LONG, D., J. BULAT, and M. SANKEY, British Geological Survey, Edinburgh, UK

Abstract: Regional Images of the Seabed Derived from 3D Exploration Seismic Data

Assessing the morphology of the seabed is essential for safe operations and to understanding the environmental setting for exploration in deepwater. Often this is by the commissioning of swath bathymetric surveys, and/or sonar surveys including long range devices such as GLORIA and TOBI. However, information on seabed morphology can be derived from 3D exploration seismic surveys. Under favourable circumstances, the seabed can be picked in a similar manner to any other horizon of interest to provide detailed images of the seafloor, thereby negating the need for expensive seabed surveys. Such data sets have a large number of data points, typically spatial and temporal sampling rates of 25m and 4ms respectively. When the horizon is picked using amplitude tracking, the "z" values are interpolated such that features of <1m height are resolved. By combining the first return from several 3D exploration surveys the regional context of the seabed morphology can be understood. The wider regional assessment is very important when considering the significance of features e.g. the rarity of a certain seafloor environment or the presence of a potential landslide upslope from an operations area.

A study for the Western Frontiers Association (a consortium of 15 oil companies, the British Geological Survey and the Health and Safety Executive set up to undertake regional geological assessments of the seabed for safe operations in deepwater west of the UK) has produced images of the seabed covering 15,000 km2 west of Shetland.This involved the integration of 15 3D datasets extending from the shelf (~120m) to more than 1600m water depth (Figs 1 and 2). Outputs comprised shaded relief, dip magnitude and dip azimuth maps. These images reveal a wide range of geological features, indicating former and modern sedimentary processes. On the shelf major moraines from an ice sheet extending from Shetland are clearly seen but between them smaller features can be interpreted, that may be the result of breaks in the retreat of ice at an annual scale. Examination of high resolution profile data reveal these to be less than 1m in height. Other parts of the shelf show extremely uneven surfaces associated with iceberg turbation. On the upper slope reworking of seabed sediments by icebergs is shown by several large alongslope troughs. Sediment transport down the slope is shown by debris flows, these indicate locations at the shelf break where the former ice-sheet was most active.The debris flows exhibit varying morphology which can be used to estimate or extrapolate the geotechnical properties of the shallow soils. The morphology also indicates that some debris flows initiated turbidity currents and eventually came to rest as base of slope fans. Modern downslope transport is demonstrated by a slope failure in the northern part of the study area. The high density of data afforded by 3D exploration seismic datasets allows fine scale imaging of this individual feature (Fig 3). It shows this Holocene slide to be a multiple event with retrogressive failure back up the slope. Other active processes are evident by the sediment waves seen in the northeast of the area (Fig 2). The recognition of low amplitude (<5m) long wavelength features is important in identifying differing environments for biological communities and also in understanding the potential of scour/burial of seabed installations.

The seabed return can be used not only to give details of the location of the seabed but also of its acoustic properties and hence it's physical characteristics. The opportunity to interpret information on such a fine scale over large areas opens new ways of interpreting.former and modern sedimentary processes. The integration of the seabed pick from several surveys allows regional features to be identified. Although combining picks from other horizons could see similar features commercial confidentiality constraints prevent such studies. As there is little chance of horizon mis-recognition and there are common aims of best practice in the fields of safety and the environment, the seabed is an excellent opportunity to share data and review morphology at a regional level and provide modern analogues for buried features.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England