Observation, theory and simulation of anisotropy in oceanic ambient noise fields and its relevance to Acoustic Daylight imaging Michael J. Buckingham † and John R. Potter Marine Physical Laboratory, Scripps Institution of Oceanography La Jolla, CA 92093-0213, USA. † also: Institute of Sound and Vibration Research, The University Southampton, SO9 5NH, England. ABSTRACT 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. INTRODUCTION For many years, anecdotal evidence has indicated that a submarine can be detected acoustically when it lies above the receiver, because it blocks some of the ambient noise created at the sea surface by breaking waves and related processes. Thus, the submarine casts an acoustic shadow, which in principle is detectable since it contrasts with the background noise field. This phenomenon, as a method of detection, was investigated by Flatté and Munk 1 at the suggestion of Dr. Allen Ellinthorpe. At about the same time, Buckingham introduced the idea that, as the ambient noise field is analogous to daylight in the atmosphere, it should be possible to create pictorial images of objects in the ocean through the acoustic "illumination" provided by the noise. If this process could be demonstrated, there would be a substantial impact both on the design and operation of man-made underwater sensing systems, and on our comprehension of the means by which echolocating marine mammals sense their environment. For man to create such an image, a multi-beam acoustic lens is required to focus the noise scattered by the object, along with appropriate signal processing and a visual display monitor to convert the acoustic information into a visual image. The image on the monitor would have much in common with a conventional photograph, although the resolution would be comparatively poor due to the relatively long acoustic wavelengths. The pixels in the image would show a one-to-one correspondence with the beams formed by the acoustic lens; and the angular beam width, which is a measure of the resolution, would be given approximately by the ratio of the wavelength to the aperture of the lens. In the first experiment on Acoustic Daylight imaging, conducted off Scripps Pier in the summer of 1991, a single-beam acoustic lens, consisting of a parabolic