Processing and interpretation of sideswipe and other external reflections from salt plugs

A study is presented of the genuine external specular reflections from salt plugs which manifest themselves on individual seismic sections as apparently unconnected foreswipe and backswipe reflections for seismic lines that cross salt plugs and as sideswipe reflections for lines that pass them by. It is shown that these external reflections can be combined, mapped and seismically migrated. An example demonstrates that this allows the depth and shape of the shoulders of a salt plug to be defined. It is also shown that sideswipe reflections on individual seismic sections can be used to determine the maximum possible distance from the seismic line to the flank of a salt plug. This is done by constructing “maximum-distance arrows” on a map; they help in locating salt plugs that are not crossed by a regular grid of regional seismic lines. Reflected refractions, internally reflected against the far salt flank, have been identified. The relation of sideswipe reflections to maximum convexity curves, and the relation between the acoustic impedance of the salt plug itself and that of the surrounding sediments have been investigated. Real seismic data which relate to salt plugs in the Norwegian-Danish Basin have been used in this investigation and as illustrations. The reflection process that gives rise to sideswipe reflections from salt plugs has been investigated by means of small-scale physical models. Routine seismic processing discriminates strongly against salt flank reflections such as foreswipe and backswipe reflections. Aspects of the special processing necessary to enhance these reflections, particularly the determination of the unusually high stacking velocities required, are reviewed. Routine seismic processing does not to the same extent discriminate against sideswipe reflections. The stacking velocities required to enhance them are also discussed.