The work of the British Geological Survey mapping the UK land mass, which began over 150 years ago, was extended in 1966 to include geological mapping of the adjacent continental shelf. A series of geophysical and geological maps (Fig. 1) are being produced from this reconnaissance survey work, complemented by a series of regional reports for which the data acquisition phase has taken just under twenty years.
An integrated approach has been adopted whereby initial geophysical surveys have been subsequently calibrated, following interpretation and site selection, by samples and shallow cores obtained using remotely operated seabed equipment and wireline coring from site investigation vessels. In the course of the survey some 200 000 km of geophysical traverses have been run using gravity meter, magnetometer, air gun or sparker, deep-towed boomer or pinger, and side-scan sonar. These data have been calibrated at more than 30000 sample stations, where seabed grab samples have been obtained together with cores of up to 6 m in length Shallow boreholes have been cored at almost 600 critical locations. As this work has progressed into deeper water it has been necessary to develop proven techniques to work to 2000 m water depth and beyond. A review is undertaken within this paper of these developments provoked by the need to operate in increasing water depths particularly on the continental slope.
The requirement to map seismo-stratigraphic units from continental shelf depth into deep water has necessitated the use of a high-resolution seismic profiling system capable of operating in a variety of water depths. The solution chosen was to develop a system capable to being deployed at a range of depths while still retaining a stable source signature In order to meet the requirements of the BGS survey programme, it was decided that the system should be designed to acquire data in water depths of 1500 to 2000 m.
In 1979 BGS purchased a Huntec deep tow boomer for operation in continental shelf water depths. It was deployed on a 300 m cable resulting in a maximum practical tow depth of 120 m. This gave excellent high-resolution records in unconsolidated and semi-consolidated sediments, achieving a resolution of better than 1 m with penetration of up to 150 m. Deep tow systems however have advantages over surface tow systems because the source and receiver are more effectively decoupled from the motion of the ship and surface waves, improving shot-to-shot correlation. Ambient noise at depth is also less, extending the survey weather window. In addition, by surveying closer to the target, signal losses are reduced and the seismic footprint is reduced, improving lateral resolution.
A particular advantage of the Huntec system is that the boomer transducer provides a wide bandwidth signal (500 Hz-6 kHz), and is pressure-compensated to ensure a uniform pulse shape at varying depth. A further feature is the incorporation of a depth sensor on the tow fish to allow adjustment of the source trigger firing to compensate for changes in the tow depth.
Fig. 1 British Geological Survey Availability of maps-1.250 000 scale U.T.M.Series (available in full paper)