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Sensitivity and stability analysis of coda quality factors at The Geysers geothermal field, California

Aglaja Blanke, Grzegorz Kwiatek, Patricia Martinez-Garzon and Marco Bohnhoff
Sensitivity and stability analysis of coda quality factors at The Geysers geothermal field, California
Bulletin of the Seismological Society of America (June 2019) 109 (3): 959-975

Abstract

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 Q (sub C) . 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 Q (sub C) (f) estimates with associated uncertainties. We investigate the sensitivity of the Q (sub C) 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 (2sigma (Q (sub C) (f)))-are found to be most sensitive factors. Testing different window lengths and lapse times results in relatively small variations of Q (sub C) . The final, highest quality mean coda Q(Q (sub C) (f)) estimates are further tested in the context of their spatiotemporal behavior in the reservoir. We found that distance and azimuthal dependence of Q (sub C) (f) are related to the observed crack-induced reservoir anisotropy, lithological, and structural features. At the northwestern The Geysers about 50% larger Q (sub C) (>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 Q (sub C) (f) estimates. Moreover, we compare the Q (sub C) (f) results with previous estimates of direct S-wave quality factors (Q (sub D) ). A match for Q (sub C) results at 7 Hz center frequency with Q (sub D) estimates is observed. However, Q (sub C) estimates show lower scattering and thus a higher stability.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 109
Serial Issue: 3
Title: Sensitivity and stability analysis of coda quality factors at The Geysers geothermal field, California
Affiliation: German Research Centre for Geosciences, Potsdam, Germany
Pages: 959-975
Published: 201906
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 30
Accession Number: 2019-062014
Categories: SeismologyEconomic geology, geology of energy sources
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch map
N38°40'00" - N38°40'00", W122°55'00" - W122°55'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201932
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