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The 1996 Seabed Environmental Survey

Introduction

The Atlantic Frontier Environmental Network (AFEN) took on the need for a regional assessment of the environment west of Shetland and provided the basis for an industry-wide co-ordination of effort. AFEN developed a new integrated approach, taking advantage of innovative techniques supplied by the Southampton Oceanography Centre.

The primary aim of the survey was to produce an integrated regional assessment of the sea floor environment in the total area licensed for hydrocarbon exploitation, sampling some 200 locations to recover seabed samples which were subsequently analysed. Thesurvey results will:

  1. provide a description of the currently existing seabed environment; and
  2. provide data against which any future changes in the environmental parameters measured could be assessed.

The sea floor environment west of Shetland is extremely varied. In the area surveyed, water depth ranges from 150 to 1500 metres - water depth being an important environmental gradient. In the west of Shetland area, the range and variation of water temperatures is amongst the most extreme found in any of the World's oceans, contrasting the relatively warm north-eastwards flowing surface water of the North Atlantic Drift with the very cold deep water flowing south-west from the Arctic Ocean and Norwegian Sea. In addition to these exceptional variations in temperature, the survey area also encompassed the full range of sea floor sediment types - from cobble pavement to soupy mud. The distribution of animal life and of the potential effects of contaminants of the marine environment is strongly controlled by even small variations in sediment type.

The survey was conducted in two phases:

  1. An initial remote sensing survey, using a sidescan sonar system to map the sea floor environment; and
  2. A detailed seabed sampling programme.

Sidescan Sonar Survey

Sidescan sonar is the most effective means of imaging the sea floor over wide areas. The principal sidescan sonar system to be used in the survey, TOBI (Towed Ocean Bottom Instrument), provides a carefully chosen compromise between resolution (picture detail) and swath width (area surveyed). TOBI is capable of imaging 100% of the sea floor, where water depth is greater than 200m. The data obtained provide an extensive general picture of the sea floor and are also being further processed in sections for detailed studies such as rig site work.

Seabed sampling survey

The survey collected seabed samples from some 200 sites which were later analysed for heavy metals, oil related hydrocarbons, total organic carbon and nitrogen, particle size analysis, and macrobenthos. The survey area was divided into sub-environments according to depth and latitude. Each sub-environment was then sampled at randomly selected locations to provide a statistically rigorous assessment. In addition, further seabed sampling was carried out at a number of selected locations to meet a number of other objectives:

  1. A depth-related transect survey to investigate the changes in environmental parameters down the continental slope.
  2. A sampler/sample inter-comparison exercise.
  3. To sample and investigate specific seabed features.
  4. To sample in the immediate vicinity of an existing well site to assess the extent of any contamination.

The sampling exercise was supplemented by seabed photography using the WASP (Wide Angle Seabed Photography) system.

The 1996 survey report is:

Bett, BJ 1997 Atlantic Margin Environmental Survey:
Seabed survey of the shelf edge and slope west of Scotland. Southampton Oceanography Centre Cruise Report No.7, 127pp and appendices.

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