Shear-Wave Splitting: An Efficient Tool to Detect 3D Fracture Patterns at The Geysers, CA

Abstract

Shear-wave splitting, observed in microearthquake records at The NW and SE Geysers geothermal fields, CA has proved to be effective in the detection of subsurface crack orientations and crack densities. Crust anisotropy due to patterns of stress-aligned fractures causes approaching shear-waves to split into fast and slow arrivals. Within the shear-wave window of a given seismic station, the measured polarization angle of the fast S-wave is in theory parallel to the predominant crack orientation beneath the station. Exceptions, however, caused by complex geometry of the crack system have been discerned. The predominant polarization directions recorded in NW Geysers range between N-S and N60E. In SE Geysers, two major sets of polarization angles are detected. The first set varies from N-S to N80E while the second set strikes generally NW. The time delay between the arrival of the fast and the slow shear-wave is proportional to the crack density (or number of cracks per unit volume) along the ray-path. When normalized to the ray length, time delays at The Geysers range typically between 8 and 40 ms/km. The sizeable collection of high-resolution shear-wave splitting parameters we compiled (polarization direction and time delay pairs) is presented in this paper. In the accompanying paper, the data is used in the inversion for crack geometry and fracture distribution within the reservoir.