Three‐dimensional Q−1 model of the Coso Hot Springs Known Geothermal Resource Area

Abstract

Observations of teleseismic P waves above geothermal systems exhibit travel time delays and anomalously high seismic attenuation, which is extremely useful in estimating the thermal regime and the potential of the system. A regional telemetered network of sixteen stations was operated by the U.S. Geological Survey in the Coso Hot Springs Known Geothermal Resources Area (KGRA) for such studies from September 1975 to October 1976. Subsequently, they deployed a portable Centipede array of 26 three‐component stations near the center of the anomaly. The seismograms of 44 events recorded by the telemetered array and nine events by the Centipede array were analyzed using the reduced spectral ratio technique to determine the differential attenuation factor δt* for the events recorded with the highest signal‐to‐noise ratio. The δt* variation observed across the Coso Hot Springs KGRA were small (<0.2 s). A three‐dimensional generalized linear inversion of the δt* observations was performed using a three‐layer model. A shallow zone of high attenuation exists within the upper 5 km in a region bounded by Coso Hot Springs, Devils Kitchen, and Sugarloaf Mountain probably corresponding to a shallow vapor liquid mixture or ‘lossy’ near surface lithology. No zones of significantly high attenuation occur between 5‐ and 12‐ km depth. Between the depth of 12–20 km a thick zone of high attenuation (Q <50) exists, offset toward the east from the surface anomaly.