Reservoir depletion at The Geysers geothermal area, California, shown by four‐dimensional seismic tomography

Intensive geothermal exploitation at The Geysers geothermal area, California, induces myriads of small‐magnitude earthquakes that are monitored by a dense, permanent, local seismometer network. Using this network, tomographic inversions were performed for the three‐dimensional V p and V p / V s structure of the reservoir for April 1991, February 1993, December 1994, October 1996, and August 1998. The extensive low‐ V p / V s anomaly that occupies the reservoir grew in strength from a maximum of 9% to a maximum of 13.4% during the 7‐year study period. This is attributed to depletion of pore liquid water in the reservoir and replacement with steam. This decreases V p by increasing compressibility, and increases V s because of reduction in pore pressure and the drying of argillaceous minerals, e.g., illite, which increase the shear modulus. These effects serendipitously combine to lower V p / V s , resulting in a strong overall effect that provides a convenient tool for monitoring reservoir depletion. Variations in the V p and V s fields indicate that water depletion is the dominant process in the central part of the exploited reservoir, and pressure reduction and mineral drying in the northwest and southeast parts of the reservoir. The rate at which the V p / V s anomaly grew in strength in the period 1991–1998 suggests most of the original anomaly was caused by exploitation. Continuous monitoring of V p , V s , and V p / V s is an effective geothermal reservoir depletion monitoring tool and can potentially provide information about depletion in parts of the reservoir that have not been drilled.