The simulation of side-scan sonar images

Abstract

This paper describes a system which applies the established ray-tracing techniques from computer graphics to the simulation of side-scan sonar images. Side-scan sonar is a widely used device in underwater searches for sunken vessels and inundated land sites in archaeology. It is also used in other fields, such as the defence and oil industries. Sonar techniques effectively use sound to ‘see’ through water, which would rapidly attenuate light, but whereas we are used to interpreting what we see with our eyes, the meaning of sonar images is often less immediately apparent. An archaeologist searching for an underwater site would benefit from being able to model the expected target and examine the images produced under varying conditions. A series of computer experiments predicting the various manifestations of a given target could save a great deal of time, and therefore money, at sea. Initially, the simulated sonar images are produced by applying the computer graphics model directly to sound. The model uses the basic principles of geometric optics, and even in this simple form produces simulated images that compare favourably with their real counterparts. As the model becomes more sophisticated it incorporates some of the many factors that degrade real sonar images, such as the beam pattern of the sonar transducer, the sea bed itself, towfish instabilities and multiple reflections. It is believed that this is the first time that ray tracing has been used in this way, and the system has the unique ability to present visual images of how a target on the sea-bed might look if the water were removed, together with simulated sonar images of that object. This would be of great value as a teaching tool, and simulated images are necessary in the testing of algorithms for, for example, distortion correction. Previous attempts at modelling the sonar process were mathematically intensive. The method developed here has the benefit of simplicity. Each ray hits a point on the seabed and that point is considered in isolation. Although millions of calculations are performed, each individual calculation is very simple. The application of ray tracing to the simulation of side-scan sonar images has proved that techniques from computer graphics can be applied to underwater acoustics, and examples are given to illustrate some of the difficulties of interpretation of side-scan sonar imagery.