Observation, theory and simulation of anisotropy in oceanic ambient noise fields and its relevance to Acoustic Daylight imaging

Acoustic Daylight is a new technique for creating pictorial images of undersea objects from the acoustic illumination provided by the ambient noise field. As in conventional photography, the directionality of the illumination affects the contrast in the image through the shadows that are cast. Three aspects of the directionality, or anisotropy, of ambient noise in the ocean are discussed in this paper, in the context of Acoustic Daylight imaging: firstly, some observations of the horizontal anisotropy of the ambient noise around Scripps Pier are reported, which indicate that the pier itself is a significant source of noise; secondly, a theoretical model of the acoustic contrast under differing degrees of noise anisotropy is described; and finally, a numerical simulation algorithm that generates Acoustic Daylight images is used to illustrate pictorially the effects of shadowing when the illumination is a representative shallow water noise field.