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

A new inversion method for P wave anisotropy [Wu and Lees, 1999a] has been applied to high-precision, microseismic traveltime data collected at Coso g eothermal 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 esti mate variations in crack density, stress distribution and permeability with in the producing geothermal field. A circular dome-like structure is observ ed at the southwestern part of the geothermal region southwest of Sugarloaf Mountain. Using a linear stress-bulk modulus relationship, deviatoric stre ss 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 comp ressional NNE-SSW and dilational WNW-ESE, coinciding with a previous, indep endent study using earthquake focal mechanisms. Following a theory on flat, elliptic cracks, residual crack density estimated from P anisotropy is sim ilar to 0.0078 assuming crack aspect ratios much greater than 1 : 60 and is similar to 0.041 when crack aspect ratios are close to 1:60. Residual crac k 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 appro ach via simple parallel fluid flow along cracks.