# Subsea Methane Hydrates: Origin and Monitoring the Impacts of Global Warming
## 5 Discussion  
Global climate warming and anthropogenic impact associated with the intensive industrial development of the Arctic shelf led to a change in the thermobaric conditions of its bottom layers. An analysis of observations and numerical simulation results demonstrates that, in recent decades, there has been a steady increase in the temperature of bottom waters. A permanent increase in the temperature of the bottom layer leads to a change in thermobaric conditions and immersion to a greater depth of the upper boundary of the gas hydrate stability zone.  
One of the undesirable consequences of this may be the thawing of permafrost, which retains gas hydrates in a solid state, and their subsequent degradation, accompanied by the release of significant volumes of methane. Therefore, the monitoring of methane hydrate deposits in order to control their condition and assess the degree of their thawing is one of the most important tasks to ensure the safe development of the shelf of the Arctic seas.  
Methane hydrates located in the stability zone are close in their physical properties to ideally elastic bodies in which there is no absorption of seismic energy. However, with an increase in temperature, thawing begins, leading to a decrease in their consolidation and, consequently, to losses of seismic energy during the propagation of elastic waves. Naturally, the closer the methane hydrates are to decomposition, the higher the absorption, and the energy losses for S-waves are much higher than for P-waves.  
The most universal and reliable method for solving inverse seismic problems is the Full Waveform Inversion (FWI). The results presented in the article confirm that its application makes it possible to restore, with sufficient accuracy, all the parameters of a viscoelastic medium--the velocity and quality factor of P- and S-waves. At the same time, the stability of the solution of the multi-parameter inverse seismic problem considered in this paper for a viscoelastic medium is achieved due to the availability of a priori information regarding the location of methane hydrate accumulations. To describe their localization, we performed predictive modeling focused on recovery of the location of the methane hydrate stability zone.  
Figure 8: A step-wise decrease in the objective functional.  
In this publication, we do not aim to solve a multi-parameter inverse seismic problem for a real geological object due to the lack of data necessary for this. Instead, we focus on substantiating the possibility of correctly solving the problem of reconstruction of absorption and velocities for a viscoelastic medium in relation to the problem of monitoring the state of gas hydrate deposits. As noted in the first section, the melting of gas hydrates leads to an increase in the fluid saturation of the studied volume. This, in turn, causes an increase in the absorption of seismic energy, as shown in a number of publications.  
Note that one of the main difficulties in solving a multi-parameter inverse problem is the coupling of the desired parameters, which, in some cases, leads to a decrease in the accuracy of the solution. (See, for example, [43]. Acoustic multi-parameter full waveform inversion based on the wavelet method in Inverse Problems in Science and Engineering, 2021, v. 29, 2, pp. 22-247).  
Thus, conducting a two-stage hybrid study allows not only localizing the stability zones of methane hydrates with acceptable accuracy but also assessing their state in order to predict possible emergency situations associated with their decomposition with methane release.