Simulation and 3D Reconstruction of Side-looking Sonar Images

Given the limited range and applicability of visual imaging systems in the underwater environment, sonar has been the preferred solution for the observation of the seabed since its inception in the 1950s (Blondel 2002). The images produced by the most commonly used side-looking sonars (side-scan and, more recently, synthetic aperture sonars) contain information on the backscatter strength recorded at every given range. This backscatter strength mainly depends on the composition and the orientation of the observed surfaces with respect to the sensor. In this chapter, the relations between surface properties (bathymetry, reflectivity) and the images resulting when the surface is observed by side-looking sonar (backscatter strength) are studied. The characterization of this sonar imaging process can be used in two ways: by applying the forward image formation model, sonar images can be synthesized from a given 3D mesh; conversely, by inverting the image formation model, a 3D mesh can be estimated from a given side-looking sonar image. The chapter is thus divided in two main parts, each discussing these forward and inverse processes. The typical imaging sensor considered here is an active side-looking sonar with a frequency of hundreds of kilohertz, which usually allows for sub-decimetre resolution in range and azimuth.