Reverberation from plankton at 330 kHz in the Western Pacific

Acoustic volume reverberation was observed in the open ocean at a frequency of 330 kHz and a depth of approximately 20 m. The instantaneous scattering volumes were small, ranging in size from 0.2 to 0.9 m3. The reverberation was observed primarily as an incoherent return from multiple scatterers (probably greater than 10/m3) but there were also discrete echoes of duration equal to that of the transmitted tone burst, 1 msec. The backscattering strength of the water calculated from the incoherent reverberation was independent of the time of day and varied from about −86 dB relative to 1 m−1 down to −96 dB and is surmised to be due to phytoplankton. The discrete echoes were more numerous at night than during the day and were produced by scatterers most of whose target strengths were on the order of −94 to −86 dB, approximately what would be expected of the small zooplankters such as copepods which were present. Subject Classification: 30.40, 30.30.