Imaging the seabed in shallow water areas (<300m) using 3D seismic surveys

This report discusses imaging the seabed from exploration 3D seismic data in shallow water areas, defined as less than 300m. The issues involved are discussed primarily with reference to data sets within the Faroe-Shetland Channel (FSC) where the bulk of the seabed imaging work has been performed and where the availability of other data such as swath bathymetry permit detailed comparison. The report then considers how seafloor conditions and survey design influence seabed reflection quality. A method for mapping seabed topography using seabed multiple energy is presented and applied to the FINA 30/14 3D survey in the Central Graben of the North Sea. The major conclusions of this report are: (1) In water depths shallower than 300-400m and in areas of high seabed velocity, the seabed reflection on conventionally processed 3D seismic volumes tends to become poorly imaged if the survey has been designed for deep targets. (2) Pre-stack processing of the seismic volume to enhance seabed multiples does appear to generate a plausible surface on the Fina survey. The first multiple is less contaminated with survey footprint artefacts than the primary reflection. The auto-correlation pick provides the most coherent surface in this study. An unresolved question is the extent to which the derived topography reflects tuning effects due to the rapidly varying thickness of the Forth Formation, as opposed to the true seabed topography. (3) The technique may usefully be applied to another, deeper water area, such as Shell’s woc96 survey in the FSC where the primary seabed reflection begins to fade. This would enable a direct comparison of surfaces generated by conventional and multiple enhancement processing as well as present an opportunity to extend the FSC seabed image.