MATCHED FIELD PROCESSING IN OCEAN ACOUSTICS

Matched field processing (MFP) is a generalized beamforming method which uses the spatial complexities of acoustic fields in an ocean waveguide to localize sources in range, depth and azimuth or to infer parameters of the waveguide itself. It has experimentally localized sources with accuracies exceeding the Rayleigh limit for depth and the Fresnel limit for range by two orders of magnitude. MFP exploits the coherence of the mode/multipath structure and it is especially effective at low frequencies where the ocean supports coherent propagation over very long ranges. This contrasts with plane wave based models which are degraded by modal and multipath phenomena and are generally ineffective when waveguide phenomena are important. MFP is a spatial matched filter where the correlation signal, or replica, is determined by the Green’s function of the waveguide. It can have either conventional or adaptive formulations and it has been implemented with an assortment of both narrowband and wideband signal models. All involve some form of correlation between the replicas derived from the wave equation and the data measured at an array of sensors. Since the replica generally lias a complicated dependence upon the source location and environmental parameters, the wave equation must be solved over this parameter space. One can view MFP as an inverse problem where one attempts to invert for these dependencies over the parameter space of the source and the environment. There is currently a large literature discussing many theoretical aspects of MFP and this is supported by numerous simulations; several experiments acquiring data for MFP now have been conducted in several ocean environments and these have demonstrated both its capabilities and some of its limitations. Consequently, there is a modest understanding of both the theory and the experimental capabilities of MFP. This article provides an overview of both.