Application of time warping to passive acoustic remote sensing of the coastal ocean

Oleg A Godin1, Tsu Wei Tan1, Boris Katsnelson2 and Michael G Brown3, (1)Naval Postgraduate School, Department of Physics, Monterey, CA, United States, (2)Professor of University of Haifa, Dept of Marine Geosciences, Haifa, Israel, (3)University of Miami, Miami, FL, United States
Abstract:
Properties of the propagation medium leave an imprint on wave fields generated by random sources. Noise interferometry separates the deterministic environmental imprint from random or unknown characteristics of spatially distributed sources. Interferometry of ambient and shipping noise in the ocean provides a way to monitor physical parameters of the water column and characterize geoacoustic properties of the seabed in a cost-effective and environmentally friendly manner, without employing any controlled sound sources. With noise interferometry, two-point cross-correlation functions of noise serve as the probing signals and replace the Green’s function measured in active acoustic remote sensing. The amount of the environmental information that can be obtained with passive remote sensing, and the robustness of the passive measurements increase, when contributions of individual normal modes are resolved in the noise cross-correlation function. In this paper, the time-warping transform is employed to isolate normal-mode components of the cross-correlation function of noise and retrieve dispersion curves of the modes, including the flow-induced non-reciprocity of normal mode travel times. The technique is applied to the data obtained in the 2012 Noise Interferometry experiment in the Straits of Florida and in the Shallow Water 2006 experiment on the continental shelf off New Jersey. In each experiment, long time series of noise were recorded on near-bottom hydrophones, and cross-correlation functions were calculated up to distances of about 8–10 km, or 100 water depths. Passively measured dispersion curves of acoustic normal modes have been inverted for geoacoustic parameters of the seabed, variations of the sound speed in the water column, and current velocity estimates. Effects of ocean non-stationarity on noise interferometry and the inversion biases that result from unresolved horizontal inhomogeneity of the waveguide, will be discussed.
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