Sound Intensity Fluctuations Caused by the Motion of Internal Wave Solitons in the ASIAEX Experiment

One of the episodes of the ASIAEX 2001 experiment (in the South China Sea) is considered, in which a large internal wave soliton 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 6-h 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 at 1.5 cph (cycles per hour) with an accuracy of 10%. The paper analyzes the causes of this phenomenon. For this, the soliton is considered within the framework of a two-layer model of the aquatic environment, and sound propagation, within the framework of mode and ray theories. According to 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 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 one, 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.