f Iterative 3D prestack depth migration in the North Sea Central trough

The North Sea Central Trough is a well es tablished hydrocarbon province. The area of this study in Norway Block 2/4 is covered by rotated sub-platforms bordered by major NW-SE trending normal faults. A 3D survey was acquired for Saga over the area in 1991 in cooperation with neighbouring operators Phillips Petroleum and BP Norway. The prospective areas in the upper Jurassic and Permian sandstones are overlain by a high velocity Chalk sequence. Although structural dips are generally mild, severe ray bending occurs as a result of the high velocity contrast, violating the simplifications which are the basis for standard processing. Time migration is therefore inadequate and it is imperative to use a depth migration method to obtain accurate imaging and positioning of the target events. Excellent results using 3D poststack depth migration have been reported by Phillips Petroleum (figure 1), Their iterative imaging procedure consisted of interpretation, model building and 3D poststack depth migration. The 17-layer velocity model was achieved based on the interpretation of depth migration results using different trial velocity models.