Towards more accurate shadow modelling for simulated SAS imagery

Shadows are a feature of particular interest in Synthetic Aperture Sonar (SAS) imagery. Shadows are cast by objects or features that are proud of the sea-floor. The shape of a shadow reveals information on the structure of the object or feature and can be used to assist in target classification. In most existing simulated imagery, the shadows are well-defined and resemble those obtained by conventional, high-frequency sonars. In practice, shadows are not well-defined or are sometimes lost in imagery obtained by low-frequency, free-towed, shallow-water SAS systems. In this paper, we show our attempts to simulate realistic SAS imagery and demonstrate some of the mechanisms responsible for the shadow degradation that is seen in practice. Our simulator uses a novel facet-based scattering model that incorporates wide-beam effects, shadowing, and multiple scattering. Using our simulator, we show that the fidelity of the shadow region is degraded by wide-beam effects, multi-path scattering from the sea-surface, and reconstruction artefacts caused by deviations from a linear tow-path.