ACOUSTIC CHARACTERIZATION AND DISCRIMINATION OF MARINE ZOOPLANKTON AND TURBULENCE

The use of high-frequency acoustic scattering techniques has become central to the study of biological and physical oceanography, yet quantitative discrimination between sound scattered by zooplankton and turbulence has long eluded researchers. (Turbulence in the presence of a temperature gradient gives rise to variations in sound speed which in turn scatter sound.) Our calculations indicate that the target strength of a 2-cm-long single shrimp can be comparable to the scattering level from 1 m(3) of turbulence in the ocean. Because of the potential ambiguities in acoustic remote sensing of the two type of scatterers, laboratory measurements were made of the acoustic scattering properties of a small patch of turbulence and a 2-cm-long decapod shrimp over a frequency range of 300-800 kHz. The data were also used to emulate echoes that one might expect in the ocean environment. The spectrum of a single echo of the laboratory turbulence and its time evolution was quite irregular and different than the more stable spectra of the echoes from the individual animals near broadside incidence which contained consistent major peaks and nulls. There were also noticeable differences between incoherent averages of the echoes as the averaged spectrum of the turbulence echo was observed to remain irregular (although with reduced variability), while that of the animals retained some regularity. These results indicate the potential for discriminating between turbulent fields and single animals.