enhancement of usgs scientific investigations in texas by ... · surface geophysical methods in a...

• Surface geophysical methodso Frequency-domain electromagneticso Time-domain electromagneticso Capacitively coupled electromagneticso Direct-current resistivityo Magnetic resonance soundingso Ground-penetrating radarThe geophysical workgroup actively participates and col-

laborates with local, State, national, and international members of the environmental geophysical community. In addition, it regularly contributes to the knowledge base through presenta-tions and publications.

Example Applications of Geophysical TechniquesGeophysical project activities of the TXWSC have steadily

increased over the past 10 years. All projects mentioned below (fig. 1) have a substantial geophysical component. Each of the projects listed was published during 2005–10. This is not a complete list of projects, but it highlights examples of several geophysical techniques and their applications.

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

Fact Sheet 2011–3037 May 2011

Enhancement of USGS Scientific Investigations in Texas by Using Geophysical Techniques, 2005–10

Printed on recycled paper

IntroductionGeophysical techniques are an increasingly important

tool for scientific investigations, environmental planning, and resource management. During 2005–10 the U.S. Geological Survey (USGS) Texas Water Science Center (TXWSC) greatly expanded its capabilities of using surface and borehole geophys-ical techniques to gain insights into how groundwater systems work and the occurrence and distribution of certain contami-nants. Geophysical techniques provide a relatively quick and inexpensive means to characterize the subsurface hydrology and lithology.

Surface and borehole geophysical methods are used to measure the physical properties of the subsurface, such as elec-trical conductivity or resistivity, dielectric permittivity, magnetic permeability, density, and acoustic velocity. Geophysical mea-surements can be influenced by chemical and physical proper-ties of soils, rocks, and pore fluids. Interpretations from these measurements can be used to image the distribution of physical properties in the subsurface. Surface and borehole geophysical methods and their application to groundwater and environmen-tal investigations are described in detail in Zohdy and others (1974), Keys (1990), and U.S. Army Corps of Engineers (1995).

This report describes the technical capabilities and proj-ect activities of the TXWSC geophysical workgroup during 2005–10. After a brief summary of the geophysical workgroup’s capabilities, the details of project activities are described. Proj-ects are grouped by the primary techniques used and include overviews, project images, and Internet links to additional project material and related publications.

Geophysical Workgroup Skills and Capabilities The TXWSC geophysical workgroup consists of geophysi-

cists and hydrologists who specialize in surface and borehole geophysical techniques. The workgroup collaborates with resi-dent natural resource scientists on a diverse range of projects. There is emphasis on technical training to keep skills current with advances in software and technology. The workgroup’s technical capabilities include but are not limited to the following:• Borehole logging techniques

o Natural gammao Electromagnetic inductiono Long and short normal resistivityo Heat pulse and electromagnetic flow metero Neutron densityo Acoustic and optical televiewero Fluid resistivity/conductivityo Full wave sonic

In cooperation with the Barton Springs/Edwards Aquifer Conservation District

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

SScciieennttiiffiicc IInnvveessttiiggaattiioonnss RReeppoorrtt 22000077––55224444

UU..SS.. DDeeppaarrttmmeenntt ooff tthhee IInntteerriioorrUU..SS.. GGeeoollooggiiccaall SSuurrvveeyy

In cooperation with the Texas Water Development Board

Application of Surface Geophysical Methods, With Emphasis on Magnetic Resonance Soundings, to Characterize the Hydrostratigraphy of the Brazos River Alluvium Aquifer, College Station, Texas, July 2006— A Pilot Study

Scientific Investigations Report 2007–5203

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



In cooperation with the San Antonio Water System

An Integrated Hydrogeologic and Geophysical Investigation to Characterize the Hydrostratigraphy of the Edwards Aquifer in an Area of Northeastern Bexar County, Texas

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


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

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


Open-File Report 2009–1017

In cooperation with the U.S. Department of Energy/National Nuclear Security Administration and Babcock & Wilcox Technical Services Pantex, LLC

Analysis of Vertical Flow During Ambient and Pumped Conditions in Four Monitoring Wells at the Pantex Plant, Carson County, Texas, July–September 2008



In cooperation with the San Antonio Water System

Lithologic and Physicochemical Properties and Hydraulics of Flow in and near the Freshwater/Saline-Water Transition Zone, San Antonio Segment of the Edwards Aquifer, South-Central Texas, Based on Water-Level and Borehole Geophysical Log Data, 1999–2007

500

600

700

800

900

1,000

GeorgetownFormation

Pers

on F

orm

atio

nKa

iner

For

mat

ion

Cyclic andmarine

members

Leached andcollapsedmembers

Regionaldense member

Grainstonemember

Kirschbergevaporitemember

Dolomiticmember

Basal nodularmember

Glen RoseLimestone

Figure 1. Map showing the cover page of each report presented in this report. Lines point to approximate location of each study.

