Nonlinear Rock + Overburden Stress = Linear Anisotropy + Vertical Velocity Gradient

The increase in seismic velocity with depth is a common property of rock, one that can be encountered practically everywhere. Overburden pressure increases vertical stress, producing nonlinear elastic responses. Application of nonlinear theory to this problem leads to transverse isotropy, with relatively simple relationships between the nonlinear constants and anisotropy elastic coefficients. These relationships can be used in velocity 'depth trend' removal and in computing offset-dependent corrections for stacking and migration. This also implies that realistic tomography models should account for elastic anisotropy as a basic feature. A proper solution for overburden stress requires a full nonlinear solution for static stress distribution. It is quite likely that anisotropy resulting from overburden pressure is a common, basic property of underground rock. Accounting for anisotropy properties requires more complex computational and imaging tools than just isotropic models, which have generally been in use up to the present. On the other hand, seismic interpretation can arrive at additional imaging rock parameters (TOE constants), which can potentially be extracted from anisotropy measurements. Additionally, the overburden-induced anisotropy is strong enough to produce shear waves by an explosion.