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 
Cover:  Brazos River at Batts Ferry Road, Brazos County, Texas, February 2007 (photograph by Jeff East,

U.S. Geological Survey).
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

By Sachin D. Shah, Wade H. Kress, and Anatoly Legchenko

In cooperation with the Texas Water Development Board

Scientific Investigations Report 2007–5203

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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reproduce any copyrighted materials contained within this report.

Suggested citation:

Shah, S.D., Kress, W.H., and Legchenko, Anatoly, 2007, 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: U.S. Geological Survey Scientific Investigations Report 2007–5203, 21 p. 
Contents

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

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

Purpose and Scope .............................................................................................................................2

Acknowledgments ...............................................................................................................................2

Site Description and Hydrostratigraphic Setting ....................................................................................2

Surface Geophysical Resistivity Methods ................................................................................................5

Time-Domain Electromagnetic Soundings ......................................................................................7

Two-Dimensional Direct-Current Resistivity ...................................................................................8

Surface Geophysical Magnetic Resonance Sounding Method ...........................................................8

Inverse Modeling Results From Surface Geophysical Methods ..........................................................9

Time-Domain Electromagnetic Soundings ....................................................................................12

Two-Dimensional Direct-Current Resistivity .................................................................................12

Magnetic Resonance Soundings ....................................................................................................14

Analysis of Results From Surface Geophysical Methods ....................................................................16

Electrical Stratigraphy and Hydrostratigraphic Framework ......................................................16

Hydrostratigraphy ..............................................................................................................................17

Hydrostratigraphic Unit Parameterization .....................................................................................19

Summary .......................................................................................................................................................19

References ...................................................................................................................................................20

Figures

1. Map showing location of the Texas A&M University Brazos River Hydrologic Field 

Research Site (study area), College Station, Texas ...............................................................3

2. Conceptual hydrostratigraphic section A-A’ across the Brazos River Hydrologic 

Field Research Site, College Station, Texas ............................................................................4

3. Map showing locations of well nests at the Brazos River Hydrologic Field 

Research Site, College Station, Texas .....................................................................................6

4. Schematic of well nest layout, Brazos River Hydrologic Field Research Site, 

College Station, Texas .................................................................................................................7

5. Graphs showing magnetic resonance sounding (MRS) inversion results for Brazos 

River Hydrologic Field Research Site, College Station, Texas: (A) phase relative to 

pulse moment, (B) depth relative to water content, (C) depth relative to hydraulic 

conductivity, and (D) pulse moment relative to signal amplitude with signal-tonoise indication ..........................................................................................................................10

6. Diagram and graphs showing: (A) location of selected wells and time-domain 

electromagnetic (TDEM) soundings, (B) drillers’ logs with static water level, 

(C) TDEM layered-earth model sounding results, and (D) robust inversion profile 

of two-dimensional direct-current dipole-dipole resistivity array, Brazos River 

Hydrologic Field Research Site, College Station, Texas .....................................................13

7. Calibration graph used to obtain optimum relaxation time (T1*) for inversion of 

magnetic resonance sounding data using the default coefficient of permeability 

(Cp) value of 7.00 x 10-9, Brazos River Hydrologic Field Research Site, College 

Station, Texas .............................................................................................................................15

iii
8. Diagram and graphs showing: (A) location of selected wells and magnetic 

resonance soundings (MRS), (B) drillers’ logs with static water level and 

locations of hydraulic conductivity measurements, (C) gridded percentage 

water content profile, and (D) gridded hydraulic conductivity profile based on 

MRS inversion results, Brazos River Hydrologic Field Research Site, College 

Station, Texas .............................................................................................................................18

Tables

1. Summary of average hydraulic conductivity of shallow, intermediate, and deep 

wells for each well nest at the Brazos River Hydrologic Field Research Site, 

College Station, Texas .................................................................................................................5

2. Approximate relations between relaxation time (T1*), mean pore size, and relative 

hydraulic conductivity .................................................................................................................8

3. Matrix parameters used in magnetic resonance sounding inversion, Brazos River 

Hydrologic Field Research Site, College Station, Texas .......................................................9

4. Time-domain inversion results, Brazos River Hydrologic Field Research Site, 

College Station, Texas ...............................................................................................................12

5. Minimum, maximum, and optimum transmissivity, saturated thickness, and 

magnetic resonance sounding (MRS)-derived hydraulic conductivity values 

obtained from default coefficient of permeability (Cp) value (7.00 x10-9), Brazos 

River Hydrologic Field Research Site, College Station, Texas ...........................................14

6. Sounding, corresponding monitoring well nest, average hydraulic conductivity 

from 1996 aquifer tests, and average monitoring well nest hydraulic conductivity 

used to calibrate coefficient of permeability (Cp), Brazos River Hydrologic Field 

Research Site, College Station, Texas ...................................................................................15

7. Corrected coefficient of permeability (Cp) value computed using magnetic 

resonance sounding (MRS)-derived hydraulic conductivity from default Cp 

value (7.00 x10-9), average monitoring well nest hydraulic conductivity, and 

correction factor, Brazos River Hydrologic Field Research Site, College Station, 

Texas ............................................................................................................................................16

8. Minimum, maximum, and optimum transmissivity and magnetic resonance 

sounding (MRS)-derived hydraulic conductivity values obtained from corrected 

coefficient of permeability (Cp) value (2.61 x10-8) compared with average 

hydraulic conductivity values from closest monitoring well nest calculated 

from 1996 aquifer tests, Brazos River Hydrologic Field Research Site, College 

Station, Texas .............................................................................................................................17

iv
Conversion Factors and Datums

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square hectometer (hm2) 2.471 acre

square meter (m2) 10.76 square foot (ft2) 

Hydraulic conductivity

meter per day (m/d) 3.281 foot per day (ft/d) 

Transmissivity

meter squared per day (m2/d) 10.76 foot squared per day (ft2/d) 

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

Datums

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

(NAVD 88).

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

v
Blank Page 
Abstract

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

Texas Water Development Board, used surface geophysical 

methods at the Texas A&M University Brazos River Hydrologic Field Research Site near College Station, Texas, in a 

pilot study, to characterize the hydrostratigraphic properties of 

the Brazos River alluvium aquifer and determine the effectiveness of the methods to aid in generating an improved groundwater availability model. Three non-invasive surface geophysical methods were used to characterize the electrical 

stratigraphy and hydraulic properties and to interpret the 

hydrostratigraphy 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 Ships clay, the alluvium of the Brazos River alluvium 

aquifer, and the underlying Yegua Formation. Magnetic 

resonance sounding (MRS), a recently developed geophysical method, was used to derive estimates 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 ground-water availability models. Stratigraphically, the 

principal finding of this study is the relation between electrical 

resistivity and the depth and thickness of the subsurface hydrostratigraphic units at the site. TDEM data defined a three-layer 

electrical stratigraphy corresponding to a conductor-resistorconductor that represents the hydrostratigraphic units—the 

Ships clay, the alluvium of the Brazos River alluvium aquifer, 

and the Yegua Formation. Sharp electrical boundaries occur at 

about 4 to 6 and 20 to 22 meters below land surface based on 

the TDEM data and define the geometry of the more resistive Brazos River alluvium aquifer. Variations in resistivity 

in the alluvium aquifer ranging from 10 to more than 175 

ohm-meters possibly are caused by lateral changes in grain 

size. Resistivity increases from east to west along a profile 

away from the Brazos River, which signifies an increase in 

grain size within the alluvium aquifer and therefore a more 

productive zone with more abundant water in the aquifer. MRS 

data can help delineate the subsurface hydrostratigraphy and 

identify the geometric boundaries of the hydrostratigraphic 

units by identifying changes in the free water content, transmissivity, and hydraulic conductivity. MRS data indicate that 

most productive zones of the alluvium aquifer occur between 

12 and 25 meters below land surface in the western part of the 

study area where the hydraulic conductivity can be as high as 

250 meters per day. Hydrostratigraphically, individual hydraulic conductivity values derived from MRS were consistent 

with those from aquifer tests conducted in 1996 in the study 

area. Average hydraulic conductivity values from the aquifer 

tests range from about 61 to 80 meters per day, whereas the 

MRS-derived hydraulic conductivity values range from about 

27 to 97 meters per day. Interpreting an interpolated profile 

of the hydraulic conductivity values and individual values 

derived from MRS can help describe the hydrostratigraphic 

framework of an area and constrain ground-water models for 

better accuracy.