Surface Geophysical Techniques

Direct Current Resistivity

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

In 1995 the Mexico Section of the International Boundary and Water Commission (IBWC) completed a study (including surface geophysics, borehole geophysics, and installation of piezometers) of subsurface conditions of the western embankment (Coahuila, Mexico) of Amistad Reservoir Dam. Technical advisors recommended that one line of similarly constructed piezometers be installed on the eastern embankment of the dam for comparison of water levels (potentiometric head) on both the western and eastern embank-ments of the dam. To provide technical assistance for the horizontal and vertical placement of piezometers on the eastern embankment of Amistad Reservoir Dam, the USGS, in coop-eration 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 charac-teristics 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 correlation of surface geophysical DC resistivity, historical lithologic data, and general trend of documented sinkholes along the eastern end of the eastern embankment profile was used to justify further exploration (drilling of piezometers). The spatial location of the piezometers and screened intervals were selected to best match the locations of the screened intervals of the western embank-ment piezometers. • USGS Scientific Investigations Report 2007–5143: http://

pubs.usgs.gov/sir/2007/5143/

Capacitively Coupled Resistivity

Two-Dimensional Resistivity Investigation Along West Fork Trinity River, Naval Air Station-Joint Reserve Base Carswell Field, Fort Worth, Texas, October 2004

The USGS, in cooperation with the Naval Air Station-Joint Reserve Base Carswell Field (NAS–JRB) at Fort Worth, Tex., conducted a 2D resistivity investigation at a site to characterize the

distribution of subsurface resistivity. Contaminants, primar-ily volatile organic compounds and metals, have entered the groundwater flow system through leakage from waste-disposal sites and manufacturing processes. The Goodland Limestone and the underlying Walnut Formation form a confining unit that underlies the alluvial aquifer. The bedrock confining unit is the zone of interest because of the potential for contaminated groundwater to enter the river through saturated bedrock. The study involved a capacitively coupled resistivity survey and inverse modeling to obtain true or actual resistivity from appar-ent resistivity. • USGS Data Series Report 178–06: http://pubs.usgs.gov/

ds/ds178/

Magnetic Resonance Soundings

Application of Surface Geophysical Methods, With Emphasis on Magnetic Resonance Soundings, to Characterize the Hydrostratigraphy of the Brazos River Alluvium Aquifer, College Station, Texas, July 2006—A Pilot Study

The USGS, in cooperation with the Texas Water Development Board, used

surface geophysical methods in a pilot study to characterize the hydrostratigraphic properties of the Brazos River alluvium aqui-fer at the Texas A&M University Brazos River Hydrologic Field Research Site near College Station, Texas, and determine the effectiveness of the methods to aid in generating an improved groundwater-availability model. Three noninvasive surface geophysical methods were used to characterize the electrical stratigraphy and hydraulic properties and to interpret the hydro-stratigraphy of the Brazos River alluvium aquifer. Two methods, time-domain electromagnetic (TDEM) soundings and two-dimensional direct-current (2D–DC) resistivity imaging, were used to define the lateral and vertical extent of the near-surface hydrogeologic zones. Magnetic resonance sounding (MRS), a recently developed geophysical method, was used to derive esti-mates of the hydrologic properties including percentage water content and hydraulic conductivity. Results from the geophysics study demonstrated the usefulness of combined TDEM, 2D–DC resistivity, and MRS methods to reduce the need for additional boreholes in areas with data gaps and to provide more accurate information for groundwater availability models.• USGS Scientific Investigations Report 2007–5203: http://


Time-Domain Electromagnetics

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

The USGS, in cooperation with the Barton Springs/Edwards Aquifer Conservation District, conducted a pilot geophysical study to use TDEM sound-

