Model with complex overburden structure
Figure 9.3-1 illustrates a complex overburden structure associated with overthrust tectonics. Such structures were formed as a result of the tectonic movements during the Lower Miocene and Upper Cretaceous, and are common in North America, South America, and the Middle East. The target horizon is the flat reflector at 2.5 km below the imbricate structures.
The velocity-depth model comprises a shallow sequence with a relatively simple structure. Underneath this shallow sequence is a shale-marl sequence with a strong vertical velocity gradient (0.5 m/s/m). Then, we have the imbricated fault structures of the carbonate sequence, and finally the target level at 2.5 km characterized as the detachment zone that separates the incompetent rock layers above from the competent rock layers below.
A total of 154 shot records was modeled using the two-way acoustic wave equation. The simulated recording geometry consists of a split-spread cable with 97 receivers and an offset range of 0-2350 m. Shot and receiver intervals are both 50 m. Figure 9.3-2a shows the CMP-stacked section. Note the traveltime distortions along the deepest reflection caused by the severe ray bending within the overburden. Time migration (Figure 9.3-2b) — whether post- or prestack, will not resolve the deleterious effect of the complex overburden structure.
Figure 9.3-2 (a) CMP-stacked section associated with the velocity-depth model in Figure 9.3-1; (b) poststack time migration.