Abstract: The wavefront of an acoustic, low-frequency source propagation a long distance in the ocean can be recovered by a holographic technique which removes the effects of ocean variability within measurements of that source at an acoustic receiving array. The method assumes that the ocean varies adiabatically in range, and that any variability along the path between a reference and the unknown source can be modeled. The phase distortion along a particular path can be corrected by combining reference and unknown signals so that their relative phases nearly cancel. Since sound propagates to long distances in the ocean over many paths and since each of these paths are distorted in a different way, a different phase correction must be provided for each path. The method that we will present has its origins in an optical technique for wavefront reconstruction in which a distorted image is corrected by placing a reference source sufficiently near the object that their fields propagate through the same media. Recording the reference and object fields together with a hologram provides the phase correction. A simple theory based on the propagation of normal modes in the ocean will be presented. Ocean variability is introduced through a range dependent sound speed profile which is incorporated into the normal modes by means of the adiabatic approximation valid for weakly, range dependent oceans. The wavefront reconstruction technique will be applied to the problem of locating an unknown source.!14
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