Phenomenon of reverberation intensity oscillation observed in shallow-water reverberation experiment

Reverberation has always been a disturbance for active sonar. In the Yellow Sea reverberation experiment conducted in July 2014, the motion of the soliton internal wave was recorded by the temperature sensor attached to the vertical array. The effect of the soliton internal wave on reverberation was investigated from both the temperature chain and the reverberation static experiment. Through the processing of the data in this section, it is found that the soliton internal wave can cause the generation of clutter and the oscillation phenomenon of the average reverberation intensity, and the subsequent research can realize the acoustic telemetry to forecast the internal wave and further improve the performance of the active sonar.


Introduction
Reverberation is a limitation for active sonar in shallow-water operations [1].The generation of reverberation is affected by the environmental conditions, the sea surface undulation and bubble layer [2], the unevenness of the seabed interface [3], different types of sedimentary layers [4], various types of mesoscale phenomena in the ocean(i.e.Kuroshio)] [5], the bubbles in the seawater as well as the scattering process caused by organisms moving in seawater could cause reverberation [6].Based on the causes of reverberation, marine reverberation can be classified into volume reverberation, surface reverberation, and bottom reverberation.The appearance of clutter in the reverberation signal interferes with active sonar.
Internal waves, as strong internal disturbances in seawater, cause mixing of water masses within a region, leading to changes in physical properties such as temperature and salinity.This, in turn, alters the magnitude and direction of sound speed in seawater, resulting in strong interference with the propagation of underwater acoustic signals and greatly affecting sonar performance and the safety of underwater vessels.Ji-xun Zhou [7] et al. studied the abnormal frequency response of summer shallow-water acoustic propagation induced by the soliton internal waves.John [8] further investigated the abnormal frequency response of shallow-water sound propagation due to internal waves and found broadband fluctuations, termed broadband fluctuations, in a 1000-kilometer sound pulse experiment conducted in the Pacific, indicating that the influence of internal waves can explain this phenomenon.Additionally, John [9] proposed a numerical simulation method for the random induced sound-speed perturbation field in deep ocean environment, constructing the realization of the sound-speed perturbation field.Dirk [10] and others studied the prediction and interpretation of the properties of sound fields in random ocean waveguides, and the variation of sound speed profiles was clearly regarded as a random process.James [11] et al. investigated the acoustic effects in a waveguide with significant curvature in the internal wave, and concluded that the effect of curvature is obvious in the modal amplitude and the angle of arrival, but the scattering mechanism is still unclear.Henyey [12] and others discussed the strong refraction phenomenon that occurs when sound waves enter the interior of solitary internal waves using ray angle simulations, which can increase the incident angle and result in high-intensity scattering with the seafloor.Soliton internal waves can produce clutter of around 10 dB, and the average reverberation intensity after clutter generation is slightly higher than before, but the underlying principles are not proposed.The generation mechanism of clutter in shallow-water channels is highly complex, and determining the cause of clutter generation has been an unresolved issue.Furthermore, shallow-water channels exhibit typical time-varying and space-varying characteristics.The channel changes caused by moving soliton internal waves have an impact on the reverberation intensity in shallow-water, which is the focus of this study.Traditional detection of ocean internal waves mainly relies on temperature chains in the sea and remote sensing of the sea surface.
In this study, reverberation intensity oscillations caused by soliton internal waves in shallow-water experiments are observed, and the dominant frequency of this oscillation is related to the speed of internal wave movement.This method provides a theoretical mechanism for obtaining internal wave velocity through active reverberation telemetry, which is beneficial for the observation of internal waves in underwater acoustics.It should be noted that further experimental research is needed to obtain more accurate internal wave velocity through reverberation intensity oscillations after the arrival of clutter.

Conditions of marine experiment
In July, 2014, a reverberation experiment was conducted in the Yellow Sea.The average depth of the water is 45m, and the depth of the sound source is 23m.The average sound velocity in the measured waters is 1500m/s, and the density of the seafloor sediment layer is 1.77g/cm 3 .The longitudinal sound speed is 1610m/s, and the longitudinal wave attenuation coefficient is 0.67.

Data processing
From the latitude and longitude of the submerged mark and the ship, calculate the spherical distance as:  For the temperature profile of the sea area where VLA1 and VLA2 are located, and from the temperature profiles, it can be obtained that there are internal waves packets that pass through VLA1 and VLA2 during the static reverberation experiments for 1.5h.Taking the front-end wave section of the internal wave and calculating it, we can get the relatively accurate actual internal wave movement speed, = 1758 1.5×3600 = 0.3255m/s (1) and the historical internal wave movement speed in the Yellow Sea is 0.3~0.4m/s; [13] The signals received in the isolated sub-internal band arriving at 16.45h were processed using the extrapolated 0.3255m/s as the speed of movement of internal waves in this sea area.
The processing object is the reverberation signal received by the vertical array of the main ship, the vertical array of the main ship has a total of 15 arrays, the first hydrophone 1.5m, spacing 3m.
The main ship carries frequency 580Hz at 16.27h, and continuously emits three groups of CW signals with pulse widths of 0.2s, 0.5s, and 1s, with a total of 10 pulses in each group of signals, and the total length of three groups of signals in the time period is 846s.Figure 2 shows the position of the internal wave from the test ship at this point in time.
The selected hydrophone the 9th hydrophone, and the depth is 19.5m As shown in figure 3, the time where the clutter is located is 1.527s, and the time without counting the direct sound time (0.2s) is 1.327s, which is more in line with the calculation of the time point of internal wave causing clutter, 1.245s, with an error of 0.082s.4, the reverberation data with the clutter appearance time conforming to the pulse widths of 0.2s, 0.5s and 1.0s were subsequently selected, in which, combined with the signal transmission time, (a) the calculated clutter time caused by the internal wave was 1.2074s, and the measured time was 1.2045s, with an error of 0.0029s; (b) the calculated clutter time caused by the internal wave was 1.1289s, and the measured time was 1.1254 s, with an error of 0.0035s; (c) the calculated time for the internal wave to cause clutter is 0.9136s, and the measured time is 0.9733s, with an error of 0.0597s.
Put the three curves on the same figure.It can be observed that with the movement of the inner wave, the position of the clutter also "drifts" with the phenomenon, as shown in figure 5.

Analysis of results
The data processing from this marine experiment was processed to obtain: Soliton internal waves can cause clutter generation; The moving soliton internal wave responds to the drift phenomenon of "clutter" on the reverberation curve; There is a correspondence between the spectrum of reverberation intensity over time and the speed of internal waves.

References
[1] Pina G, EvaMarie N, Erin O, "Tracking time differences of arrivals of multiple sound sources in the presence of clutter and missed detections," The Journal of the Acoustical Society of America, vol.150, no.5, pp.3399-3416, 2021, doi: 10.1121/10.0006780.

Figure 1 .
Figure 1.Position of submarine markers(VLA1 VLA2) and test ship.For the temperature profile of the sea area where VLA1 and VLA2 are located, and from the temperature profiles, it can be obtained that there are internal waves packets that pass through VLA1 and VLA2 during the static reverberation experiments for 1.5h.Taking the front-end wave section of the internal wave and calculating it, we can get the relatively accurate actual internal wave movement speed, =

Figure 2 .Figure 3 .
Figure 2. Position of the internal wave from the test ship