A mode space energy detector for acoustic source depth discrimination

Abstract

A likelihood ratio test based on a newly defined statistic, the modal scintillation index (MSI), is presented for acoustic source depth discrimination in a shallow-water waveguide. The MSI is defined as the variance in the estimated modal excitation normalized by its expected value over some observation interval. The approach represents, in effect, a mode space energy detector. In a shallow-water waveguide, the acoustic modes are nearly sinusoidal, and thus all share a zero-crossing and maximum derivative at z=0. Consequently, surface source modal excitation levels will be very sensitive to low bandwidth source depth modulation due to surface wave interaction. The hypothesis herein is that a surface source will be characterized by a high scintillation index across all modes, while a source at depth will exhibit a low scintillation index for at least one acoustic mode. The test statistic is self-normalizing, so knowledge of source level and source range is not required to separate the two source classes. Estimation of the modal excitation statistics requires knowledge of the water depth and the sound-speed profile at the array. Stable estimates of the MSI require observation intervals on the order of a few minutes. Performance predictions based on KRAKEN simulations will be presented and both vertical and horizontal array configurations will be considered. It is believed that the proposed technique may have potential as a classifier or clutter management tool, particularly in a littoral environment. [Work sponsored in part by SPAWAR, under Air Force Contract F19628-95-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the U.S. Air Force.]