Anisotropy parameters in a VTI medium are obtained by anisotropic velocity analysis performed on short-spread or long-spread P-wave reflection-seismic data, in combination with check-shot or well-log data. Analysis of four reflection-traveltime approximations to the actual reflection traveltime in weakly anisotropic media shows that each reflectiontraveltime approximation has its own requirements for spread length and subsurface anisotropic parameters. The accuracy of the estimated Thomsen anisotropy parameter δ depends not only on the accuracy of the picked NMO velocity but also on the subsurface anisotropy parameters. The smaller the value of (ε - δ), the higher the accuracy of the estimated δ value. The results of the four reflection-traveltime inversions by semblance analysis for synthetic seismic examples demonstrate that in estimating δ , the nonhyperbolic and the shifted-hyperbolic estimations are better than the three-term Taylor-series method, which, in turn, is better than hyperbolic estimation. Only the nonhyperbolic approximation can be used to estimate the anisotropy parameter ε accurately.
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