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Scientific Investigations Report 2007–5203

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

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

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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 ground-water 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-resistor-conductor 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 range 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.

Version 1.0

Posted January 2008


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

Introduction

Purpose and Scope

Acknowledgments

Site Description and Hydrostratigraphic Setting

Surface Geophysical Resistivity Methods

Time-Domain Electromagnetic Soundings

Two-Dimensional Direct-Current Resistivity

Surface Geophysical Magnetic Resonance Sounding Method

Inverse Modeling Results From Surface Geophysical Methods

Time-Domain Electromagnetic Soundings

Two-Dimensional Direct-Current Resistivity

Magnetic Resonance Soundings

Analysis of Results From Surface Geophysical Methods

Electrical Stratigraphy and Hydrostratigraphic Framework

Hydrostratigraphy

Hydrostratigraphic Unit Parameterization

Summary

References


For additional information contact:
Director, Texas Water Science Center
U.S. Geological Survey
8027 Exchange Drive
Austin, Texas 78754-4733

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