On the stability of long-range sound propagation through a structured ocean

Several acoustic experiments show a surprising degree of stability in wave fronts propagating over multi-megameter ranges through the ocean's sound channel despite the presence of random-like, sound speed fluctuations. Previous works have pointed out the existence of chaos in simplified ray models incorporating structure inspired by the true ocean environment. A ``predictability horizon'' has been introduced beyond which stable wave fronts cease to exist and point-wise, detailed comparisons between even the most sophisticated models and experiment may be limited for fundamental reasons. We find, by applying one of the simplified models, that for finite ranges, the fluctuations of the ray stabilities are very broad and consistent with lognormal densities. A fraction of the rays retain a much more stable character than the typical ray. This may be one of several possible mechanisms leading to greater than anticipated sound field stability. The lognormal ray stability density may underlie the recent, experimentally determined, lognormal density of wave field intensities [J. Acoust. Soc. Am. 105 (1999) 3202--18].