Traveltime inversion of multi-azimuthal walkaway VSP data for a model with dipping TI layers

Abstract

Summary Walkaway vertical seismic profiling (WVSP) data potentially provides the most reliable anisotropy estimates. In many cases, a 1D depth velocity model is not adequate for WVSP data inversion. If the walkaway first break function is not symmetrical with respect to the well then a velocity model with lateral velocity changes is needed. In this paper, I present a first break constrained traveltime inversion approach for a 3D tilted transverse isotropy (TTI) parameter estimation that takes into account lateral velocity changes above the shallowest receiver. A new methodology, based on the constrained first break inversion, was developed to estimate anisotropic parameters in a 3D model. This method includes residual source statics calculation, walkaway first break correction due to zero-offset misfit for different walkaway lines, lateral velocity changes above the shallowest receiver, dipping boundaries and visual uncertainty analysis of estimated anisotropic parameters. Dipping boundaries, interval velocities and anisotropic TI parameters are simultaneously estimated through the least squares method with adaptive stabilizing weights. This approach was applied to a four-line walkaway dataset. The results showed small residual time differences between actual and synthetic first breaks with a standard deviation of 2 msec.