Poisson's ratio and porosity at Coso geothermal area, California

High-resolution, three-dimensional, compressional and shear wave velocity m odels, derived from microearthquake traveltimes, are used to map the distri bution of Poisson's ratio and porosity at Coso Geothermal Area, Inyo County , California. Spatial resolution of the three-dimensional Poisson's ratio a nd porosity distributions is estimated to be 0.5 km horizontally and 0.8 km vertically. Model uncertainties, +/- 1% in the interior and +/- 2.3% aroun d the edge of the model, are estimated by a jackknife method. We use pertur bations of r = V-p/V-s ratio and Psi = V-p.V-s, IS product to derive distri butions of Poisson's ratio, sigma, and porosity, which are then used to con strain and delineate possible zones of intense heat, fracture accumulation and fluid saturation. Poisson's ratio at Coso ranges from 0.15 to 0.35 with an average of 0.223, lower than the crustal average of 0.25. High Poisson' s ratios are more extensive in shallower depths (< 1.5 km) while lower Pois son's ratios are found in the deeper section (1.5-3.0 km) of the target are a. Two major features with low Poisson's ratio are identified at geothermal production depth (1-3 km) around stations S2-S6 and S1-S3-S4. The two low- sigma features are separated by a northwest-southeast-trending high-sigma b elt with variable width of 1-3 km. A high-Psi body is found around S2 and S 6, and extends down in depth. A circular, low-Psi belt corresponding to the high-a belt, is located around S2-S6 and is linked to a previously reporte d structure in V-s tomography. This low-Psi (highly porous) belt is probabl y a horizontal conduit/reservoir of geothermal fluid. A vertical, low-sigma and high-Psi channel beneath triangle S1-S3-S4 corresponds to a high-atten uation, dome-like feature. We propose an upwell-and-spread magma intrusion model for the last major magmatism in the Coso region. The magmatic upwelli ng is centered in the S1-S3-S4 area. The model predicts potential geotherma l resources to the south and west of triangle S1-S3-S4 based on local fault ing patterns. (C) 2000 Elsevier Science B.V. All rights reserved.