PROCESSING OF MARINE SEISMIC DATA FROM AREAS WITH LARGE AND RAPID VARIATION IN WATER DEPTH

Abstract

The offshore search for oil and gas is progressively moving further out to sea as near-shore structures are delineated and drilled. Prospects that overlap the edge of the continental shelf and slope will more than likely present problems in the processing of marine seismic data because of large and rapid variation in water depth.

Magellan Petroleum encountered such difficulties in the digital computer processing of its East Gippsland Basin Prospect which is located some 50 miles southeast of the Victoria coastline.

A series of problems developed when an integrated computer program sequence or "package" was applied to the data. It was found that first break suppression schedules, deconvolution design gates, band-pass filter application gates and velocity functions could not be changed often enough due to program restrictions.

Where the water bottom topography was rough, the restriction of submitting only three or four water depths to vary the velocity function and subsequent calculation of normal move-out corrections resulted in questionable accuracy for the corrected results.

Sometimes, water bottom variations required individual trace static corrections which were not available in this particular "package" processing.

Water bottom multiple periods vary as rapidly as the surface that generates them. A meticulous selection of the parameters of deconvolution programs is necessary to attenuate multiples under such conditions. Also close examination of the purposes and consequently methods of deconvolution computer programs is necessary to maximize the effectiveness of this powerful processing tool.

Diffractions are frequently generated at points on an irregular sea bottom surface. Such detractions mask true water bottom reflections in deeper water and thus decrease the geophysicist's ability to process data accurately where computer programs require true water bottom depth.

Presentation of record sections which illustrate problems and their probable solutions comprise a major part of this paper.