Sub-bottom Permafrost and Shallow Gas in Pechora Sea

Abstract Joint research Russian-Finnish expeditions have been carried out to eastern Pechora Sea, in order to investigate the setting of Pleistocene-Holocene sediments and to locate areas with hazardous shallow permafrost and accumulation of free gas. In the obtained seismo-acoustic data, there are reflective events from deep Mesozoic horizons to shallow Holocene layers with signs on shallow gas and permafrost. The stratigraphy iscontrolIed by penetration testing and core sampling. Samples are analyzed for their physical, mechanical, and chemical properties according to both Russian and Western methods and classification charts. Furthermore, the main differences between these GOST and ASTM standards are demonstrated. Introduction Engineering-geological investigations have carried out in the Pechora Sea by SE AMIGE, "Arctic Marine Engineering Geological Expeditions" since 1980. The organizationoperates four vessels which are designed for shallow drilling and seismo-acoustic profiling. Detailed surveys are carried out at known structures. The drilled and penetration tested shallowsoil sequences to a depth of 150 m at the individual sites are characterized and correlated with each other by regional geophysical profiles. In these offshore investigations carried out in Russian arctic seas, soils with temperatures below zero centigrade containing pore water at least partly in solid phase, i.e. ice in different states, has been found under seabed. Frozen soils are observed in central part of Kara Sea, along Yamal Peninsula and in Pechora Sea (Fig. 1) at a depth of 10 - 30 m under seabed (Ref. I). In the frame of the Russian-Finnish program of scientific and technical co-operation to develop hydrocarbon resources of the arctic seas, the Finnish-Russian Offshore TechnologyWorking Group has carried out joint environmental research expeditions to the Pechora Sea since 1992. The engineering geological research is focused on the setting and soil propertiesof Pleistocene-Holocene sediments and to location of areas with hazardous shallow permafrost and gas. Special emphasis is put on analyzing the thermal and mechanical properties of subaqueous permafrost. Finnish scientists have been participating in engineering geological expeditions carried out on Russian survey vessels from Murmansk to Pechora Sea (Refs. 2, 3). In order to improve and to enlarge the knowledge of sub-bottom soil properties, some western instruments and equipment areutilized on Russian survey vessels. The joint investigations, i.e., the geophysical profiles with engineering-geological interpretations and the engineering-geological soil sampling with adjoining in-situ testing results and laboratory analysis are analyzed together. Furthermore, the analysis of drilling and soil sampling of engineering-geological boreholes with the adjoining penetration tests points out some major differences between the Russian and Western soil testing and classification standards. Permafrost in Pechora Sea According to several evaluations, e.g. Ref. 4, the continental shelf of Pechora Sea to a water depth of 120-140 m was submerged during Pleistocene transgression relatively recently, from 7 000 to 18 000 years ago. Before this, the area developed rather long time in a subaerial arctic regime. Palaeoclimatic reconstruction5 shows that the thickness of permafrost formed at that time can reach a thickness of 500 m.