Attenuation structure of Coso geothermal area, California, from wave pulse widths

Abstract

Pulse width data are used to invert for attenuation structure in the Coso geothermal area, California. The dataset consists of pulse width measurements of 838 microseismic events recorded on a seismic array of 16 downhole stations between August 1993 and March 1994. The quality factor Q correlates well with surface geology and surface heat flow observations. A broad region of low Q (≈ 30 to 37) is located at 0.5 to 1.2 km in depth below Devil's Kitchen, Nicol Prospects, and Coso Hot Springs. A vertical, low Q (≈ 36 in contrast with surrounding rock of 80) region is interpreted as a channel through which hydrothermal energy is transported from depth to the surface. The location of the channel is between stations S1 and S4, and its dimension is about 1 km. At the deep end of the channel, a large, broad body of low Q is also located at 3 km in depth 2 to 4 km to the southwest of Nicol Prospects and Devil's Kitchen. Since it lies at the bottom of the target region and beyond the scope of seismicity, further research is needed to constrain its extent. Numerical modeling with a pseudospectral method is also done to investigate the applicability of the inversion scheme to fractured regions. A linear relationship between pulse width broadening and travel time is upheld, and the proportional constants are estimated.