# Subsea Methane Hydrates: Origin and Monitoring the Impacts of Global Warming
###### Abstract
Increased methane emissions are also expected as a result of the intensified destabilization of gas hydrates and degradation of permafrost on the Arctic shelf in the future [14]. Observational data for the years 1920-2009 in the shallow part of the shelf and the coastal zone of the Laptev Sea and the East Siberian Sea reflect a significant increase in the bottom temperature (up to 2.1 degrees), which began in the mid-1980s [15]. Several events of unprecedented warming in bottom waters thought to be on verge of freezing all year round have been observed on the central shelf in winter [16; 17]. Three-dimensional numerical modelling of the Arctic Ocean state in this century has shown an increase in the temperature of the surface and bottom layers of the Siberian Arctic Seas [18; 19]. One of the possible consequences of this fact is the degradation of gas hydrates associated with an increase in the temperature of the bottom water. This increase can lead to the destabilization of gas hydrate deposits, followed by the rise of a huge amount of gas. In turn, this can provoke the formation of huge gas bubbles with a diameter of several hundred meters, which can threaten both engineering structures [20] and shipping. Thus, the temperature dynamics of bottom waters near gas hydrate deposits should be monitored carefully. The process of methane hydrates thawing is inevitably accompanied by their transition from a solid to a gaseous state, which entails an increase in the absorption of seismic energy, particularly for S-waves. Modern industrial methods of reflection seismic are successfully used to detect gas hydrates. Due to the high velocity of seismic waves in these deposits, gas hydrates appear on wave images as special objects, called Bottom Simulating Reflectors or BSRs, characterized by reversed polarity [21; 22]. The BSR amplitude is determined by the acoustic impedance contrast between gas hydrates and formations in the lower part of Section [23; 24]. Seismic methods are widely used in the detection of gas hydrates and have demonstrated their effectiveness in a number of cases. However, they do not allow recovery of the petrophysical parameters of methane hydrate deposits due to their extremely complex microstructure. To overcome the limitations of seismic methods in describing the microscale structure of gas hydrate deposits, many measurements and numerical experiments have been performed to elucidate the relationships between the micro- and macroscale properties of gas hydrate accumulations. Thus, in [25], an increase in the absorption of seismic waves with an increase in the fluid saturation of rocks was established. Consequently, during the thawing of gas hydrates, which causes an increase in fluid saturation in rocks, the absorption of seismic waves will increase as well. It is this property that we propose to use to assess the state of gas hydrates. Therefore, the study of climate change and, as a consequence, possible variations in P-T conditions, together with the localization of gas hydrate accumulations and monitoring of their properties, such as P- and S-quality factors are necessary for reliable risk prediction in relation to various parts of the shelf. In this paper, we consider the following questions:  
* Investigation of the influence of climate warming up to 2200 on the degradation of subsea permafrost and the violation of the conditions for the stable state of methane hydrates in bottom sediments on the example of the East Siberian shelf. * Estimation of the state of permafrost and MHSZ using a numerical model of heat transfer in shelf bottom sediments and comparison of the results with standard seismic research methods based on the theory of seismic wave propagation in media with attenuation. * Monitoring of the state of methane hydrate deposits using multi-parameter Full Waveform Inversion (FWI) of seismic data, prioritizing a reliable reconstruction of attenuation. We use numerical modeling to assess the state of permafrost and the conditions for the stability of hydrates in bottom sediments.