Browsing by Subject "Ocean acoustics"

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Inference of the ocean sound speed profile using ambient vertical beam noise
(2023-04-20) Taylor, Robert Thomas, Ph. D.; Hartman, John W., active 2023; Hamilton, Mark F.; Keenan, Ruth E; Haberman, Michael R; Wilson, Preston S
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.
Quantifying three dimensional effects in acoustic rough surface scattering
(2011-05) Joshi, Sumedh Mohan; Hamilton, Mark F.; Isakson, Marcia J.
Interface roughness can have a significant effect on the scattering of sound energy, and therefore an understanding of the effects of roughness is essential to making predictions of sound propagation and transmission underwater. Many models of roughness scattering currently in use are two dimensional (2D) in nature; three dimensional (3D) modeling requires significantly more time and computational resources. In this work, an effort is made to quantify the effects of 3D scattering in order to assess whether or under what conditions 3D modeling is necessary. To that end, an exact 3D roughness scattering model is developed based on a commercially available finite element package. The finite element results are compared with two approximate scattering models (the Kirchhoff approximation and first order perturbation theory) to establish the validity and regimes of applicability of each. The rough surfaces are realizations generated from power spectra measured from the sea floor. However, the surfaces are assumed to be pressure release (as on an air-water interface). Such a formulation is nonphysical, but allows the assessment of the validity of the various modeling techniques which is the focus of this work. The comparison between the models is made by calculating the ensemble average of the scattering from realizations of randomly rough surfaces. It is shown that a combination of the Kirchhoff approximation and perturbation theory models recovers the 3D finite element solution.