Combined parametric synthetic and interferometric sonar for the detection of buried objects

Both parametric and synthetic aperture sonar can provide low frequency and high resolution "virtual arrays" either thanks to the medium nonlinearity (parametric) or to extra processing including sonar platform trajectory (SAS). They can thus be relevant tools for the detection of buried objects (need of low frequency for bottom penetration) with a "reasonably sized" array. The SITAR project is an EC funded project for the detection and characterization of buried waste in the Baltic Sea. It requires an acoustic tool for area surveys and detection of "suspicious zones". In this framework, both parametric and synthetic aperture techniques were used concurrently in order to get the best resolution for both bottom and subbottom images. A prototype of parametric SAS has been developed in this project and tested in tank and at sea. The primary frequencies of the prototype are around 100 kHz leading to a secondary one ranging from 10 to 20 kHz. The difference in penetration (between primaries and secondary frequency ranges) is used to discriminate proud from buried targets. Two lines of receiving array were used in order to provide SAS interferometric mapping. Six SAS images and three maps were thus obtained from the same spot, at the same time and with comparable resolution (two high frequency images and a low frequency one for both top and bottom arrays). After a presentation of the prototype and of the experiments, this paper will concentrate on processed images obtained at sea and on the understanding of the physical phenomena involved (in particular their frequency dependence) in order to identify buried objects.