Jm. Augustin et al., CONTRIBUTION OF THE MULTIBEAM ACOUSTIC IMAGERY TO THE EXPLORATION OF THE SEA-BOTTOM, Marine geophysical researches, 18(2-4), 1996, pp. 459-486
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.