The detection by sonar of difficult targets (including centimetre-scale plastic objects and optical fibres) buried in saturated sediment

This paper reports on a laboratory study into the use of sonar to detect objects, two of which exhibit a poor acoustic impedance mismatch with the water-saturated sediment in which they are buried to depth of about 30 cm. The targets are solid cylinders having diameters of 20–25 mm and 50 cm length, made of polyethylene, of telecommunications optical fibre, and of steel. Steel spheres are included for comparison. A poor acoustic impedance mismatch between the target and the host sediment is one factor that can make buried targets difficult to detect with sonar, but such detection is increasingly becoming an issue in a range of applications from archaeology to defence to telecommunications. Attention is paid to those signal processing techniques which could be of potential benefit. For this range of test objects, comparisons are made between use of optimal filtering and synthetic aperture sonar. In addition, the potential of a range of acousto-optical effects (optical time domain reflectometry, Raman and Brillouin scattering, and fibre optic hydrophones) is assessed in the Appendix for the particular application of detecting non-metallised fibre optic telecommunications cables. A web page dedicated to this paper hosts movies and reports at http://www.isvr.soton.ac.uk/fdag/uaua/target_in_sand.htm.