Statistical Behavior of Echoes from Swimbladder-Bearing Fish at 2-4 kHz

Measurements of broadband acoustic backscattering by swimbladder-bearing fish have recently been made over Georges Bank, a shallow-water region off of Cape Cod, Massachusetts. A seismic source, towed by a ship just above the schools of fish, was used to transmit chirp (linear frequency modulated) signals downward toward the fish. The system, which is most sensitive over the range 2-4 kHz, swept through the range of frequencies, 1-10 kHz. The echoes were especially strong at frequencies near the swimbladder resonance of 3 kHz. The measurements were done in conjunction with net tows in order to ground truth the echo data. The echoes were processed with a matched filter, achieving high signal-to-noise ratios and range resolutions of approximately 20 cm. As a result of this resolution, fish very near the seafloor were resolved. The filtered echoes were partitioned into various spatial windows and analyzed both in the time domain and frequency domain. Of particular interest was the statistical behavior of the echoes. The schools were highly patchy and the statistics of the echoes varied according to whether a set of echoes were from within a patch, or spanned across the patches. The echoes are shown to be strongly non-Rayleigh when the sonar is sweeping across the patches. Net tows and swimbladder-resonance data indicate that all targets are mono-size and mono-specific. Thus, the fluctuations in the echoes are a result of a combination of interference between random phase echoes from individual targets, local variability in numerical density of targets, and effect of beam pattern associated with individual targets or patches of targets smaller than a sonar footprint