For weak anisotropy as defined by Thomsen and according to his scheme, approximations of the moveout velocity and traveltime of PP and SVSV waves reflected at the bottom of a single layer are presented in the form of an offset-dependent polynomial. The moveout velocity does not only differ from the vertical ray velocity as already pointed out by Thomsen, but varies with offset depending on the anisotropy coefficients. This variation gives rise to non-hyperbolic traveltime curves. If this effect is not taken into account, stacking will likely deteriorate. The variation is most pronounced in the SVSV -case, especially if the medium is characterized by a negative value of the difference of two of Thomsen's anisotropy coefficients, (ε - δ). This behaviour also explains the results of Levin's case study, since interpretation must consider the presence of anisotropy. But by using, the regression method on the approximate coefficients, additional information on the SV -anisotropy can be obtained.
View full article as PDF (0.50 Mb)