Dissertation
Underwater Acoustical Scattering by Cubes and of Helical Beams
Doctor of Philosophy (PhD), Washington State University
01/2017
Handle:
https://hdl.handle.net/2376/111459
Abstract
Acoustic scattering is the study of the reflections of sound by objects, which is used to detect and identify said objects underwater. This dissertation has two parts; the first part discusses the topological properties of helical beams. An acoustic helical, or vortex, beam has a null down its axis of propagation and a phase ramp. In this dissertation, the modulation and direct measurement of the beam's helicity, and the forward and back-scattering by a small solid sphere were examined. Moreover, for the forward scattering a helicity sensitive detector was used, and for the nearly monostatic backscattering the source transducer was turned into a helicity sensitive receiver in post-processing. Since there is a rapid phase change across the null of the beam, subwavelength imaging of said null is possible. The experimental results were compared to a Kirchhoff-integration based simulation of the beam and its backscattering.
The second part covers the backscattering by solid cubes. Two cubes, one brass and one steel, were insonified underwater in a tank and their backscattering measured using Synthetic Aperture Sonar (SAS). Cubes were chosen as the target due to their sharp corners and flat faces, making it easy to measure aspect dependent backscattering, such as Rayleigh waves. Rayleigh waves couple into the face of the cube at a material dependent angle, called the Rayleigh angle, and are retroreflected back to the source. Similar, to the helical beam, the backscattering of the cube was compared to Kirchhoff-integration simulations. The specular response was isolated from elastic effects using the simulations, which determined the cause of a splitting of the backscattered signal from the face of a forward tilted cube. Target images were also constructed using Fourier back-projection methods from the SAS scans.
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Details
- Title
- Underwater Acoustical Scattering by Cubes and of Helical Beams
- Creators
- Viktor Bollen
- Contributors
- Philip L Marston (Advisor)Fred Gittes (Committee Member)Mark G Kuzyk (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Department of Physics and Astronomy
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 217
- Identifiers
- 99900581428601842
- Language
- English
- Resource Type
- Dissertation