Successful geothermal reservoir development requires knowledge of subsurface structures such as the location and orientation of fracture systems and possible fluid flow pathways. While techniques exist for characterizing the subsurface in oil and gas applications, technical capabilities must be augmented for the EGS application, given the strong heterogeneity and time-varying nature of the typical geothermal reservoir. The induced seismicity that occurs during the production history of a geothermal reservoir provides significant clues about the location and orientation of newly created faults and fractures. Accurate locations of the time-varying microseismicity can give field operators information about fracture location, spacing and orientation. Equally important is the seismic P and S velocity structure, since subtle changes in the Poisson
