By Paolo Franchi
Simulating seabed characterisation using the CMRE high-resolution low-frequency synthetic aperture mine-hunting sonar. Nielsen, Peter L.; Hollett, Reginald D.; Troiano, Luigi; Canepa, Gaetano. CMRE-FR-2015-023. December 2015.
Reliable sonar performance estimates and probability of mine burial prediction are two very important, not yet completely solved, problems in mine hunting planning and operations. Both sonar performance and mine burial depend on the seabed properties, which often are considered as the most difficult underwater environmental information to obtain. Buried mines are particularly difficult to detect and classify, because of the complex interaction between the acoustic field, seabed and mine. The Centre for Maritime Research and Experimentation (CMRE) has established a quay-side test facility to evaluate a unique low-frequency mine hunting sonar for seabed characterisation. The sonar is wide band and the monostatic source-receiver units are composed of a transducer matrix where the elements are partly operating individually. A frequency invariant shading technique is applied to both source and receive units to obtain constant main lobe amplitude and vertical beamwidth with minimum side lobes. This beamforming technique reduces the impact of multi-path arrivals and allows for direct backscatter measurements from frequency independent patch sizes of the seabed. A technique for acoustic remote sensing of the seabed properties is proposed in this report. The technique is based on traditional matched-field processing of direct path backscattering from a fluid seabed, and the algorithm provides a best estimate of the seabed properties with associated uncertainties. The acoustic backscattered field is calculated by a state-of-the-art low-frequency backscattering model called BLASST, developed at CMRE. The estimated seabed properties and their uncertainties are obtained from synthetically generated scenarios to evaluate the seabed information contents in the sonar backscattered intensity. The results are considered as guidelines in preparation of measurements at the quayside rail facility in 2016. The intention is to apply the algorithm to data acquired by a prototype version of the sonar system installed at the quayside rail, and the performance of the seabed characterisation algorithm will be assessed and recommendations of future development and procedures will be proposed. Ground truth environmental surveys have been conducted at the quayside rail, and the data have been analysed and are presented in this report to support the results from the proposed environmental characterisation algorithm.