Sensitivity and stability analysis of coda quality factors at the geysers geothermal field, California

Over 700 induced seismic events recorded between June 2009 and March 2015 at different parts of The Geysers geothermal field, California, are used to estimate local S-wave coda quality factors (QC). Recorded by the 31-station short-period Berkeley-Geysers seismic network, the events have duration magnitudes 1 <MD < 3, depth ranges of 1 and 4 km, and epicentral distance ranges of 0.7–19 km. We apply the coda analysis technique of Phillips (1985) to find QC. Using a sequence of overlapping time windows, the average power spectral density of the coda is calculated. We extract the signal amplitudes at fixed octave-width frequency bands, measure their decay with time and fit for QC f estimates with associated uncertainties. We investigate the sensitivity of the QC results to different input parameters, including lapse time, magnitude range, moving window width, total coda length, and seismic-sensor components. The choice of quality criteria—signal-to-noise ratio and coda Q uncertainties (2σ QC f)—are found to be most sensitive factors. Testing different window lengths and lapse times results in relatively small variations of QC. The final, highest quality mean coda Q (QC f) estimates are further tested in the context of their spatiotemporal behavior in the reservoir. We found that distance and azimuthal dependence of QC f are related to the observed crack-induced reservoir anisotropy, lithological, and structural features. At the northwestern The Geysers about 50% larger QC >40 Hz estimates are obtained compared with the southwest. In contrast, geothermal production rate variations, analyzed for a tight cluster in the northwest, do not influence the QC f estimates. Moreover, we compare the QC f results with previous estimates of direct S-wave quality factors (QD). A match for QC results at 7 Hz center frequency with QD estimates is observed. However, QC estimates show lower scattering and thus a higher stability. © 2019, Seismological Society of America. All rights reserved.