Resistivity Imaging in Eastern Nevada Using the Audiomagnetotelluric Method for Hydrogeologic Framework Studies

Inversion of audiomagnetotelluric (AMT) sounding data collected in eastern Nevada shows significant structure within the upper kilometer of the subsurface that defines the geologic framework from which hydrologic models will be developed. We collected AMT data along two profiles in Spring and Cave valleys in 2004-2005, using the Geometrics StrataGem EH4 system, a four-channel, natural and controlled-source tensor system recording in the range of 10-92,000 Hz. Profiles were 12 and 3 km in length with station spacing of 200-400 m. Two-dimensional inverse models show detailed structure within the alluvial basin including clear transitions between unsaturated and saturated alluvium/volcanic rocks, highly-resistive (>1000 ohm-m) carbonate rocks, and the locations of range-front and intra-basin faults. In addition, our results define the shape of and the depth to the basement surface, which correlates well with depth to basement estimates derived from the inversion of gravity data.