AN OVERVIEW OF MATCHED-FIELD METHODS IN OCEAN ACOUSTICS

Recent array processing methods for ocean acoustics have utilized the physics of wave propagation as an integral part of their design. The physics of the propagation leads to both improved performance and to algorithms where the complexity of the ocean environment can be exploited in ways not possible with traditional plane wave based methods. 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 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. One can view MFP as an inverse problem where one attempts to ''invert'' the wave equation 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 including 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.