The Effect of Anisotropy in Seismic Shear Wave Kinematics and Nonhyperbolic Shear Wave Normal Moveout for the VTI Media

Shear waves have been used for oil and gas exploration for decades. Reflected seismic shear waves (SV-SV and SH-SH) have shown good capability in gas-cloud imaging and geological interpretation. Compared with the longitudinal wave, shear waves are more sensitive to anisotropy. The SV-SV wave traveltime may significantly deviate from the hyperbolic form in anisotropic media. Conventional hyperbolic normal moveout (NMO) correction is inadequate for handling the far-offset SV-SV wave data and is insufficient to recover the true model parameters. VTI (transverse isotropy with a vertical axis of symmetry) is one of the most common types of anisotropy in sedimentary basins. Nonhyperbolic analysis in VTI media is important, but the influence of VTI on the SV-SV wave kinematics is still ambiguous. To solve this problem, we first analyze the influence of varied anisotropy parameters and velocity ratio (the ratio of the vertical P-wave velocity to the vertical S-wave velocity) on SV-SV wave traveltime in VTI media. Second, we evaluate three SV-SV wave approximate traveltime equations in terms of their accuracy with parameters and offset-depth ratio (x/z). Finally, a nonhyperbolic NMO correction method for SV-SV wave in VTI media is established based on the two-parameter (NMO velocity V(S2 )and effective anisotropic parameter zeta) approximate equation, in which both parameters are inverted from the reflection events. It is applied to a seismic shear wave dataset which shows obvious VTI characteristics acquired from the Qaidam Basin in China. The reflection events in common-middle-point gather show a typical "hockey stick" phenomenon after a conventional hyperbolic NMO correction, while the proposed method can flatten reflection events at full offset, with the NMO velocity V-S2 inverted from the near-offset reflection events, and the effective anisotropic parameter zeta inverted from the far-offset reflection events. The following stacked seismic imaging using the proposed method also shows great improvement. Besides, more far-offset information can be retained, and this has a significant impact on AVO analysis and interpretation of shear wave seismic data.