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Horizontal Coherence Function for Ambient Noise and Helicopter Sound in Water


The radiated sound from an airborne source result in a head wave when it is incident on the sea bottom at the critical angle. The head wave travels along the sea bottom and exits at the same critical angle, and the pressure from the head wave can only be detected within a narrow range window due to geometrical spreading. Computing the horizontal coherence function from the data allows for the inference of the speed of sound in the sediment. The theoretical expression that was derived previously was validated in a new experiment that used a line array with a wider aperture of 15 m as compared to a 3 m separation used in an earlier experiment. The least mean squares method was proposed for better estimation of the sediment sound speed instead of extracting the zero crossings when the data is noisy. Multiple ocean noise models were used for comparison with the coherence function computed for the ambient noise. The best fit for the ambient noise data consists of a convex combination of noise fields in order to account for the zero crossing locations and the amplitude of the coherence function at higher frequencies.

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