We present the results of the modeling of a 3-D differential attenuation structure (δQ-1) beneath the Northwest Geysers geothermal field. A set of 480 high-quality microearthquakes distributed evenly throughout the field and with a minimum of 10 P-wave arrivals was selected for the study. We constructed spectral ratios by dividing each spectrum with a reference spectrum from each observing station. The reference spectrum was derived from the average spectrum of all events that were recorded at that station to correct for the strong site dependence of the observed spectra. We then estimated the differential attenuation operator from the slopes of the spectral ratios. The velocity models and the raypaths for all events are known from a previous velocity inversion study. The inversion for the differential attenuation structure was carried out using a modified progressive inversion method. The observed δQ-1 structure correlates well with mapped geologic units. High δQ-1 and lower velocities correlate with Franciscan melange, while lower attenuation and higher velocities correspond to metagraywacke units. High P-wave δQ-1 also underlies the southern region between 2 and 3 km depth where low Vp/Vs values suggest undersaturation of the reservoir rocks. Most of the steam entries also occur within this region and probably delineate the steam reservoir. These anomalies may be explained by high rock temperatures and the presence of steam and other gases.

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