Lake Worth

West Fork

Tri nity R iver

and 3 ea

0.5 MILE0 0.25

EXPLANATION

NAS–JRB boundary

Boundary of resistivityinvestigation site

Concentration of TCE, in micrograms per liter

5 to 50

50.1 to 500

500.1 to 1,000

1,000.1 to 5,000

5,000.1 to 15,000

More than 15,000.1

Base map USGS Digital Orthophoto Quarter Quadrangle - 2002 State Plane Coordinate System, North Central Zone, NAD 83

32o46'

97o25'

32o47'

97o24'

MWA2-4MWB2-4MWC3-4

45

50

55

60

65

70

35

40

45

50

55

60

65

70

25

30

35

40

45

50

55

60

65

70

50 100 150 200 250 300 350 400 450 500 550

DISTANCE, IN METERS

BRA09BRA100

BRA110BRA160BRA170

BRA180BRA60

BRA70BRA80

BRA90BRA01

BRA05BRA40

BRA50

DC resistivity profile

MWA2-4MWB2-4

MWC3-4

746,700 746,800 746,900 747,000 747,100 747,200

3,38

2,80

0

3,382,900

EASTING, IN METERS

NO

RTH

ING

, IN

ME

TER

S

BRA05BRA09BRA100

BRA110BRA160

BRA170BRA40

BRA50

BRA60

BRA70

BRA80

BRA90BRA180

BRA01

lessthan 10 20 32 54 89

greaterthan 175

Direct-current resistivity, in ohm-metersClay

Sand and gravel

Water level

Direct-current resistivity profile

Time-domain electromagnetic soundingand identifier

EXPLANATION

Time-domain electromagneticlayered-earth model sounding

BRA170

MWC3-4

Lithology

Monitoring well and identifier Shale

)D()C()B(

ALT

ITU

DE

, IN

ME

TER

S A

BO

VE

NAV

D 8

8

APPROXIMATE LAND SURFACE

500 ft

200

500 ft

240

500 ft

245

500 ft

247

500 ft

250

500 ft

260

ALLT

ITU

DE,

INFE

ETAB

OVE

NG

VD29

A

B

-100

0

100

200

300

400

500

600

700

0 50 100150200250

0 50 100 0 50 0 50 100 0 50 0 50

EXPLANATIONTaylor Group

Austin Group

Eagle Ford Group

Buda Limestone

Del Rio Clay

Georgetown Formation

Edwards Group

Regional dense member TDEM apparent resistivity,in ohm-meters

Edw

ards

aqui

fer

Upp

erco

nfin

ing

unit

TDEM soundingand identifier

1,000 0 1,000 FEET

-100

0

100

200

300

400

500

600

700

Top of Edwards aquifer

Approximate land-surface elevation

ALTI

TUD

E,IN

FEET

ABO

VEN

GVD

29

200

240

245

247

250

260

C6 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

240245

250260

247

DC resistivity profile

Spring

2

Spring

3

GG15

GG17

200 m

200 m

W EA70E

A70W

A72E

A72W

A74E

A74W

A76E

A76W

A80E

A80W

LA-6

LA-7

LA-8L

LA-10

LA-11

LA-12L

LA-14

LA-15

LA-16L

Profile 1

Profile 2

240

250

260

270

280

290

300

310

320

330

340

DISTANCE, IN METERS

ELE

VA

TIO

N, I

N M

ETE

RS

AB

OV

E N

AV

D 8

8

less than75 166 256

greater than365

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 limestoneTop of limestoneArgillaceous fossiliferous zone

Section trace line

EXPLANATION

Selected core hole and identifier

Piezometer and identifier—Underlined identifier indicates logged well

Spring and identifier

Sinkhole and identifier

.

A

B

feet or meter

F other3servZM

latitude 29 40 58.4

deltat matrix

44

mud tem

pN

Ares m

ud filtrate

elev kbN

Alog m

eas fromLSD

sys version3.59T

temp gradient

0m

ud resN

A

file type id

township

NA

time circ stopped

NA

eng units or cps E

recorded byTH

OM

AS

field office TWSC - AUSTIN

depth driller320 field

CSSA

rangeN

A

mean surface tem

p

casing typeSTEEL

casing bottom45

tool serial num

deltat fluid

longitude -098 38 18.8

fluid viscosity

SERVICE COMPANY USGS

bit size6.125

01.

tni

elp

mas

gol

Uno

itce

rid

gol

elect cutoff9999

elev perm datum

1179

countyB

EXA

R

truck cal num.599

mud sam

ple sourceN

A

temp m

ud cake

LOC

ATIO

N

casing diameter

10.

