Anisotropic P-wave velocity analysis and seismic imaging in onshore Kutch sedimentary basin of India

The long-offset P-wave seismic reflection data has observable non-hyperbolic moveout, which depend on two parameters such as normal moveout velocity (Vnmo) and the anisotropy parameter(η). Anisotropy (e.g., directional dependence of velocity at a fixed spatial location in a medium) plays an important role in seismic imaging. It is difficult to know the presence of anisotropy in the subsurface geological formations only from P-wave seismic data and special analysis is required for this. The presence of anisotropy causes two major distortions of moveout in P-wave seismic reflection data. First, in contrast to isotropic media, normal-moveout (NMO) velocity differs from the vertical velocity; and the second is substantial increase of deviations in hyperbolic moveout in an anisotropic layer. Hence, with the help of conventional velocity analysis based on short-spread moveout (stacking) velocities do not provide enough information to determine the true vertical velocity in a transversely isotropic media with vertical symmetry axis (VTI media). Therefore, it is essential to estimate the single anisotropic parameter (η) from the long-offset P-wave seismic data. It has been demonstrated here as a case study with long-offset P-wave seismic data acquired in onshore Kutch sedimentary basin of western India that suitable velocity analysis using Vnmo and η can improve the stacking image obtained from conventional velocity analysis.

Research highlights
► Velocity anisotropy plays a crucial role in seismic imaging and cannot be ignored.
► Long-offset P-wave surface seismic data is used to estimate anisotropy in VTI-media.
► Anisotropic velocity analysis has been made to estimate coefficient of anisotropy η.
► The anisotropic seismic image obtained is superior to its isotropic counterpart.
► The anisotropic seismic image is an important indicator of lithology – sands/shales.