Introduction

The Brazos River alluvium aquifer is used primarily 

as a source of supply for drinking water and agriculture in 

Texas and is defined by the Texas Water Development Board 

(TWDB) as a minor aquifer (Ashworth and Hopkins, 1995). 

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 

By Sachin D. Shah1, Wade H. Kress1, and Anatoly Legchenko2

1

 U.S. Geological Survey.

2

 Bureau de Recherches Géologiques et Minières, Orleans Cedex, France. 
A projected doubling of the Texas population by the year 

2050, as well as the constant threat of drought, necessitates the 

development of effective water-management plans for aquifers to meet future water needs. Effective planning, however, 

requires accurate characterization of both the geology and 

hydrostratigraphy of the aquifer. A ground-water availability 

model (GAM) for the Brazos River alluvium aquifer needs 

valid hydraulic conductivity, transmissivity, storage coefficients and other properties for estimating ground-water 

availability to simulate changes in water levels in the aquifer 

due to pumping and drought conditions. The GAM includes 

information on hydrostratigraphic characteristics such as 

hydraulic conductivity, specific capacity, and transmissivity (Shah and Houston, 2007); however, numerous data gaps 

exist throughout the extent of the defined aquifer and require 

collection of additional information on the aquifer. Standard 

methods of data collection include installing wells, examining 

borehole logs, or conducting aquifer tests, but these methods 

can be time-consuming and expensive. An important aspect in 

filling in these data gaps is reducing the time and complexity 

of collecting valid data in areas that lack hydrostratigraphic 

information. In July 2006 the U.S. Geological Survey (USGS), 

in cooperation with the TWDB, conducted an integrated 

geophysical pilot study at the Texas A&M University Brazos 

River Hydrologic Field Research Site (BRHFRS) near College 

Station, Tex. (fig. 1), using two surface geophysical methods, time-domain electromagnetic (TDEM) soundings and 

two-dimensional direct-current (2D–DC) resistivity imaging, 

to measure the thickness, extent, and lateral variation in the 

resistivity of the hydrostratigraphic units—the Ships clay, the 

alluvium of the Brazos River alluvium aquifer, and the Yegua 

Formation. A third, more recently developed surface geophysical method, magnetic resonance sounding (MRS), was used 

to estimate the hydraulic properties, specifically percentage 

water content, transmissivity, and hydraulic conductivity at the 

BRHFRS. These data were integrated to identify the relations 

between the distribution of resistivity and hydraulic properties 

in the Brazos River alluvium aquifer and to identify hydrostratigraphic boundaries of the Ships clay, the alluvium of the 

Brazos River alluvium aquifer, and the Yegua Formation. 

Purpose and Scope

The purpose of this report is to document the application 

of MRS and other surface geophysical methods and to characterize the hydrostratigraphy of the Brazos River alluvium aquifer. A general overview of the TDEM and 2D–DC resistivity 

methods is given, an extensive explanation of the methods and 

interpretation of MRS data is provided, and inverse modeling results are presented to describe how the data obtained 

can be applied to validate aquifer properties for ground-water 

flow modeling. The MRS part of the pilot study was done to 

compare previously collected hydraulic conductivity data from 

aquifer tests in the study area with MRS-derived hydraulic 

conductivity data. The report documents an integrated surface 

geophysical approach in which the hydrostratigraphy of the 

Brazos River alluvium aquifer at the BRHFRS was interpreted 

from surface geophysical methods.

Acknowledgments

The authors thank Bruce D. Smith, USGS Crustal Imaging and Characterization Team, for his assistance with this 

report and explanation of the MRS applications. The authors 

thank Jean Bernard of Iris Instruments in Orleans, France, for 

his insight and technical support with processing and interpreting the MRS data.

Site Description and 

Hydrostratigraphic Setting

The BRHFRS encompasses 8.5 square hectometers of the 

Brazos River floodplain about 15 kilometers (km) southwest 

of College Station and 200 meters (m) west of the Brazos 

River (fig. 1). The BRHFRS was initially established in 1993 

at the Texas A&M University Research Farm to study groundwater flow and agricultural chemical transport in the Brazos 

River alluvium aquifer (Munster and others, 1996). Nests of 

wells at the BRHFRS were installed to monitor water quality 

and to assess horizontal and vertical ground-water gradients 

in the Brazos River alluvium aquifer. Numerous ground-water 

research projects have been conducted at the BRHFRS including river-aquifer interactions (Alden and Munster, 1997a), 

atrazine transport in ground water (Chakka and Munster, 

1997), bacteriophage transport studies (Corapcioglu and others, 2006), and an in-situ permeable ground-water flow sensor 

study (Alden and Munster, 1997b).

From oldest to youngest, the geologic units at the 

BRHFRS are the Tertiary Yegua Formation (fig. 2), a shale 

that functions as the basal confining unit of the alluvium 

aquifer at an average depth of about 21 m below land surface. 

The Yegua Formation is overlain by the Quaternary Brazos 

River alluvium, which is divided into two hydrostratigraphic 

units—an alluvium aquifer and an upper leaky confining unit 

(fig. 2). The Brazos River alluvium aquifer is characterized 

by a fining-upward sequence of coarse sand and gravel at the 

base to fine sand at the transition zone between the aquifer 

and the upper leaky confining unit. The upper leaky confining 

unit (locally named the Ships clay) varies in thickness from 

about 5 m in the western part of the site to 9 m near the Brazos 

River (Wrobleski, 1996). The transition from the Ships clay to 

the Brazos River alluvium aquifer is very abrupt with only a 

0.3- to 0.6-m transition zone consisting of a sandy clay layer 

(Munster and others, 1996). The water table in the aquifer generally is immediately below the clay at a depth of about 9 m 

during the summer. For this study, water-level measurements 

were obtained during surface geophysical data collection by 

electronic sensor. An approximate water table for BRHFRS is 

shown in figure 2. 

   Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Figure 1. Location of the Texas A&M University Brazos River Hydrologic Field Research Site (study area), College Station, Texas.

0 0 0 , 3 3 8 , 3 5 0 ,7 2 3 8 , 3 0 5 0 , 2 3 8 ,3

100 0 100 METERS

IN METERS , NG HI RT NO

EASTING, IN METERS

746,500 746,750 747,000 747,250 747,500

Base from U.S Department of Agriculture National Agricultural Imagery Program Digital Ortho Photo Image, August 2006Universal Transverse Mercator projection, Zone 14North American Datum of 1983

EXPLANATIO

N

Direct-current resistivity profile

Monitoring well nest and identifier (includes well numbers 1 through 4; see fig. 4)

Magnetic resonance sounding and identifierTime-domain electromagnetic sounding and identifier

A A’ Location of hydrostratigraphic section shown in figure 2

Study area boundary

MWC1

MRS5

BRA180

Brazos River

Drainage ditch

College Station

TEXAS

LOCATION MA

P

A

A’

MWA1

MWA2

MWA3

MWB1

MWB2

MWB3

MWC1

MWC2

MWC3

BRA60

BRA180 BRA170 BRA160

BRA110

BRA100BRA09

BRA90

BRA80

BRA70

BRA50

BRA40BRA05

BRA01

MRS5

MRS8

MRS12

MRS11

MRS7

Site Description and Hydrostratigraphic Setting 
Figure 2. Conceptual hydrostratigraphic section A-A’ across the Brazos River Hydrologic Field Research Site, College Station, Texas (modified from Wrobleski, 1996).

BRAZOS RIVER

ell MW W

A2-4

Well MWB2-4

Well MWC2-4 Constructed levee

h ainage ditc Dr

BURLESON COUNTY

BRAZOS COUNTY

0 100 METERS Vertical scale greatly exaggerated Location of section in figure 1

VD 88 TITUDE, IN METERS ABOVE NA AL

A

A’

Yegua Formation

(confining unit)

Alluvium

(Brazos River alluvium aquifer)

Ships clay

(leaky confining unit)

70

65

60

55

50

45

APPROXIMATE WATER TABLE (JULY 26, 2006)

 Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Nine well nests were drilled at the site to monitor groundwater flow (Munster and others, 1996) (fig. 3). A detailed 

analysis of the geology at the site could not be completed 

because of poor core recovery of saturated sands in the alluvium aquifer during drilling (Wrobleski, 1996). However, a 

hydrostratigraphic section was constructed on the basis of the 

drillers’ logs during the installation of the monitoring wells at 

the BRHFRS (Munster and others, 1996). Each well nest has 

four monitoring wells with 152-millimeter-long screens set 

at average depths of 7.2 m (not in the saturated zone), 11 m 

(shallow), 14.8 m (intermediate), and 18.3 m (deep) (fig. 4). 

