Wavefront folding, chaos, and diffraction for sound propagation through ocean internal waves

Abstract

This work presents an analysis of the effects of ocean internal waves on long-range acoustic pulse propagation from the geometrical-optics point of view. The chaotic behavior of rays and the microfolding of timefronts are investigated. The extent of the region of the timefront in which strongly chaotic rays appear, and the strength of the rays’ sensitivity to initial conditions, are found to depend on the average (range-independent) sound-speed profile, on the range from the source to the receiver, and on the internal-wave spectral model, but not on the specific realization of the internal waves. For a particular experiment (SLICE89), it is concluded that the observed depth diffusion of energy in the late-arriving portion of the timefront is a result of refraction (of geometrical-optics rays), not diffraction. It is found that internal-wave effects cause an upper turning point of a ray to be spread to the extent of 10 km horizontally and 100 m vertically, which affects the resolution of ocean-acoustic tomography. The validity and usefulness of ray-based, semi-classical (WKB) waveforms to represent received arrivals are evaluated by comparing with waveforms generated with multifrequency, parabolic-equation simulations. Center frequencies of 250 Hz and 1000 Hz with 100-Hz bandwidth are used. The semi-classical waveforms reproduce the correct arrival time and temporal shape of almost all arrivals, even those that are made of dozens of microrays induced by the internal waves. The overall intensities of the 1000-Hz arrivals are reasonably accurate, while the 250-Hz intensities show differences of order 5 dB.