ACOUSTIC IMAGING USING AMBIENT NOISE - SOME THEORY AND SIMULATION RESULTS

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.TM'' 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. The possible performance envelope of an acoustic daylight system as a function of object shape, composition, degree, and orientation of anisotropy in the illuminating field is discussed. 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 employs the Helmholtz-Kirchhoff integral with far-field approximations, evaluated by the stationary phase technique, which results in an analytical approximation for the energy density scattered near the specular angle by an object for a single source-object-receiver geometry. The expression is applied in a numerical implementation that sums over many sources and segments of a modeled 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 simulations of various objects 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.