Effective anisotropy of layered media

In-situ seismic anisotropy can be estimated using a variety of techniques. Since they require data that are typically collected over some finite intervals of inevitably heterogeneous earth, one usually needs to separate the influence of anisotropy on the measurements from that of heterogeneity. It is not an easy task because, depending on the frequency of propagating waves, heterogeneity and anisotropy might mimic each other. The well-known example of this is a finely layered isotropic solid that effectively behaves as a vertically transversely isotropic (VTI) rock when probed by sufficiently long seismic waves (Backus, 1962). Similar transitions from heterogeneity to anisotropy and back occur over the whole range of seismic frequencies. For instance, Grechka and Tsvankin (2002) recently showed that a layered isotropic structure also looks like a VTI one at high frequencies, when the ray tracing and Dix-type averaging of the normal moveout (NMO) velocities can be applied.