SAMI: A low-frequency prototype for mapping and imaging of the seabed by means of synthetic aperture

The objective of the Synthetic Aperture Mapping and Imaging (SAMI) project was to develop and to test at sea a wide-band synthetic aperture sonar prot otype, capable of providing high-resolution seafloor images together with b athymetry maps. This system used the motion of a physically small array in order to synthesize a longer array, providing images with an across-track r esolution independent of both range and transmit frequency. Such systems ar e clearly very relevant to the high-precision long-range (low-frequency) im aging of the sea bottom. The project has led to the construction of a prototype tested at sea on sev eral well-known areas for comparison with existing images and maps. These a reas included several types of sea bottom, depths, and geological structure s. The results obtained in real time, on-board ship, have shown the relevance of the proposed wide-band techniques. The many profiles produced have provi ded high-resolution images and maps of various seafloors, Interpretation by geologists showed that the system was capable of providing the same or fin er detail than a deep-sea short-range, high-frequency system and maintained a higher resolution over a wider swath. The sea data processed have shown that the system provided maps with a cubi c meter voxel, The resolution cell is constant over the whole range (50 to 2500 m) thanks to the dynamic focusing of the synthetic aperture. Postprocessing of a part of the data stored during the experiments has been carried out in the laboratory. This work has shown that techniques such as autofocusing can give an increase in resolution (i.e., gain in contrast an d resolution of about 3 dB), The results displayed in the paper show the relevance of the techniques dev eloped to the provision of a complete highperformance imaging tool for the oceanographic community.