## 5 Discussion
Even though infragravity wave amplitudes decrease with increasing ocean depth, the signal remains significant to depths of many kilometers below the seabed depending on the frequency and the elastic parameters as a function of depth [41]. At shallow depths, gravity and infragravity waves can drive motions directly, tilting the instrument [41; 55; 56]. However, it is difficult to estimate the relative contribution of the direct tilting of the instrument to the total level of recorded long-period noise. The HVSR time-frequency plots presented in Figure 6 show the reduced level of HVSR in the low-frequency band of 0.2-2 Hz during the period of time when the sea was free of ice. This makes the high-frequency peaks above 2 Hz more pronounced. This can be explained by the fact that an increased level of seismic noise in the frequency range 0.2-2 Hz, caused by the wind waves of the sea, is observed both on the horizontal and vertical components of seismometers. Therefore, the ratios of the smoothed spectral curves have low values in this frequency range. In the same time period, when there was ice on the sea and the seismic noise caused by the wind waves was reduced, the HVSR curve becomes flatter over the entire time and frequency range. The signal maximum at 9 Hz, which corresponds to the coupling effect, is also well pronounced on the HVSR curves. HVSR can also be used to estimate the ellipticity of the fundamental mode Rayleigh wave, which has the advantage of defining the total depth of the soft sediments [57]. The principal problem of retrieving ellipticity from HVSR spectral ratios is to correct for the energy of body and Love waves present in the ambient vibration recordings [58,59]. This could be provided by classical polarization analysis in the frequency domain or by identifying P-SV wavelets along the signal and computing the spectral ratio from these wavelets only [59]. In the framework of this work, these methods were not considered. Based on the characteristics of the region where the OBSs were deployed, we assumed that the sub-bottom reflecting boundary, most likely, would be the upper surface of the permafrost, and the layer of soft sediments above it would consist of silty soils. Since the exact values of soil densities and seismic wave velocities for the study area are unknown, their values were taken from reference materials. All this together leads to a significant uncertainty in the assessment of the depth of the reflecting boundary and the results of the HVSR inversion. In the case of the Typ2 site, the inversion is further complicated by the absence of a resonant peak in a representative frequency range of the HVSR curve. Therefore, the results of the inversion for the Type2 site should be treated as very approximate. It should also be noted that due to the fact that the inversion is based on the Monte Carlo method, the result obtained can be considered as only one of the possible solutions. Many papers present formulas that correlate cover thickness with the frequency of the HVSR main peak [49,60,61,62,63,64]. All of them have the following general form:  
\[d=af_{reson}^{b} \tag{3}\]  
Table 11 shows the values of the parameters \(a\) and \(b\) obtained from different sources, along with corresponding values of calculated \(d\) for the St3 site. It can be seen that the thickness values vary significantly from 11.7 to 28.9 m, which is quite understandable, because these empirical formulas were obtained in different regions and for different parameters of soils. In the present study, a thickness value of 17.7 m was obtained. The formulas from [61,62] give the closest estimate.