Abstract
Three-dimensional seismic travel-time tomography of The Geysers geothermal
area, in the coast ranges of northern California, shows a strong
(-9\%) anomaly in VP/VS , the ratio of the compressional and shear
wave speeds, that is not evident in VP alone and corresponds closely
to the most intensively exploited part of the geothermal reservoir.
This anomaly probably indicates low pore pressure and relatively
dry conditions, caused partly by boiling of pore water as steam is
extracted. Steam pressure decreases over the last decade have probably
caused seismologically measurable changes in wave speeds. Tomographic
measurement of VP/VS is a promising technique both for identifying
geothermal resources and for monitoring them during exploitation.
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