IMAGING THE OCEAN WITH AMBIENT NOISE

For many years, the principal means of probing the ocean using sound has been through the use of '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 we report a new method of using sound in the ocean, which is neither passive nor active. It relies on the naturally occurring, incoherent ambient noise field in the ocean-which can be thought of as 'acoustic daylight'-as the sole source of acoustic illumination. By focusing the sound scattered by an object immersed in the noise field, it should be possible to produce a visual image of the object on a television monitor. We have tested this concept by conducting a simple experiment in the ocean, with a parabolic reflector acting as an acoustic lens, and our results confirm that objects illuminated only by ambient noise can indeed be 'seen' at frequencies between 5 and 50 kHz.