High resolution underwater acoustic imaging

Abstract

A high resolution underwater imaging system using sparse array technology has been developed and tested in turbid waters around Australia. High resolution 3D imaging implies many voxels. The present system can image 10/sup 3//spl times/10/sup 3//spl times/4/spl times/10/sup 3/ voxels although faster images can be obtained by zooming to a smaller volume of interest. With 3D images, precise dimensions of complex shapes such as pipe joints can be obtained. Potential applications in offshore platform inspection are discussed. The longer wavelengths of MHz acoustic signals produce less scattering than light while array technology allows the imager to take advantage of pulsed ensonification to further reduce backscatter fogging of the image. The advantage of acoustics is most pronounced in very turbid waters. The MHz frequencies allow millimeter resolution with array sizes compatible with ROVs. Digitization takes place on the acoustics tile that make up the array and the signals are then passed by fiber optical cable to the above surface processor to reduce the volume and underwater weight of the system. There are a number of obstacles to producing an innovation in underwater imaging. Worries about adequate coherence lengths to support arrays capable of adequate resolution were found to be groundless. One impediment to success was the computing loads implied by multi-element arrays and large voxel numbers. To attack this problem international groups were funded to provide potential solutions and a decreasingly smaller number of groups supported at each review point. This technique has produced a lower cost approach for the underwater acoustic camera design.