Anisotropy estimation using VSP and surface Seismic data

Abstract

Anisotropic pre-stack depth migration is becoming a common imaging practice in oil and gas industry because of its enhanced imaging capabilities. It is a slow, iterative process that greatly depends on a-priory knowledge of anisotropy in the imaged subsurface. Here a quick and robust workflow of estimation of anisotropy is presented. This workflow utilizes knowledge of lateral velocity variations, which can mask themselves as anisotropy, from the isotropic interval velocities of the surface seismic data and combines this with the VSP data. Acquisition of a simple zero-offset Vertical Seismic Profile (VSP) provides the starting place for validation of these seismic velocities, and hence input into the isotropic 2D velocity model, as well as a well tie that can be used in estimation of anisotropy from a CMP gather at the well location. The CMP gather is thus corrected for normal moveout using an isotropic 2D velocity model and then artificially moved out using an isotropic 1D model derived from VSP measurements. The resulting CMP gather can be used in interactive 1D anisotropy estimation. The results of this anisotropy estimation can then be used to provide a seed point for a 3D model for anisotropic pre-stack depth migration. This approach can also be extended to evaluate azimuthal variations in anisotropy.