Using MBES backscatter strength measurements for assessing a shallow water soft sediment environment

Shallow water naval operations require detailed knowledge of the environmental characteristics. In this context, the BP'07 experiment was carried out in the Mediterranean Sea, south-east of Elba Island, in 2007. Measurements that were taken during this experiment employ a large set of sensors, thereby providing all information required to fully describe the environment. Water depths as measured by multi beam echo sounders (MBES) are found to range from 0 to about 130 meter. The flnescale topography reveals that areas of different bottom morphology are present. Information about the physical sediment properties is obtained by bottom grab samples. They indicate the seafloor in the area to be composed of very fine-grained sediments with mean grain sizes ranging from 0.5 to 8 micrometer. In addition, the MBES also allows for classifying the seafloor. The MBES classification approach taken discriminates between sediments in the most optimal way by applying the Bayes decision rule for multiple hypotheses. It employs the MBES backscatter data, averaged per beam, which are assumed to be normally distributed. For shallow water situations, this assumption no longer holds due to the limited number of independent scatter pixels in the beam footprint. Averaging over a series of pings has been applied to restore this assumption. The application of the method results in a map of the acoustic classes in this area, indicating the presence of four different seafloor types. A comparison with other results indicates a correlation between seafloor type and the presence of specific bottom features.