drl meas from

NA

permanent datum

LSD

Nam

e Prn AP

UN

IQU

E WELL ID

neutron matrix

DO

LOM

ITE

fluid typeH

2O

fluid ph

company

CSSA

- U.S. G

EOLO

GIC

AL SU

RV

EY

elev dfN

A

stateTX

PRO

VIN

CE

temp m

ud filtrate

file type ORIGINAL

sys serial1

remarks1

AM

BIEN

T

other1servZE

log bottom 328.60

time

12:13:

sectionN

A

density matrix

2.85

logging unit302

remarks2

WL = 156.46

casing thick0 elev gl

1179

log top 2.90

well

I10-4

DA

TE02/17/11

other2servZI

mag declination

0

res mud cake

NA

fluid density

Depth

1ft:120ft

GAM(NAT)

0 150CPS

RES(FL)

0 35OHM-MSP

75 175MV

RES(16N)

1 1000OHM-MRES(64N)

1 1000OHM-M

TEMP

68 73DEG F

LATERAL

1 1000OHM-M

DEL TEMP

-1 1DEG FCALIPER

0 10INCHCCL

60000 63000CPS

IND. Res

1 1000OHM-MAP-COND

0510 MMHO/M

SP COND

300 350US/CMTool.Image-NM

°0°0 180°90° 270°

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

300

310

320

Depth

1ft:120ft

GAM(NAT)

0 150CPS

RES(FL)

0 35OHM-M

SP

75 175MVRES(16N)

1 1000OHM-M

RES(64N)

1 1000OHM-MTEMP

68 73DEG F

LATERAL

1 1000OHM-MDEL TEMP

-1 1DEG F

CALIPER

0 10INCH

CCL

60000 63000CPS

IND. Res

1 1000OHM-M

AP-COND

0510 MMHO/M

SP COND

300 350US/CM

Tool.Image-NM

°0°0 180°90° 270°

NORTH SOUTH

NNN

27 103 206 308 411 513 616 718 821 923

RESISTIVITY, IN OHM-METERS

EXPLANATION

Time-domain electromagnetic sounding site and identi�er

Site–12

Fault projection (Stein and Ozuna, 1995)

Interpreted fault location fromgeophysical data

Indicates change in location of fault, basedon interpretation of geophysical data

Interpreted karst feature fromgeophysical data

K

Geologic unit (hydrostratigraphic zone) (table 1)Kkd (VII)

Site–12 Site–13

Kkd (VII)

Kkd (VII)

Bat Cave fault(Stein and Ozuna, 1995)

Kkd (VII)

Bat Cave faultinterpreted from

surface geophysics

Kplc (III)

Kplc (III) Kprd (IV)

Bat Cave fault(Stein and Ozuna, 1995)

Bat Cave faultinterpreted from

surface geophysics

KK

ALT

ITU

DE,

IN M

ETER

SA

BOVE

NAV

D 8

8

245255265275285295305315325

CAPACITIVELYCOUPLEDSURVEY

CAPACITIVELYCOUPLEDSURVEY

Site–13

Site–12

LINE

Bat Cave fault(George, 1952; Stein and Ozuna, 1995)

Unnamed fault

Geologic units, hydrostratigraphic zones, and fault locations interpretedfrom geophysical data and hydrogeologic mapping for this report

125 0 125 METERS

Vertical scale greatly exaggerated

Altitude of log datum (ft above NAVDof 1988) 3539.11Casing depth (ft) 613

Log datum LSD

Well name PTX06–1044County Carson

Borehole bit size (in) 7.865Borehole depth (ft) 622

Casing diam. (in) 4

Latitude N35 deg. 19' 44"Date of log 7/25,8/11,9/24/08

Site ID 351944101324201Owner Pantex

State TexasLongitude W101 deg. 32' 42"

Depth ConstructionEMFM Amb Before

-5 5Gal/Min

Neutron

0 50CPS

Natural Gamma

0 150CPS

RES(FL) Amb Before

15 20OHM-MEMFM Pu Before

-5 5Gal/Min

WL Depth Before

0Ft005 54WL Depth After

0Ft005 54Trans. Before

0.5 500Ft^2/D

TEMP Amb. Before

63 73DEG FTEMP Pu Before

63 73DEG F

RES(FL) Pu Before

15 20OHM-MEMFM Amb After

-5 5Gal/MinTrans. After

0.5 500Ft^2/DStatic WL

0 1

EMFM Pu After

-5 5gpm

TEMP Amb After

63 73DEG F

RES(FL) Amb. After

15 20OHM-MRES(FL) Pu After

15 20OHM-M

400

450

500

550

600

473

482

481

481

481

473

9/24/08

34 Lithologic and Physicochemical Properties and Hydraulics of Flow, Freshwater/Saline-Water Transiti on Zone

Estimated �ow less than0.1 gallon per minute (gpm)

