Measured three-dimensional effects of mode-1 and mode-2 nonlinear internal waves on broadband acoustic wave propagation in shallow water

Horizontal shadowing effects and frequency shifts of acoustic intensity level curves were measured with a bottomed horizontal array in the East China Sea during the summer of 2008. Low-frequency acoustic pulses were transmitted by two fixed sources at 33 km (270–330 Hz LFM) and 20km (450–550 Hz LFM) range. Strong shadowing effects were observed when mode-1 nonlinear internal wave fronts were nearly parallel to the acoustic propagation path. Numerical studies indicated that shadowing effects are more complex for mode-2 nonlinear internal waves due to acoustic-mode dependent focusing and defocusing. These effects were further analyzed using 3-D PE simulations for more dynamic mode-2 nonlinear internal waves observed on the US New Jersey Shelf. The shadowing was less pronounced for internal waves with curved wave-fronts and small amplitudes. However, regular and irregular frequency shifts were still present. The experimental observations coupled with 3-D PE simulations suggest that nonlinear internal waves may be sensed and characterized via low-frequency acoustic signals. [Work supported by the Office of Naval Research.]