A review of recent results on ocean acoustic wave propagation in random media: basin scales

Abstract

Abstract-Measurements of basin-scale acoustic transmissions made during the last four years by the Acoustic Thermometry of Ocean Climate (ATOC) program have allowed for the study of acoustic fluctuations of low-frequency pulse propagation at ranges of 1000 to 5000 km. Analysis of data from the ATOC Acoustic Engineering Test conducted in November 1994 has revealed new and unexpected results for the physics of ocean acoustic wave propagation in random media. In particular, use of traditional 3; 8 methods (using the Garrett-Munk (GM) internal wave model) to identify the wave propagation regime for early identifiable wavefronts predict the saturated regime, whereas observations of intensity probability density functions, intensity variance, and pulse time spread and wander suggest that the propagation is more likely near the border between the unsaturated and partially saturated regimes. Calculations of the diffraction parameter 3 are very sensitive to the broad-band nature of the transmitted pulse, with CW calculations differing from a simplistic broad-band calculation by 10 3 ! A simple model of pulse propagation using the Born approximation shows that CW and broad-band cases are sensitive to a random medium very differently and a theoretical description of broad-band effects for pulse propagation through a random media remains a fundamental unsolved problem in ocean acoustics.The observations show that, at 75-Hz center frequency, acoustic normal mode propagation is strongly nonadiabatic due to random media effects caused by internal waves. Simulations at a lower frequency of 28 Hz suggest that the first few modes might be treated adiabatically even in a random ocean. This raises the possibility of using modal techniques for ocean acoustic tomography, thereby increasing the vertical resolution of thermometry.Finally, the observation of unsaturated or partially saturated propagation for 75-Hz broad-band transmissions, like those of ATOC, suggests that ray-based tomography will be robust at basin-scales. This opens up the possibility of ray-based internal wave tomography using the observables of travel time variance, and vertical and temporal coherence. Using geometrical optics and the GM internal wave spectrum, internal wave tomography for an assortment of parameters of the GM model can be formulated in terms of a mixed linear/nonlinear inverse. This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wave energies as a function of depth for the SLICE89 experiment. However, this Monte Carlo approach demonstrated, for the SLICE89 experiment, that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean. Until a new theory for the forward problem is advanced, internal wave tomography utilizing the signal from strong mode coupling can only be carried out using time-consuming Monte Carlo methods.

Effects of internal waves on low frequency, long range, acoustic propagation in the deep ocean

“…Second, for both regimes it has been shown that the edges of the shadow zones of the wavefront are significantly extended in depth, and in time [19,20][26]This extension of the shadow zone shows that the effect of scattering in long range low frequency ocean acoustic propagation is to introduce a significant bias into the wavefront intensity pattern; that is to say, the acoustic fluctuations cannot be considered a zero mean effect superimposed upon an otherwise deterministic wavefront pattern. Finally, in spite of the large fluctuations in the wavefront finale the time stability of the phase is surprisingly large and close to that of the wavefront region [22,79,30].…”

“…Second, there is the assumption that the ray path has very little curvature, and thus the straight ray result is applied locally; and assumption that is clearly violated exactly at the upper turning point, where the slope is zero and the curvature is maximum. [44] and [20] have shown that this approximation places too much of the scattering strength near the ray upperturning point. Thirdly, the MZ theory is inherently narrowband, and thus issues of signal bandwidth in the observations needs to be addressed.…”

“…Previous work on single UTP propagation has been entirely at frequencies of 1000-Hz or more (MATE Experiment [35], AFAR Experiment [40]), and is not directly related to the low frequency cases [20].…”

Capacity Analysis of Ocean Channels

“…The power in energy per time units P trans is given by multiplying P tot by the sample rate W . In the same manner, the mutual information is given in 'nats per sample' -for a 'bits per sec' measure, the mutual information must be multiplied by the sample rate W and divided by ln (2). The existence of receiver CSI allows us to consider the fading state as an additional channel output.…”

“…The UW medium has been characterized by communication aspects of the fading intensity, and spatial/temporal/spectral coherence properties [1], [2], [3]. Previous attempts to characterize the ocean channel capacity have, to the best knowledge of the authors, used oversimplified models of the acoustic propagation disregarding frequency selectiveness characteristics of the channel.…”