundings (voltage data from which resistivity is 

calculated). For each sounding, voltage data from five collected datasets were averaged and evaluated statistically—that 

is, voltages with standard deviation greater than 3 percent 

were deleted before inverse modeling. 

Inverse modeling is the process of creating an estimate of 

the true distribution of subsurface resistivity from the measured apparent resistivity obtained from TDEM soundings. A 

smooth model is a vertical delineation of calculated apparent 

resistivity that represents a (non-unique) estimate of the true 

resistivity. A smooth model for each sounding site was created 

using an iterative software program. Iterations were continued 

until the root mean square (RMS) error between measured 

and calculated apparent resistivity was less than 0.1 percent 

between iterations.

 The effectiveness of using TDEM soundings to delineate 

the transition zone was indicated by comparing the distribution 

of resistivity in the aquifer with the distribution of DS concentrations in the aquifer along the profiles. TDEM sounding 

data show that, in general, the Edwards aquifer in the study 

area is characterized by a sharp change in resistivity from west 

to east. The western part of the Edwards aquifer in the study 

area shows higher resistivity than the eastern part. The higher 

resistivity regions correspond to lower DS concentrations 

(fresh­water), and the lower resistivity regions correspond to 

higher DS concentrations (saline water). 

On the basis of reasonably close matches between the 

inferred locations of the freshwater/saline-water transition 

zone in the Edwards aquifer in the study area from resistivities and from DS concentrations in three of the four profiles, 

TDEM sounding appears to be a suitable tool for delineating 

the transition zone. The fact that the Edwards aquifer lies 

about 200–500 ft below land surface in the area of the transition zone did not appear to adversely affect TDEM measurements. The ability to identify the transition zone from TDEM 

measurements was not hindered by the presence of relatively 

low-resistivity lithologic units in the upper confining unit of 

the aquifer. Whether urban features adversely affected the 

sounding data could not be determined. Urban features might 

have adversely affected data from the two soundings near 

enough to urban features to have been potentially affected. 

Judicious site selection to avoid urban features to the extent 

possible is called for in future TDEM applications in the Barton Springs segment of the Edwards aquifer.

20 Geophysical Delineation of the Freshwater/Saline-Water Transition Zone, Barton Springs Segment, Edwards Aquifer
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