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

Abstract

High-resolution, three-dimensional, compressional and shear wave velocity models, derived from microearthquake traveltimes, are used to map the distribution of Poisson's ratio and porosity at Coso Geothermal Area, Inyo County, California. Spatial resolution of the three-dimensional Poisson's ratio and porosity distributions is estimated to be 0.5 km horizontally and 0.8 km vertically. Model uncertainties, ±1% in the interior and ±2.3% around the edge of the model, are estimated by a jackknife method. We use perturbations of r= V p /V s ratio and Ψ= V p · V s product to derive distributions of Poisson's ratio, σ, and porosity, which are then used to constrain 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 0.224, lower than the crustal average of 0.25. High Poisson's ratios are more extensive in shallower depths (<1.5 km) while lower Poisson's ratios are found in the deeper section (1.5–3.0 km) of the target area. 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- σ features are separated by a northwest–southeast-trending high- σ belt with variable width of 1–3 km. A high- Ψ body is found around S2 and S6, and extends down in depth. A circular, low- Ψ belt corresponding to the high- σ belt, is located around S2–S6 and is linked to a previously reported structure in V s tomography. This low- Ψ (highly porous) belt is probably a horizontal conduit/reservoir of geothermal fluid. A vertical, low- σ and high- Ψ channel beneath triangle S1–S3–S4 corresponds to a high-attenuation, dome-like feature. We propose an upwell-and-spread magma intrusion model for the last major magmatism in the Coso region. The magmatic upwelling is centered in the S1–S3–S4 area. The model predicts potential geothermal resources to the south and west of triangle S1–S3–S4 based on local faulting patterns.