Abstract

Sound provides a natural means for exploring the ocean but current sonar systems, as used for example in swath-mapping applications, do not provide directly pictorial images of the ocean depths. Such systems are more akin to radar, which relies on travel-time information to map the environment. A new acoustic technique for providing real-time visual images of the interior of the ocean is currently under development, and the results from initial experiments at sea provide evidence in support of the concept. The imaging process relies on ambient noise, or "acoustic daylight", as the source of illumination, the underlying idea being analogous to photography in the atmosphere with daylight illuminating the subject. An object in the noise field scatters the incident sound, and the scattered field is focussed with an acoustic lens to form an image on an array of transducers. After signal processing, the acoustic image is displayed as a pictorial image on a television monitor. Acoustic "colour", characterizing the spectral reflectivity of the object, could be represented as artificially generated optical colour in the display, and a rapid refresh rate should yield moving images much like those from a conventional video camera.