PROCEEDINGS, Twenty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 29-31, 2001 SGP-TR-168 SHEAR WAVE SPLITTING AND FRACTURE PATTERNS AT THE GEYSERS (CALIFORNIA) GEOTHERMAL FIELD D. Erten, M. Elkibbi and J.A. Rial Wave Propagation Laboratory, Department of Geological Sciences, University of North Carolina at Chapel Hill , NC 27599 ABSTRACT Shear-wave splitting analyses from recorded microearthquakes at The Geysers (California) indicate that subsurface predominant fractures in the field are oriented N10 o E to N50 o E under the NW Geysers seismic array and N40-50 o E and N40-50 o W under the SE Geysers array. These crack alignments seem consistent with a local structure characterized by a pull-apart block structure limited by two right-lateral strike-slip faults oriented NW-SE and regionally controlled by the San Andreas fault. INTRODUCTION Shear-wave splitting due to patterns of stress- aligned cracks in the crust has been widely ob- served in a variety of earthquake settings and controlled-source seismic data. It has also been recognized that the polarization of the leading split shear wave is usually parallel to the local strike of crack systems (or normal to the direction of the minimum horizontal stress). In addition, the time delay between fast and slow shear waves is directly related to the number of cracks per unit volume (crack density) in the medium (Crampin, 1987; Crampin and Lovell, 1991). Therefore, the interpretation of shear-wave splitting is an im- portant diagnostic tool to determine the direction and evaluate the bulk density of subsurface frac- tures in hydrocarbon and geothermal reservoirs. METHODOLOGY The shear-wave splitting method has been de- scribed extensively (e.g., Crampin, 1981; Lou and Rial, 1997; Lou et al., 1997). Although straightforward, the analysis of shear-wave splitting for the purpose of crack detection must be based on a large set of shear-wave seismo- gram data. This is in part because the behavior of shear waves in cracked media is usually as complex as the distribution of cracks may be along the ray joining the source and the re- ceiver. In addition, the source time history, me- dium heterogeneity, a weathered surface layer, the presence or absence of fluids in the cracks, and the surface topography tend to obscure the effect of crack-induced anisotropy. Ideally, the approaching S-wave is split into a fast wave polarized along the predomi- nant crack direction, and a slow wave, polarized perpendicular to it and arriving a few tens of milliseconds later. Polarization diagrams are used to detect the marked switch in polarity of the two arrivals, which provide the clearest indi- cation of medium anisotropy (Figure 1). It is the assumption here that the splitting of shear waves in The Geysers is indeed induced by the pres- ence of oriented fractures in an otherwise homo- geneous medium. Typically, the orientation of the fast wave polarization can be measured with errors not exceeding 10%. The time delay between the fast and slow shear waves is measured directly after the seismogram is rotated to orient the fast and slow arrivals along the instrument’s horizontal com- ponents. This operation cleanly separates the two shear wave arrivals allowing direct and ac- curate measurement of the time delay. Occa- sionally, rotation of all three components is performed to align the vertical component with the approaching ray. Most delay times recorded at The Geysers range from 20 to 70 ms (see Fig- ure 1). The smallest time interval that can be measured is 2.1 ms (480 samples/sec). At each receiver polarization data are collected in equal area projection diagrams and rose diagrams. Numerous measurements per station (Figures 2, 3, 4, 5) allow for a statisti- cally robust determination of the preferred di- rections. One important restriction to the shear- wave splitting analysis is that the arriving rays need to be within the shear wave window de-
SHEAR WAVE SPLITTING AND FRACTURE PATTERNS AT THE GEYSERS (CALIFORNIA) GEOTHERMAL FIELD
May 23, 2023
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Related Documents
