Abstract

Oceanographic variability creates a weak random propagation medium for acoustic waves. The impact on acoustic transmission is becoming increasingly appreciated as the deterministic modelling of sound propagation in the ocean has become tractable and better understood. Beyond the near-field (where phase fluctuations are yet weak) and the far field (where the scintillation index becomes saturated) multiple scattering theory predicts random focusing will greatly enhance the acoustic energy density over small volumetric regions, which we call 'ribbons'. In 1986 an experiment was carried out in the eastern Mediterranean to test this prediction using acoustic propagation along distinct, resolvable ray paths. This experiment is one of the few to map the spatial structure of acoustic intensity with such a large vertical aperture, and as far as the authors are aware it remains the only experiment to attempt to detect the 2D structure of the predicted focussed ribbons for individual energy paths. Renewed impetus to publish the results has been provided by the recent focus on moderate to high-frequency acoustics in near-shore and shallow-water environments. The experiment is described and high-intensity regions consistent with the theoretical predictions are reported. A 3.5 kHz pulsed signal was transmitted over ranges of 11-23 km and sampled over a vertical aperture of 250-350 m and horizontal apertures of 4-4.5 km. The acoustic signals travelling along individual ray paths developed randomly focused regions of 6-18 dB over regions with a vertical dimension of about 20 m and whose horizontal length could possibly be up to 1 km. The understanding of these features allows systems limitations to be quantitatively estimated and opens up the way to their constructive tactical use. The results are applicable to many systems from towed array sonars to high frequency bathymetric sidescan and minehunting.