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

PSTM and the PSDM volumes in this study. I derived coherence from the PSTM volume
and analysis of the coherency revealed problems with the migration. After about one
second the migration began to fall apart. Before one second, faults and channels were
low coherency events and the continuous strata were high coherence events (Figure 72).
After one second the faults are imaged as more coherent and the coherent strata as less
coherent and began producing a "wormy" pattern due to poor migration (Figure 73).
Degradation in the coherency cube from a poor PSTM motivated me to use PSDM. I
used the application of attributes as a QC tool in two aspects of the study. The first to
determine the need for PSDM and the second was to QC two separate approaches to
velocity modeling. Velocity modeling is a critical step in the PSDM process. The two
approaches I used in this study, using a velocity model derived from well logs and using
a velocity model derived from seismic data, are different and required a method of QC to
determine which method produced a higher fidelity image.
After the migrations of each of the initial sediment velocity models, I applied two passes
of edge preserving smoothing to the data and coherence volumes were output. Since
problems were apparent in the PSTM volume at about one second two way travel time,
only 8000 feet of data were migrated in the PSDM primarily to save time. I compared
depth slices from both PSDM (Figure 74) in the area of compartmentalized faults used in
Chapter 4. Since the two volumes were migrated with different velocities, there is a
difference of about 200 feet in the positioning of events. I examined both direct
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