Deducing environmental mismatch from matched-mode processing ambiguity surface sidelobes

Abstract

The application of non-linear time sampling to broadband acoustic signals propagating in shallow water has made modal filtering possible using a single hydrophone. As a result, incoherent matched-mode processing (MMP) techniques are now practical using only single-hydrophone data. When MMP ambiguity surfaces are constructed from pairs of modes and plotted as a function of range and frequency, both the mainlobe and sidelobes form striations that embed information about the type and amount of environmental mismatch present between the modeled and true environment. Thus the degree of symmetry that a two-mode frequency-averaged ambiguity surface displays around the mainlobe provides a metric for identifying environmental mismatch. Acoustic invariant theory, combined with simulations, demonstrate how mismatched waveguide replicas can be used to (1) estimate the true bottom interface sound speed, (2) determine whether a downward-refracting waterborne profile is present in data, and (3) estimate waterborne sound speed profile slopes. The theory also explains why certain mismatched environments can generate very high MMP correlations, even with broadband signals. In a shallow water environment with a downward-refracting sound speed profile, taking full advantage of this approach requires reformulation of non-linear time-sampling methods to extract refractive modes. [Work sponsored by the North Pacific Research Board.]