P wave anisotropy, stress, and crack distribution at Coso geothermal field, California

Abstract

A new inversion method for P wave anisotropy [Wu and Lees, 1999a] has been applied to high‐precision, microseismic traveltime data collected at Coso geothermal region, California. Direction‐dependent P wave velocity and thus its perturbation, are represented by a symmetric positive definite matrix A instead of a scalar. The resulting anisotropy distribution is used to estimate variations in crack density, stress distribution and permeability within the producing geothermal field. A circular dome‐like structure is observed at the southwestern part of the geothermal region southwest of Sugarloaf Mountain. Using a linear stress‐bulk modulus relationship, deviatoric stress is estimated to be 3–6 MPa at geothermal production depths (1–2 km), assuming all the anisotropy is related to stress. The stress field is compressional NNE‐SSW and dilational WNW‐ESE, coinciding with a previous, independent study using earthquake focal mechanisms. Following a theory on flat, elliptic cracks, residual crack density estimated from P anisotropy is ∼0.0078 assuming crack aspect ratios ≫ 1:60 and is ∼0.041 when crack aspect ratios are close to 1:60. Residual crack orientation distribution is related to velocity anisotropy. On the basis of anisotropic part of crack density distribution function, the anisotropic part of permeability distribution may be calculated by a statistical approach via simple parallel fluid flow along cracks.