Abstract

This article is concerned with the theory and simulation of passive imaging of quiet objects using the ambient noise field as illumination, a method known as 'Acoustic Daylight'. A great body of work exists on active and passive acoustic systems, but the principle of using ambient noise as the sole illumination for acoustic imaging is almost unexplored. We investigate the possible performance envelope of such a system as a function of object shape, composition, degree and orientation of anisotropy in the illuminating field. With such a large number of controlling variables a simple scattering model is required in the first instance for the problem to remain tractable. The theoretical development begins with the Helmholtz-Kirchoff integral with far-field approximations, evaluated by the stationary phase technique. The result gives an analytical approximation for energy scattered near the specular angle by an object for a single source-object-receiver geometry. The expression is then used in a numerical implementation which sums over many sources and segments of a modelled object to give the total field. The model can be used for a wide range of object shapes and materials immersed in various ambient noise fields of practical interest. Some example simulations of various objects placed in a homogeneous shallow-water duct are given. It is found that useful images can be formed provided either the ambient noise field is not isotropic in all three dimensions or the object is not perfectly reflecting.