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. Copyright 1999 by the American Geophysical Union.