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Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 4: experimental investigations into the acoustic detection of objects buried in saturated sediment

By R.C.P. Evans

Abstract

This report is the fourth 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. This particular report aims to review the<br/>signal processing requirements of the system and to investigate the relative<br/>performance of a number of different detection algorithms.<br/>In order to do this, some basic signal processing concepts are presented and a number<br/>of different approaches to detection processing were discussed, noting that the<br/>detection system should be optimised to the class of object being sought. To this end,<br/>the scattering characteristics of spheres and cylinders are calculated. These were used<br/>to aid in the selection of an optimal frequency range for later incorporation into the<br/>detection algorithms.<br/>The study examines the use of waveform dependent filtering, processing in a number<br/>of ways (forms of optimal filter and Synthetic Aperture Sonar, SAS) the same<br/>scattered data obtained with incident FM pulse compression waveforms. (There is a<br/>preliminary study which examines a simple scattered power measurement, using<br/>pulsed AM waveforms, but this can only be seen as rudimentary preparatory work<br/>because: (i) it cannot be compared with the waveform dependent filtering methods<br/>because it uses a different incident waveform; and (ii) the designers of a realistic<br/>system would almost certainly consider some form of matched filter, at the very least<br/>an attempt would be made to match the filter input bandwidth to that of the signal).<br/>The algorithms presented in this report are applied in an experiment and prove to be<br/>very successful in detecting objects buried at depths of between 25 and 30 cm in the<br/>saturated sediment of the test tank. In every case, either 60 or 300 sample points were<br/>measured over a series of planes extending vertically into the sediment. It was found<br/>that with 60 points (having a sample spacing of 5 cm) the resolution was not high<br/>enough to provide conclusive detection results. Conversely, with 300 points (having a<br/>sample spacing of around 2 cm) the buried objects could readily be detected.<br/>xi<br/>Simple matched filtering is shown to be useful in an environment dominated by noise.<br/>However, the optimal filter is shown to be more successful in dealing with the<br/>cluttered seabed environment. Target optimisation techniques had mixed success.<br/>When the target scattering responses (for both rigid and the elastic scattering) were<br/>incorporated into the filters, qualitative improvements in target localisation were<br/>observed. However, these were not accompanied by an increase in the average values<br/>of the signal-to-noise ratio.<br/>The investigation into Synthetic Aperture techniques was limited by the positional<br/>error in the laboratory apparatus and the small number of measurement positions used<br/>to form the synthetic aperture. As a result of these experimental limitations, no<br/>significant performance improvement was actually observed.<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: GC, QC, TC
Publisher: University of Southampton
Year: 2007
OAI identifier: oai:eprints.soton.ac.uk:46561
Provided by: e-Prints Soton

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