# Subsea Methane Hydrates: Origin and Monitoring the Impacts of Global Warming
###### Abstract  
The East Siberian Arctic shelf is the area where the largest natural gas reserves are concentrated. The formation of permafrost of the Arctic shelf during the Ice Age contributed to the emergence of a zone of stable existence of gas hydrates in the sedimentary layer, and subsequent flooding of the shelf led to its gradual degradation, the thawing of gas hydrates and the subsequent emissions of methane into the atmosphere. In the first part of the paper, we use mathematical modeling to study the processes of the formation of subsea permafrost on the Arctic shelf considering changes in the sea levels over the past 200 thousand years. Numerical simulations show the influence of climate warming up to 2200 on the degradation of subsea permafrost and the violation of the conditions for the stable existence of methane hydrates in bottom sediments using the example of the East Siberian shelf. The second part of the paper proposes a method for seismic monitoring of the state of gas hydrates based on a solution of multi-parameter inverse seismic problems. In particular, the degree of attenuation of seismic energy is one of the objective parameters for assessing the consolidation of gas hydrates: the closer they are to the beginning of decomposition, the higher the attenuation and, hence, the lower the quality factor. In this publication, we do not solve a multi-parameter inverse seismic problem for a real geological object. This would be impossible due to the lack of necessary data. Instead, we focus on substantiating the possibility of correct solutions for the problem of the reconstruction of the absorption and velocities for a viscoelastic medium in relation to the problem of monitoring the state of gas hydrate deposits. As noted in a range of publications, the thawing of gas hydrates leads to an increase in the fluid saturation of the geological medium followed by an increase in the absorption of seismic energy--that is, a decrease in the quality factor. Thus, the methods of seismic monitoring of the state of gas hydrates to predict the possibility of developing dangerous scenarios should be based on solving a multi-parameter inverse seismic problem. This publication is devoted to the presentation of this approach. viscoelasticity; seismic attenuation; generalized standard linear solid; multi-parameter inverse problem; subsea permafrost; Arctic shelf; methane hydrate stability zone +
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1 Sobolev Institute of Mathematics SB RAS, 630090 Novosibirsk, Russia 1  
and  
22 Trohnik Institute of Petroleum Geology and Geophysics SBhydrates typically appear in bottom sediments at water depths of more than 500 m; however, in the Arctic Ocean, due to low temperatures, gas hydrates can be stable at depths starting from 250 m. The shelf of the Siberian Seas (Kara, Laptev, East Siberian and Chukchi) is an area where the existence of permafrost is possibly due to the suitable P-T stability conditions. Reliable information on the feasibility of these conditions is obtained from drilling data [4; 5]. The presence of permafrost in the bottom sediments provides conditions for the formation of a gas hydrate stability zone at a water depth of less than 100 m [6; 7; 8; 9]. The gas hydrate deposits could have been formed during ice ages under subaerial conditions as a result of the lowering of the ocean level, the freezing of exposed bottom sediments and their subsequent flooding. The reserve of relict gas hydrates of the Arctic shelf is estimated at 2-1400 Gt CH4 [10; 11]. These relict hydrates are sensitive to climate change, since the temperature of the bottom sediments rises after the shelf is flooded. An increase in temperature or a decrease in pressure due to changes in the sea level lead to a violation of the stability conditions for the stability of methane hydrates, which can lead to the release of large amounts of dissolved and gaseous methane [12; 13].