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Diffractions can supplement reflected waves in anisotropic velocity analysis because they increase the aperture and may illuminate parts of the model that do not produce strong reflections. However, enhancement of diffractions and their separation from the more intensive reflections remains a challenging task, especially if the velocity model is not sufficiently accurate. Here, we construct dip-angle common-image gathers (CIGs) for transversely isotropic (TI) media using Kirchhoff migration. Dip-angle gathers can be conveniently used to generate diffraction-based depth images if a sufficiently accurate velocity model is available. We also analyze the moveout distortions of diffraction and reflection events in dip-angle CIGs in the presence of errors in the TI parameters. The residual moveout of diffractions computed from dip-angle gathers can be employed in migration velocity analysis (MVA) to refine the anisotropic velocity model.

Seismic diffractions, which are produced by heterogeneities smaller than seismic wavelength, have proved useful in a number of 2D and 3D applications. Among the challenges hampering the use of diffractions in seismic data processing is the sensitivity of diffraction separation after prestack migration to velocity errors. Therefore, it is essential to properly account for velocity anisotropy in diffraction separation and imaging.Here, we analyze the signature of diffractions in orthorhombic models, which are typical for many subsurface formations. Tip and edge diffraction events are modeled for a tetrahedron embedded in an orthorhombic medium. We construct 3D dip-angle common-image gathers (CIGs) as a function of azimuth and migration dip using the actual orthorhombic velocity field as well as a purely isotropic velocity model. The modeling results provide insight into the influence of velocity errors on the diffraction moveout in different azimuths in the dip-angle domain. This work should aid in constructing diffraction-based depth images using 3D anisotropic velocity models.

Imaging diffracted waves can provide useful information about complex subsurface geology and fracture networks. Separation of diffractions from typically more intensive reflected waves can be based on specularity, which measures deviation from Snell's law. Here, we analyze two formulations of specularity and their applicability to diffraction processing for anisotropic media. We show that the most common definition of specularity, originally introduced for pure modes in isotropic media, remains valid for both pure and converted waves in arbitrarily anisotropic models. The other formulation operates directly with the difference between the slowness projections onto the reflector for the incident and reflected waves. Testing on a VTI (transversely isotropic with a vertical symmetry axis) diffraction ramp model demonstrate that both formulations produce satisfactory results for anisotropic media with appropriate tapering of the specularity gathers.

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