HIGH-FREQUENCY ACOUSTIC VOLUME BACKSCATTERING IN THE GEORGES BANK COASTAL REGION AND ITS INTERPRETATION USING SCATTERING MODELS

High-frequency (120 and 420 kHz) sound was used to survey sound scatte rers in the water over Georges Bank, In addition to the biological sou nd scatterers (the plankton and micronekton), scattering associated wi th internal waves and suspended sediment was observed, Volume backscat tering was more homogeneous in the vertical dimension (with occasional patches) in the shallow central portion of the Bank where there is si gnificant mixing. In the deeper outer portion of the Bank where the wa ter is stratified, volume backscattering was layered and internal wave s modulated the vertical position of the layers in the pycnocline, The internal saves typically had amplitudes of 5-20 m, but sometimes much higher, Species composition and size data from samples of the animals and suspended sediment used in conjunction with acoustic scattering m odels revealed that throughout the region the animals generally domina te the scattering, but there are times and places where sand particles (suspended as high as up to the sea surface) can dominate. The source of the scattering in the internal waves is probably due to a combinat ion of both animals and sound-speed microstructure. Determination of t heir relative contributions requires further study.