Nonhyperbolic reflection moveout of -waves: An overview and comparison of reasons |
A transversely isotropic model with a vertical symmetry axis (VTI medium) is the most commonly used anisotropic model for sedimentary basins, where the deviation from isotropy is usually attributed to some combination of fine layering and inherent anisotropy of shales. One of the first nonhyperbolic approximations for the -wave reflection traveltimes in VTI media was proposed by Muir and Dellinger (1985) and further developed by Dellinger et al. (1993). Thomsen (1986) introduced a convenient parameterization of VTI media that was used by Tsvankin and Thomsen (1994) to describe nonhyperbolic reflection moveouts.
We begin with an overview of the weak-anisotropy approximation for -wave velocities in VTI media and use it for analytic derivations throughout the paper. First, we consider a vertically heterogeneous anisotropic layer. For this model, we compare the three-parameter approximation for the -wave traveltimes suggested by Tsvankin and Thomsen (1994) with the shifted hyperbola (Malovichko, 1978; Castle, 1988; de Bazelaire, 1988). Next, we examine -wave moveout in VTI media above a curved reflector. We analyze the cumulative action of anisotropy, reflector dip, and reflector curvature, and develop an appropriate three-parameter representation for the reflection moveout. Finally, we consider models characterized by weak lateral heterogeneity and show that it can mimic the influence of transverse isotropy on nonhyperbolic moveout.
Nonhyperbolic reflection moveout of -waves: An overview and comparison of reasons |