Matched-field processing (MFP),an approach for solving inverse problems by comparing acoustic data with solutions of the wave equation,has been developed for localizing underwater acoustic sources[i] and for estimating ocean acoustic parameters.[2] For source localization problems,MFP estimates source range and depth by matching acoustic fields measured at an array of sensors with replica fields computed for a grid of possible source locations using a numerical propagation model.Localization requires good knowledge of the physical properties of the ocean environment,which strongly affects the propagation of acoustic signals.If the environment is not sufficiently accurate,in a situation referred to as mismatch,MFP can break down even if the signal-to-noise ratio is high.