Every expert tends to exaggerate the importance of his special field in proportion to the length of time spent on it and to the difficulties overcome.— Max Planck (1858–1947)
In this chapter, we consider seismic-velocity anisotropy and how it depends on frequency. We restrict our comments to vertical transverse isotropy (VTI), a kind of anisotropy that represents velocity variation with respect to the vertical axis in a horizontally layered earth (Backus, 1962; Thomsen, 1986; Berryman et al., 1999; Bakulin, 2003). Laboratory-scale rock samples can show significant velocity anisotropy. When it occurs on this fine scale, we call this intrinsic anisotropy to distinguish it from apparent anisotropy, which results in layered media (Figure 1). Intrinsic anisotropy is not a function of frequency, and it represents a rock property such as density or bulk modulus.