Seismic Refraction and Basement Temperature Investigation of the East Mesa Known Geothermal Resources Area, Southern California

Abstract

Fifteen refraction shots located primarily in the East Mesa Known Geothermal Resources Area (KGRA) of southern California were detonated in March of 1976. These explosions, along with recordings stations, were arranged to provide a distribution of directions and areal coverage to determine the nature of the Tertiary sedimentary section and the velocity of, and depth to, the underlying crystalline basement rocks of the East Mesa KGRA. Measured velocities varied from 1.70 to 5.40 km/sec and depth to basement was found to be 4.15 km. Although basement is 1.1 km deeper, the velocity is comparable to the 5.5 km/sec obtained 10 km to the north. Comparisons with published refraction profiles located in the Salton Trough and with continuous velocity logs from six wells (ranging in depth from 1.82 to 3.07 km) indicate similar velocity structure for the sedimentary section. By combining the refraction results with eh measure bottom hole temperatures from five geothermal wells, and the background gradient of 75°C/km, temperatures are predicted to range from 325°C to 365.C at the sediment basement interfaces underlying the East Mesa KGRA.