Dissertation
Imaging and spectral analysis of acoustical scattering by underwater objects located near flat interfaces: evanescent wavefields and orientation effects
Doctor of Philosophy (PhD), Washington State University
01/2015
Handle:
https://hdl.handle.net/2376/108643
Abstract
The detection and classification of objects in an underwater environment using sound has been an active area of research for over a century. Wide bandwidth acoustic signals scatter from objects and the results are recorded for analysis. For certain frequency ranges the scattering is dependent on both the shape and material characteristics of the target, making it a valuable tool for target identification. Effort has been made to understand the physical mechanisms whereby sound interacts with a target, with the goal of creating acoustic ``fingerprints'' that allow objects to be distinguished. The scattering situation is complicated when the object is located near a boundary. In this work flat fluid-fluid interfaces are used as a stand-in for more complicated conditions. As the sound is incident on the boundary between two media with different physical properties, its reflection and refraction are dependent on both geometry and those properties. An object with axial symmetry sitting on a flat interface has many more mechanisms for scattering sound than one in the water column (free-field). If the axis of that object is not parallel to the inteface, but instead tilted vertically with respect to the interface, the geometry governing these additional mechanisms changes in a predictable way. These predictions were confirmed in experimental work at an air-water interface. If the sound speed of the second fluid is higher than that of the first, a critical angle exists beyond which total reflection occurs. This produces an exponentially decaying wavefield in the second fluid. An object located in the evanescent wavefield may still scatter sound in a measurable manner. An evanescent wavefield near a water-oil interface is simulated using wavenumber integration techniques and the scattering from elastic objects is studied experimentally. Acoustic images of the objects were also produced in these experiments. Finally, complications arise if the source/receiver is not located sufficiently distant from the object; a method for converting data taken in the nearfield to that which would have been obtained in the farfield is demonstrated.
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Details
- Title
- Imaging and spectral analysis of acoustical scattering by underwater objects located near flat interfaces: evanescent wavefields and orientation effects
- Creators
- Daniel Stewart Plotnick
- Contributors
- Philip L Marston (Advisor)Matthew D Duez (Committee Member)David B Thiessen (Committee Member)Steven G Kargl (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Physics and Astronomy, Department of
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 348
- Identifiers
- 99900581729201842
- Language
- English
- Resource Type
- Dissertation