Time-dependent three-dimensional tomography of reservoir structure may provide an important tool for monitoring changes in operating geothermal reservoirs. The method may also be useful for oil and gas reservoirs, and for active volcanoes. Temporal changes in structure may be studied either using repeat, independent tomographic inversions or by inverting two epochs simultaneously for changes. We studied The Geysers geothermal field using MEQ data for the years 2008 - 2014. We obtained data from the Northern California Earthquake Data Centre (NCEDC) catalogue. The database comprises 1657 earthquakes, with 35,278 P-wave arrival-time measurements and 9,452 S-wave arrival-time measurements. We selected earthquakes for quality and chose events with at least ten P- and three S-wave arrival time measurements, station azimuthal gaps of less than 180˚, and RMS arrival-time residuals smaller than 0.05 s. Production was stable during the period studied, whereas the injection rate increased in 2010 and decreased in 2014. Net production decreased between 2008 and 2010 and increased from 2010 to 2014. We detected a negative Vp/Vs anomaly throughout the period studied in the central Geysers and a positive anomaly in the NW Geysers. We do not detect anomaly growth of the same kind as reported for the period 1991-1998. This may be because a) large-scale water injections into the reservoir have arrested the fluid depletion that caused the anomaly growth in the period 1991-1998, or b) because the catalog seismic data we used are not of sufficient quality to detect the subtle anomaly growth. This paper is a preliminary report. On-going work will include a) hand-processing of selected datasets, and b) application of a more powerful inversion program (TOMO4D) that is not susceptible to errors introduced by variations in experimental setup from year to year.

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