Imaging beneath shallow gas anomalies in the Gulf of Thailand
The fifth 2-D case study for structural inversion is from the Gulf of Thailand. This case study deals with the deleterious effect of shallow gas anomalies on a multileveled reservoir zone along growth faults. The velocity-depth model is characterized by near-horizontal layers with structure-independent velocity variations. Shown in Figure 10.5-1 is the CMP-stacked section. The shallow gas anomalies are represented by the high-amplitude piecewise-continuous reflections within the first 500 ms of the section.
The shallow gas anomalies give rise to fluctuations in the stacking velocity field as seen in Figure 10.5-2. For a horizon below the shallow anomaly zone, CMP raypaths at some locations travel through an overburden with laterally varying velocities associated with the shallow gas anomalies. As a result, traveltimes associated with deeper reflections are distorted.
Time migration (Figure 10.5-3) using a smoothed form of the stacking velocity field (Figure 10.5-2) produces a seemingly accurate image of the subsurface. Note, for instance, the fault below CMP 2170 within 1-2 s time window, and the v-shaped structure below CMP 1370. The subsurface appears to be so simple that one may be tempted to convert this section to depth using vertical raypaths and Dix-converted stacking velocities. However, the anomalous behavior of the stacking velocities will cause Dix conversion to yield meaningless interval velocities.
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