1

Corrected �owstation 05/19/2005



gpm

gpm

gpm

Zone of in�ow or out�ow

Depth

1 in

ch:1

00 fe

et

Acoustic televiewer

0° 0°180°90° 270°

1000 cps

GAM(NAT)

122 inch

CALIPER

-1

1-1

1-1Corrected �owstation 11/06/2007

gpm 1-1

Stratigraphic unit

24.2 29.2

COND 05/19/2005

TEMP 06/20/2006

TEMP 09/13/2006

23 28

COND 09/13/2006

10000 35000COND 11/06/2007

7000 32000

TEMP 11/06/2007

25 30

deg C

deg C

deg C

deg C

10000 35000µS/cm

µS/cm

µS/cm

Gen

eral

ized

cal

iper

pro

�le

Conductance (COND), inmicrosiemens

per centimeter (µS/cm)Temperature (TEMP), indegrees Celsius (deg C)Natural gamma (GAM(NAT)), in

counts per second (cps)

TEMP 05/19/2005

23.8 28.8µS/cmµS/cm

COND 06/20/2006

10000 35000

23.7 28.7

COND 11/08/2002

deg C25000 50000µS/cm

TEMP 11/08/2002

µS/cmµS/cmµS/cm

600

700

800

900

Note: “Corrected” indicates that EM �owmeter data sets were corrected by zeroing out values measured in intervalswhere no �ow was expected, such as in the casing, and by subtracting those values from �owmeter measurements madein open borehole; this adjustment allowed for more consistent �owmeter data sets and well hydraulic interpretations.

GeorgetownFormation

Pers

on F

orm

atio

nKa

iner

For

mat

ion

Cyclic andmarine members

Leached andcollapsedmembersRegional

dense member

Grainstonemember

Kirschbergevaporitemember

Dolomiticmember

Basal nodularmember

570

575

580

585

590

595

600

JAN

FEB

MA

RA

PRM

AY

JUN

EJU

LYA

UG

SEP

OCT

NO

VD

ECJA

NFE

BM

AR

APR

MA

YJU

NE

JULY

AU

GSE

PO

CTN

OV

DEC

JAN

FEB

MA

RA

PRM

AY

JUN

EJU

LYA

UG

SEP

OCT

NO

VD

EC

700260025002

ALT

ITU

DE,

IN F

EET

ABO

VE N

ORT

H A

MER

ICA

N V

ERTI

CAL

DA

TUM

OF

1988

EXPLANATION

Geophysical data collected May 2005

Geophysical data collected June 2006

Geophysical data collected September 2006

Geophysical data collected November 2007

Daily mean equivalent freshwater head, in feet above North AmericanVertical Datum of 1988.Water-level data providedby the San Antonio WaterSystem

DATE

Figure 8. Daily mean equivalent freshwater head and borehole geophysical data in Kyle 4 well (LR–67–02–105), San Antonio segment of the Edwards aquifer, south-central Texas, 2005–07.

ing to delineate the freshwater/saline-water transition zone of the Edwards aquifer in Travis and Hays Counties, Tex.. 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- to 3-mile-long profiles in a study area overlying the transition zone. The soundings yielded 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. On the basis of reasonably close matches between the inferred locations of the freshwater/saline-water transition zone from resistivities and from dissolved-solids concentrations in three of the four profiles, TDEM sounding appears to be a suit-able tool for delineating the transition zone.• USGS Scientific Investigations Report 2007–5244: http://


Frequency-Domain Electromagnetic Profiling

An Integrated Hydrogeologic and Geophysical Investigation to Characterize the Hydrostratigraphy of the Edwards Aquifer in an Area of Northeastern Bexar County, Texas

