COHERENCE MODULATED ACOUSTIC SPECKLE INTERFEROMETRY

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Sophisticated mines buried in the sea bed on littoralcoast lines pose a threat to any vessel in the vicinity.Their danger comes largely with their camouflage, astheir presence in the sand could easily be mistaken fora rock, coral or even man made material.Despite elo-quent seabed mapping techniques, it is difficult to detectsuch a mine without having a high probability of falsepositives.A method of characterising these objects inorder for detection to be made possible is devised anddetailed 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, opticalmethods 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 anacoustic context, where such inhomogeneities are lessof an obstacle.The idea is to send a sound wave froman underwater sounder to a target object and record itsreflection using a linear array of sensors.The variationin amplitude over the array, particularly the interferencepatterns due to the partial coherence of the signal, shouldallow a classification of the structure of the reflectingsurface.A challenge which is faced in the design of this tech-nique is determining the source signal required for thedistinction of given surfaces.For the classification offiner details on the object surface, a signal with a shapedspectrum may be necessary, for which the parametersneed to be investigated and determined.Further to this,there is the problem of developing a software model foracoustic surface reflections, particularly when the sur-face is irregular and the source has a complex spectralstructure.This document describes the plan of research for thisproject including the current work from which can bedrawn.This includes previous work done on the topicas a direct predecessor to the Ph.D. The plans are out-lined for developing the software tool, which will beimportant for much of the research, and designing thealgorithms for determining surface roughness from theinterferograms based on similar technologies used formapping.