| Popis: |
Broadband midfrequency acoustic measurements of fish backscattering were made with two complementary sonar systems in September 2008 at the northern slopes of Georges Bank in the Gulf of Maine. One system, towed near the surface, was downward‐looking (short‐range: < 0.2 km) and was used to image fish at high vertical resolution throughout the water column. The other system was horizontal‐looking (relatively long‐range: 1–10 km), consisting of a vertical source array and a horizontal receive array, and was used to image fish at high horizontal resolution. In this talk, the focus is on characterizing the longer‐range data. Spectrogram analysis showed that the echoes had resonance frequencies in the 2–4 kHz band, consistent with scattering by Atlantic herring, while examination of normalized match‐filter output revealed strong, spatiotemporally variable clutter. The associated probability‐density functions (PDFs) exhibited variable non‐Rayleigh behavior. Several PDF models were fit to the data, with the K and Poisson‐Rayleigh PDF models providing not only good fits, but physical insights into the clutter process (such as estimates of the expected number of discrete scatterers per unit area). These results provide an improved understanding of both fish behavior and the characteristics of fish aggregations as long‐range clutter fields. [Work supported by ONR.] |
