Sound field fluctuations in shallow water due to nonlinear internal waves and determination of their parameters

Fluctuations of the sound intensity are studied in the presence of moving nonlinear internal waves (NIW). Prior work revealed the existence of peaks in the spectrum of the sound field intensity due to mode coupling. As an example, results of experiment ASIAEX 2001 are considered. Episodes are analyzed when soliton-like NIW moves approximately along acoustic track ∼30 km during ∼6 h. The depth of the ocean changes from ∼350 m near the source up to ∼120 m near the receiver (Vertical Line Array). The source, placed near the bottom, transmitted pulses (M-sequences) with the frequency 224 Hz. Numerical modelling shows that peak frequencies correspond to the most strongly interacting pairs of modes: in given case pairs 2-3, 3-4. Due to narrowing the channel velocity v of soliton should decrease; note that change of the predominating frequency as a function of range (or time) was observed while soliton was moving. Using speed v as the fitting parameter between experiment and modelling it is possible to estimate v of real soliton and its variation, that gives values of v from 1.8 to 0.8 m/s, this is in good agreement with experimental data.