## 5 Discussion  
A description of seafloor seismic noise features is necessary for HVSR modeling and interpretation. Only by taking into account the characteristics of seismic noise is it possible to adequately select a frequency confidence range for further searching for resonant peaks associated with a contrasting reflective boundary under the seabed surface. In the present study, such a confidence frequency range was between 1 and 6 Hz. It was this frequency range that was used for the HVSR inversion (Figure 11). The level of seismic noise at frequencies below 1 Hz at the studied shelf sites strongly depended on the intensity of the wind waves of the sea, and therefore on the wind speed and the presence of ice cover. The maximum level of seismic noise during the period of time when the sea was ice-free was in the frequency ranges 0.1-0.2 Hz and 0.3-1.5 Hz (Figures 2 and 3), related to primary and secondary microseisms, which are recorded everywhere and are the result of processes related to sea gravity waves and atmospheric pressure [41; 44; 45]. When the sea was covered with ice, this low-frequency seismic noise level decreased, but nevertheless remained, periodically increasing at certain intervals under the influence of an increase in wind speed and a drop in atmospheric pressure (Figures 2 and 3). The wind effect on seismic noise has also been noticed for on-land seismographs. It is present in the low frequency range and is caused by natural wind directly blowing on the sensor [46; 47; 48; 49]. The high-frequency seismic noise at frequencies above 6 Hz with the clear maximum at 9-10 Hz is highly distorted by the parasitic oscillations in the system \"station-ballast-soft sediments\" (coupling effect), which is usually observed in the high-frequency range of 5-10 Hz [41]. While the signal amplification at 10 Hz was present all the time at any wind strength, the additional parasitic oscillations maxima appear at 18 and 30 Hz with an increase in wind speed (Figure 4). There is a clear correlation between sea level fluctuation in the 30 s-5 min spectral periods range related to infragravity sea waves [40; 50; 51], ambient seismic noise and wind speed, as seen in Figure 5. This pattern may be due to the supposed similarity of the mechanisms behind the generation of the ocean infragravity waves and seismic noise [51]. Long-period horizontal noise levels for sensors on the seafloor are higher than vertical noise levels [41], as also seen in Figure 2. Interestingly, sea level fluctuations in the spectral range related to infragravity waves correspond to fluctuations in wind speed better than ordinary gravity waves. Perhaps this is due to the fact that the infragravity waves have a larger length and are more stable in time. The ordinary wind-generated gravity waves and swell are less stable, since they have a shorter period (5-8 s), while the infragravity waves are more inert. This is important, especially since the reanalysis data was measured hourly. Separately, it is necessary to note the excellent agreement between the reanalysis data and the observational data. Long-period seismic noise with spectral periods larger than 30 s is associated with both infragravity waves and seafloor currents [41]. Current-induced tilt noise is caused by seafloor currents flowing past the instrument and in eddies spun off the back of the instrument [52; 53; 54]. The current noise at deep seafloor sites is usually orders of magnitude larger than infragravity wave noise [41], but at shallow depths this could be the reverse, especially in view of the fact that, according to the data obtained, no obvious dependence of seismic noise on tides was revealed, i.e., characteristic periodicity, while Figure 5 demonstrates significant correlation between the amplitudes of infragravity waves and seafloor seismic noise. Thus, it can be assumed that at the considered sites, the effect of infragravity waves on long-period seismic noise is predominant.