Scientific Investigations Report 2007-5244
 
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 |
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.
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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Last modified: Thursday, 01-Dec-2016 19:55:29 EST
