| Abstract |
Temporal changes in seismic wave speeds in geothermal areas have been reported, detected using time-dependent seismic tomography. However, despite the potential value of such measurements for monitoring exploited geothermal reservoirs, most reports of changes in wave speeds are of questionable reliability. This is because changes in calculated structure are expected, even if the real structure did not change, as a result of different experimental conditions and observational errors from epoch to epoch. The difficulties in detecting true changes in seismic wave speeds can be greatly reduced by inverting data from different epochs simultaneously, imposing regularization constraints to minimize the differences between derived models. This method suppresses spurious changes that are not required by the data. We report on the application of this method to data from the exploited Coso geothermal area in southeastern California. The Coso geothermal area is the seventh largest exploited geothermal area in the world. It is 28 km2 in area, and has a water-dominated reservoir with temperatures up to ~ 345ºC. It has been under production for two decades, and now has ~ 180 boreholes up to 4 km deep and produces 300 Mw of electricity, sufficient for the needs of ~1 million people. We performed tomographic inversions for the years 1996, 2006, 2007, 2008, 2010, and 2012. The epochs 1996-2006 and 2007-2012 were studied in detail. Vp, Vs and Vp/Vs mostly increased during the first epoch in the geothermal field but were more varied during the second epoch. We conclude that different parts of the field have different reservoir characteristics, and that operational activities changed with time. These may have involved increasing water saturation in some areas as a result of increased water injection in recent years. |