# Study of Low-Frequency Hydroacoustic Waves' Behavior at the Shelf of Decreasing Depth
## 3 Experimental Data Processing  
Each device used in the experiment was \"tied\" to the exact time clock, providing an accuracy of 1 ms. During pre-processing, spectral processing of the obtained synchronous data of the hydrophone and the laser strainmeter was carried out. It was found that in the data of both receiving systems there were oscillations at a frequency of 22 Hz when operating at stations P1-P7. As an example of the stable reception of the transmitted signal by the hydrophone and the laser strainmeter, Figure 3 shows the sections of spectra of simultaneous records by the hydrophone and the laser strainmeter during operation at station 3, in which powerful spectral components at the frequency of 22 Hz are singled out. During measurements at each station, the transmitter operated for about 15 min in a continuous mode of transmitting harmonic signals at a frequency of 22 Hz. The measurements by the hydrophone were carried out every 1 m from the bottom to the surface. At each horizon, the hydrophone recorded a signal for 1 min. From the obtained hydrophone data, the amplitude of the received signal was determined at a frequency of 22 Hz. Based on this data, a curve was constructed that describes the  
Figure 3: Sections of spectra of synchronous data from the hydrophone (**left**) and the laser strainmeter (**right**). level of the received hydroacoustic signal, starting from the surface to the bottom. For clarity, Figure 4 present the curves of the obtained experimental data (corresponding model curves are also given, which will be discussed in \"model calculations\"). Figure 4: Vertical profiles of the distributions of hydroacoustic pressure at a frequency of 22 Hz at stations P1–P7. For better visualization, the experimental curves presented in Figure 4 were grouped into one figure, which is presented in Figure 5. As we can see in Figure 5, the pressure wave at point P1 has a pronounced section of increased amplitude at a depth of 18 m, associated with the position of the emitter's acoustic axis at the same depth. However, when passing through the points P2-P4, the curves become flatter, which can be explained by energy loss during interaction with the bottom. When passing the critical depth, a sharp drop in the pressure amplitude is seen at the measuring point P5, after which the pressure wave at points P6 and P7 abruptly changes its profile to the opposite. Then, for each experimental curve, a polynomial equation of the most suitable degree was constructed. The experimental data of the laser strainmeter was recorded on the working computer with a sampling frequency of 1000 Hz. When operating at each transmission station, the recording of the laser strainmeter was divided into successive sections of 65536 points which subsequently underwent spectral processing. With such processing, 12-13 sections of the laser strainmeter records were singled out during the operation at each transmission station and, in accordance with this, 12-13 signal amplitudes were obtained at a frequency of 22 Hz, from which the average amplitude was subsequently determined. The results described in this paragraph are shown in Table 1.  
In the calculations to be presented below, the following data were used: \(\rho_{\mathrm{w}}\) = 1000 kg/m\({}^{3}\) (density of water), \(c_{\mathrm{w}}\) = 1500 m/s (sound velocity in water), \(\rho_{\mathrm{b}}\) = 2100 kg/m\({}^{3}\) (rock density of the upper layer of the earth crust), \(c_{\mathrm{b}}\) = 2300 m/s (Rayleigh-type wave velocity at the \"earth crust-air\" boundary [3]), f = 22 Hz (frequency), \(\theta\) = 9.4\({}^{\circ}\) (angle between the axis of the laser strainmeter and the \"transmitter stations P1-P7\" line), \(\omega\) = 2\(\pi\)f, R = 1780 m (distance from the point of transmission to the nearest point of the laser strainmeter). Figure 5: Vertical profile of the hydroacoustic pressure distribution at a frequency of 22 Hz at stations P1–P7.