The naming convention for the wells is shown in figures 3 and 

4. The well nest identifier (MWA1, 2, or 3; MWB1, 2, or 3; 

MWC1, 2, or 3) precedes a well number (1 through 4) for the 

shallowest to deepest wells. 

A series of aquifer tests were done at the BRHFRS 

in 1996 to obtain the hydraulic properties of the aquifer 

(Wrobleski, 1996). A summary of the average horizontal 

hydraulic conductivity values (referred to as hydraulic conductivity in this report) for each of the three depth zones (shallow, intermediate, deep) is listed in table 1. The average for 

each well is based on multiple aquifer tests for each interval.

Hydraulic conductivity values generally are largest in row C 

wells and smallest in row A wells. All hydraulic conductivity 

data from the 1996 aquifer tests were compared to the MRSderived hydraulic conductivity data.

Surface Geophysical Resistivity 

Methods 

Two surface geophysical methods were used to characterize the electrical stratigraphy of the BRHFRS. These methods 

were used to measure the thickness, extent, and lateral variation in the resistivity of the subsurface, which could then be 

used to define the correlation between electrical stratigraphic 

units and hydrostratigraphic units. The survey was designed 

so that multiple methods could be used to achieve a more comprehensive analysis of the subsurface at BRHFRS. Fourteen 

TDEM soundings and one 2D–DC resistivity profile were 

collected along an east-west profile about 480-m long (fig. 1) 

in proximity to monitoring wells at the site.

Surface geophysical resistivity methods can be used to 

detect changes in the electrical properties of the subsurface 

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

and rocks are determined by water content, porosity, clay 

content, and conductivity (reciprocal of electrical resistivity) 

of the pore water (Lucius and others, 2007). Typically, the 

resistivity of water has a large effect on the bulk resistivity of 

the subsurface. Two of the most commonly used methods for 

estimating subsurface variations in resistivity are TDEM and 

2D–DC resistivity, which can be used to determine the depth 

and lateral extent of clay, sand, and gravel; depth to water; and 

bedrock. Interpretations from these measurements can be used 

to image the distribution of physical properties in the subsurface. Comprehensive descriptions of TDEM and 2D–DC resistivity methods, as well as tables of the electrical properties of 

earth materials, are in Zohdy and others (1974), Fitterman and 

Labson (2005), and Lucius and others (2007). 

TDEM and 2D–DC resistivity instruments are used to 

measure the voltage response of the earth from a current field 

that is applied to the earth through direct-current injection or 

electromagnetic induction. The raw data collected by these 

instruments are filtered statistically to remove poor quality 

(noisy) data and are then used to calculate apparent resistivity 

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

material (Keller and Frischknecht, 1966). To determine the 

resistivity of non-uniform earth material, inverse modeling 

Table 1. Summary of average hydraulic conductivity of shallow, intermediate, and deep wells for each well nest at the Brazos River 

Hydrologic Field Research Site, College Station, Texas (modified from Wrobleski, 1996).

[--, not determined]

Well 

nest

Well 

identifier 

(shallow) 

(fig. 1)

Average 

hydraulic 

conductivity 

(meters per day)

Well identifier 

(intermediate) 

(fig. 1)

Average 

hydraulic 

conductivity 

(meters per day)

Well 

identifier 

(deep) 

(fig. 1)

Average 

hydraulic 

conductivity 

(meters per day)

Average hydraulic 

conductivity for 

each well nest 

(meters per day)

A1 MWA1–2 -- MWA1–3 -- MWA1–4 -- --

A2 MWA2–2 63 MWA2–3 57 MWA2–4 55 58

A3 MWA3–2 -- MWA3–3 -- MWA3–4 68 68

B1 MWB1–2 66 MWB1–3 67 MWB1–4 63 65

B2 MWB2–2 64 MWB2–3 65 MWB2–4 62 64

B3 MWB3–2 66 MWB3–3 69 MWB3–4 -- 68

C1 MWC1–2 82 MWC1–3 80 MWC1–4 79 80

C2 MWC2–2 77 MWC2–3 -- MWC2–4 93 85

C3 MWC3–2 73 MWC3–3 75 MWC3–4 75 74

Surface Geophysical Resistivity Methods 
Figure 3. Locations of well nests at the Brazos River Hydrologic Field Research Site, College Station, Texas.

0 85 382, 3, 800 82, 3 3, 0 75 382, 3, 0 70 82, 3 3,

746,800 746,850 746,900 746,950 747,000 747,050

25 50 METERS 0

S THING, IN METER NOR

EASTING, IN METER

S

MWA1

MWA

2

MWA

3

MWB1

MWB2

MWB3

MWC1

MWC2

MWC3

EXPLANATIO

N

Well nest and identifier

MWC1

Base from U.S Department of Agriculture National Agricultural Imagery Program Digital Orthophoto Image, August 2006Universal Transverse Mercator projection, Zone 14North American Datum of 1983

   Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
software is used. Inverse modeling theory is described for 

TDEM data in Fitterman and Labson (2005) and for 2D–DC 

resistivity data in Loke (2000) and in Advanced Geosciences, 

Inc. (2007). The methods used for TDEM and 2D–DC resistivity data acquisition and processing as well as the application 

of inverse modeling methods used in this investigation are 

described in detail in Kress and others (2006) and Shah and 

others (2007), respectively.

Time-Domain Electromagnetic Soundings

Fourteen TDEM sounding sites were selected to provide a uniform distribution of data to define the framework 

of the electrical stratigraphy across the BRHFRS (fig. 1). A 

gap between soundings BRA01 and BRA40/BRA05 occurs 

because of the difficulty in deploying the TDEM system with 

minimal disturbance in a mature corn field. The Geonics Protem-47 and -57 systems used nine 20-square-meter (m2

) and 

five 40-m2 transmitter loops to collect the TDEM soundings 

(Geonics Ltd., 2005). IX1D, a program developed by Interpex 

Ltd. (2006), was used to visually analyze and perform smooth 

and layered-earth inverse modeling of the apparent resistivity 

data. Inverse modeling, using the smooth modeling technique 

based on Occam’s inversion principle (Constable and others, 

1987), was used to estimate the electrical stratigraphy for each 

sounding along the profile. A mul­tiple-iteration, smoothmodel inversion was computed until the root-mean-square 

(RMS) error reached an acceptable limit of about 5 percent or 

less. The smooth-model inversion technique mini­mizes model 

roughness subject to the constraint that the model fits the data 

to a desired tolerance (Interpex Ltd., 2006). 

A simple layered-earth forward model was constructed 

by comparing inflections observed in the smooth modeling 

results to the number of hydrostratigraphic units observed in 

the drillers’ log data from the BRHFRS monitoring wells. 

Figure 4. Schematic of well nest layout, Brazos River Hydrologic Field Research Site, College Station, Texas (modified from Wrobleski, 

1996).

Surface Geophysical Resistivity Methods 

WELL 1

WELL 2

SHALLOW

WELL 3

INTERMEDIATE

WELL 4

DEEP

Alluvium

(Brazos River alluvium aquifer)

Ships clay

(leaky confining unit)

Yegua Formation

(confining unit)

5.2-9.1 m

9.8-12.2 m

13.7-15.8 m

16.8-19.8 m

water table

LAND SURFACE
Inversion of the forward model was done using the ridge 

regression inverse modeling method (Inman, 1975), which 

attempts to best-fit the data while minimizing the change in 

the model.

Two-Dimensional Direct-Current Resistivity

The 2D–DC resistivity survey was conducted using the 

IRIS Instruments Syscal Pro system (Iris Instruments, 2006) 

that incorporates 96 electrodes spaced 5 m apart. A 480-m 

2D–DC resistivity profile was collected to measure the subsurface distribution of electrical properties using the dipole-dipole 

array. The dipole-dipole array is good for mapping vertical 

structures and yields a high horizontal resolution dataset, but 

the array is highly sensitive to horizontal changes in resistivity 

(Loke, 2004). 

