Random focusing of sound into spatially coherent regions

Oceanographic variability creates a weak random propagation medium for acou stic waves. The impact on acoustic transmission is becoming increasingly ap preciated as the deterministic modelling of sound propagation in the ocean has become tractable and better understood. Beyond the near field (where ph ase fluctuations are weak) and the far field (where the scintillation index becomes saturated) multiple-scattering theory predicts that random focusin g will greatly enhance the acoustic energy density over small volumetric re gions, which we call 'ribbons'. In 1985 an experiment was carried out in th e 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 experimen t to attempt to detect the two-dimensional structure of the predicted focus ed ribbons for individual energy paths. Renewed impetus to publish the resu lts has been provided by the recent focus on moderate- to high-frequency ac oustics in near-shore and shallow-water environments. The experiment is des cribed and high-intensity regions consistent with the theoretical predictio ns 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 aper tures of 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 t o 1 km. The understanding of these features allows system limitations to be estimated quantitatively and opens up the way to their constructive tactic al use. The results are applicable to many systems from towed array sonars to high-frequency bathymetric sidescan and minehunting.