TTI Anisotropic Depth Migration: Which Tilt Estimate Should We Use?

Summary We perform a series of numerical modelling and migration experiment with different homogeneous TTI anisotropic media, characterized by tilt axis, polar velocity and anisotropy parameters. In the case of structurally conformable media, where the tilt of the medium coincides with the dip of the structure, great simplifications arise in the decoupling of the anisotropy parameters. In particular, positioning and short spread focusing become decoupled from long-spread behaviour. We show that in this case, the tilt of the medium can be observed with sufficient accuracy on an image obtained by isotropic or VTI elliptic migration with an educated estimate of the Thomsen parameter delta. Introduction Anisotropic velocity model building tools and migration techniques have been in use for many years and have gained industry acceptance for at least simple VTI anisotropic media. Slightly more complex anisotropic media are now gradually being used, they are commonly referred to as TTI. TTI stands for Tilted Transverse Isotropy and it characterizes a medium exhibiting polar anisotropy around an arbitrary tilt axis (polar axis). The polar anisotropy, with respect to the tilt axis, is entirely determined as a function of the polar velocity (velocity along the pole axis) and the well-known Thomsen parameters ε and δ (Thomsen, 1986). As it proves next to impossible to determine, from surface seismic alone, the exact and spatially varying orientation of the polar axis of such TTI media, a simplifying assumption is usually made that the polar axis of the TTI medium coincides with the dip field of the reflecting subsurface structure. We will henceforth in our study referred to this special TTI medium as a Structurally Conformable TTI, or STI, medium. In such an STI medium seismic propagation can be fully described with the same parameters as for polar anisotropy with the addition of a reflector dip field. A potential problem with STI media is that the reflector dip field has to be extracted from a seismic image, which must be produced using a nonSTI/TTI velocity model. Our paper therefore seeks to answer the question: what is the best velocity model that can be used to compute the structural dip field for an ensuing TTI (STI) anisotropic depth migration? To answer this question we use a simple numerical modeling and