Open AccessThe Geysers geothermal field: results from shear‐wave splitting analysis in a fractured reservoir
Author(s) -
Elkibbi Maya,
Rial J. A.
Publication year - 2005
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2005.02698.x
Subject(s) - shear wave splitting , geology , geothermal gradient , seismology , shear (geology) , microseism , shear waves , borehole , crust , geophysics , petrology , geotechnical engineering
SUMMARY Clear shear‐wave splitting (SWS) is observed in 1757 high signal‐to‐noise ratio microearthquake seismograms recorded by two high density seismic arrays in the NW and the SE Geysers geothermal fields in California. The Geysers reservoir rocks within the study area are largely composed of lithic, low‐grade metamorphism, well‐fractured metagraywackes which commonly lack schistosity, warranting the general assumption that shear‐wave splitting here is induced solely by stress‐aligned fracturing in an otherwise isotropic medium. The high quality of observed shear‐wave splitting parameters (fast shear‐wave polarization directions and time delays) and the generally good data spatial coverage provide an unprecedented opportunity to demonstrate the applicability and limitations of the shear‐wave splitting approach to successfully detect fracture systems in the shallow crust based on SWS field observations from a geothermal reservoir. Results from borehole stations in the NW Geysers indicate that polarization orientations range between N and N60E; while in the SE Geysers, ground surface stations show polarization directions that are generally N5E, N35E‐to‐N60E, N75E‐to‐N85E, and N20W‐to‐N55W. Crack orientations obtained from observed polarization orientations are in good agreement with independent field evidence, such as cracks in geological core data, tracer tests, locally mapped fractures, and the regional tectonic setting. Time delays range typically between 8 and 40 ms km −1 , indicating crack densities well within the norm of fractured reservoirs. The sizeable collection of high resolution shear‐wave splitting parameters shows evidence of prevalent vertical to nearly vertical fracture patterns in The Geysers field. At some locations, however, strong variations of SWS parameters with ray azimuth and incident angle within the shear‐wave window of seismic stations indicate the presence of more complex fracture patterns in the subsurface.