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

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