Scintillation of short duration records

We have investigated propagation of sound in the ocean using the theory of wave propagation in random media. The theory classifies specific source/receiver geometries as either unsaturated, partially saturated, or saturated. In the unsaturated regime the variance of the intensity fluctuations will generally be quite small such that the scintillation index is less than unity. The partially saturated regime can give rise to scintillation indices larger than unity. In the fully saturated regime the scintillation index asymptotically approaches unity. We have conducted an experiment in which the two-dimensional sound-speed field was measured directly using a towed CTD chain, and relatively short acoustic records were captured using a number of source/receiver pairs. The length scales and index of refraction variation place the experiment in the saturated regime. However, measured scintillation indices are much less than 1. We provide an explanation for this difference and utilize local current measurements to bring the theory into agreement with the data. Our results are applicable to operational sonar systems, where short duration records are ordinarily utilized to transmit information or detect targets. [Work sponsored by ONR Code 321US.]