Scattering from a fault interface in the Coso geothermal field

Abstract

Large-amplitude, secondary arrivals are modeled as scattering anomalies near the Coso, California, geothermal field. Polarization and ray tracing methods determine the orientation and location of the scattering body. Two models are proposed for the scatterer: (1) a point scatterer located anywhere in a one-dimensional (1-D), layered velocity model; and (2) a dipping interface between two homogeneous half spaces. Each model is derived by non-linear, grid search inversion for the optimal solution which best predicts observed travel times. In each case the models predict a nearly vertical scatterer southwest of stations S4 and Y4, each southeast of Sugarloaf Mountain, a large rhyolite dome. The interface model includes five unknown parameters describing the location and orientation of the interface in addition to the S-wave velocity of the half space. The S-wave velocity, 3.25 km/s, agrees with independently derived 1-D models in this area. The large amplitude, vertical impedance contrast interface coincides with steep gradients of heat flow measured near the surface and with structural boundaries observed in surface geology. The reflector is most probably the sharp boundary between the northern part of the field where there is significant fluid flow and the southern part where hydrothermal fluids are absent. The interface coincides with geological boundaries and faults recently observed in this region, most likely representing the hydrothermal barrier which channels hot fluids northward.