Relations entre la réponse acoustique (5 kHz) et la nature lithologique des fonds marins

Sub-bottom profilers which were worked out in the sixty years did not first come into general use. These last few years, some new devices have been developed with most sediment penetration, which are to be more systematically used in sedimentological prospecting. After reviewing the sub-bottom profilers development, with laying stress on their originality, the different points of view of the interpretation are considered: layers geometry, lithological correlations, relations between the reflected picture and the depositional dynamics. In a bounded area, about 15 km², 90 bucket dredged samples and 12 core samples allowed a detailed mapping of the facies. This map is compared with different data from the seismic reflection investigation (5 kHz). A first approach concerns the distribution of some typical sismofacies selected on the whole seismic prospection. Thus five echotypes were picked out. By comparing, we can make a correlation between the character and surface feature of the sediment, and the corresponding seismic echo. In a second stage, the reflectivity change is considered in relation with the different sediment facies. The relative reflection coefficients are measured, either on the first peak, or on a longer time interval (1 ms), by integrating the seismic pulse. Both ways result in two maps of the relative reflection coefficient distribution.