Double-difference seismic attenuation tomography method and its application to The Geysers geothermal field, California

Abstract:

We have developed a new double difference seismic attenuation (DDQ) tomography method for imaging high-resolution 3-D subsurface attenuation structure. Our DDQ method includes two main parts: (1) the inversion of differential t* data using spectral ratio data from pairs of events at common stations using a newly developed spectral ratio method, and (2) 3-D attenuation tomography with the obtained differential t* data. Due to the cancellation of instrument and site responses, event-pair spectral ratio method can determine more accurate differential t* data than the traditional method using individual spectra. Due to high quality differential t* data and the cancellation of model uncertainties along the common paths outside the source regions, the DDQ tomography method can achieve higher resolution, especially in the source regions, than the tradition method using absolute t* data. We tested the effectiveness and robustness of our spectral ratio method using both synthetic and real earthquake spectra data. We then applied the DDQ method to The Geysers geothermal field, California. P-wave data from 2937 events in the year 2011 observed at 34 stations were used. With updated earthquake locations and Vp model, we determined a new P-wave attenuation (Qp) model, which shows fine scale structures in the geothermal reservoir. Synthetic tests show that the DDQ method can better recover the model than the method using absolute t* data. Our Qp and Vp models show variations in the normal temperature reservoir at depths of ~0.5-2 km in different parts of The Geysers, which can be explained by variations in permeability or pore fluid saturation. At depths of ~2-4 km where a high temperature reservoir exists in the northwest part of The Geysers, our results show a low-Qp and low-Vp zone that is associated with very active seismicity. This area is likely partially saturated with injected fluids. We have also determined a preliminary Qp model of The Geysers for the year of 2005. The difference between the 2011 and 2005 Qp models shows significant change in and near the deep low-Qp, low-Vp, and seismically active zone. Decreases in Qp may indicate increased fracturing whereas increases in Qp may indicate the cooling of hot reservoir rocks due to water injection.