• Leighton, Timothy

Abstract

This report is the last in a series of five, designed to investigate the detection of<br/>targets buried in saturated sediment, primarily through acoustical or acoustics-related<br/>methods. Although steel targets are included for comparison, the major interest is in<br/>targets (polyethylene cylinders and optical fibres) which have a poor acoustic<br/>impedance mismatch with the host sediment. Whilst previous report in this series<br/>have considered techniques for the detection of difficult targets in general when they<br/>are buried in saturated sediment, this particular report discusses one specific type of<br/>target: the optic fibre. The underlying theory, and a laboratory test, are investigated in<br/>order to assess the feasibility of its practical use.<br/>The use of optic fibres as distributed sensors is discussed, the most notable example<br/>of such a system being the OTDR. Non-linear optical processes that could be<br/>exploited in a distributed sensor are also discussed. The Brillouin effect is given<br/>special consideration, and it is noted that the associated Brillouin frequency shift<br/>depends on variations in temperature and, of particular interest, variations in strain.<br/>The related, acousto-optic effect, is also investigated, and it is shown that both length<br/>and refractive index changes (in response to an applied strain) can cause a change in<br/>the optical phase within a fibre. Moreover, pressure sensitivity is shown to be related<br/>to the elastic properties of the fibre jacket material. A greatly enhanced sensitivity is<br/>predicted if the cladding has a high compressibility (which is expected to be the case<br/>for non-metallised fibre optic cables).<br/>An experiment is performed to investigate whether a conventional OTDR could be<br/>used to detect acoustically-generated stresses within an optic fibre. A change in the<br/>back-scattered power in the optical window of the detector is found to occur. These<br/>results are discussed in terms of the practicality of a working system.<br/>This series of reports is written in support of the article “The detection by sonar of<br/>difficult targets (including centimetre-scale plastic objects and optical fibres) buried<br/>in saturated sediment” by T G Leighton and R C P Evans, written for a Special Issue<br/>of Applied Acoustics which contains articles on the topic of the detection of objects<br/>buried in marine sediment. Further support material can be found at<br/>http://www.isvr.soton.ac.uk/FDAG/uaua/target_in_sand.HTM.

Topics

• impedance spectroscopy
• polymer
• phase
• theory
• experiment
• steel