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

in geology and depth, but more importantly, I maintained the relative position and
spacing of each horizon to one another (Figure 49).
Using the V z velocity gradient, I converted the horizons to depth. With the seven
geologic horizons in depth, I plotted the wells with respect to the horizons to determine
where the logs intersected the horizons. Logs extend to different depths so not all logs
penetrated the same horizons. With points of known depth from the sonic logs, I flexed
the horizons to fit the logs. I adjusted horizons I interpreted from the seismic data to
match the sonic logs keeping the relative spacing between horizons. I applied the sonic
log velocities at the point of intersection between the horizon and the log. Then I
calculated a sloped interval velocity between horizons by using the velocity at the
intersection from the top horizon to the next horizon down. By extrapolating velocities
from the wells, I built the velocity cube (Figure 50). Variations in layer thickness
resulted in a nonlinear extrapolation.
My initial QC of the model by comparison with known structures helped validate the
model. Smoothing and extrapolation initially removed all structure from the horizons I
used to build the model; yet, in the velocity cube there are faults as can be seen in Figure
51. By comparing the velocity model and the PSTM seismic volume, I confirmed that
the structures were real. Even with sparse sampling of well logs, this technique was
sensitive enough to include structure into the initial sediment velocity model.
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