VERY HIGH-RESOLUTION 3D MARINE SEISMIC DATA-PROCESSING FOR GEOTECHNICAL APPLICATIONS

The processing of a small-scale, very high resolution (VHR) shallow ma rine 3D data volume is described. The data were acquired over a small clay diapir, on the river Schelde, in 1990. Using an array of 12 dual- channel microstreamers towed from a catamaran, a network of 1 m x 1 m bins could be produced over an area of 50 m x 180 m (< 100 m penetrati on). Positioning was performed with an auto-tracking laser ranging sys tem, assuring an absolute accuracy of a few decimetres. Preliminary pr ocessing steps included tidal correction and multiple removal. An impo rtant step concerned the application of 3D prestack Kirchhoff depth mi gration. Indeed this processing allows easy handling of the exact posi tions of both source and receivers as the latter were not set out on a conventional regular grid due to navigation difficulties. Because of the restricted data volume and the more-or-less stratified medium, a 1 D velocity model could be used. This allowed a considerable simplifica tion of the migration algorithm, based on summation. Traveltimes were calculated only once, using a 2D time grid with 0.1 m intervals. This migration method proved very efficient, greatly improving the seismic image, and involved only limited CPU time on a small computer (Sparc 1 0 workstation). It clearly demonstrates that advanced seismic processi ng can form a valuable and economically feasible tool for VHR shallow subsurface 3D seismics, as long as the velocity field is not too compl ex. This method should therefore no longer be restricted to large comp uters and hydrocarbon exploration, but should also become a routine fo r VHR 3D shallow seismic work.