Pockmarks are craters formed in the soft seabed by gas and in some cases liquid expulsion. They were first described on the Scotian Shelf in 1970 (King and MacLean, 1970) and have since been mapped in a range of shallow seas including the North Sea and the Arabian Gulf. Pockmarks range in size from less than one metre to about 200m across and up to 20m deep.
There are assumed to be three phases of pockmark development:
1) The pressure build-up phase, whereby gas accumulates and builds up local pressure below the seabed.
2) The eruption phase, where by gas, liquids and solids are ejected into the water colum, and
3) The post eruption phase, which can either be a dormant phase or one where gas is continually seeping through the pockmark floor.
Triggering of pockmark eruptions are discussed. They may be triggered by earthquakes or seabed pressure perturbations caused by tidal or gravity waves or, in deep water, by a combination of tidal waves and low atmospheric pressure and storm waves.
Pockmarks are of concern in relation to seabed construction.
Pockmark avoidance and crossing with pipelines is discussed together with methods of artificial gas drainage away from construction sites.
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