The USGS, in cooperation with the San Antonio Water System, did a hydrogeologic and geophysical investigation to characterize the hydrostratigraphy (hydrostratigraphic zones) and karst features (features such as sinkholes and caves) of the Edwards aquifer in a 6.2-square-mile area undergoing urban development. Existing hydrostrati-graphic information, enhanced by local-scale geologic mapping in the area, and surface geophysics were used to associate ranges of electrical resistivities, obtained from capacitively coupled resistivity surveys, frequency-domain electromagnetic (FDEM) surveys, TDEM soundings, and 2D-DC resistivity surveys, with each of seven hydrostratigraphic zones of the Edwards aquifer. The principal finding of this investigation is the relation between electrical resistivity and the contacts between the hydrostratigraphic zones of the Edwards aquifer and the underlying Trinity aquifer in the area. • USGS Scientific Investigations Report 2008–5181: http://


Borehole Geophysical Techniques

Analysis of Vertical Flow During Ambient and Pumped Conditions in Four Monitoring Wells at the Pantex Plant, Carson County, Texas, July–September 2008

The USGS, in cooperation with B&W Pantex, made a series of flowme-ter measurements and collected other borehole geophysical logs during July–September 2008 to analyze vertical flow

in screened intervals of four selected monitoring wells at the Pantex Plant. Hydraulic properties (transmissivity values) of the section of the High Plains (Ogallala) aquifer penetrated by the wells also were computed. Geophysical data were col-lected under ambient and pumped flow conditions in the four monitoring wells. Unusually large drawdowns occurred at two monitoring wells while the wells were pumped at relatively low rates. A decision was made to redevelop those wells, and logs were run again after redevelopment in the two monitoring wells. • USGS Open-File Report 2009–1017: http://pubs.usgs.

gov/of/2009/1017/

Lithologic and Physicochemical Properties and Hydraulics of Flow in and near the Freshwater/Saline-Water Transition Zone, San Antonio Segment of the Edwards Aquifer, South-Central Texas, Based on Water Level and Borehole Geophysical Log Data, 1999–2007

The freshwater zone of the Edwards aquifer in south-central Texas is bounded

to the south and southeast by a zone of transition from fresh-water to saline water (hereinafter, the transition zone). The boundary between the two zones is the freshwater/saline-water interface (hereinafter, the interface), defined as 1,000-mil-ligrams per liter of dissolved solids. This report presents the findings of a study, done by the USGS in cooperation with the San Antonio Water System, to obtain lithologic properties and physicochemical properties and to analyze the hydraulics of flow in and near the transition zone of the Edwards aquifer on the basis of water level and borehole geophysical log data collected from 15 monitoring wells in four transects during 1999–2007. A hydraulic connection between the transition zone and the freshwater zone is indicated by similar patterns in the hydrographs of the 15 transect monitoring wells in and near the transition zone and three county index wells in the fresh-water zone during 1999–2007. The data for this report support, in part, a conceptualization of regional flow in and near the transition zone in which water enters the transition zone from the freshwater zone in the western part of the Edwards aquifer, flows toward the northeast, and discharges to the freshwater zone in the eastern part of the aquifer. The data for this report support the hypothesis that the interface is likely to remain laterally and vertically stable over time. • USGS Scientific Investigations Report 2010–5122: http://

pubs.usgs.gov/sir/2010/5122

References Cited

Keys, W.S., 1990, Borehole geophysics applied to ground water investigations: U.S. Geological Survey Techniques of Water-Resources Investigations, book 2, chap. E2, 150 p.

U.S. Army Corps of Engineers, 1995, Geophysical exploration for engineering and environmental investigations: Engineer-ing Manual 1110-1-1802, chap. 4, 57 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 Investiga-tions, book 2, chap. D1, 116 p.

Geophysical Workgroup Contact InformationGregory P. Stanton Jason D. PayneGroundwater Specialist Geophysicist1505 Ferguson Lane 944 Arroyo StreetAustin, Tx 78754 San Angelo, Tx 76903512-927-3558 325-944-4600 x-28

Andrew P. Teeple Jonathan V. ThomasHydrologist Geophysicist944 Arroyo Street 2775 Alta Mesa Blvd.San Angelo, Tx 76903 Fort Worth, Tx 76133325-944-4600 x-17 817-263-9545 x-228

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For additional information, contact DirectorUSGS Texas Water Science Centerhttp://tx.usgs.gov/ [email protected]

Time-domain electromagnetic sounding layout and operation

Transmitter loop

Protem-47 transmitter

Eddy currents

Receiver loopReceiver

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