1. An apparatus for processing acoustic signals received on a horizontal line array for creating range-bearing images of a target, the apparatus comprising: a receiver array for receiving an acoustic signal from the target; a processor for determining a beam covariance matrix; said processor capable of determining a replica field for a range-bearing of the target, said processor being used for further determining a replica beam from the replica field; and said processor capable of further processing the beam covariance matrix and the replica beam to determine range-bearing ambiguity surface and estimate depth ambiguity for selected peaks of the ambiguity surface.