A Deterministic Evaluation of eismic Fidelity using Velocity Modeling ...

PSDM is the best current method for addressing strong lateral variations in velocity
(Judson et al., 1980). Greater accuracy in the velocity model relates directly to enhanced
fidelity so that development of an accurate velocity model is essential in subsurface
imaging (Fliedner et al., 2002, Liu et al., 2002, Popovici et al, 2003, Schultz, 1999). I
built a sediment velocity model using sonic logs, a well-based model, and compared it to
the current practice of building the initial sediment model using seismic migration
velocities. I will refer to the velocity model derived from the sonic logs as the well-based
model, and I will refer to the velocity model derived from the seismic data as the prestack
time migrated velocity model.
There were two reasons why I used the well-based approach. One is that the seismic data
used to derive a prestack time migrated velocity model is prone to be noisy, especially
with land data. Sources of noise in land data range from variable receiver coupling,
cultural noise, static problems and a variety of other issues that are not as profound in
marine data. Much of the noise that is inherent to land data is coherent and difficult to
eliminate so it can have an adverse impact on velocity analysis, which in turn causes the
PSDM to produce lower fidelity subsurface images. Another reason for using sonic logs
to build the initial velocity model is the minimal manipulation of the velocity data. Sonic
logs record velocities as depth interval velocities; and depth interval velocities are what I
used for the PSDM velocity model. Seismic velocities are RMS time velocities. It is a
two-step process to convert RMS time velocities to depth, once to an interval velocity
and then to a depth interval velocity. Differences between land data and marine data
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