Study, Design and Concept of Low Frequency Sas

Abstract

In the MCM (Mine Counter Measure) context, a lot of effort has been put in imagery systems and especially in high frequency sonar. The latest generation of sonar, SAS (Synthetic Aperture Sonar) systems, has been developed in the last 15 years and provides a new powerful tool for mine 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 centre frequency) but, despite the extremely good quality of the SAS images, ambiguities between mine-like objects cannot always be resolved. This study investigates how low frequency SAS systems (LF-SAS) could address the two main limitations of the present systems linked essentially to the sound absorption at high frequencies: providing imagery inside the target and 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 limiting factor in the interpretation of sonar images. A computation of the RL has been done using the small perturbation theory on a rough seabed and we demonstrate that the first resonant echoes are still visible relative to the RL.