Sub-bottom permafrost is wide spread phenomenon of Arctic offshore areas. It has been identified in the Pechora, Kara, Laptev and other Russian Arctic seas. Offshore permafrost can be an important geohazard and constraint for various marine and near-shore constructions and facilities. According to general opinion, frozen soils formed during the last glaciation period when the surface was exposed and subsequently covered by seawater following the marine transgression. Currently permafrost melts because of heating from both the top and bottom of the stratum. Investigation of this phenomenon in the Russian Arctic Offshore has been ongoing since 1980s. The study is based on a combination of geotechnical and geophysical methods of site and regional surveys for oil and gas industry.

1. Introduction
1.1 Sub-bottom permafrost origin

Permafrost is a result of low temperatures freezing the soil - actually pore water in general. It is very common in high latitudes (in Europe, Asia, North America). Estimated thickness of permafrost can be up to 0.7km (Xie and Mathews, 2011) and possibly 1km in Russian East Siberia. (Over 50% of Russia land is frozen.) Frozen offshore soils may be a result of old relict onshore permafrost now submerged, or may be part of new generation of frozen soil. The latter occurs when sea-bottom temperature is below freezing point and pore water is less saline than the overlying seawater. It is reported occasionally in Arctic seas, but it is a rare occurrence in general. Vast areas of the Northern Hemisphere have become covered by seawater (marine transgression) after the last glaciation. Although global sea levels rose by about 120m due to local effects of isostasy, the actual submergence may have varied between 80m and 200m. Thus for this water depth in the Arctic Ocean, relict permafrost can be possible.

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