Summary

Most of the available anisotropic inversion methods from walkaway (WVSP) or 3DVSP are valid only for flatlayered or laterally homogenous velocity models. Anisotropic parameters in the presence of laterally inhomogeneous geological structures such as salt diapirs and faults often must be estimated for accurate seismic imaging. 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 scheme) is employed to efficiently and accurately calculate first break (FB) times in 2D or 3D grid-based laterally inhomogeneous anisotropic velocity models. Next, 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. Both numerical and field WVSP data examples are used to demonstrate the feasibility of the methodology in estimating anisotropic parameters in laterally inhomogeneous anisotropic models from WVSP or 3DVSP surveys.

Introduction

Transverse isotropy with a vertical symmetry axis (VTI) is a common type of anisotropy observed in most sedimentary rocks. Accurately estimating VTI anisotropic parameters (e and d) is a critical step in the work flow for seismic anisotropic migration. Walkaway VSP (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 (Gelinsky and Parkin 1999, Horne and Leaney 2000, Zhou and Kaderali 2006, Blias 2010).

Most of the available anisotropic inversion methods are valid only for flat-layered or lateral homogenous velocity models. However, anisotropic parameters often must be estimated in areas that contain laterally inhomogeneous geological structures such as salt diapirs and faults. Therefore, a new methodology was developed to estimate anisotropic parameters in laterally inhomogeneous velocity models using the FB times measured from WVSP/3DVSP surveys.

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