Seismic tomography for field VSP surveys in an inhomogeneous and anisotropic medium

Abstract

Direct waves generated as a result of field VSP surveys are used to obtain tomographically reconstructed images in anisotropic and inhomogeneous media. A method is described to determine the anisotropic characteristic of velocity which assumes that velocity changes with depth and arrival angle. There is no need to assume any form of anisotropy. The form of velocity anisotropy can be determined by the results of the velocity inversion. Then a method is outlined for velocity tomography with an anisotropic correction based on the inverse theory of Newton's non-iterative solution. Instead of using 2-D or 3-D ray tracing to deal with strongly vertical inhomogeneous media, the method uses vertical changes and anisotropic velocity determined from the velocity analysis as an a priori model of reference velocity. The method circumvents image distortion caused by straight line rays and the ray divergence caused by ray tracing. The method was designed for 3-D Vertical Seismic Profiling (VSP) application and to give a 3-D volume tomographic image of velocity disturbance by displaying 2-D cross-section slices in 3-D space. Two field VSP data sets have been analysed, one which is the result of flat layers with obvious anisotropy and the other with no obvious anisotropy but obvious horizontal velocity variation. Strata logs from the two sites confirmed the tomographic results at each well position. The method described in this paper can be conveniently used in 2-D and 3-D VSP and cross hole seismic surveys for the petroleum and mining industry.