Fracture-induced Q-anisotropy: Inversion for fracture parameters

Abstract

Aligned vertical fractures are common in the Earth’s crust due to tectonic stresses and/or hydraulic fracturing. These fractures lead to azimuthally varying velocities and attenuations of P-, SVand SH-waves. We have extended Schoenberg’s elastic linear slip model of fractured medium to anelastic case. We invert synthetic data on velocities and attenuations in the cases of two anisotropic models, that is Transversely Isotropic with Horizontal symmetry axis (HTI) model and Orthorhombic (ORT) model. The first model, HTI, corresponds to a single set of parallel vertical fractures embedded in isotropic background. The second model, ORT, is for the same set of fractures, which are embedded in VTI background. We find fracture parameters – complex-valued weaknesses ∆N and ∆T – by inverting HTIand ORT-data on phase velocities and attenuations of P-, SVand SH-waves. Simulation examples are given for the inversion of VSP-type synthetic data. The aim of this work is to clarify how much would be the errors in the inversion for the fracture weaknesses if one uses HTI model instead of the more complicated but more realistic ORT model of fractured formation. The inversion results have clearly shown that both real and imaginary parts of normal weakness ∆N occurred to be extremely high in the case of the HTI-model employing instead of the actual ORT model. This may lead to false conclusion on dry fractures, whereas they were actually fluid-filled.