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A Strategy for Interpretation of Microearthquake Tomography Results in the Salton Sea Geothermal Field Based upon Rock P

. Geophysical Setting. . Microearthquake Data. . State 2-14 Borehole. Microearthquake stations. Microearthquakes. CalEnergy Data. . Reservoir

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A Strategy for Interpretation of Microearthquake Tomography Results in the Salton Sea Geothermal Field Based upon Rock P

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    1. A Strategy for Interpretation of Microearthquake Tomography Results in the Salton Sea Geothermal Field Based upon Rock Physics Interpretation of State 2-14 Borehole Logs Brian Bonner Lawrence Hutchings Paul Kasameyer Lawrence Livermore National Laboratory September, 2006 GRC, San Diego

    5. Purpose of Project Utilize CalEnergy microearthquake data Analyze borehole data to identify strategies for interpretation of tomography results Develop interpretations that can be obtained from surface recordings Use rock physics interpretations Identify permeability and alteration in the reservoir Obtain higher resolution of geologic structure Improve resolution for drilling targets

    8. Rock Physics Observations Closing of cracks (filling) due to mechanical pressure and chemical alteration Increase of velocity with depth due to closing of small cracks Extreme temperature gradient works to decrease velocity with depth Variations in sediment properties with depth

    9. Lithology Elders and Sass, 1988; Paillet, 1988 Four Mineralogic Zones: Near-surface unaltered sediments; 0 < depth < 1200 m, 100o–190o C Illite zone; 1200 < depth < 1900 m, 190o–250o C Chlorite zone; 1900 < depth < 2500 m, 250o–300o C Feldspar zone; 2500 < depth < 3220 m, >300o C

    10. Poisson’s Ratio Longitudinal strain divided by transverse strain Obtained by relationship between Vp and Vs Perfectly elastic material: PR = 0.5 There is not a linear relation between Vp/Vs and PR

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    13. We observe a monotonic decrease in PR due to consolidation We hypothesize that the anomaly is due to a further decrease is PR due to mechanical effects Crack closure caused by an increase in effective pressure tends to increase PR and is a purely mechanical (Bonner and Schock, 1982) Therefore, crack opening due to fractures counters this and causes a decrease in PR, i.e. the anomaly

    15. There is evidence that Poisson’s ratio may be an indicator of permeability in the Salton Sea geothermal field High resolution hypocenter locations may also provide definition of permeable zones High resolution tomographic inversion results may provide supportive evidence for identifying permeability We hope these studies will improve likelihood of success in choosing drilling targets

    16. Gail Wigget – California Energy Commission Brian Berard - CalEnergy Dennis Kaspereit – CalEnergy Tom Daley – Lawrence Berkeley National Laboratory Bill Foxall – Lawrence Livermore National Laboratory

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