Turning point filters

Modal group delays in ocean acoustics are a potential observable, but their interpretation is ambiguous for signals with sufficient bandwidth for the inherent modal frequency dispersion to be significant. An inability to use the late-arriving energy in an inverse solution for ocean properties motivates this paper. We describe an alternate approach based on the WKBJ approximation for extracting observables from the final crescendo. We have called this approach a turning point filter. The turning point filter offers a unified approach to underwater acoustics, joining what are often two disparate schools of thought, rays and modes. The turning-point filter is an intermediate step between linear beamforming and a full matched field processor. Linear beamforming requires knowledge of the sound speed at the center of the receiving array to extract ray travel-times. Matched field processing attempts to match a measured arrival pattern with a hypothesized arrival pattern, by adjusting the entire sound-speed field between the source and receiver. Turning-point filtering only requires the sound-speed profile across the aperture of the receiving array to extract robust observable peaks.