ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

5aUW10. Acoustic scintillation and random focusing effects in the ocean.

John R. Potter

Marine Phys. Lab., Scripps Inst. of Oceanogr., La Jolla, CA 92093-0238

Small-scale oceanographic variability becomes increasingly relevant to acoustic propagation as the frequency of interest exceeds a few kHz. The intensity effects are generally unspectacular at very close or distant ranges; little scattering has yet occurred in the near field and stochastic limits apply in the far field. At intermediate ranges random focusing can greatly enhance the intensity in small volumetric regions. These predicted high-intensity ``ribbons'' result from an accrual of multiple forward scattering of individual wavefronts and appear in addition to multipath effects. An experiment was performed in the eastern Mediterranean during 1986 using 3.6-kHz pulses transmitted over ranges of 11--23 km to investigate this effect. Observations were made over vertical and horizontal apertures of 200--250 m and 4--4.5 km, respectively. The experiment is described and the random focusing is confirmed. The acoustic signals traveling along individual ray paths are observed to develop randomly focussed regions 6--18 dB above normal levels over regions some 1 km long and 20 m deep. The strength and size of these anomalies are consistent with predictions from multiple scattering theory. Understanding these features creates the possibility of inversion for a statistical description of the small-scale oceanography in addition to permitting constructive tactical use and improved quantitative estimates of system performance. [Work supported by ONR.]