Barra Fan Project : Geophysical operations report - project 92/02

There is significant geochemical, geological and biological interest in the source and fate of methane in marine sediments. Where production rates are high in the near seabed sediments, or pathways through the sediment column allow the migration of deep biogenic or thermogenic gas, methane may escape into the bottom water where gas plumes may be detected by geophysical techniques. Such plumes were potentially identified on seismic records obtained by BGS at the foot of the Barra Fan, west of the Hebrides during surveys in 1985. A multidisciplinary project has been established to examine geological, chemical and biological aspects . of active gas seeps specifically in this Barra Fan area. The project is funded by the Natural Environment Research Council and involyes scientists from British Geological Survey, Marine Biological Association and Dunstaffnage Marine Laboratory. The geological objectives of the project include: searching for and mapping shallow gas and methane hydrate deposits and gas seepage areas on the continental slope east of the Rockall Trough and defining their geological and oceanographic setting; determination of the source of the gas and comparison of the physical and geotechnical properties of sediments at the seep sites with those from the surrounding areas. Survey methodology comprised: an initial survey of the area with seismic and sonar equipment to locate seeps and gas charged sediments; sediment coring at identified control sites; photography of the seabed; CTD profiles and subsampling of the cored samples for geological, chemical, physical, microbiological and infaunal measurements. The wide range of equipment and large numbers of scientists required to undertake the various components of this multidisciplinary project presented severe logistic and accommodation problems. It was therefore decided that the most cost efficient way of meeting the objectives of the project was to undertake the geophysical component of the programme as a separate exercise. This was made possible by extending the charter of the vessel mv Kommandor Michael which was already on contract to BGS undertaking geophysical survey work in the Rockall Trough and Rockall Continental Margin areas. The vessel was already fully mobilised and operational and requiring minimal transit time to the project area. A total of 17 lines were surveyed on 5 and 6 July 1992, representing a total of 201 line km. Bathymetry and pinger data were acquired on all lines and deep-tow boomer information was acquired concurrently on the first 11 lines. The weather throughout the survey period was excellent. This summary operations report contains brief details of the survey including a daily log of events, details of the lines surveyed and a description of the equipment used. The financial contribution from Statoil towards the cost of the geophysical survey is gratefully acknowledged.