Incoherent Imaging With Ambient Noise

Abst rac t Traditionally, two acoustic techniques familiarly known a s “active” and “passive” are used for probing the ocean. With an active system, a pulse of sound is transmitted and the presence of a n object inferred from the echo it produces. The passive approach involves simply l istening for the sound tha t a n object itself emits. A third method of using sound in the ocean, which is neither active nor passive, relies on ambient noise generated by breaking waves and other naturally occurr ing phenomena a t the sea sur face to “illuminate” objects in the ocean. The radiation scattered by an object is focussed to create a pictorial image of the object on a visual display monitor. This new concept has been designated “acoustic daylight” by analogy with daylight in the atmosphere: the ambient (acoustic) noise field and the (optical) daylight field both consist of incoherent radiat ion travell ing randomly in all di rect ions. Recent exper iments conducted off Scripps pier, in which a parabolic reflector was used a s an acoustic lens for focussing the scattered noise, indicate tha t objects in the ocean can indeed be detected using the acoustic daylight technique .