Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 2: design and commissioning of test tank

Abstract

This report is the second in a series of five, designed to investigate the detection of
targets buried in saturated sediment, primarily through acoustical or acoustics-related
methods. Although steel targets are included for comparison, the major interest is in
targets (polyethylene cylinders and optical fibres) which have a poor acoustic
impedance mismatch with the host sediment. This particular report details the
construction of a laboratory-scale test facility. This consisted of three main
components. Budget constraints were an over-riding consideration in the design.
First, there is the design and production of a tank containing saturated sediment. It
was the intention that the physical and acoustical properties of the laboratory system
should be similar to those found in a real seafloor environment. Particular
consideration is given to those features of the test system which might affect the
acoustic performance, such as reverberation, the presence of gas bubbles in the
sediment, or a suspension of particles above it. Sound speed and attenuation were
identified as being critical parameters, requiring particular attention. Hence, these
were investigated separately for each component of the acoustic path.
Second, there is the design and production of a transducer system. It was the intention
that this would be suitable for an investigation into the non-invasive acoustic
detection of buried objects. A focused reflector is considered to be the most costeffective
way of achieving a high acoustic power and narrow beamwidth. A
comparison of different reflector sizes suggested that a larger aperture would result in
less spherical aberration, thus producing a more uniform sound field. Diffraction
effects are reduced by specifying a tolerance of much less than an acoustic
wavelength over the reflector surface. The free-field performance of the transducers
was found to be in agreement with the model prediction. Several parameters have
been determined in this report that pertain to the acoustical characteristics of the water
and sediment in the laboratory tank in the 10 – 100 kHz frequency range.
Third, there is the design and production of an automated control system was
developed to simplify the data acquisition process. This was, primarily, a motordriven
position control system which allowed the transducers to be accurately
positioned in the two-dimensional plane above the sediment. Thus, it was possible for
the combined signal generation, data acquisition and position control process to be coordinated
from a central computer.
This series of reports is written in support of the article “The detection by sonar of
x
difficult targets (including centimetre-scale plastic objects and optical fibres) buried
in saturated sediment” by T G Leighton and R C P Evans, written for a Special Issue
of Applied Acoustics which contains articles on the topic of the detection of objects
buried in marine sediment. Further support material can be found at
http://www.isvr.soton.ac.uk/FDAG/uaua/target_in_sand.HTM.

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Identifiers

ORCID for T.G. Leighton: orcid.org/0000-0002-1649-8750

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