CONTRIBUTION OF THE MULTIBEAM ACOUSTIC IMAGERY TO THE EXPLORATION OF THE SEA-BOTTOM

Since early 1991, the EM12 dual multibeam echo sounder mounted on RN L 'Atalante has been mainly used for regional mapping and more particula rly for EEZ exploration. The acoustic imagery data, which is collected at the same time as the multibeam soundings, and positioned with the same accuracy, illustrates the variability of the acoustic interface. The working principles of the multibeam echo sounder are at first pres ented. The backscattering phenomena are then considered, where the ech o structure is interpreted in terms of insonified area, propagation lo ss and backscattering strength; the latter, especially important for t he seafloor acoustical characterization, is described as contributions from the interface roughness and the volume inhomogeneities. Problems associated with the seafloor type interpretation from the backscatter ed level data are briefly evoked. Then, the various operations associa ted with 'mosaic' computations are presented, describing the transitio n from the original time signals to the final image usable by geologis ts. Three examples, selected from a large set of data collected by IFR EMER for SOPAC (SOPACMAPS Project) and the Territory of New Caledonia (ZoNeCo Project), both in late 1993, show that there are well differen tiated acoustic reflectivity levels. After a description of their regi onal geological environment, the three examples are presented with the main results. These reflectivities, not yet calibrated in terms of fi eld truth, are tentatively related to the previous existing data on th e nature of the bottom and to the basic rules of marine sedimentology.