Three-dimensional seismic image of a geothermal reservoir: The Geysers, California

Three-dimensional seismic travel-time tomography of The Geysers geothermal area, in the coast ranges of northern California, shows a strong (−9%) anomaly in V_P/V_S, the ratio of the compressional and shear wave speeds, that is not evident in V_P alone and corresponds closely to the most intensively exploited part of the geothermal reservoir. This anomaly probably indicates low pore pressure and relatively dry conditions, caused partly by boiling of pore water as steam is extracted. Steam pressure decreases over the last decade have probably caused seismologically measurable changes in wave speeds. Tomographic measurement of V_P/V_S is a promising technique both for identifying geothermal resources and for monitoring them during exploitation.