Non-Rayleigh Echoes From Resolved Individuals and Patches of Resonant Fish at 2–4 kHz
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In this paper, the statistics of the echo envelope measured from fish is studied over a range of scattering geometries in the frequency band 2-4 kHz. In this frequency band, not only are these particular swimbladder-bearing fish resonant (at about 3.7 kHz) resulting in strong echoes, but also their target strength is narrowly distributed at these frequencies. In addition, their size distribution is narrow. Thus, it can be assumed that the echo variability as measured through the receiver of the sonar system is principally due to beampattern effects of the sonar, spatial variability of the fish, and interference between overlapping echoes. The measurements were conducted over Georges Bank near Cape Cod, MA, using a broadband downward-looking echosounder in a short-range, direct-path geometry. Echoes from sequential pings were grouped according to whether they 1) involved resolved echoes from individual fish, 2) were entirely within a single patch of fish, 3) spanned the edge of a single patch of fish, or 4) spanned multiple patches of fish. The resultant distributions of echoes were compared with theoretical probability density functions (pdfs). The echoes are determined to be strongly non-Rayleigh, with beampattern effects a major factor, for all geometries except for the case when all echoes are from within a single patch. Recommendations are made for the appropriate echo pdf for these scattering geometries.