Julian, B.R. and G.R. Foulger, Changes in Three-Dimensional Seismic Structure 1996 – 2002 at the Coso Geothermal Area, California: A Possible Monitoring Tool for Engineered Geothermal Systems, 29th Stanford Workshop on Geothermal Reservoir Engineering, California, 26-28 January 2004.
Changes in Three-Dimensional Seismic Structure 1996 – 2002 at the Coso Geothermal Area, California: A Possible Monitoring Tool for Engineered Geothermal Systems
U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States, [email protected]
Univ. Durham, Science Laboratories, South Rd., Durham, DH1 3LE United Kingdom, [email protected]
Local-earthquake tomography of the Coso geothermal area, conducted using data from the permanent seismometer network of the U. S. Navy, shows evidence of changes in physical properties in the geothermal reservoir between 1996 and 2002. A shallow region of high seismic-wave speeds, which correlates closely with the well field, became more pronounced during this period, while the lower wave speeds surrounding it did not change detectably. These changes extend from the surface down to 0.5 – 1.0 km below sea level (about 2 to 2.5 km below the surface). The shear-wave speed, Vs, increased more than the compressional-wave speed, Vp, so the Vp/Vs ratio, which was initially low, became more so. The replacement of water by steam alone is unlikely to cause the changes because this process would have the opposite effect from that observed, i.e., it would reduce wave speeds, and affect Vp more than Vs. Both pressure decrease and drying of argillaceous minerals are candidate processes that might be consistent with the observations, including the sign of the wave-speed changes and the relative effects on Vp and Vs. The repeat tomography may thus be detecting and mapping pressure reduction and/or drying of clay minerals in the central part of the exploited reservoir, as a result of the removal of geothermal fluids. These candidate interpretations may be checked using well-head pressure data and perhaps data from borehole logging. data and perhaps data from borehole logging.
last update 3rd February, 2006