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The formation of pockmarks and their potential influence on offshore construction

, Statoil, P.O. Box 300, N-4001 Stavanger, Norway

Pockmarks are craters formed in the soft seabed by gas and, in some cases, liquid expulsion. They were described on the Scotian Shelf in 1970 (King & 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 200 m across and up to 20 m deep.

There are assumed to be three phases of pockmark development:

1. the pressure build-up phase, in which gas accumulates below the seabed;
   
2. the eruption phase when liquids and solids are ejected into the water column;
   
3. the post-eruption phase, which can either be a dormant phase or one where gas is continually seeping through the pockmark floor.
   

The paper discusses triggering of pockmark eruptions which may be by earthquakes, by 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. The relevance of pockmarks in relation to seabed construction sites is also considered.

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