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ACOUSTIC DETECTION OF MICROBUBBLES AND PARTICULATE MATTER NEAR THE SEA SURFACE
NAVAL POSTGRADUATE SCHOOL MONTEREY CA
A pulse-echo system using an electrostatic transducer and steel reflector was operated over the frequency range 24-192 Kcps in the sea and in the laboratory. Excell attenuations from 0.15 to 1.1 dbM due to absorption and scattering by inhomogeneities were made over the full frequency range with a maximum error of less than 0.05 dbmeter. Backscattering at sea was detected in the frequency range 24-60 Kcps with an average scattering cross-section per unit volume of 400 sq cm cu m at a depth of 10 feet. Scattering as not detected above 60 Kcps. Variation of sound velocity with frequency was measured from 24- 192 Kcps. The accuracy of these values was 0.03 for frequencies over 40 Kcps. Results are compared and it is postulated that 1 below 60 Kcps the absorption and scattering was due to microbubbles of radius greater than 50 microns, and 2 above 60 Kcps minute plankton caused absorption without significant scattering.
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