Seismic surveys
Seismic survey techniques are used to build up an image of the rock formations beneath the seabed during the first part of the exploration phase. Later in the life of a potential oil field further seismic surveys are undertaken to gain more detailed information about the geology around proposed well sites.
In general terms, seismic surveying equipment works on the same principle as radar or sonar devices. Pulses of low-frequency sound are released at regular intervals, which pass through the water and the seabed to be reflected by the geological layers beneath the surface of the earth. The reflected echoes are recorded and analysed, and the results are used to create a computer-generated image of the geology far below the bottom of the sea.
Survey engineers use an array of underwater airguns towed behind the survey ship to release a 'pop' of compressed air. These brief pulses occur approximately every ten seconds. The echoes coming back from the subsurface rock formations are measured by a towed array of cables fitted with hydrophone sound detectors while the survey vessel sails along a pre-determined line several kilometres long.
Each line sailed yields data which is used to produce a two-dimensional image of the earth in cross-section. These cross-sections can indicate the geology of rock layers down to 20,000 ft below the bottom of the ocean. If enough closely-spaced seismic survey lines are sailed, analysts can use the data to create a three-dimensional image of the subsurface rock by layering the two-dimensional cross-section images side by side.
Seismic survey activity and cetaceans
West of Britain operators acknowledge that seismic survey activities must not harm marine wildlife. There has been some public concern about the effects of low-frequency noise on some marine animals, particularly cetaceans -- whales, dolphins and porpoises.
However experience has shown that, with care, seismic research can be managed to minimise any potential impact on the marine environment. The industry's record of environmental sensitivity has been achieved through observance of guidelines drawn up by the UK Government agency, the Joint Nature Conservation Committee (JNCC). The guidelines are intended to minimise acoustic disturbance to marine mammals. They state that:
- Survey vessels will not start up a survey line if cetaceans are seen within 500m
- Survey vessels wait for 20 minutes after the last sighting before proceeding
- Airgun firing begins with a slow build up of power to allow cetaceans to leave the area
JNCC data indicates that there is no evidence of any cetacean suffering injury as a result of seismic operation in UK waters.
Cetacean survey
The oil industry is funding what is believed to be the world's most extensive cetacean monitoring programme. The results of this programme will be used to enhance existing environmental protection measures for all industry activities.
The four-year study is called the Acoustic Array Cetacean Monitoring programme. It is co-ordinated by the UK Joint Nature Conservation Committee (JNCC), in conjunction with Cornell University in the US.
The programme uses US military fixed array sonar equipment -- the Integrated Undersea Surveillance System -- to track the movements of large sea mammals. This equipment, developed to track shipping, enables more comprehensive data to be accumulated on the species, populations and migration routes of large cetaceans (particularly blue, fin and humpback whales). The equipment is sensitive enough to track different species -- even on occasion individual animals -- by identifying whale 'songs' over large distances.
Existing survey data reveals that two large baleen whale species -- the fin whale and humpback whale -- undertake regular seasonal migrations along the edge of the UK Continental Shelf in spring and autumn. Other cetaceans observed include minke whales, harbour porpoises, white beaked dolphins and orcas. During spring and summer months the reported numbers of cetaceans increases.
Seismic survey activity and fish
There is a large amount of fishing in shallower waters along the UK Continental Shelf to the West of Shetland. Fishing takes place all year round, with peak levels of activity in May and September to December. West of Britain operators will ensure that this vital local industry is not affected by seismic survey activity. For example, all seismic surveys are preceded by the issue of a Notice to Mariners to avoid interference with fishing fleets and other users of the sea.
Fish, like cetaceans, are also largely unaffected by seismic survey activity. Last summer scientists from the Fisheries Research Services Marine Laboratory combined a tagging programme with the use of underwater TV cameras to observe the behaviour of fish in the vicinity of a seismic survey airgun as it discharged.
The researchers found that while the fish displayed a brief involuntary fright reaction (similar in neurological terms to the blink of a human eyelid), this was no more than would be observed if tapping on the glass of an aquarium window. The moment the stimulus passed, the fish resumed their activities, their intended track apparently unaltered. The long-term day to night movements of the shoals observed were also uninterrupted by the presence of the airgun releasing low-frequency pulses in their path.
In contrast, the presence of a grey seal near the shoals produced an identifiable disturbed behaviour pattern which lasted for about an hour.
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