Echo Statistics of Aggregations of Scatterers in a Random Waveguide Application to Biologic Sonar Clutter
Jones, Benjamin A.
Colosi, John A.
Stanton, Timothy K.
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Reverberation, echoes from the water column boundaries and non-target scatterers within the water column, often limits active sonar system performance. Fish can constitute a significant portion of sonar reverberation under certain conditions, such as a horizontally looking sonar system operating at mid-frequencies (19 kilohertz), where scattering by fish with resonant gas-filled swimbladders can dominate over bottom or surface reverberation. This study examines the contributions of various elements of an active sonar system ensonifying aggregations of water column scatterers to determine relative contributions of various physical factors to producing target-like echoes (i.e., clutter). Elements of the system that are considered include 1) a shallow-water waveguide, 2) a stochastic, range-dependent sound-speed profile, 3) a directional acoustic source, 4) a variable scattering response, and 5) an extended scattering volume. Numerical modeling predictions are compared with analytical models and observations. Modeling is validated with long-range, broadband (1.5 to 5 kilohertz) sonar measurements of aggregations of fish in the Gulf of Maine. Key results include the classification of a mixed assemblage of swimbladder-bearing fish, with target-sized aggregations, that produce a significant number of echoes that are at least 15 decibels above background levels. These sparsely distributed scatterers produce highly non-Rayleigh distributions of echo magnitudes that are well-modeled by a computationally efficient, physics-based model.
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