Vertical heterogeneity is another reason for nonhyperbolic
moveout. We start this section by reviewing well-known results for
isotropic media. Although these results can be interpreted
in terms of an effective anisotropy, we show that it has different properties
than those for the VTI model. We then extend the theory to vertically heterogeneous
VTI media and perform a comparative analysis of various three-parameter
nonhyperbolic approximations.