Tk. Stanton et al., ACOUSTIC CHARACTERIZATION AND DISCRIMINATION OF MARINE ZOOPLANKTON AND TURBULENCE, ICES journal of marine science, 51(4), 1994, pp. 469-479
The use of high-frequency acoustic scattering techniques has become ce
ntral to the study of biological and physical oceanography, yet quanti
tative discrimination between sound scattered by zooplankton and turbu
lence has long eluded researchers. (Turbulence in the presence of a te
mperature gradient gives rise to variations in sound speed which in tu
rn 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 ambi
guities in acoustic remote sensing of the two type of scatterers, labo
ratory measurements were made of the acoustic scattering properties of
a small patch of turbulence and a 2-cm-long decapod shrimp over a fre
quency 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 sin
gle echo of the laboratory turbulence and its time evolution was quite
irregular and different than the more stable spectra of the echoes fr
om the individual animals near broadside incidence which contained con
sistent 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 re
duced variability), while that of the animals retained some regularity
. These results indicate the potential for discriminating between turb
ulent fields and single animals.