The Laptev Sea shallow water area represents the key site where submarine permafrost has been predicted by geological onshore investigation and subsequently found by both the seismic and drilling methods (Rachor, 1997). This area is a key location for understanding permafrost evolution processes (incl. its rate and variability) under different climatic conditions. However, the main parameters of offshore permafrost are known insufficiently. During last decade mathematical modeling on the Laptev Sea permafrost evolution was conducted by the team headed by N.Romanovsky.

The High Resolution Seismic data were obtained in 2005 during the “LAPEX-2004/Transdrift - X” expedition (r/v “Yakov Smirnitskiy”). The investigations were focused on the seismic research that might give the base for the selection of the places for further shallow drilling. Multi-channel low frequency (_100 to 400 Hz) seismic acoustic profiling was accompanied by high frequency (5 kHz) profiling. Side scan profiling was carried out simultaneously. Total 1200 km of the seismic lines were obtained during expedition.

The interpretation of the HRS data shows the following features of the seismic images:

1. The strong lowermost reflector with uneven topography have been identified as permafrost table (PT). The unequal seismic signal penetration beneath the PT in the different part of polygon led us to assumption that there are different types of permafrost (in terms of ice content, lithology, generation etc). Generally, the PT has hummocky topography, characterized by flat top banks, surrounded by basins with very step flanks. Banks has lateral size of 200 to 2000 m and distribute at the depth of 40 to 50 m below sea level. All PT banks are slightly inclined to WNW. At the SE part of studied area the PT arises up to the sea floor and locates at the depth of -24 m below the modern sea level.

2. Within the grid area 7 separated basins filled by sediment were found. These basins are surrounded by banks of PT. The basins have a width of 800 to 1000 m , length of 1000 to 2000 m and depth up to 12 m . They are elongated at the NE and NNW direction. The basins have very steep flanks. The NW flanks are mostly steeper.

Basins completely filled by stratified sediment, which should be interpret as lacoustrine sediments. It made possible to suggest that here could have existed a few isolated (lakes) or partly isolated (lagoons) basins with quiet hydrodynamic regime. The lacustrine sediment, are overlayed by upper seismic unit, consist of marine sediments. There are a lot of buried ploughmarks spread within the upper subsequence.

3. A lot of gas seeps were found in the studied area. No any regularity in the gas seeps distribution have been found.

4. The most remarkable phenomena obtained during interpretation of the HRS data is transparent zones at the western flank of basins. They have an acoustic transparent seismic pattern and sharp vertical or negative angle boundary, cutting the normal sedimentation strata. These zones are located at the W-SW flank of the basins only. The top of these zones is flat and locates at the same depth level as a top of the nearby PT banks. All of these features led us to assumption that the origin of these zones can be explained in terms of postsedimentary refreezing of the sediments.

But several questions still remain: Why refreezing zone distribute only near the W-SW flanks of the basins? When this refreezing took place? How does high frequency seismic energy penetrate the permafrost zone and reflect from the subjacent strata?

References

Rachor, E. (Ed., 1997): Scientific Cruise Report of the Arctic Expedition ARK-XI/1 of RV “Polarstern” in 1995. Reports on Polar Research, 226.