Seismic Imaging of the Velocity Structure and the Location of a Hydrofrac in a Geothermal Reservoir

The Los Alamos Hot Dry Rock Reservoir is an experimental geothermal project in north-central New Mexico. A fractured zone was created within otherwise impermeable igneous and metamorphic rock by injecting water into a borehole under high pressure, at about 3.5 km depth. During the injection process, the seismic waves created by the fracturing events were recorded by seismometers located in four nearby boreholes. A subset of the arrival times from these microearthquakes is iteratively inverted for the three-dimensional P-wave and S-wave velocity structures and the hypocenter parameters, using the separation of parameters technique. The inversion results indicate that the P-wave and S-wave velocities decrease by at least 20% within the fractured zone. Also, the hypocenters are rotated into a more compact distribution ,elative to the initiallocations found using a homogeneous velocity model, suggesting that the hypocenter locations are significantly improved.