Characterization of Subsurface Fracture Patterns in the COSO Geothermal Reservoir by Analyzing Shear-Wave Splitting of Microearthquake Seismograms

Abstract

A large number of microearthquake seismograms have been recorded by a downhole, three-component seismic network deployed around the COSO, California geothermal reservoir. Shear-wave splitting induced by the alignment of cracks in the reservoir has been widely observed in the recordings. Over 100 events with body wave magnitude greater than 1 .O from microearthquakes recorded since March of 1992. have been processed. The results show that most events with paths within the critical angle that defines the shear-wave window, display clear shear-wave splitting, and the fast shear waves have predominant polarization directions for most stations. The rose diagrams of fast shear-wave particle motion suggest that there are three predominant fracture strikes (or directions of maximum horizontal stresses) in the COSO geothermal field: N 4Oo-68E, N 0?-29E, and N 2 9 - 3 9 W, which are consistent with photographically or magnetically mapped alignments on the surface. From the delay time of split shear waves, we estimate that the crack density in the most active geothermal reservoir area (above 3.00km depth) ranges between 0.030 and 0.055. values commonly found in other hydrocarbon or geothermal reservoirs.