Intrinsic and layer‐induced VTI anisotropy

Abstract

Seismic anisotropy observed in the field is a cumulative result of various mechanisms. Subsurface rocks often possess intrinsic anisotropy approximated by a transversely isotropic solid with a vertical symmetry axis (VTI). Alternatively, in the long-wavelength limit, ”layer-induced” VTI anisotropy may arise even in a stack of thin isotropic constituent layers. In most practical cases these two effects occur simultaneously. We develop weak anisotropy and weak contrast approximations to understand the contributions of intrinsic and layer-induced anisotropy. When the contrast in elastic parameters between the constituents is small and their anisotropy is weak, then (to the first order) layering-induced anisotropy is insignificant whereas intrinsic anisotropy produces effective Thomsen parameters equal to the thickness-weighted average of the interval anisotropy parameters. This conclusion considerably simplifies upscaling of finely-layered VTI media because to find the effective Thomsen parameter 2 (or δ or γ) one needs to know only constituents 2’s (or δ’s or γ’s correspondingly).