Acoustic daylight: Imaging the ocean with ambient noise.

Abstract

For many years the principal means of probing the ocean using sound has been through the use of either ‘‘active’’ or ‘‘passive’’ techniques. With an active system, an object is illuminated by a pulse of sound and its presence inferred from the echo it produces, whereas the passive approach involves simply listening for the sound that the object itself emits. Here a new method of employing sound in the ocean is reported, which is neither passive nor active. It relies on the naturally occurring, incoherent ambient noise field in the ocean as the sole source of acoustic illumination. By analogy with daylight in the atmosphere, ambient noise scattered from an object in the ocean can be focused onto an image plane, and from this acoustic image, with appropriate signal processing, a visual image can be produced on a television-type monitor of the object space in the ocean. To test this concept, several simple experiments have been conducted in the ocean off Scripps pier, with a single parabolic reflector acting as an acoustic lens, which confirm that objects illuminated only by ambient noise can indeed be ‘‘seen’’ over a frequency band between 5 and 50 kHz. [Work supported by ONR.]