Apparent 2D–DC resistivity data were inverted using 

RES2DINV version 3.55 (Loke, 2004). Both the smooth and 

blocky inversion techniques were used to invert the dipoledipole data collected at the BRHFRS. The smooth inversion 

technique uses an L2 norm-based least-squares optimization 

method that attempts to minimize the sum of squares of the 

spatial changes in the resistivity model while the blocky or L1 

norm optimization method attempts to minimize the sum of 

the absolute values of spatial changes in the resistivity model 

(Loke and others, 2003). After analyzing apparent resistivities 

and evaluating the inversion results of both the smooth and 

robust inversion techniques, the robust inversion technique was 

judged best for modeling the apparent resistivity data because 

of the sharp contrasts in the data. The smooth inversion 

technique yields more accurate results where gradual changes 

occur in subsurface resistivity, whereas the blocky inverse 

modeling technique produces substantially better results where 

the rocks are semi-homogeneous and sharp boundaries exist 

between different areas (Loke and others, 2003).

Surface Geophysical Magnetic 

Resonance Sounding Method

MRS is a recently developed surface geophysical method 

designed solely for direct ground-water detection. Using MRS 

methods might alleviate the need to drill additional boreholes 

and wells at a site. An in-depth discussion of the technique 

and data collection method is given in this report because 

MRS currently (2007) is not widely used. MRS can be used as 

a non-invasive method to measure the vertical distribution of 

water content, hydraulic conductivity, and the depth and thickness of aquifers in the Earth’s subsurface (Legchenko and others, 2002). The correlation of measured geophysical contrasts 

that reflect changes in hydrogeology of the subsurface primarily is empirical and depends on the quality of the results.

The MRS system consists of a high-powered electrical 

transmitter and receiver system. A single loop of wire, used as 

both the transmitter and receiver antenna, is deployed on the 

surface. The MRS system generates pulses of variable electrical energy through the transmitter creating an external electromagnetic field to the subsurface. The intensity of the pulse (the 

“pulse moment,” which equals current intensity times current 

duration) controls the depth of investigation. The applied 

electromagnetic field excites the hydrogen protons of the 

water molecules at the Larmor frequency (Lieblich and others, 

1994). When the energizing field is turned off, the hydrogen 

protons of the water molecules resonate back to their static 

state and emit a signal at a frequency equal to the Larmor 

frequency. This signal is then recorded at the surface by the 

receiver after a turn-off time (Legchenko and others, 2002). 

During this turn-off time, three parameters of the MRS signal 

are measured (Lubczynski and Roy, 2004): amplitude (Eo), 

relaxation time (T1*), and phase (φ). The amplitude of the 

magnetic field produced during this time is proportional to the 

water content, whereas the phase is influenced by the electrical conductivity of the hydrostratigraphic layer and affects the 

attenuation of the MRS signal (Lubczynski and Roy, 2004). 

Relaxation time is directly related to the mean pore size (and 

therefore hydraulic conductivity) in the formation (table 2) 

and provides a general estimation of free water or bound water 

(Legchenko and others, 2002). Free water usually has a longer 

T1* and therefore less water is bound to sediment. Typically, 

the shorter the T1*, the more tightly the water molecules are 

bound (Bernard, 2003). T1* typically varies from less than 

30 milliseconds (ms) in clay to about 400 to 600 ms in gravel 

(Legchenko and others, 2002). 

NumisPLUS magnetic resonance equipment (Iris Instruments, 2006) was used to collect five soundings along a single 

profile across the study area (fig. 1). The square-eight antenna 

was used to improve the signal-to-noise ratio thus minimizing 

the effects of nearby power lines that run parallel to the profile 

where four of the soundings were collected (Legchenko and 

others, 2004). Because of a lack of space and increased signalto-noise values, sounding MRS5 used a 50-m2

 square antenna 

to alleviate some of the high-noise measurements. The Larmor 

frequency was calculated by the system on the basis of the 

Table 2. Approximate relations between relaxation time (T1*), 

mean pore size, and relative hydraulic conductivity (modified from 

Iris Instruments, 2006).

T1*

 (milliseconds) Mean pore size

Relative 

hydraulic 

conductivity

1,000 Lake, river (free water)

High

500 Gravel

200 Coarse-size sediment

Medium

100 Medium-size sediment

50 Fine-size sediment

Low

20 Clay (bound water)

 Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Earth’s magnetic field in the study area and was set to 2,076.9 

hertz (Hz) throughout the BRHFRS. The duration of current of 

the pulse was set to 40 ms. The recording time of the receiver 

was set to 240 ms to ensure that the voltage decay would be 

recorded until the voltage decreased below the background 

noise level of the area. Using these parameters, high-quality 

MRS data were acquired owing to a favorable signal-to-noise 

ratio indicated by the low mean noise relative to the signal 

amplitude (fig. 5D).

Prior to MRS data inversion, a matrix for each sounding 

site was created. A matrix in this context is the linear filter 

used for inversion of MRS data. The matrix was constrained 

by the following model parameters: antenna type, magnetic 

field inclination of the study area, maximum depth of the 

matrix, resistivity of layered-earth models obtained from the 

TDEM data, and calculated maximum pulse moment (table 

3). The magnetic field inclination, used in the relaxation time 

calculation, was calculated using the geospatial coordinates of 

each sounding and was about 60 degrees for the entire study 

area. The maximum depth of the matrix was set at 75 m as 

limited by the size of the antenna loop used. In general, the 

depth of penetration equals about 1.5 times the length of one 

side of the antenna loop.

Samovar, a program developed by Iris Instruments 

(2006), was used for the MRS data inversion process. The 

MRS inverse modeling technique is based on the Tickhonov 

regularization method (Legchenko and Shushakov, 1998) 

and was used to estimate the depth, thickness, water content, 

transmissivity, and T1* for each water-saturated layer for each 

sounding along the profile. The inversion technique minimizes 

model roughness subject to the parameters that constrain the 

model to fit the data to a desired tolerance. 

Inverse Modeling Results From Surface 

Geophysical Methods

The TDEM and 2D–DC resistivity inverse modeling 

results collected in the study area produced models with a 

similar resistivity structure. The MRS method showed a clear 

response from the water content, and the results yielded a 

smooth model depicting the water-saturated boundaries and 

hydraulic conductivities in the hydrostratigraphic units of the 

study area. 

Table 3. Matrix parameters used in magnetic resonance sounding inversion, Brazos River Hydrologic Field Research Site, College 

Station, Texas.

Sounding 

identifier

(fig. 1)

Antenna 

type

Antenna 

side length 

(meters)

Resistivity 

layer

Depth to 

bottom of layer 

(meters below land surface)

Resistivity 

(ohm-meters)

Calculated pulse 

moment maximum 

(ampere-milliseconds)

MRS5 Square 50

1 2.80 4.50

10,682.80

2 16.70 35.30

3 29.50 10.10

4 75.00 4.20

MRS7 Square eight 50

1 4.30 4.10

7,323.20

2 18.10 29.40

3 38.10 9.60

4 75.00 3.00

MRS8 Square eight 50

1 2.70 3.20

7,296.00

2 17.60 37.60

3 38.20 7.50

4 75.00 3.00

MRS11 Square eight 50

1 2.80 26.70

7,000.00

2 10.10 5.10

3 22.70 24.30

4 44.50 6.90

5 75.00 2.70

MRS12 Square eight 50

1 3.30 4.80

7,000.00

2 16.40 31.80

3 31.60 9.00

4 75.00 4.20

Inverse Modeling Results From Surface Geophysical Methods 
Figure 5. Magnetic resonance sounding (MRS) inversion results for Brazos River Hydrologic Field Research Site, College Station, 

Texas: (A) phase relative to pulse moment, (B) depth relative to water content, (C) depth relative to hydraulic conductivity, and (D) pulse 

moment relative to signal amplitude with signal-to-noise indication. 

