The survey of continental slopes with shallow and deep sea multibeam systems

Continental margins and slopes are of important interest for the EEZ exploitation. However, their exploration is not easy and much survey work has yet to be done. The steepness and the wide range of depths in such areas require a good positioning as well as efficient echo sounders. The new wide swath multibeam echo sounders (MBES) provide accurate bathymetry and imagery and therefore they are the required tools for surveying the continental margin slopes. The multibeam ranges are either limited by their maximum depth (about 800 m or 300 m for high frequencies MBES i.e. 100 kHz or 200 kHz) for the shallow water systems, or, on shallowest depths, by lack of accuracy for the low frequencies MBES. This paper describes the comparison between a shallow water MBES and a deep water one, in their common depth range, i.e. 100 m to 800 m, using data gathered simultaneously, therefore ruling out discrepancies due to ship positioning or ray bending. The two systems have different ping rates, frequencies, signal sampling rates and beam widths and these differences influence bathymetry and imagery performances according to the sea floor morphology and the direction of the survey tracks. Depending on the accuracy of the representation of the sea floor, or the cost of the survey, one may chose different parameters so as to have wider swath or increased density.