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Abstract

Seismic line 74-6 was shot 250 km (155 mi) northeast of Shetland near the boundary between United Kingdom and Norwegian waters in the northern North Sea (Figure 1).

The major features of the section are the Magnus Ridge, in which lies the BP Magnus Field (De'Ath and Schuyleman, 1981) and the North Shetland Trough. These are examples from a series of similar structures which extend northeast from the Shetland Islands basement platform. They result from the reactivation of Caledonide lineaments by Middle Jurassic to Early Cretaceous dilational tectonics which were ultimately to lead to the opening of the Atlantic Ocean (Threlfall, 1981; Hay, 1978). The effect of these movements has been to produce half-grabens where deeply buried Jurassic Kimmeridge Clay formation source rocks alternate with potentially trapping ridges, the whole sealed by onlapping Cretaceous rocks. Such Mesozoic tilted fault blocks provide the trapping structures for most of the major northern North Sea oil fields.

The field data for line 74-6 included strong water bottom multiples, particularly beneath the Base Cretaceous reflection, and the deeper part of the North Shetland Trough was masked by long period multiples generated in the shallower section above. There was also a significant level of low velocity noise.

The processing sequence for the section shown here is given in the "Geophysical Title Block." Array simulation was used early to attenuate low velocity noise. This also improved the performance of some of the later stages by simplifying the identification of reflection events. in addition to use of deconvolution to attenuate multiples, a process was introduced which involved dip filtering of CDP gathers. The CDP tack was weighted to improve the continuity of deep reflections. Finally, thesection was migraed to collapse diffractions and move dipping reflections to their correct position.

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