A

B

C

D

EXPLANATION (D) Free induction delay inversion

Mean noise

Free induction delay amplitude

0 5 10 15 20

80

60

40

20

0

DEPTH, IN METERS

0 4,000 8,000 12,000

-80

-40

0

40

80

120

PHASE, IN DEGREES

0 50 100 150 200 250

80

60

40

20

0

DEPTH, IN METERS

0 4,000 8,000 12,000

0

50

100

150

200

250

300

0 5 10 15 20

80

60

40

20

0

0 4,000 8,000 12,000

-80

-40

0

40

80

120

0 50 100 150 200 250

80

60

40

20

0

0 4,000 8,000 12,000

0

50

100

150

200

250

300

0 4,000 8,000 12,000

0

50

100

150

200

250

300

0 50 100 150 200 250

80

60

40

20

0

0 5 10 15 20

80

60

40

20

0

0 4,000 8,000 12,000

-80

-40

0

40

80

120

PULSE MOMENT, IN AMPERE-MILLISECONDS

WATER CONTENT, IN PERCENT

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

PULSE MOMENT, IN AMPERE-MILLISECONDS

SIGNAL AMPLITUDE, IN NANOVOLTS

SOUNDING MRS5 SOUNDING MRS7 SOUNDING MRS8

10 Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Figure 5. Continued.

EXPLANATION (D) Free induction delay inversion

Mean noise

Free induction delay amplitude

A

B

C

D

0 5 10 15 20

80

60

40

20

0

0 4,000 8,000 12,000

-80

-40

0

40

80

120

0 50 100 150 200 250

80

60

40

20

0

0 4,000 8,000 12,000

0

50

100

150

200

250

300

0 5 10 15 20

80

60

40

20

0

DEPTH, IN METERS

0 4,000 8,000 12,000

-80

-40

0

40

80

120

PHASE, IN DEGREES

0 50 100 150 200 250

80

60

40

20

0

DEPTH, IN METERS

0 4,000 8,000 12,000

0

50

100

150

200

250

300

SOUNDING MRS11 SOUNDING MRS12

SIGNAL AMPLITUDE, IN NANOVOLTS

PULSE MOMENT, IN AMPERE-MILLISECONDS

WATER CONTENT, IN PERCENT

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

PULSE MOMENT, IN AMPERE-MILLISECONDS

Inverse Modeling Results From Surface Geophysical Methods 11
Time-Domain Electromagnetic Soundings

For each sounding, the voltage data were averaged and 

evaluated statistically. The raw field data (voltage data) first 

were checked for uncertainty by computing the standard deviation of the voltage data from each duty cycle. The raw voltage 

data were then averaged over each duty cycle for each gate 

for each frequency using TEM2IX1D (Interpex Ltd., 2006). 

TEM2IX1D is a program used to analyze and average the duty 

cycles for each sounding. The data were then imported into 

the inverse modeling software IX1D version 3 (Interpex Ltd., 

2006). Voltages with standard deviation greater than 5 percent 

were deleted before modeling, which eliminated data from 

late-time gates that yielded the highest signal-to-noise ratios.

The apparent resistivity data were then plotted as a function of time on a logarithmic scale. Data points that deviated 

severely (a judgment decision) from the curve were deleted 

before inverse modeling. A smooth model consisting of 25 

layers with a minimum depth of 1 m, a maximum depth of 60 

to 75 m, and a starting resistivity of 10 ohm-meters (ohm-m) 

were used to approximate the measured resistivity points in 

the starting model.

Smooth model and layered-earth model inversion results 

had final RMS errors that ranged from 1.15 to 6.31 percent 

and from 1.09 to 6.27 percent, respectively. These values, 

used to evaluate the precision of each sounding, are listed 

in table 4. Any sounding with an RMS error greater than 5 

percent was given less weight than the others because of the 

uncertainty in the data. After each sounding was inverted in 

IX1D, the layered-earth model was imported into a geodatabase to compare with other soundings along the same profile. 

Inversion results depicted a distinct electrical contrast between 

the Ships clay (clay), the alluvium of the Brazos River 

alluvium aquifer (sand and gravel), and the Yegua Formation 

(shale) (fig. 6C). 

Two-Dimensional Direct-Current Resistivity

The Syscal Pro system was used to collect a 2D profile 

of apparent resistivity data consisting of 2,374 data points 

using the dipole-dipole array. The raw field data (current and 

voltage data) first were checked for uncertainty by evaluating 

the standard deviation of the computed apparent resistivity 

data using Prosys II version 2.10.02 (Iris Instruments, 2006). 

Apparent resistivity data with standard deviation less than 

0 were removed, resulting in the removal of 92 data points 

related to low signal-to-noise values. After filtering, 10 additional (three on data level 1 and seven on data level 9.1) data 

points were removed because of lack of data per data level 

(Loke, 2000). The data were then filtered by doing a sliding 

average on all depth levels with a span of 10 m. The sliding 

average dampens noisy data especially at deeper depths. The 

final apparent resistivity dataset was imported into the 2D 

inverse modeling software RES2DINV version 3.55 (Loke, 

2004). Apparent resistivity data were inverted using the blocky 

inverse modeling technique by selecting the robust constraint 

option in RES2DINV. After inversion the RMS error between 

the measured and calculated apparent resistivity data was 2.2 

percent. Generally RMS errors less than 5–10 percent can be 

expected.

Table 4. Time-domain inversion results, Brazos River Hydrologic Field Research Site, College Station, Texas.

[RMS, root-mean-square]

Sounding 

identifier 

(fig. 1)

Transmitter antenna 

side length 

(meters)

Geonics Protem System 

(Geonics Ltd., 2005) 

transmitter type

Smooth model 

RMS error 

(percent)

Number of 

smooth model 

layers

Layered-earth 

model RMS 

error 

(percent)

Number 

of layered-earth 

model layers

BRA01 20 47 1.15 25 1.09 5

BRA05 40 47,57 2.34 25 2.45 4

BRA09 40 47,57 2.91 25 3.41 4

BRA40 20 47 2.52 25 2.53 4

BRA50 20 47 1.78 25 1.80 4

BRA60 20 47 2.31 25 2.35 4

BRA70 20 47 1.67 25 1.66 4

BRA80 20 47 2.34 25 1.86 4

BRA90 20 47 2.33 25 1.87 4

BRA110 20 47 1.93 25 1.82 3

BRA160 40 47, 57 3.81 25 4.44 4

BRA170 40 47, 57 3.85 25 3.82 4

BRA180 40 47, 57 6.31 25 6.27 4

12 Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Figure 6. (A) Location of selected wells and time-domain electromagnetic (TDEM) soundings, (B) drillers’ logs with static water 

level, (C) TDEM layered-earth model sounding results, and (D) robust inversion profile of two-dimensional direct-current dipole-dipole 

resistivity array, Brazos River Hydrologic Field Research Site, College Station, Texas.

MWC3-4 MWB2-4 MWA2-4

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

BRA09

BRA100

BRA160 BRA110

BRA170

BRA180

BRA60

BRA70 BRA80 BRA90

BRA01

BRA05

BRA40

BRA50

DC resistivity profile

MWB2-4 MWA2-4

MWC3-4

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

3,382,800

3,382,900

EASTING, IN METERS

NORTHING, IN METERS

BRA05 BRA09 BRA100

BRA160 BRA110

BRA170

BRA40

BRA50

BRA60

BRA70

BRA80

BRA90 BRA180

BRA01

less

than 10 20 32 54 89 greater

than 175

Direct-current resistivity,

Clay in ohm-meters Sand and gravel

Water level

Direct-current resistivity profile

Time-domain electromagnetic sounding

and identifier

EXPLANATION

Time-domain electromagnetic

layered-earth model sounding

BRA170

MWC3-4

Lithology

Monitoring well and identifier Shale

(B) (C) (D)

ALTITUDE, IN METERS ABOVE NAVD 88

APPROXIMATE LAND SURFACE

Inverse Modeling Results From Surface Geophysical Methods 13
Magnetic Resonance Soundings

Because this was a pilot study, the MRS modeling and 

interpretations are preliminary. Further testing of model 

parameters and the collection of additional ground-truthing 

data would be necessary to compare and contrast various 

parameters used to derive hydraulic conductivity from MRS 

data. For each MRS sounding there are several constraints 

that can be either automatically set by the software program 

(default values) or manually set.

Standard default parameters first were used to derive 

hydraulic conductivity values. However, exclusively using 

these default parameters during inversion resulted in hydraulic 

conductivity values that were substantially greater (5 to 10 

times) than those computed from aquifer tests. Consequently, 

an alternate inversion method described by Legchenko and 

others (2004) was used. This method is explained below.

