COHERENCE MODULATED ACOUSTIC SPECKLE INTERFEROMETRY

Sophisticated mines buried in the sea bed on littoral
coast lines pose a threat to any vessel in the vicinity.
Their danger comes largely with their camouflage, as
their presence in the sand could easily be mistaken for
a rock, coral or even man made material.Despite elo-
quent seabed mapping techniques, it is difficult to detect
such a mine without having a high probability of false
positives.A method of characterising these objects in
order for detection to be made possible is devised and
detailed in this report.
Speckle interferometry is an optical process that al-
lows the analysis of an object's roughness.The tech-
nique involves exposing the object to a laser, and if its re-
flection scatters and interferes with itself, a characterisa-
tion can be made on its surface texture.However, optical
methods of identification are unreliable in ocean envi-
ronments due to inhomogeneities in the medium.For ex-
ample, small particles, fluctuations in density and salin-
ity, or sea life.The technique is then translated into an
acoustic context, where such inhomogeneities are less
of an obstacle.The idea is to send a sound wave from
an underwater sounder to a target object and record its
reflection using a linear array of sensors.The variation
in amplitude over the array, particularly the interference
patterns due to the partial coherence of the signal, should
allow a classification of the structure of the reflecting
surface.
A challenge which is faced in the design of this tech-
nique is determining the source signal required for the
distinction of given surfaces.For the classification of
finer details on the object surface, a signal with a shaped
spectrum may be necessary, for which the parameters
need to be investigated and determined.Further to this,
there is the problem of developing a software model for
acoustic surface reflections, particularly when the sur-
face is irregular and the source has a complex spectral
structure.
This document describes the plan of research for this
project including the current work from which can be
drawn.This includes previous work done on the topic
as a direct predecessor to the Ph.D. The plans are out-
lined for developing the software tool, which will be
important for much of the research, and designing the
algorithms for determining surface roughness from the
interferograms based on similar technologies used for
mapping.