Anisotropy Estimation in Lateral Inhomogeneous Velocity Model from Walkaway or 3DVSP Surveys

Walkaway (WVSP) or 3DVSP provides a unique and highly valuable way to measure seismic anisotropy, because WVSP/3DVSP geometry is generally characterized by wide seismic-ray angle coverage and well-determined vertical velocities and first break (FB) times. Most of the available anisotropic inversion methods are valid only for flat-layered or lateral homogenous velocity models. However, anisotropic parameters in laterally inhomogeneous geological structures such as salt diapirs and faults must sometimes be estimated. Therefore, a new methodology was developed to estimate anisotropic parameters in lateral inhomogeneous velocity models. First, an anisotropic Eikonal solver (referred to as a fast marching (FM) scheme) is employed to efficiently and accurately calculate first break (FB) times in 2D or 3D grid-based inhomogeneous anisotropic velocity models. Then, a constrained global optimization algorithm known as simulated annealing (SA) is used to efficiently search and invert the anisotropic parameters based on the least square errors between calculated and observed FB times. A numerical example is used to demonstrate the feasibility of this methodology in estimating anisotropic parameters in lateral inhomogeneous anisotropic models from WVSP or 3DVSP survey.