The constraints necessary for inversion include processing time, regularization parameters E (observed relaxation 

time) and T1* (longitudinal relaxation time), and the coefficient of permeability (Cp). Default parameters maintained by 

the Samovar program for inversion include the processing time 

(198.4 ms), which corresponds to the NumisPLUS measurement 

time window (Iris Instruments, 2006) and bandpass filter (10 

Hz), The inversion program provides the best solution for T1* 

on the basis of the total response of the magnetic resonance 

signal. 

If necessary after inversion, T1* can be optimized to 

make the inversion solution for each sounding more definitive in terms of changes with depth in hydraulic conductivity, 

transmissivity, or signal-to-noise ratio (Legchenko and others, 

2004). Using the default Cp value of 7.00 x 10-9 assigned by 

the Samovar program, T1* is calibrated by running a series of 

inversions with different values (13 in this application) of T1*

in the range of 1 to 1,000. Using transmissivity as an example, 

the resulting transmissivity values and fitting error for each 

inversion were then normalized (divided by the respective 

maximum). The normalized values were summed for each 

inversion and graphed relative to T1* (fig. 7). The graph (and 

a similar one for each sounding) shows a curve with a flat 

segment corresponding to essentially equivalent solutions. The 

optimal solution is chosen to be the center of the flat segment 

and defined as a regularized solution that yields a value of T1*

with a reasonable fitting error. The maximum, minimum, and 

optimum transmissivity values for each MRS sounding are 

listed in table 5. The saturated thickness is then determined for 

each sounding and divided by the optimum transmissivity to 

yield an MRS-derived hydraulic conductivity (table 5).

To more accurately represent the hydraulic conductivity of the soundings collected in the Brazos River alluvium 

aquifer in the study area, a Cp value that is more representative of the study area was manually calibrated. Data from 

two soundings and four monitoring well nests were used to 

calibrate Cp and then to calculate hydraulic conductivity. Well 

nests MWB3 and MWB2 (fig. 3) correspond to MRS sounding 7 data and well nests MWC3 and MWC2 correspond to 

MRS sounding 12 data. Only MRS soundings 7 and 12 were 

used to calibrate Cp because of their proximity to monitoring 

well nests in the study area with previously obtained hydraulic 

conductivity values that can be compared directly with MRSderived hydraulic conductivity values. First, the hydraulic conductivities obtained from the 1996 aquifer tests (Wrobleski, 

1996) for well nests MWB3 and MWB2 were combined and 

averaged, and then those for well nests MWC3 and MWC2 

were combined and averaged to obtain two hydraulic conductivity values (table 6). 

This method assumes that MRS results are averaged 

over a large area (as defined by the loop size) and that aquifer 

tests provide average results over a large volume (Legchenko 

and others, 2004). Two correction factors (table 7) are then 

computed by dividing the average monitoring well hydraulic 

conductivity corresponding to MRS soundings 7 and 12 (table 

6) by the MRS-derived hydraulic conductivity obtained by 

using the default Cp value (table 5) for both soundings. The 

average of these correction factors then is multiplied by the 

default Cp value (7.00 x 10-9 ) to obtain a corrected Cp value 

of 2.61 x10-8. The corrected Cp value is then used in the 

inversion. 

Table 5. Minimum, maximum, and optimum transmissivity, saturated thickness, and magnetic resonance sounding (MRS)-derived 

hydraulic conductivity values obtained from default coefficient of permeability (Cp) value (7.00 x10-9), Brazos River Hydrologic Field 

Research Site, College Station, Texas.

Sounding 

identifier 

(fig. 1)

Minimum 

transmissivity 

(meters squared 

per day)

Maximum 

transmissivity 

(meters squared 

per day)

Optimum 

transmissivity 

(meters squared 

per day)

Saturated 

thickness 

(meters)

MRS-derived 

hydraulic conductivity 

(meters per day)

MRS5 147 181 164 15 11

MRS7 225 294 242 14 17

MRS8 302 510 415 16 26

MRS11 104 156 121 17 7

MRS12 294 389 328 15 22

14  Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Figure 7. Calibration graph used to obtain optimum relaxation time (T1*) for inversion of magnetic resonance sounding data using the 

default coefficient of permeability (Cp) value of 7.00 x 10-9, Brazos River Hydrologic Field Research Site, College Station, Texas.

Table 6. Sounding, corresponding monitoring well nest, average hydraulic conductivity from 1996 aquifer tests, and average 

monitoring well nest hydraulic conductivity used to calibrate coefficient of permeability (Cp), Brazos River Hydrologic Field Research 

Site, College Station, Texas.

Sounding

identifier

(fig. 1)

Monitoring 

well nest

Average hydraulic 

conductivity from 

1996 aquifer test 

(meters per day)

Monitoring 

well nest

Average hydraulic 

conductivity from 

1996 aquifer tests 

(meters per day)

Average monitoring 

well nest hydraulic 

conductivity 

(meters per day)

MRS7 MWB3 68 MWB2 64 66

MRS12 MWC2 85 MWC3 74 80

0 200 400 600 800 1,000

T1*

0

0.4

0.8

1.2

1.6

2

NORMALIZED PARAMETERS

EXPLANATION

Sum of normalized transmissivity and fitting error

Normalized fitting error

Normalized transmissivity

Inverse Modeling Results From Surface Geophysical Methods 15
After the inversion process, a series of calculated data 

outputs—raw decay curves, water content, hydraulic conductivity, and signal amplitude—were generated (fig. 5). 

The results from MRS inverse modeling (fig. 5) can assist in 

estimating the percentage water content and hydraulic conductivity of the hydrostratigraphic units. 

A final hydraulic conductivity value for each sounding 

based on the MRS results was derived and compared with the 

previously calculated values from the 1996 aquifer tests (table 

8). Analyses of the results from these derived hydraulic conductivity values are explained in the “Analysis of Results From 

Surface Geophysical Methods” section of this report.

The output data for each sounding generated from 

the MRS inversion include phase relative to pulse moment 

(fig. 5A); depth relative to water content (fig. 5B); depth relative to hydraulic conductivity (fig. 5C); raw voltage decays for 

each signal amplitude observed in the field and best-fit line to 

each decay (shown in red) (fig. 5D). 

Analysis of Results From Surface 

Geophysical Methods

Prior knowledge of resistivity of the subsurface and data 

from aquifer tests are necessary to both interpret and analyze 

the MRS data. General hydrostratigraphic data about the 

Ships clay, the alluvium of the Brazos River alluvium aquifer, 

and the Yegua Formation were required for data comparison.

An integrated interpretation can be made from the TDEM, 

2D–DC resistivity, and MRS inversion results. Creating the 

electrical stratigraphy of the geology from TDEM and 2D–DC 

resistivity data and the hydrostratigraphy using MRS data 

further enhances the understanding of the hydrostratigraphy 

of the Brazos River alluvium aquifer. MRS-derived hydraulic 

conductivity values also can provide input for ground-water 

models to further examine the ground-water flow and availability of the aquifer.

Electrical Stratigraphy and Hydrostratigraphic 

Framework

Stratigraphically, the principal finding of this study is 

the relation between electrical resistivity and the depth and 

thickness of the subsurface hydrostratigraphic units at 

BRHFRS. Not only could thicknesses and extents of these 

units be defined to a greater level than previously interpreted, 

but lateral variations in resistivity within the Brazos River alluvium aquifer also could be detected. The MRS soundings 

have added supporting data to the 2D–DC and TDEM resistivity profiles allowing for improved understanding of the 

hydrostratigraphic framework and the related depositional 

environments. 

The TDEM shows a three-layer model in which there is 

a conductor-resistor-conductor pattern. This correlates with 

the hydrostratigraphic units within the study area: Ships clay 

(conductor), alluvium of the Brazos River alluvium aquifer 

(resistor), and Yegua Formation (conductor). Sharp electrical boundaries that range from 4 to 6 m and from 20 to 22 m 

below land surface, based on the TDEM data, define the more 

resistive alluvium of the aquifer. The thickest part of the more 

resistive alluvium of the aquifer is in the middle of the study 

area between TDEM soundings BRA110 and BRA90 where 

the thickness is about 17 m (fig. 6C). This is interpreted to be 

an ancestral channel deposit of the Brazos River that has not 

been identified previously. The interpretation is based on correlating lithology to resistivity and comparisons of lithology 

to the 2D–DC and MRS soundings. The higher resistivities 

indicate coarse sediments (sand and gravel) shown in the drillers’ logs of figures 6B and 8B.

