Seismic Attenuation Imaging of The Geysers Geothermal Field, USA

Abstract:

The Geysers geothermal field is located in northern California and is the world's largest producer of geothermal energy. Previous studies found spatio-temporal variations in seismic velocity that are likely related to fluid injections and distributions within the reservoir. Because seismic attenuation may exhibit more sensitivity to fluid saturation and to the thermal state of the medium, we obtained for the first time 2D intrinsic (Qi^-1) and scattering (Qs^-1) attenuation models and 3D total-Q^-1 attenuation structure. These models have been obtained by fitting waveform envelopes to diffusion model and coda-normalization method, respectively. 2D intrinsic and scattering models were derived by applying the diffusion model which is an approximation of the general energy transport theory developed by Wegler and Lühr (2001) and Wegler (2003). We use energy envelopes of the seismograms, to obtain intrinsic attenuation coefficient and diffusivity coefficients. In a subsequent step, we applied a new representation method based on numerically estimated space-weighting functions (Del Pezzo et al., 2016), to estimate inverse quality factors. We have used over 35000 waveforms recorded at 34 seismic stations to estimate the 3D attenuation structure of the reservoir. Seismic ray tracing were based on through the 3D velocity model by Gritto et al., 2013. The spectral ratios obtained with the coda normalization method, were subsequently inverted to obtain total-Q^-1 values with 3-km grid spacing. Both 2D maps and 3D attenuation structure are in good agreement with the velocity model: the low velocity zones being consistent with regions featuring high attenuation effects and the high velocity zones with regions featuring low attenuation effects. Low attenuation and low Vp regions can be interpreted as region with steam concentration, which have been observed in geothermal reservoirs and volcanic regions (De Siena, 2010).