Application of the Phase Closure Technique to Passive Acoustic Imaging Through Inhomogeneous Media
We consider the problem of passive acoustic imaging of a spatially uncorrelated noise source through an inhomogeneous medium. The measurement system consists of an array of transducers whose signals are processed in a manner similar to that used in radio interferometry of celestial sources. Using a philosophy similar to that of radio astronomers in the field of very long baseline interferometry, we make use of a quantity known as the closure phase. Under certain assumptions, this quantity is independent of the effect of phase shifts in the intervening medium, but yet contains useful information about source structure. There is, of course, an important distinction between the acoustic case and radio astronomy. In the former case, the distorting medium is present in the entire region between the source and the imaging system, whereas in the latter case, the distorting medium is the earth’s atmosphere, representing a thin sheet of material at one end of the propagation path. Nevertheless, in the acoustic case, the closure phase is still a useful quantity for imaging purposes, provided that the time delay for a given path depends primarily on the receiver position and is independent of (or weakly dependent on) source position transverse to the line of sight, and that the medium does not change appreciably during the observation time. To investigate the applicability of the phase closure technique to the acoustic case, we have performed a series of numerical experiments in which a simple source is viewed through an inhomogeneous medium. These calculations support the theoretical predictions.
KeywordsCoherence Deconvolution Acoustics
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