Time to Move to Anelliptic Time Processing
Stephen J. Pharez1, R. Siliqi1
(1) Compagnie Generale de Geophysique, Brentford, Middlesex, United Kingdom
Standard time processing attempts to focus seismic data step by step, from pre-stack to zero-offset, the goal being to preserve the major part of the recorded reflectivity. Because the sole purpose is to focus the pre-stack energy at the correct location, the only input needed is the well-known VRMS velocity field. However assumptions of small incidence angles and isotropic media are now stretched to the limit. Long streamers and an anisotropic earth means that focusing the far offset data, especially dipping events, through an anisotropic subsurface cannot be achieved using only the VRMS field.
Moving from standard time processing means opening the focusing process to the anellipticity of the effective media. We demonstrate here a Dix-type effective medium, where two velocities could describe the full time processing. The ratio of these two velocities points out the anellipticity strength, which is represented by the well-known fphi parameter. Effective fphi encompasses two travel path effects: vertical inhomogeneity and transverse isotropy. In the case of anelliptic media the velocity picks, sparse or dense, are extended on two parameters. Interpolated and filtered parameter fields can be estimated thanks to Dix properties of stacking and anelliptic velocities. Normal and Dip Move Out as well as Pre-Stack Time Migration operators are made using an anelliptic shifted hyperbola approach. We will demonstrate the application of this A+ anelliptic time processing approach through a series of examples illustrating the key steps. This homogeneous description of the focusing process now allows for time processing to take into account the anellipticity of the media.