Seismic velocity and attenuation structure of the Geysers geothermal field, California

The Geysers geothermal field is located in northern California and is one of the world's largest producers of electricity from geothermal energy. The resource consists of primarily dry steam which is produced from a low, porosity fractured graywacke. Over the last several years steam pressure at the Geysers has been dropping. Concern over decline of the resource has prompted research to understand its fundamental nature. A key issue is the distribution of fluid in the matrix of the reservoir rock. In this paper we interpret seismic compressional-wave velocity and attenuation data at the Geysers in terms of the geologic structure and fluid saturation in the reservoir. Our data consist of approximately 300 earthquakes that are of magnitude 1.2 and are distributed in depth between sea level and 2.5 km. Using compressional-wave arrival times, we invert for earthquake location, origin time, and velocity along a three-dimensional grid. Using the initial pulse width of the compressional-wave, we invert for the initial pulse width associated with the source, and the one-dimensional Q structure. We find that the velocity structure correlates with known mapped geologic units, including a velocity high that is correlated with a felsite body at depth that is known from drilling. The dry steam reservoir, which is also known from drilling, is mostly correlated with low velocity. The Q increases with depth to the top of the dry steam reservoir and decreases with depth within the reservoir. The decrease of Q with depth probably indicates that the saturation of the matrix of the reservoir rock increases with depth.

[1]  A. Linde,et al.  Anelastic damping of acoustic and seismic pulses , 1975 .

[2]  David Oppenheimer,et al.  Induced seismicity in The Geysers Geothermal Area, California , 1984 .

[3]  Douglas R. Toomey,et al.  Tomographic inversion of local earthquake data from the Hengill‐Grensdalur Central Volcano Complex, Iceland , 1989 .

[4]  Fracture detection using P-wave and S-wave vertical seismic profiling at The Geysers , 1988 .

[5]  Active high‐resolution compressional wave attenuation tomography at Newberry Volcano, Central Cascade Range , 1992 .

[6]  Clifford H. Thurber,et al.  Earthquake locations and three‐dimensional crustal structure in the Coyote Lake Area, central California , 1983 .

[7]  G. Nolet,et al.  Seismic wave propagation and seismic tomography , 1987 .

[8]  Einar Kjartansson,et al.  Constant Q-wave propagation and attenuation , 1979 .

[9]  Francis T. Wu,et al.  Empirical Green's functions from small earthquakes: A waveform study of locally recorded aftershocks of the 1971 San Fernando earthquake , 1990 .

[10]  Paul G. Richards,et al.  The apparent attenuation of a scattering medium , 1983 .

[11]  Arthur Frankel,et al.  The effects of attenuation and site response on the spectra of microearthquakes in the northeastern Caribbean , 1982 .

[12]  A. R. Gregory.  FLUID SATURATION EFFECTS ON DYNAMIC ELASTIC PROPERTIES OF SEDIMENTARY ROCKS , 1976 .

[13]  D. Eberhart‐Phillips,et al.  Three-dimensional velocity structure in northern California Coast Ranges from inversion of local earthquake arrival times , 1986 .

[14]  A. Nur,et al.  Compressional and shear waves in saturated rock during water-steam transition , 1979 .

[15]  J. R. Evans,et al.  Active high-resolution seismic tomography of compressional wave velocity and attenuation structure at Medicine Lake Volcano, northern California Cascade Range , 1988 .

[16]  J. Brune.  Tectonic stress and the spectra of seismic shear waves from earthquakes , 1970 .

[17]  Guust Nolet,et al.  Seismic tomography : with applications in global seismology and exploration geophysics , 1987 .

[18]  L. Johnson,et al.  Progressive inversion for hypocenters and P wave and S wave velocity structure: Application to the Geysers, California, Geothermal Field , 1991 .

[19]  Frank D. Stacey,et al.  Anelastic degradation of acoustic pulses in rock , 1974 .

[20]  T. Narasimhan,et al.  On fluid reserves and the production of superheated steam from fractured, vapor‐dominated geothermal reservoirs , 1982 .

[21]  E. L. Majer,et al.  SEISMOLOGICAL INVESTIGATIONS AT THE GEYSERS GEOTHERMAL FIELD , 1979 .