• Evans, R. C. P.
• Leighton, Timothy

Abstract

This report is the second 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 details the<br/>construction of a laboratory-scale test facility. This consisted of three main<br/>components. Budget constraints were an over-riding consideration in the design.<br/>First, there is the design and production of a tank containing saturated sediment. It<br/>was the intention that the physical and acoustical properties of the laboratory system<br/>should be similar to those found in a real seafloor environment. Particular<br/>consideration is given to those features of the test system which might affect the<br/>acoustic performance, such as reverberation, the presence of gas bubbles in the<br/>sediment, or a suspension of particles above it. Sound speed and attenuation were<br/>identified as being critical parameters, requiring particular attention. Hence, these<br/>were investigated separately for each component of the acoustic path.<br/>Second, there is the design and production of a transducer system. It was the intention<br/>that this would be suitable for an investigation into the non-invasive acoustic<br/>detection of buried objects. A focused reflector is considered to be the most costeffective<br/>way of achieving a high acoustic power and narrow beamwidth. A<br/>comparison of different reflector sizes suggested that a larger aperture would result in<br/>less spherical aberration, thus producing a more uniform sound field. Diffraction<br/>effects are reduced by specifying a tolerance of much less than an acoustic<br/>wavelength over the reflector surface. The free-field performance of the transducers<br/>was found to be in agreement with the model prediction. Several parameters have<br/>been determined in this report that pertain to the acoustical characteristics of the water<br/>and sediment in the laboratory tank in the 10 – 100 kHz frequency range.<br/>Third, there is the design and production of an automated control system was<br/>developed to simplify the data acquisition process. This was, primarily, a motordriven<br/>position control system which allowed the transducers to be accurately<br/>positioned in the two-dimensional plane above the sediment. Thus, it was possible for<br/>the combined signal generation, data acquisition and position control process to be coordinated<br/>from a central computer.<br/>This series of reports is written in support of the article “The detection by sonar of<br/>x<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
• surface
• polymer
• laser emission spectroscopy
• steel
• two-dimensional