Imaging Crack Systems in The Geysers with Shear-Wave Splitting

Clear shear-wave splitting (SWS) is observed in 1757 high signal-to-noise ratio microearthquake seismograms recorded in The Geysers geothermal field, CA. The high quality observations of shear-wave splitting parameters (fast shear-wave polarizations and time delays) and the good data azimuthal coverage provide a unique opportunity to test the observability of shear-wave splitting and its usefulness for accurate subsurface fracture modeling. Shear-wave splitting parameters are highly sensitive to the anisotropic fabric of the medium through which shear-waves propagate and constitute the basic dataset to invert for 3D crack geometry and crack density in the subsurface. Fracture inversion results from simultaneous minimization of polarization and time delay residuals show that the most common patterns of fracture-induced anisotropy in The Geysers can be simulated by horizontally transversely isotropic (HTI) media or rocks with vertical to steeply-dipping systems of parallel cracks, which in general strike parallel to the N-to-NE direction of maximum compressive stress. The average crack density is about 4%. Deviations from horizontal transverse isotropy conditions in The Geysers are modeled by non-vertically dipping crack systems or intersecting crack systems. The SWS data also show strong evidence of frequency-dependence. Observed time delays, which are used to measure shear-wave velocity anisotropy, show a consistent decrease as the dominant shear-wave frequency increases. We attribute such frequency-dependence to fluid-flow effects in subsurface macro-fractures and describe a method that allows estimation of fracture size from detailed measurements of frequency-dependent double shear-wave splitting.