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

decreased impedance contrast of the salt flank in the PSDM. To investigate this I crossplotted
impedance with depth. I calculated the impedance of the sediments from sonic
and density logs. Using a velocity of 15,000 ft/sec and a density of 2.2 gm/cm 3 I
calculated the salt impedance to be 33,000 and because the velocity and density of salt
remain constant, the impedance does also. Using the cross-plot (Figure 69) and
extrapolating the trend line shows that the sediments in the Vinton Dome survey begin to
approach the same impedance as salt at about 7000 feet, the same as where the salt
impedance contrast begins to diminish.
I interpreted compartmentalized faults on a traverse that was not along a dip line to the
faults. To further investigate and compare the fidelity of the different migrations, I
compared the dip lines through the fault blocks (Figure 70). Interestingly, what these
indicate is that the well-based PSDM is higher frequency and images the faults and salt
flanks better, but the PSTM has imaged the shallower faults almost as well. This further
demonstrates the power of the PSDM is in areas of strong lateral velocity changes.
A final test was to compare synthetic seismic derived from well logs and compared to the
data. Typically, synthetics are stretched and squeezed to fit the data but for this test I
choose not to adjust the synthetics.
By not adjusting the synthetics to match the seismic
data, I could evaluate which most closely matched the synthetic. In some areas the
differences were very compelling and in most the well-based PSDM most closely
matched the synthetics (Figure 71)
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