Three high-precision gravity and GPS surveys have been conducted over The Geysers geothermal reservoir and surrounding area. These surveys, in September 2000, April 2001, and September 2001, provide an initial baseline for future imaging studies of spatial mass changes from the Santa Rosa Effluent Pipeline currently under construction. Analysis of the three surveys provides a measure of the seasonal and annual gravity and deformation signals currently occurring at The Geysers. GPS results show an average subsidence of 3±2 cm over the production field from September 2000 to April 2001, although some stations have significant deviations from this average. The deformation signal between September 2000 and September 2001 indicates an annual subsidence rate across the field of 2±2 cm/year. Gravity changes between April and September 2001 show an average of -38±8 mGal; a magnitude consistent with seasonal and annual production and ground water effects. Gravity change between April and September 2001 show an average change of -10±8 mGal. Spatial variation in the gravity changes may represent local injection and ground water responses.

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