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Geoelectric survey of the Granite Gravel aquifer, Llano Uplift, Central Texas, to determine locations for water wells

November 1, 2021

An electrical geophysical survey was completed within a small area of the Llano Uplift of central Texas to determine locations to install two water wells in the Granite Gravel aquifer (GGA). Electrical resistivity tomography (ERT) was performed along two 188-m long profiles that intersected at the approximate center of a 100-m by 100-m self-potential (SP) map. The ERT survey was completed to map two-dimensional (2D) electric resistivity distributions in the GGA and the underlying Precambrian Town Mountain Granite (TMG) bedrock, whereas SP mapping was performed to delineate apparent streaming potential anomalies at the land surface that appeared correlated to the subsurface resistivity distributions, which exhibited strong lateral heterogeneity in the upper 35 m of the weathered layer that comprises the GGA. The depth to TMG bedrock, as shown by the resistivity distributions, varied substantially over relatively small profile distances and surface areas; however, the general electrical structure showed resistivity increasing with depth, beginning with a thin electrically conductive layer at the surface characterized by resistivity in the range of about 30–100 Ω-m, followed by a resistivity increase to 300–500 Ω-m at a depth that coincided with the water-table depth observed in the installed water wells. Resistivity of the TMG bedrock was generally greater than 500 Ω-m and exceeded 1000 Ω-m in some locations of the tomograms. Electrical structure beneath the survey area, as shown by the 2D resistivity distributions beneath the ERT profiles, delineated a relatively thick weathered section of GGA that spatially aligned with a conspicuous negative anomaly observed in the SP map after electrode-drift and terrain corrections were made. The combination of ERT and SP mapping guided selection of locations for two productive water wells within the small survey area despite substantial heterogeneity in the weathering profile of the GGA beneath the survey area.

Publication Year 2021
Title Geoelectric survey of the Granite Gravel aquifer, Llano Uplift, Central Texas, to determine locations for water wells
DOI 10.1016/j.jappgeo.2021.104479
Authors Scott Ikard, David S. Wallace, Andrew Teeple, Gregory Stanton
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Applied Geophysics
Index ID 70227796
Record Source USGS Publications Warehouse
USGS Organization Texas Water Science Center