Estimation of ocean bottom scattering strength using discrete eigenray matching in shallow water

A new approach for processing active sonar data for the estimation of ocean bottom (sediment) acoustic scattering strength is presented. This approach uses an algorithm that compares statistical parameters derived from the output of sonar beamformed time series to the modeled eigenstructure of the ocean environment. Events (echoes) are recognized in the time series of the reverberation. Time of arrival, duration, measures of energy and higher-order moments, and an initial estimate of the georeferenced location of the echo recomputed. These derived and computed data are compared to the modeled eigenstructure of the ocean environment. The eigenstructure is computed by discretization of the ocean environment in depth and range intervals determined by sonar design characteristics. The resulting eigenstructure is sorted by time and then compared to events in the observed time series. Typically, a given arrival time observed in the time series will have multiple matches in time in the eigenstructure file. A system of tests and rules are applied to the matching eigenray set to resolve ambiguities. If the rulebased ambiguity resolution algorithm falls to identify a particular eigenray as the probable source of dominant energy, then the event (echo) is flagged and not used in the scattering strength calculation. Unique features of this automated algorithm are (1) the ability to match observed events in the time series with a specific propagation path in the ocean, (2) the ability to identify the launch angle at the sonar, (3) the ability to identify the grazing angle at the ocean bottom, (4) the ability to resolve eigenray (multipath) arrival ambiguities in shallow water, and (5) a potential to make high-angle measurements on beam pattern side lobes. Results are presented in shallow water, where the ability to resolve particular eigenrays in a multipath shallow-water environment has been previously thought to be difficult or impossible. The results also demonstrate the ability to generate georeferenced scattering strength as a function of grazing angle, utilizing this technology