Broad-band acoustic propagation through moving internal solitary wave packets in shallow water

Propagation of acoustic signals in coastal seas are strongly influenced by internal solitary wave (ISW) packets which are often generated by local tides during seasons of strong density stratification. During the 1995 Shallow Water Acoustics in a Random Medium (SWARM95) experiment, amplitude fluctuations and correlation times of broad-band (216-232 and 350-450 Hz) acoustic signals were measured in such an environment. The authors numerically simulate broad-band acoustic signal propagation to explain the experimentally-observed fluctuations and correlation times at 42 kilometer propagation range. Broad-band signals are calculated by using Fourier synthesis of continuous wave (CW) numerical solutions of the parabolic wave equation (PE) for the entire SWARM95 propagation path. In the case in which the strong range dependency can be localized within an ISW packet, a computationally efficient one-way coupled mode model is formulated to calculate the acoustic field at a distant receiver array. Internal solitary wave packets are modeled as mode coupling structures moving along the acoustic propagation path. Frequency-dependent coupling matrices are calculated by using PE numerical solutions for each acoustic mode propagating through the soliton packet. A similar approach has been used recently for the simulations of CW signal fluctuations, T. F. Duda et al. (1999). In the present paper, strong fluctuations and rapid decorrelation of broad-band signals are attributed to the interaction of moving soliton packet with the interference patterns of the dominant acoustic modes. The model predictions are consistent with the ~15 dB signal fluctuations and ~2 min decorrelation times observed in the SWARM experiment in addition to the prediction of a partial recorrelation at longer time scales (>20 min)