Seismic Refraction And Downhole Velocity Surveys For Investigation Of The Tinemaha Dam Inyo County, California

Abstract

Seismic refraction and downhole velocity surveys were conducted as part of a geotechnical investigation of the Tinemaha Dam. Thirteen P and S-wave velocity surveys were conducted concurrently with cone penetration testing along the crest and downstream toe of this dam. One additional velocity survey was conducted from a borehole on the crest of the dam using a downhole geophone array. The shear wave velocities estimated for the dam and its underlying alluvium ranged from 345 to 2008 ft/sec. A depth profile of these velocities indicates areas within the dam and its foundation where the shear wave velocities are less than 600 ft/sec. These velocity estimates were used with other geotechnical data, to calculate the dynamic elastic moduli for the dam and its foundation, to evaluate the stability of the dam in the event of a local earthquake on the Owens Valley fault. To further investigate the location of this active strike-slip fault, seismic refraction surveys were conducted at two locations. The first refraction survey was positioned south of the dam across the expected trend of the Owens Valley fault. A velocity analysis of the upper refracting horizon was conducted using the generalized reciprocal method. This revealed changes in lateral velocity more » that were interpreted as due to a fault-transition from saturated alluvium to volcanic bedrock. The second survey was positioned along the dam`s crest at the west abutment to help locate a possible fault beneath this area. The interpreted depth section from this survey locates a point where the deeper bedrock unit appears to be truncated by faulting. This information will help locate additional boreholes to evaluate lithologic conditions. « less