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

Measurements of basin-scale acoustic transmissions made during the last fou r years by the Acoustic Thermometry of Ocean Climate (ATOC) program have al lowed for the study of acoustic fluctuations of low-frequency pulse propaga tion 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 Lambda, Phi methods (using the Garrett- Munk (GM) internal wave model) to identify the wave propagation regime for early identifiable wavefronts predict the saturated regime, whereas observa tions of intensity probability density functions, intensity variance, and p ulse time spread and wander suggest that the propagation is more likely nea r the border between the unsaturated and partially saturated regimes. Calcu lations of the diffraction parameter Lambda 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 propa gation 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. Thi s raises the possibility of using modal techniques for ocean acoustic tomog raphy, 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-b ased tomography will be robust at basin-scales. This opens up the possibili ty of ray-based internal wave tomography using the observables of travel ti me variance, and vertical and temporal coherence, Using geometrical optics and the GM internal wave spectrum, internal wave tomography for an assortme nt of parameters of the GM model can be formulated in terms of a mixed line ar/nonlinear inverse, This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wa ve energies as a function of depth for the SLICE89 experiment. However, thi s Monte Carlo approach demonstrated, for the SLICE89 experiment, that the G M model failed to render a consistent inverse for acoustic energy which sam pled the upper 100 m of the ocean, Until a new theory for the forward probl em is advanced, internal wave tomography utilizing the signal from strong m ode coupling can only be carried out using time-consuming Monte Carlo metho ds.