The 2D–DC resistivity profile provides a good resolution 

for determining lateral variation of resistivity. According to 

the 2D–DC resistivity profile, variations in the Brazos River 

alluvium aquifer range from 10 to more than 175 ohm-m 

(fig. 6D). These variations are possibly caused by lateral 

changes in grain size and help define the geometry of the 

subsurface hydrostratigraphic units (Kress and others, 2006). 

Table 7. Corrected coefficient of permeability (Cp) value computed using magnetic 

resonance sounding (MRS)-derived hydraulic conductivity from default Cp value 

(7.00 x 10-9), average monitoring well nest hydraulic conductivity, and correction 

factor, Brazos River Hydrologic Field Research Site, College Station, Texas.

Sounding

identifier

(fig. 1)

MRS-derived 

hydraulic 

conductivity 

(meters per day)

Average monitoring well nest 

hydraulic 

conductivity 

(meters per day)

Correction 

factor

Average 

correction 

factor

Corrected 

Cp

MRS7 17 166 3.82

3.73 2.61 x10-8

MRS12 22 280 3.63

1

 Well nests MWB3 and MWB2.

2

 Well nests MWC2 and MWC3.

16 Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
Resistivity increases from east to west along the profile (fig. 

6D) away from the Brazos River toward the interpreted ancestral Brazos River channel. Typically, an increase in resistivity 

signifies an increase in grain size in the alluvium aquifer, and 

therefore a more productive aquifer (more water). The highest resistivities, from about 100 to 175 ohm-m, occur over 

a distance of 200 m. This zone of high resistivity (shown in 

blue in figure 6D) occurs between TDEM soundings BRA110 

and BRA90 and is the thickest section of coarse sediment 

in the ancestral channel. The zones of lowest resistivity (or 

high conductivity) occur at the top of the 2D–DC resistivity 

profile from land surface to about 7 m below land surface and 

at the base of the profile. These upper and lower zones of low 

resistivity correlate with the Ships clay and Yegua Formation, 

respectively. By combining TDEM and 2D–DC resistivity 

data, information on the aquifer geometry and lateral variations in resistivity were obtained. These data helped build the 

hydrostratigraphic framework into which the MRS data were 

integrated. Using this joint interpretation of the resistivity 

and MRS also helped improved the accuracy of the derived 

hydraulic conductivity values. 

Hydrostratigraphy

MRS data can help delineate the subsurface hydrostratigraphy and identify the geometric boundaries of the hydrostratigraphic units by indicating changes in the free water content, 

transmissivity, saturated thickness, and hydraulic conductivity 

(Lubczynski and Roy, 2004). Typically, this is only possible if 

there is a high signal-to-noise ratio (fig. 5D). If the signal-tonoise ratio is too low, it might not be possible to distinguish 

hydrostratigraphic boundaries at depth (Lubczynski and Roy, 

2004). The aquifer geometry in this application encompasses 

the lateral extent of porous and permeable materials. On the 

basis of the gridded MRS-derived water content and hydraulic 

conductivity data, most of the soundings show that the most 

productive parts of the Brazos River alluvium aquifer occur 

from about 15 to 20 m below land surface (fig. 8) in the western part of the study area and become slightly more productive 

in the eastern part of the area (toward the Brazos River). The 

profile indicates that the hydraulic conductivity in this productive zone is between 90 and 250 meters per day (m/d) (fig. 

8D). Zones of high water content and high hydraulic conductivity occur mostly between and adjacent to MRS soundings 

12 and 7 with the highest percentage water content occurring 

around MRS sounding 7. 

The higher values of water content and hydraulic conductivity are consistent with the geology based on the TDEM and 

2D–DC resistivity data in which the thickest part of the Brazos 

River alluvium aquifer is in the middle of the study area. The 

2D–DC resistivity data (fig. 6) show a gradual change in resistivity toward the west where the high resistivities indicate an 

increase in grain size and, therefore, a higher percentage water 

content and pore space. As the water content and hydraulic 

conductivity increase farther below land surface, coarser material such as sand and gravel (Brazos River alluvium aquifer) 

increases. 

The TDEM layered models (fig. 6) and the MRS gridded 

water content profile (fig. 8) verify the aquifer geometry. The 

abrupt changes in resistivity shown in the TDEM soundings 

correlate with the depth and thickness of the areas of high and 

Table 8. Minimum, maximum, and optimum transmissivity and magnetic resonance sounding (MRS)-derived hydraulic conductivity 

values obtained from corrected coefficient of permeability (Cp) value (2.61 x10-8) compared with average hydraulic conductivity values 

from closest monitoring well nest calculated from 1996 aquifer tests, Brazos River Hydrologic Field Research Site, College Station, 

Texas.

Sounding 

identifier 

(fig. 1)

Minimum 

transmissivity 

(meters squared 

per day)

Maximum 

transmissivity 

(meters squared 

per day)

Optimum 

transmissivity 

(meters squared 

per day)

Saturated 

thickness 

(meters)

MRS-derived 

hydraulic 

conductivity 

(meters per day)

Average hydraulic 

conductivity from 

closest monitoring 

well nests

(meters per day)

MRS5 548 677 612 15 40 180

MRS7 838 1,096 902 14 64 266

MRS8 1,128 1,901 1,547 16 97 373

MRS11 387 580 451 17 27 461

MRS12 1,096 1,450 1,225 15 82 580

1

 Average for well nests MWC2 and MWC3; about 110 meters from MRS5.

2

 Average for well nests MWB3 and MWB2; about 20 meters from MRS7.

3

 Average for well nests MWB2, MWB3, MWC2, and MWC3; about 60 meters from MRS8.

4

 Average for well nests MWA2 and MWA3; about 120 meters from MRS11.

5

 Average for well nests MWC2 and MWC3; about 30 meters from MRS12.

Analysis of Results From Surface Geophysical Methods 17
Figure 8. (A) Location of selected wells and magnetic resonance soundings (MRS), (B) drillers’ logs with static water level and 

locations of hydraulic conductivity measurements, (C) gridded percentage water content profile, and (D) gridded hydraulic conductivity 

profile based on MRS inversion results, Brazos River Hydrologic Field Research Site, College Station, Texas.

MWC3-4 MWB2-4 MWA2-4

45

50

55

60

65

70

Group, formation, or member

MRS11

MRS5 MRS12

MRS8 MRS7

MWB2-4 MWA2-4

MWC3-4

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

3,382,800

3,382,900

Brazos River

WEST EAST

MRS11 MRS5 MRS12 MRS8 MRS7

35

40

45

50

55

60

65

70

MRS11

MRS5 MRS12 MRS8 MRS7

35

40

45

50

55

60

65

70

50 100 150 200 250 300 350 400 450 500 550

DISTANCE, IN METERS

Shale

A

B

C

D

Monitoring well and identifier

Magnetic resonance sounding

and identifier

EXPLANATION

Clay

Sand and gravel

Water level, in feet

below land surface

Lithology

Location of hydraulic

conductivity measurements

MWC3-4

MRS12

0 5 10 15 greater

than 20

Water content, in percent

Hydraulic conductivity, in

meters per day

less

than 5 25 85

greater 

than 250

(B) (C)

(D)

ALTITUDE, IN METERS ABOVE NAVD 88

EASTING, IN METERS

NORTHING, IN METER

S

APPROXIMATE

LAND SURFACE

APPROXIMATE

LAND SURFACE

18  Application of Surface Geophysical Methods, Brazos River Alluvium Aquifer, College Station, Texas, July 2006
low percentages of water content, particularly in the alluvium 

of the Brazos River alluvium aquifer. At most of the MRS 

soundings, the static water level (measured on the same day 

that the MRS soundings were made) is about 10 m below land 

surface and therefore very little to no water (shown in yellow 

in figure 8C) was detected above that depth by the MRS. The 

minimal water detection above this depth correlates with the 

compact, clay-rich material (Ships clay) above the water table. 

At the base of both profiles (figs. 8C and 8D), percentage 

water content and hydraulic conductivity decrease where the 

Yegua Formation occurs. Very little or no pore space exists in 

that unit to hold or transmit water. 

