Inference of the ocean sound speed profile using ambient vertical beam noise

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2023-04-20

Authors

Taylor, Robert Thomas, Ph. D.

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Abstract

This dissertation introduces a novel passive acoustic method for estimating the water column sound speed using the directionality of the ambient noise field measured with a vertical line array. In the absence of anthropogenic and biological sources, wind/wave sea surface interaction is the dominant source of mid-frequency noise insonifying the undersea environment. The resulting vertical noise field is anisotropic, with the directionality of the noise environment dependent on the paths of noise from the surface to the receiver. Changes in receiver depth, and associated sound speed, affect the level of ambient vertical noise at shallow elevation angles, providing the sensitivity necessary to acoustically estimate the in situ sound speed profile. Application of a clustering algorithm to an historical set of measured sound speed profiles allows for the characterization of common regional profiles. The scale and resulting undersampled nature of the ocean environment prompts the development of a novel interpolation scheme that retains acoustically important sound speed features in the region between available measurements. Motivated by the vertical stratification and motion of the water column, the interpolation scheme determines an appropriate vertical shift in depth to align sound speed features, enabling persistent features in the interpolated range between measurements. The developed method for inferring the sound speed profile combines sparse measurements of the vertical noise environment with an historical set of measured sound speed profiles and the vertical shifting method to provide an estimate of the in situ sound speed profile, with modeled results having error on the order of common active acoustic inversion methods. Finally, the method is applied to a low-frequency vertical line array data set, with results suggesting the array aperture was insufficient, and that further experimentation is needed.

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