Time to Move to Anelliptic Time Processing
By
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.