## 4 Discussion of the Results
In the conditions of the deep sea, the main loss in sound propagation is the medium itself. However, when passing the hydroacoustic signal to depths commensurate with the wavelength, the influence of the bottom increases. As the depth of the sea decreases, the fraction of energy that is released from the water layer to the sea bottom increases, passing into elastic vibrations of the water-bottom interface and seismoacoustic waves of the bottom soil of longitudinal and transverse polarization. According to [13], the density of energy transferred into elastic surface and volume waves at a point of a plane waveguide can be written as  
\[\varepsilon(x,z)=\frac{\left[P(x,z)\right]^{2}}{2\rho c^{2}}=\frac{2P_{0}^{2}} {\rho cH^{2}(f_{0}^{2}-f^{2})^{\frac{1}{2}}x}\sin^{2}(\alpha z)\sin^{2}(\alpha z _{1})e^{-\frac{4\pi}{c}(f_{0}^{2}-f^{2})^{\frac{1}{2}}x}, \tag{2}\]  
Figure 6: Low-frequency hydroacoustic radiator recording spectra, obtained from the control hydrophone at station No. 1 (**a**), station No. 1 (**b**), station No. 3 (**c**), and station No. 4 (**d**). where \(H\) is the depth of the sea, \(P_{0}\) is the acoustic pressure at the radiation point, \(\alpha=2\pi f_{0}/c\) is the vertical wave number, \(z\) is the depth at the measurement point, \(z_{1}\) is the depth of the radiation source, \(\rho\) is the density of the medium, \(c\) is the speed of sound, \(f\) is the frequency of the radiated signal, and \(f_{0}\) is the critical frequency, described by the expression  
\[f_{0}=\frac{c}{4H\sqrt{1-n^{2}}}, \tag{3}\]  
where \(n\) is the refractive index, equal to the ratio of the sound speed in water (\(c\)) to the sound speed in the sea bottom. From expression (2), it can be seen that the maximum of the energy transferring to the solid half-space will be reached at \(f=f_{0}\). Thus, from (3), each critical frequency will correspond to its critical depth H, at which the energy will be completely transferred to the elastic vibrations of the bottom. It follows from the above that the hydroacoustic signal should propagate in the water along the shelf and, when reaching the critical depth corresponding to its frequency, should completely transfer into seismoacoustic oscillations. In our case, the critical depth for the 22 Hz frequency is 34 m, which explains well the large amplitudes of the seismoacoustic signal at stations No. 1 and No. 2 in the first experiment, since the hydroacoustic signal is practically not delayed in the water and quickly reaches the critical depth, after which it is completely transformed into elastic vibrations of the  
Figure 7: Distribution of the hydrological parameters of the environment by depth during the experiment. bottom. However, this does not explain in any way the sharp decrease in the amplitude at station No. 3 and its reverse increase at station No. 4. Thus, we are left with the only rational explanation of this phenomenon, namely the influence of the bottom relief and its geological structure at station No. 3. Several assumptions can arise here. The first of these is that, due to the complex relief of the seabed, there was an absorption of the hydroacoustic energy due to its dissipation during propagation, with only a small part of the energy transferred to the surface and seismoacoustic waves. In the second case, it can be assumed that, due to the geological structure of the seabed at this station, most of the hydroacoustic energy passed through the sedimentary rocks up to the granite base and propagated bypassing the laser strainmeter. Let us return to the second experiment. As mentioned above, the values of the seismoacoustic energy at stations No. 1 and No. 4 differ from the others. This is due to the design features of the laser strainmeters, namely, their directional diagram, which is a dipole, and the width of its main lobes is about 90-100 degrees. Thus, the maximum sensitive zone of these devices is in the direction of the measuring arm; in the case of the two presented experiments, it is the North-South direction.