3-DIMENSIONAL SEISMIC IMAGE OF A GEOTHERMAL RESERVOIR - THE GEYSERS, CALIFORNIA

Three-dimensional seismic travel-time tomography of The Geysers geothe rmal 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 wav e V-p alone and corresponds speeds, that is not evident in V-p closely to the most intensively exploited part of the This anomaly probably i ndicates low pore pressure and relatively dry conditions, caused partl y by boiling of pore water as steam is extracted. Steam pressure decre ases over the last decade have probably caused seismologically measura ble changes in wave speeds. Tomographic measurement of V-p/V-s is a pr omising technique both for identifying geothermal resources and for mo nitoring them during exploitation.