Calibration target for extraction of frequency dependency in synthetic aperture sonar

Synthetic aperture sonar (SAS) can provide high resolution images of the seafloor backscatter. A current research topic is whether wider bandwidth and lower frequencies can support better discrimination of different materials. In principle, the frequency dependence of the backscatter can be estimated at high resolution over the image. We suggest a calibration target that can provide simultaneous ground truth on both strength and frequency dependence of the backscatter. We develop an extended target consisting of objects with different combinations of scattering strength and frequency dependence. In order to provide deterministic frequency dependencies, we use solid geometrical shapes in the form of spheres, cylinders/cones and corners as reflectors. The scattering strengths of these reflectors are functions of frequency to the power of 0, 1 and 2, respectively. These three frequency dependencies are combined with different scattering strengths by using replicas of different sizes. We try to limit the acoustic footprint of the targets to a point where possible, in order to support processing at the maximum SAS resolution. We mount the objects on a metal plate, and elaborate on the choice of material and thickness needed to provide the required acoustic properties at the frequencies under investigation. Finally, we present SAS images of a prototype target measured both at and outside of the design frequency. The suggested calibration target should be well suited to validate algorithms extracting the frequency dependence, and their ability to isolate the frequency dependence from the scattering strength.