Geothermal Methods:
Application of Time-Dependent Tomography to Detect Changes in Structure at Long Valley Caldera and the Coso Geothermal Area, California

Abstract

Structural changes in active volcanic and producing geothermal systems are expected because of changes in the distribution of fluids, gases and cracks in the host rocks. Such changes have traditionally been studied using seismic tomography where two independent inversion results are differenced. A new tomography program tomo4d, inverts two epochs simultaneously, imposing constraints to minimize the structural differences calculated between different epochs. This method suppresses spurious changes not required by the data. Both methods were applied to data from Long Valley caldera and the Coso geothermal area, and the results compared.

Long Valley caldera, California, has been seismically active since 1978. In particular, a region to the south of the resurgent dome (the “south moat”) and Mammoth Mountain have experienced multiple swarms involving hundreds of thousands of earthquakes. Inverting data from 1997 and 2009/10 using tomo4d detected changes with weaker anomaly strengths compared to those of simul2000A. Some changes imaged using simul2000A are thus not required by the data. Variable changes in Vp, Vs and Vp/Vs were detected and are interpreted as pore pressure decrease and/or drying of minerals, CO2 depletion and flooding during the tectonically active period.

The Coso geothermal area, California, is highly seismogenic, with thousands of earthquakes occurring each year. Time-dependent seismic tomography was performed for the years 1996, 2006, 2007, 2008, 2010 and 2012 using both simul2000A and tomo4d. The epochs 1996-2006 and 2007-2012 were studied in detail. During the first epoch, Vp, Vs and Vp/Vs mostly increased in the geothermal field whereas during the second epoch changes were more varied and less extreme. It is concluded that different parts of this tripartite field have different reservoir characteristics, and that operational activities changed with time. These likely involved increasing water saturation in some areas as a result of increased water injection in recent years.