Industrially induced changes in Earth structure at The Geysers geothermal area, California

Industrial exploitation is causing clearly-measurable changes in Earth structure at The Geysers geothermal area, California. Production at The Geysers peaked in the late 1980s at similar to 3.5 x 10(3) kg s(-1) of steam and 1800 MW of electricity. It subsequently decreased by about 10% per year [Barker et at, 1992] because of declining reservoir pressure. The steam reservoir coincides with a strong negative anomaly (similar to 0.16, similar to 9%) in the compressional-to-shear seismic wave speed ratio nu(P)/nu(S),consistent with the expected effects of low-pressure vapor-phase pore fluid [Julian ed at, 1996]. Between 1991 and 1994 this anomaly increased in amplitude by up to about 0.07 (similar to 4%). This is consistent with the expected effects of continued pressure reduction and conversion of pore water to steam as a result of exploitation. These unique results show that nu(P)/nu(S) tomography can easily detect saturation changes caused by exploitation of reservoirs, and is a potentially valuable technique for monitoring environmental change. They also provide geophysical observational evidence that geothermal energy is not a renewable energy source.