The Ocean Acoustics Program is divided into three major focus areas or "thrusts", representing three scientifically interesting, limiting regimes: the high-frequency acoustics, shallow-water acoustics, and long-range propagation thrusts consist of investigations on the interaction of sound with the ocean environment in the limit of high-frequencies, multiple boundary interactions, and long transmission ranges, respectively. In high-frequency acoustics the emphasis is acoustic frequencies from a few kHz to 1000 kHz, and therefore the focus is on acoustic interaction with ocean fine structure (e.g., turbulence, zooplankton, bubbles, and small-scale seafloor roughness). In shallow-water acoustics the emphasis is multiple acoustic interactions with the waveguide boundaries (i,e., air-sea interface and seafloor), and therefore the typical scenario is sound transmission of several Hz to a few kHz at ranges greater than a few kilometers within littoral (shallow) waters. In long-range propagation the focus is on the scattering and propagation of sound over transmitted distances of several hundred to several thousand kilometers .
Long-Range Propagation
The thrust in long-range propagation is unique in that it primarily involves the deep-water ocean environment. Because the ocean boundaries (e.g., air-sea interface and seafloor) are much less important in deep than in shallow water, and because there exist reasonable models for deep-water ocean processes at the acoustically-relevant space and time scales, there is a real potential for developing a solid scientific understanding of the relation between the ocean and the acoustic fluctuations in this thrust. Additionally, since this thrust deals with sound propagation across ocean basins, there is a unique opportunity for simultaneous multi-national participation.
It is a myth that ocean acoustics in deep water is uninteresting and well understood, especially in the limit of long-range propagation. Listed below are several topics in long-range propagation having fundamental scientific issues that have been identified as unresolved, important, and interesting.
･ scattering/fluctuations due to internal waves & mesoscale structure
- frequency dependence
- range dependence
- geographic dependence
- 3-dimensionality (azimuthal scattering & coherence)
･ predictability/sensitivity of the acoustic wavefield (e. g., wave chaos)
･ normal modes vs rays (e. g., connection between mode coupling & ray chaos)
･ character of ambient noise (generation & propagation)
- directional spectra (granularity) and temporal behavior
･ inverse methods (tomography, thermometry & localization)
- full-field inversion
- stochastic inversion
- inversion limitations (e. g., imposed by internal waves)
