Jm. Lees et Ht. Wu, P wave anisotropy, stress, and crack distribution at Coso geothermal field, California, J GEO R-SOL, 104(B8), 1999, pp. 17955-17973
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.