The work presented here is a comprehensive study of using time reversal to detect objects located in an inhomogeneous environment using backscattered signals with an emphasis on littoral environments. Time reversal of acoustic signals in the ocean has been studied for more than two decades with the emphasis on the use of the forward scattered field. All studies share similar geometries where both the acoustical source and an adjacent array of transducers are placed in the water column. This configuration, known as a time-reversal mirror (TRM), is not practical when detecting an object that is located in a different environment than the TRM, such as beneath the ocean floor. Little work has been done to study the efficacy of a single transceiver performing the time-reversal operation on the backscattered signals from targets buried beneath the ocean floor. Here, I start by presenting the theory for such a system in both time and frequency domains for scattering by a sphere. Then by using simulations I show that time reversal of backscattered signals provides a robust method to detect targets buried in an acoustically inhomogeneous sediment using a point transceiver in the water column several meters above the sea floor. Effects of the time-reversal window (TRW) on the iterative time-reversal operation are also presented. I define a signal-to-noise ratio (SNR) that treats the return with the sphere as the signal and the return without the sphere as noise to quantify improvements to the sphere returns. I consider two different sediment models and angle of incidence to show that the TRO operates independently of the sediment type and transceiver orientation. Theoretical analysis reveals that the time-reversal of backscattered signals converges to a subset of waveforms defined by the target and time-reversal window, not the initial pulse. Analysis further reveals that the time-reversal operator detects the sphere after only two iterations of the TRO, with more iterations enhancing the sphere return through the non-linear filtering property of the TRO. Through this work, I demonstrate that time reversal is a robust method to detect objects.
| Identifer | oai:union.ndltd.org:NCSU/oai:NCSU:etd-01122004-083659 |
| Date | 28 January 2004 |
| Creators | Pierson, David Michael |
| Contributors | Dr. Michael Paesler, Dr. Hans Hallen, Dr. David Aspnes, Dr. Tony Clark |
| Publisher | NCSU |
| Source Sets | North Carolina State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://www.lib.ncsu.edu/theses/available/etd-01122004-083659/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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