Investigation of Preferential Groundwater Seepage in the Ellenburger – San Saba Aquifer Using Geoelectric Measurements

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The USGS Texas Water Science Center (TXWSC) is completing a geophysical pilot study adjacent to Hamilton Creek in Burnet County, central Texas.  The pilot study is intended to test whether electrical geophysical methods can provide information regarding the locations of dissolution cavities and preferential groundwater flow within in the Ellenburger San-Saba aquifer.  Data from this study will give the Central Texas Groundwater Conservation District (GCD), who is responsible for management of the groundwater resources in Burnet County, a more thorough understanding of the potential for exchanges of groundwater and surface-water between Hamilton Creek and the Ellenburger - San Saba aquifer.

Two-dimensional (2D) electric resistivity tomography (ERT) and self-potential (SP) profiling are being utilized in this pilot study.  These methods are low-frequency electrical geophysical methods and are frequently used to map preferential groundwater flow patterns in shallow aquifers. Their utility for such applications is attributable to the physical relationship between the electric resistivity and the hydraulic gradient within the Earth, and the electric field at the surface of Earth.  Electric resistivity and the electric field are measured directly by the ERT and SP methods, respectively, whereas the hydraulic gradient, while directly measurable as well, can be either qualitatively or quantitatively assessed from the SP data and interpreted from the combined geophysical data through mathematical modeling.

 

Geophysical methods

ERT is an active, near-surface geophysical method that is sensitive to the spatial variations of the electrical property of resistivity of soils, rocks, and groundwater.  The term “active” indicates that the geophysicist supplies energy into the Earth, in the form of a direct electric current, and then measures the Earth’s response to that provided energy to discern the spatial variations of resistivity in the subsurface. 

On the other hand, SP is a passive, near-surface geophysical method that is sensitive to spatial variations in the hydraulic gradient, which causes groundwater to flow. The term “passive” indicates that the geophysicist does not supply an energy source to trigger a measurable geophysical response from the Earth. Instead, measurements of the natural electrical field are made at the Earth’s surface, which include the effects of hydraulically-driven electric current flows in the subsurface.  The SP method is somewhat unique among geophysical methods in this sense, because of the passive nature of the measurement and the direct sensitivity of the method to preferential groundwater flow patterns.