Study, design and concept of low frequency SAS

In the MCM (Mine Counter Measure) context, a lot of effort has been put in imagery systems and specially in high frequency sonar. The last generation of sonar, SAS (Synthetic Aperture Sonar) system, has been developed in the last 5-10 years and provides a new powerful tool for mines detection, identification and classification. The main advantage of SAS systems is a resolution close to the wavelength even at long range. A high frequency SAS system has been developed at NURC, the MUSCLE vehicle (300kHz center frequency). But despite the extremely good quality of the SAS images, ambiguities between mine-like objects cannot always be suppressed.
This study investigates low frequency SAS systems (LF-SAS) in order to address the two main limitations of the present system linked essentially to the sound absorption at high frequencies: providing imagery inside the target, detecting buried targets. Synthetic SAS images have been generated and studied in order to demonstrate the capability of LF-SAS to provide information of the inside of targets. We demonstrate that the inner resonances of objects are visible and exploitable in these LF-SAS images. Most of the time the reverberation level (RL) of the seafloor is the main limitation factor in sonar images. In order to take into account the RL limitation and buried targets, we have introduced the seafloor boundary into the problem. The backscattering echo can be computed thanks to an approximation of the Helmotz-Kirchoff equation. The synthetic SAS images have been compared to real SAS images produced in the HWU (Heriot Watt University) test tank.