Electrical Characterization Of Success Dam In Porterville, California

Abstract

An integrated geophysical investigation of Success Dam, near Porterville, California, evaluated the foundation of a proposed new dam and delineated zones of underground water seepage. In fall 2005, the U.S. Geological Survey (USGS) conducted seismic refraction tomography, direct current (DC) resistivity, self-potential (SP), and audio-magnetotelluric (AMT) surveys of the base of the current Success Dam. The seismic refraction tomography delineated alluvium and bedrock and mapped the bedrock surface. The DC resistivity survey delineated more electrically conductive zones (ground water) and less conductive (unsaturated) zones within the alluvium. The DC survey also mapped resistive bedrock and indicated subsurface displacement east of the spillway. Interpretation of self-potential data confirmed the fault by modeling it as a sheet flow source. A repeat SP survey, performed along the dam face in June 2006 during the high water levels, indicated much stronger seepage along the eastern end of the dam. The AMT survey provided corroborating evidence of the conductive zones representing water in unconsolidated materials. Integration of the geophysical information has helped resolve ambiguities and brought out subtleties in data sets that might have been overlooked if another data set with similar, but physically different, information were not present. These geophysical investigations have been successful in characterizing variations in lithology and geologic structure that should be of assistance to the designers of the new Success Dam under development.