Individual hydraulic conductivity values derived from 

MRS were consistent with those from the 1996 aquifer tests. 

Average hydraulic conductivity values from the aquifer tests 

for the closest monitoring-well nests are about 61 to 80 m/d 

(Wrobleski, 1996), whereas, the MRS-derived hydraulic 

conductivity values are about 27 to 97 m/d (table 8). The 

highest hydraulic conductivity values indicated by the gridded 

hydraulic conductivity profile are between MRS soundings 12 

and 7 (fig. 8D). The alluvium aquifer is very heterogeneous 

in the areas of MRS soundings 5 and 11 based on MRSderived hydraulic conductivity values that differ greatly from 

the surrounding hydraulic conductivity values. MRS soundings 5 and 11 also show the greatest discrepancy between the 

MRS-derived hydraulic conductivity and the average hydraulic 

conductivity computed from the 1996 aquifer tests (table 8), 

but these soundings are farthest from the well nests. Interpreting both the gridded profiles and individual hydraulic conductivity values derived from MRS can help generate a conceptualization of the hydrostratigraphy and constrain groundwater models for better accuracy. Collecting supporting data 

might be necessary to further define the hydrostratigraphy of 

the Brazos River alluvium aquifer at BRHFRS.

Hydrostratigraphic Unit Parameterization

Aquifer and confining-unit properties for ground-water 

modeling usually are obtained from aquifer tests or calculated 

from known variables. However, MRS also can be used to 

obtain hydrostratigraphic data for input into ground-water 

models. Because of the large volume of material MRS is able 

to measure, properties such as transmissivity, water content, 

and hydraulic conductivity can be estimated over a larger area 

and depth, whereas aquifer tests yield data from a discreet 

point in the aquifer. The MRS soundings allow for many 

more data points to supplement aquifer-test sites, thus providing more comprehensive coverage of aquifer properties. In 

this Brazos River alluvium aquifer study, MRS helps define 

the hydrostratigraphic units and vertical aquifer boundaries 

essential for input into ground-water models (Plata and Rubio, 

2006). 

On the basis of historical literature, Brazos River Basin 

regional hydraulic conductivity ranges from about 2 m/d north 

of the study area to about 130 m/d south of the study area 

(Shah and Houston, 2007). The MRS data collected at the site 

are well within this range and confirm that the MRS method 

is capable of obtaining hydrostratigraphic unit properties at 

the BRHFRS. On the basis of data collected at the site, MRS 

could be used in areas in the Brazos River alluvium aquifer 

where data are lacking and can be used in conjunction with 

ground-water availability modeling. In comparison to smallscale hydrologic property measurements and expensive aquifer 

tests, MRS has been shown to be an effective method to 

investigate large volumes of the subsurface (based on the size 

of the loop). For ground-water models, MRS can provide a 

comprehensive distribution of properties for model cells (Roy 

and Lubczynski, 2003). 

Summary

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

Texas Water Development Board, conducted an integrated 

surface geophysical pilot study at the Texas A&M University 

Brazos River Hydrologic Field Research Site (BRHFRS) in 

July 2006 to characterize the hydrostratigraphy of the Brazos 

River alluvium aquifer. Two methods, time-domain electromagnetic (TDEM) soundings and two-dimensional (2D) 

direct-current (DC) resistivity imaging, were used to define the 

lateral and vertical extent of the Ships clay, the alluvium 

of the Brazos River alluvium aquifer, and the underlying 

Yegua Formation for the BRHFRS. Magnetic resonance 

sounding (MRS), a recently developed surface geophysical 

method, was used to derive estimates of hydrostratigraphic 

unit properties including percentage water content, transmissivity, and hydraulic conductivity. This innovative experimental method combines non-invasive surface geophysical 

methods for estimation of hydrostratigraphic unit properties 

to better characterize an aquifer with few wells. Results from 

the geophysics study demonstrated the usefulness of combining TDEM, 2D–DC resistivity, and MRS methods to reduce 

the need for additional boreholes in areas with data gaps and 

to provide information for use in ground-water availability 

models.

Fourteen TDEM sounding sites were selected to provide distinct electrical boundaries of the hydrostratigraphic 

units—the Ships clay, the alluvium that constitutes the Brazos 

River alluvium aquifer, and the underlying Yegua Formation—

at the BRHFRS. The Geonics Protem-47 and -57 systems, 

using nine 20-m2 and five 40-m2 transmitter loops, were used 

to collect the TDEM soundings. Inverse modeling, using the 

smooth modeling technique based on Occam’s inversion principle, was used to identify the hydrostratigraphic units for each 

sounding along the profile. A layered-earth forward model 

was constructed by comparing inflections observed from the 

smooth inverse modeling results to the vertical boundaries of 

hydrostratigraphic units observed in the drillers’ log data from 

the BRHFRS monitoring wells. A 480-m 2D–DC resistivity 

profile was collected to measure the subsurface distribution 

Summary 19
of electrical properties using the dipole-dipole array. After 

analyzing apparent resistivities, the robust inverse modeling method was determined the best model for the apparent 

resistivity data because of sharp contrasts in the data. Five 

MRS soundings were collected to estimate water content, 

transmissivity, and hydraulic conductivity at different depths 

in the Brazos River alluvium aquifer for potential input into 

a ground-water availability model. Because this was a pilot 

study, the MRS modeling and interpretations are preliminary. 

Further testing of model parameters and collection of additional ground-truthing data would be necessary to compare 

and contrast various parameters used to derive hydraulic 

conductivity from MRS data. 

By combining TDEM and 2D–DC resistivity data, 

information on the aquifer geometry and lateral variations in 

resistivity were obtained and used to build the hydrostratigraphic framework into which the MRS data were integrated. 

The TDEM soundings show that there is a conductor-resistorconductor pattern in the modeled data. This pattern correlates 

well with the Ships clay (conductor), alluvium of the Brazos River alluvium aquifer (resistor), and Yegua Formation 

(conductor). The abrupt boundaries that define the alluvium 

aquifer range from 4 to 6 m below land surface at the top of 

the aquifer and from 20 to 22 m below land surface at the 

base. The thickest part of the more resistive alluvium aquifer 

is in the middle of the study area between TDEM soundings 

BRA110 and BRA90. This is interpreted to be an ancestral 

channel deposit of the Brazos River that has not been identified previously. The interpretation is based on correlating 

lithology to resistivity and comparisons of lithology to the 

2D–DC and MRS soundings. The higher resistivities indicate 

coarse sediments similar to those shown in drillers’ logs. 

According to the 2D–DC resistivity profile, variations in the 

Brazos River alluvium aquifer range from 10 to more than 175 

ohm-m over a distance of 200 m. The zones of lowest resistivity occur from land surface to about 7 m below land surface 

and at the base of the profile and correlate with the Ships clay 

and Yegua Formation, respectively. 

On the basis of the MRS soundings, the most productive parts of the Brazos River alluvium aquifer occur from 15 

to 25 m below land surface in an ancestral channel deposit in 

the western part of the BRHFFRS. The hydraulic conductivity in this zone is between 90 and 250 m/d and occurs mostly 

between and adjacent to MRS soundings 12 and 7 with the 

highest percentage water content occurring around MRS 

sounding 7. The higher values of water content and hydraulic 

conductivity collected with MRS correlate to the greatest saturated thickness derived from the TDEM and 2D–DC resistivity 

data. As the water content and hydraulic conductivity increase 

farther below land surface, coarser material such as sand and 

gravel increases. 

The TDEM layered models and the MRS gridded watercontent profile confirm the aquifer geometry and boundaries 

of the upper confining unit, alluvium aquifer, and underlying 

confining unit. The hydraulic conductivity values calculated 

from MRS (27 to 97 m/d) correlate well with values from 

1996 aquifer tests (61 to 80 m/d). On the basis of the data 

obtained at BRHFRS, MRS can be used for hydrostratigraphic 

unit parameterization for input into ground-water models 

because of the large volume of material MRS is able to 

measure. The combination of TDEM, 2D–DC resistivity, 

and MRS can provide geographically extensive hydrostratigraphic characterization—that is, thickness, extent, lateral 

variability, transmissivity, water content, and hydraulic 

conductivity. 

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References 21

Prepared by the USGS Lafayette Publishing Service Center.

Information regarding water resources in Texas is available at 

http: //tx.usgs.gov/