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CLOUDS of suspended particulate matter in the ocean have been observed by satellite1, sensed by nephelometers2, and sampled by water bottles. We present here evidence that it may be possible to observe oceanic suspended particulate matter acoustically. The particulate matter discussed is thought to have arisen from a dredging operation. It is also thought that the suspended matter is present at lower concentrations than any in any other case recorded acoustically3. Certain questions, as yet unresolved, regarding acoustic scattering strengths are also discussed.

Spires or humps of particulate material extending from the bottom of the oceanic mixed layer to near the ocean's surface have been observed in the Florida Current and in the regions of other current systems, using an ultrasensitive, 20-kHz acoustical echo sounding system. These spires may be the result of oceanic circulation rather than a purely biological phenomenon.

PULSED high frequency acoustics has been shown to have unique applications in the detection and measurement of oceanic phenomena such as internal waves1–3 and dispersion of ocean-dumped sewage sludge4,5 or dredge material6. With moderate to high pulse repetition rates, a virtually continuous record of water column scattering strength is available to depths of the order of 500 m from a ship travelling at speeds of 4 or 5 knots. We used this feature to detect acoustically the interfaces associated with an intrusion of cold New England shelf water into the warmer slope water during a 9-d period in June 1976.