Sound intensity fluctuations caused by the motion of internal wave solitons in the ASIAEX experiment

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Abstract

One of the episodes of the ASIAEX 2001 experiment (South China Sea) is considered, in which a large soliton of internal waves moved along two stationary acoustic paths 32 and 19 km long, and associated fluctuations in the intensity of low-frequency sound (224 and 300 Hz) were observed. During the study, the phenomenon of constancy of the dominant frequency of fluctuations over time was discovered. For example, during a six-hour soliton motion along a long path, where the sea depth changed three times (from 350 to 120 m), and the soliton velocity – two times (from 2 to 1 m / s), the dominant frequency of fluctuations remained approximately constant and equal to 1.5 c / h with an accuracy of 10%. The paper analyzes the causes of this phenomenon. For this purpose, the soliton is considered within the framework of a two-layer model of the aquatic environment, and sound propagation – within the framework of the mode and ray theories. According to the ray theory, the dominant frequency of fluctuations is determined by the ratio of the soliton velocity to the ray cycle responsible for the dominant fluctuations. In the mode theory, a similar expression is obtained, where the role of the ray cycle is played by a combination of spatial beat periods of several pairs of modes. It is shown that with a change in the sea depth, the soliton velocity and the ray cycle change almost proportionally, as a result of which the dominant frequency of fluctuations remains constant. The described phenomenon may be universal and not limited to the ASIAEX water area. The constancy of the dominant frequency allows, in particular, to determine the variable soliton velocity as a function of time or distance, which is successfully demonstrated in the work and can be used for acoustic monitoring of solitons.

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V. А. Grigoriev

Voronezh State University; A.M. Prokhorov General Physics Institute of the Russian Academy of Sciences

Author for correspondence.
Email: grig4@yandex.ru
Russian Federation, 394018, Universitetskaya sq. 1, Voronezh; 119991, st. Vavilova 38, Moscow

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Supplementary files

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2. Fig. 1. The ASIAEX experiment area with bathymetry. Depths of 200, 500, 2000 m are shown. Solitons were born in the Luzon Strait and two days later, having passed through the deep part of the sea, were observed in the ASIAEX area.

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3. Fig. 2. Soliton with an amplitude of 150 m at a sea depth of 350 m, recorded on 05/09/2001 in the ASIAEX experiment using the E1 thermistor chain.

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4. Fig. 3. A typical satellite image of a soliton taken on 05.05.2001 during the passage of the ASIAEX acoustic paths. The paths are highlighted in red. The white line crossing the paths is the soliton front.

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5. Fig. 4. ASIAEX experiment (07.05.2001): (a) – top view, (b) and (c) – side view of path 1 (S1–VLA) and path 2 (S2–VLA), (d) – unperturbed profile of sound speed in water [19]. Blue line in Fig. 4a – soliton front at 12:20. Blue line in Fig. 4b and 4c – sound speed isolines (1525 and 1535 m/s) corresponding to maximum soliton amplitude (100 and 55 m) at 09:45 and 12:40 (horizontal and vertical scales are respected). Red dots – thermistor chains E1–E5. Blue dots – VLA receiving antenna, sources S1, S2. The location of all elements corresponds to real coordinates.

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6. Fig. 5. Recordings on thermistor chains E1–E4 (07.05.2001), converted to the speed of sound. Each panel vertically corresponds to the sea depth. The horizontal dotted lines show the boundaries of the chains. The values ​​above and below these boundaries are reconstructed from the unperturbed profile of the speed of sound (Fig. 4d). The vertical dotted lines show the times of soliton arrival at the chains.

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7. Fig. 6. Average experimental, theoretical and measured soliton velocities on paths 1 and 2.

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8. Fig. 7. The experimental spectrograms of intensity fluctuations on paths 1 and 2 are shown with a color scale. The patterns are normalized to the maximum on each vertical. The yellow stripe corresponds to the dominant fluctuations. The black lines are the theoretical dispersion curves of 1–3 and 1–6 orders for paths 1 and 2.

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9. Fig. 8. Verification of proportionality between the theoretical soliton velocity and the beam cycle on path 1. The beam cycle was calculated within the framework of mode theory based on dispersion curves of 1, 2 and 3 orders.

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10. Fig. 9. Histograms of the distribution of local maxima determined on each vertical in the spectrograms.

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11. Fig. 10. Red solid lines are the boundaries of the experimental confidence intervals of the dominant frequency. Blue solid lines are the boundaries of the theoretical confidence intervals of the dominant frequency obtained for a given number of modes. The intervals are maximally close at 7 on both traces.

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12. Fig. 11. Dependence of the modal amplitude modules on the distance on paths 1 and 2. The images are normalized to the maximum on each vertical.

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13. Fig. 12. (a) – Spatial periods of mode beats with numbers m and m+1 for path 1. (b) – Ray cycle determining the dominant fluctuations, obtained within the framework of mode and ray theories for path 1.

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14. Fig. 13. Analysis of rays on path 1. (a) – Angular intensity spectrum of proper rays connecting the source and receiver, depending on the exit angle from the source. (b) – Trajectory of the ray responsible for the dominant fluctuations (the grazing angle at the exit from the source is 0.18 rad).

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