INTERNAL-WAVE EFFECTS ON 1000-KM OCEANIC ACOUSTIC PULSE-PROPAGATION - SIMULATION AND COMPARISON WITH EXPERIMENT

A recent 1000-km acoustic pulse transmission experiment in the Pacific revealed unexpected fluctuations On received wavefronts, including a dominant rapid variation, called the broadband fluctuation, with time scales less than 10 minutes and spatial scales of less than 60 m; a distinct breakdown of the geometrical optics wavefront pattern and broadening of the wavefront near the transmission finale; and a coherent wavefront motion with a timescale near the semi-diurnal tidal period. Parabolic-equation numerical simulations have been carried out which utilize environmental data and which take into account internal-wave-induced sound-speed perturbations obeying the Garrett-Munk (GM) spectral model. It is shown that the effects of internal waves can account for the broadband fluctuations, the breakdown of the geometrical optics pattern, and the wavefront broadening. The sensitivity of these fluctuations to internal-wave energy and modal content is examined. The spectral energy in the GM model at tidal periods proves insufficient to explain the tidal period coherent fluctuations, strongly suggesting the influence of an internal tide during the experiment. The simulations allow the estimation of the average travel-time bias caused by internal waves. The simulation results for travel-time wander and bias are compared with analytic calculations based bn the path-integral technique.