Abstract:

Broadband acoustic propagation data have been obtained at ranges up to 5 Mm and for frequencies as low as 28 Hz. The arrival patterns measured on the sound channel axis generally have well resolved steep ray arrivals, unstable or chaotic shallow angle arrivals, and a sharp SOFAR cutoff. The wander and spread of the steep ray arrivals increase with range roughly as predicted. The effect of internal waves on the propagation are evident: In most cases the shallow-angle acoustical energy has been severely scattered by internal waves, rendering the use of these data problematical. In addition, the internal waves cause a diffusion of energy away from the sound channel axis. At 28 Hz, more stability in the arrival patterns is evident, including the shallow-angle portion. The internal waves scatter the energy associated with the earlier, resolvable ray vertically and temporally, but with less effect than on the latest acoustic arrivals. Acoustic propagation at lower frequency apparently reduces the detrimental effect of internal waves on the coherence of the signals. Acoustical receptions at very deep receivers have ``ray-like'' arrivals in the shadow zone of the predicted time front. These ``ray-like'' arrivals are associated with the cusps or caustics of the time front, but they cannot be explained by the full wave equation or by internal wave scattering.