Compressional And Shear Wave Seismic Refraction Tomography At Success Dam, Porterville, California

Success Dam is an earthen embankment located near Porterville, Calif., and is of interest due to earthquake-related collapse potential. In order to characterize the geologic setting of the current dam and to help guide planning for a replacement dam, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, has undertaken a multi-method geophysical investigation of the site. The seismic component of this investigation consists of three transects, each with compressional (P) and horizontally polarized shear (S) wave coverage. Along each transect, we calculated tomographic velocity models of P-wave velocity (Vp) and S-wave velocity (Vs). These velocity models agree with available well information and show the transition from unconsolidated sediment at the surface to consolidated sedimentary rock to crystalline basement at depth. Weak zones such as soft sand in the upper sediments and highly weathered/fractured rock are manifested as low-velocity zones. Because Swave velocity is closely tied to engineering properties such as shear strength, low-velocity zones in the S-wave velocity models are of particular interest. We identify several zones of concern using calculated Vp/Vs ratios and Poisson’s ratio; these results will be used to guide future plans at the site.