Seismological investigations in geothermal regions

Abstract

Seismological methods, including studies of microearthquakes, P- and S-wave velocities and P-wave attenuation were investigated as tools for the exploration and delineation of geothermal resources. Seismograms from explosions and microearthquakes were examined for changes in frequency content and relative arrival times across a known geothermal area, The Geysers, California, and a potential geothermal region, Grass Valley, Nevada. Microearthquakes within the two regions were examined for evidence of spatial variations in radiated P- and S-waves. Additional information concerning Basin and Range structure was provided by regional refraction studies. Detailed structural analysis in Grass Valley was obtained by commercial reflection and refraction work. Heat flow modeling, consistent with structure inferred by seismological techniques, was used to discriminate between conductive and convective heat flow anomalies in Grass Valley. Concentrated observations in Grass Valley around Leach Hot Springs revealed moderate microearthquake activity on a trend crossing the southern end of the valley, with occasional swarms in the area of high heat flow (4 to 6 hfu) at the north end of the 1915 Pleasant Valley earthquake (mag = 7.5) fault trace. Studies in The Geysers steam field reveal significant velocity and attenuation anomalies associated with the production zone. It is concluded that with propermore » sampling in space and time, P- and S-wave velocity and attenuation data can detect and delineate significant anomalies associated with the static properties of a geothermal resource. Microearthquake data are useful for monitoring the dynamic strain relief processes associated with fluid movement, temperature and pressure gradients in geothermal environments.« less

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