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

• Published in: IEEE journal of oceanic engineering 1999-04, Vol.24 (2), p.138-155
• Main Authors: Colosi, J.A., Baggeroer, A.B., Birdsall, T.G., Clark, C., Cornuelle, B.D., Costa, D., Dushaw, B.D., Dzieciuch, M.A., Forbes, A.M.G., Howe, B.M., Menemenlis, D., Mercer, J.A., Metzger, K., Munk, W., Spindel, R.C., Worcester, P.F., Wunsch, C.
• Format: Article
• Language: English
• Subjects: Acoustic measurements; Acoustic propagation; Acoustic pulses; Acoustic waves; Acoustics; Computer simulation; Frequency; Internal waves; Marine; Mathematical models; Monte Carlo methods; Oceans; Pulse measurements; Random media; Tomography; Wave propagation
• ISSN: 0364-9059; 1558-1691
• DOI: 10.1109/48.757267

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 /spl Lambda/, /spl 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 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 /spl 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/sup 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 chi 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 w