Enhancing bottom tracking for AUVs by using sound diffraction

In this paper, we present a method of seabed detection based on the use of a single electro-acoustic transducer (sounder). This method uses both acoustic diffraction and bottom backscattering coefficients. Firstly, we present the acoustic sensor and its geometry. When dimensions of the aperture of the transducer are not very large in comparison with the sound wavelength, diffraction must be taken into account in the estimation of beam geometry. We determine the beam geometry by application of the Huygens-Fresnel principle. We explain how the shape and dimensions of its aperture will influence the diffraction of the acoustic beam (diffraction patterns are presented). We also detail the bottom backscattering properties and their influence on the backscattered acoustic energy of main and secondary beams. In the second part, we introduce a new method aiming to extract seabed features from the received acoustic echo. This method is based on the shape of the received acoustic echo and aims to work out the seabed slope. In the last part, we present results obtained during simulations and during experimentations conducted in the open sea.