Sound transmission through a fluctuating ocean (1979) and ocean acoustic tomography (1995): An intertwined history.

Abstract

Early attempts at understanding scintillations in sound transmission through the ocean assumed that ocean fine structure is homogeneous and isotropic. It is, in fact, dominated by internal waves, which are neither homogeneous nor isotropic. Sound Transmission Through a Fluctuating Ocean (Flatté et al., Cambridge University Press, Cambridge, England 1979) combined recently developed internal-wave models with path-integral methods to predict the fluctuations of resolved acoustic multipaths. At that time, Ocean Acoustic Tomography (Munk et al., Cambridge University Press, Cambridge, England 1995), which uses ray travel times to determine large-scale ocean structure, had just been proposed. The wideband acoustic sources and receivers required to make travel-time measurements for tomography also provided the technology needed to quantify acoustic fluctuations. Conversely, internal-wave-induced scattering limits travel-time measurement precision. Tomography experiments at 25-km range provided some of the earliest tests of path-integral predictions. Subsequent tomography experiments provided data at ever-increasing ranges and decreasing frequencies. The Acoustic Thermometry of Ocean Climate project made measurements at megameter ranges with 1400-m vertical line array receivers. The next step is a modular distributed VLA capable of spanning the full water column that is under development to enable separation of acoustic modes using spatial filtering and to fully characterize deep-water acoustic time fronts.