Fig. 1. Structural elements of the southern North Sea, after Glennie and Bougner (1981) and Dunning (1985). Fig. 2 at about 0.9 to 1.0 sees TWT. The corresponding depths are about 800 to 900 metres, and there is virtually no structuring at or above this level. The Chalk extends to rather more than 1.4 sees TWT at the left hand side of Fig. 2. Seismic energy travels through carbonates at much higher velocities than through Tertiary shales, and the Chalk thickness at this point is about 600 metres. The strong reflection from its base is, however, severely distorted by geometrical effects and particularly by diffraction hyperbolae generated by discontinuities at the edges of fault blocks. The way in which these features are produced on seismic sections is well understood but where several intersect it is extremely difficult to 'see through' to the true geological structure. Fortunately the processing technique known as migration can be used to correct for such distortions (Fig. 3). A single seismic section can be successfully migrated only if the seismic line crosses the geological structures at right angles to their strike. This condition The inversion of sedimentary basins to form structural highs has long been recognised as a feature of the geology of southern England. The classic example is the Weald Basin of Sussex and Kent, which suffered inversion early in the Tertiary. More recently, inversion structures have been mapped in the course of exploration for oil and gas in the North Sea. In the southern North Sea the major inverted feature is the Sole Pit Basin, which occupies much of the subsea region between the UK and the Netherlands (Fig. 1; cf Glennie & Boegner, 1981). The Sole Pit inversion extends almost to the southern limit of North Sea basinal structures. This limit is marked by the northern flank of the London-Brabant Platform, which was a positive and intermittently emergent structural feature from Early Palaeozoic times until the Cretaceous. Numerous seismic surveys have covered the region between the Platform and the Sole Pit, many of the most recent having been shot in anticipation of the UK Ninth Round allocations in late 1984. The Sole Pit Basin formed in the Permian as a large half-graben, subsiding along the Dowsing Fault Zone which formed its southwest margin. Rotation took place about a horizontal axis along the Swart Bank Hinge to the NE (Fig. 1). The hinge converges with the Dowsing Fault near the UK-Netherlands median line off the coast of East Anglia . Associated faulting produced numerous small basins which are shown by seismic reflection surveys to have also suffered Cretaceous inversion. The small inverted basins are preserved beneath a thick pile of younger sediments. Late Cretaceous seas which transgressed across the London-Brabant platform deposited a rather uniform layer of Chalk in the region close to the present-day Norfolk coast . Further to the northeast, and crossing the Swart Bank Hinge , the West Netherlands Basin developed late in the Cretaceous and continued to subside and receive sediments throughout the Tertiary. The Tertiary and youngest Cretaceous rocks have suffered little or no deformation and are clearly imaged on 'Final Stack' seismic sections (cf Fig. 2). The strongest reflectors are the top of the Chalk and a thin high-velocity Eocene bed. Together these give rise to the complex of high-amplitude events , seen on o ! km 50 [ [ I r