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Scientific Investigations Report 2007-5244

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

By Jason D. Payne, Wade H. Kress, Sachin D. Shah, James E. Stefanov, Brian A. Smith, and Brian B. Hunt

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Scientific Investigations Report
2007-5244 PDF (5.06 MB)
Abstract

During September 2006, the U.S. Geological Survey, in cooperation with the Barton Springs/Edwards Aquifer Conservation District, conducted a geophysical pilot study to determine whether time-domain electromagnetic (TDEM) sounding could be used to delineate the freshwater/saline-water transition zone in the Barton Springs segment of the Edwards aquifer in Travis and Hays Counties, Texas. There was uncertainty regarding the application of TDEM sounding for this purpose because of the depth of the aquifer (200–500 feet to the top of the aquifer) and the relatively low-resistivity clayey units in the upper confining unit. Twenty-five TDEM soundings were made along four 2–3-mile-long profiles in a study area overlying the transition zone near the Travis-Hays County boundary. The soundings yield measurements of subsurface electrical resistivity, the variations in which were correlated with hydrogeologic and stratigraphic units, and then with dissolved solids concentrations in the aquifer. Geonics Protem 47 and 57 systems with 492-foot and 328-foot transmitter-loop sizes were used to collect the TDEM soundings. A smooth model (vertical delineation of calculated apparent resistivity that represents an estimate [non-unique] of the true resistivity) for each sounding site was created using an iterative software program for inverse modeling. The effectiveness of using TDEM soundings to delineate the transition zone was indicated by comparing the distribution of resistivity in the aquifer with the distribution of dissolved solids concentrations in the aquifer along the profiles. TDEM sounding data show that, in general, the Edwards aquifer in the study area is characterized by a sharp change in resistivity from west to east. The western part of the Edwards aquifer in the study area shows higher resistivity than the eastern part. The higher resistivity regions correspond to lower dissolved solids concentrations (freshwater), and the lower resistivity regions correspond to higher dissolved solids concentrations (saline water). On the basis of reasonably close matches between the inferred locations of the freshwater/saline-water transition zone in the Edwards aquifer in the study area from resistivities and from dissolved solids concentrations in three of the four profiles, TDEM sounding appears to be a suitable tool for delineating the transition zone.

Version 1.0

Posted December 2007


Suggested citation:

Payne, J.D., Kress, W.H., Shah, S.D., Stefanov, J.E., Smith, B.A., and Hunt, B.B., 2007, Geophysical delineation of the freshwater/saline-water transition zone in the Barton Springs segment of the Edwards aquifer, Travis and Hays Counties, Texas, September 2006: U.S. Geological Survey Scientific Investigations Report 2007–5244, 21 p., 6 appendixes.



Contents

Abstract

Introduction

Purpose and Scope

Previous Investigations

Description of the Barton Springs Segment of the Edwards Aquifer, Confining Units, and Study Area

Acknowledgments

Methods and Assessment of Data

Geodatabase Development

Geologic Sections

Site Selection

Time-Domain Electromagnetic Sounding

Borehole Geophysics

Data Processing and Quality

Inverse Modeling

Delineation of Transition Zone Using Time-Domain Electromagnetic Soundings

Profile 2

Profile 3

Profile 4

Profile 5

Evaluation of Findings

Summary and Conclusions

References

Appendix 1—Locations and Dissolved Solids Concentrations of Existing Wells

Appendix 2—Geophysical Borehole Logs, Swinney Well

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

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

Appendix 5—Graphs Showing Time-Domain Electromagnetic Resistivity From Field Measurements as a Function of Time and Inverse Modeling Results (Smooth Models)

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

 


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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