On acoustic tomography of internal waves

With the planned acoustic instrumentation of several of the worlds ocean basins, the possibility exists for measuring the strength of the internal wave field as a function of depth, time, and geographical position. The variability of internal-wave energies and spectra over these parameters will provide information on the sources and sinks of internal waves. In the early eighties Flatte formulated an analytic, ray-based framework for internal-wave tomography. Analysis of the SLICE89 experiment combined with supercomputer numerical simulations have recently shown that internal waves have even stronger effects on signals transmitted over 1000 km than were expected from the early ray-based theory plus the Garrett–Munk (GM) spectrum. It was found that the ray-based theory is adequate for identifiable wavefronts that turn at depths below 100 m; energy that reaches very shallow depths and energy that stays near the sound-channel axis do not behave so simply. The numerical simulations establish an internal-wave strength below 100 m of 0.5±0.25 the reference GM level, and point to known limitations of the GM spectrum in the upper ocean. Simulations to 3000 km have quantified the range dependence of internal-wave effects, and extensions of ray-based theory attempt to explain these effects.