Abstract
Intensive geothermal exploitation at The Geysers geothermal area,
California, induces myriads of small-magnitude earthquakes that are
monitored by a dense, permanent, local seismometer network. Using
this network, tomographic inversions were performed for the three-dimensional
Vp and Vp/Vs structure of the reservoir for April 1991, February
1993, December 1994, October 1996, and August 1998. The extensive
low-Vp/Vs anomaly that occupies the reservoir grew in strength from
a maximum of 9\% to a maximum of 13.4\% during the 7-year study period.
This is attributed to depletion of pore liquid water in the reservoir
and replacement with steam. This decreases Vp by increasing compressibility,
and increases Vs because of reduction in pore pressure and the drying
of argillaceous minerals, e.g., illite, which increase the shear
modulus. These effects serendipitously combine to lower Vp/Vs, resulting
in a strong overall effect that provides a convenient tool for monitoring
reservoir depletion. Variations in the Vp and Vs fields indicate
that water depletion is the dominant process in the central part
of the exploited reservoir, and pressure reduction and mineral drying
in the northwest and southeast parts of the reservoir. The rate at
which the Vp/Vs anomaly grew in strength in the period 1991-1998
suggests most of the original anomaly was caused by exploitation.
Continuous monitoring of Vp, Vs, and Vp/Vs is an effective geothermal
reservoir depletion monitoring tool and can potentially provide information
about depletion in parts of the reservoir that have not been drilled.
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