Time-frequency analysis of acoustic modal intensity fluctuations induced by internal solitary wave packets

Modal intensity fluctuations caused by a moving nonlinear internal wave packet in a shallow water waveguide are investigated in this paper. An expression of the modal intensity is derived based on the coupled mode theory in a simple waveguide model. Theoretical calculations indicate that the time-varying modal interferences induced by mode coupling effects and mode stripping (different attenuation rates between different modes) give rise to the modal intensity fluctuations. The temporal variation of the oscillatory amplitude of an individual modal interference component is found to be dependent on the relationship between the attenuation rate of the observed mode arriving at receivers and that of the incident modes entering the internal solitary wave packet. The modal intensity fluctuations, characterized by the modal interferences with different frequency components and time-varying oscillatory amplitudes, are expected to have visible patterns in the time-frequency plane. The reassigned spectrogram with high time-frequency concentration is proved to be a desirable representation of the features of modal intensity fluctuations.