Abstract:

Waveguide invariants [e.g., G. A. Grachev, Acoust. Phys. 39(1), 33--35] predict the local behavior of interference maxima/minima of acoustic intensity in the frequency-range plane. These invariants are independent of the detailed environment of the waveguide. As the Bartlett matched-field processor (MFP) ambiguity surface has a physical interpretation in terms of a time-reversed acoustic field, with the sidelobes analogous to local interference maxima, these invariant concepts are reformulated for application to MFP. The theory predicts that when a series of perfectly matched range-independent Bartlett surfaces computed over small increments of frequency are stacked and then sliced along a constant depth, the resulting sidelobe contours in the range-frequency plane display interference patterns that converge to the correct range. Indeed, these patterns are demonstrated to exist in simulated data, broadband source tows, and blue whale vocalizations recorded off San Miguel Island last year. Using the Radon transform, it is even possible to localize a source at the correct range and depth using only the trajectories of the sidelobes of the MFP surfaces. The robust nature of the range-frequency invariant suggests that incoherent postprocessing based on this concept might yield results insensitive to environmental mismatch. [Work sponsored by ONR.]