ИСТИНА

Significant reserves of CH4 in hydrate and free gas forms are held in the Arctic seabed, but the release of CH4 to the overlying ocean and, subsequently, to the atmosphere has been believed to be restricted by impermeable subsea permafrost, which has sealed the upper sediment layers for thousands of years. It has been suggested by IPCC that destabilization of shelf Arctic hydrates could lead to large-scale enhancement of aqueous CH4, but this process was hypothesized to be negligible on a decadal?century time scale .Consequently, the continental shelf of the Arctic Ocean has not been considered as a possible source of CH4 to the atmosphere until very recently: it was described by Shakhova et al. showing that the key area of the Arctic Ocean for atmospheric venting of CH4 is the East Siberian Arctic Shelf (ESAS). Thus, this is an emergent need to use array of convenient geophysical techniques to map the subsea permafrost table with focus on talik zones. Here, we show several applications of electromagnetic technique in the ESAS and in the Kara Sea. It is shown that the Kara Sea could be also a source of bubbling methane as the result of pingo-explosions. Traditionally, seismic methods are used to map the subsurface in marine geophysics. It allows to obtain images of geological structures by analyzing reflected elastic waves. However, gassy rocks in the upper part of the section often produce very bright reflections, thus creating a shielding effect for the study of underlying sediments. In the case of studies talik zones in the arctic seas with saline frozen sediments, seismic methods do not always give satisfactory results - the presence of ice does not provide a significant increase in acoustic stiffness, and the reflectivity of the boundary between frozen and thawing areas is insufficient to clearly distinguish these zones. As a result, seismic data often do not ��see�� permafrost in subaquatic conditions. To solve this problem, we elaborated the Time Domain Electromagnetic method (TDEM), which allows to obtain data on the electrical resistivity of rocks. A distinctive feature of TDEM is that even in saline frozen rocks the electrical resistivity differs significantly from that in thawed rocks, which allows us to identify the boundaries of frozen layers. Thus, the electrical parameters, unlike the velocities of elastic vibration propagation, are more sensitive to the presence of ice and its distribution in the rocks, which makes the TDEM method particularly valuable for mapping perennially frozen shelf rocks and identifying local talik zones that are pathways for release of deep gases. Verification of the results of TDEM by drilling, carried out in the period from 2012 to 2020, has clearly demonstrated the advantage of TDEM over traditional seismic in mapping subsea permafrost and taliks associated with methane ebullition. The obtained data confirm that the application of the TDEM method allows to significantly improve the accuracy of geophysical interpretation, reduce the geo-hazards associated with the instability of hydrates and their outburst.