Seismic anisotropy estimation in TTI media using walkaway VSP data

Abstract

Walkaway VSP data are commonly used to estimate P-wave anisotropic parameters for subsurface formations that exhibit transverse isotropy (TI). When vertical transverse isotropy (VTI) is assumed for dipping formations where the anisotropic symmetry axis is normal to the bedding, errors can occur in the estimated anisotropic parameters. Using a simple layered model, we examine the errors in estimated Thomsen parameters e and δ as a function of dip angle and anisotropic magnitude. The results show that errors in e in general are smaller than those measured for δ. When formation dips are greater than 5° and the anisotropic parameters are greater than 0.05, the use of a tilted TI assumption is necessary to correctly recover the anisotropic parameters from the traveltime data. We then investigate the discrepancies between P-wave velocity in the symmetry direction, V0, and vertical interval velocities, Vv, from check-shot surveys using a tilted TI model. We find that if Vv is used as V0 without proper adjustments, significant errors can result in the estimated anisotropic parameters. Finally using numerical simulations we demonstrate a new method that simultaneously estimates Thomsen parameters and updates the velocity V0 for general TI formations from walkaway VSP traveltime data. Our results show that the anisotropic parameters are well resolved for the test models using this inversion method.