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Dual-frequency acoustic lens sonar system developments for the detection of both buried objects and objects proud of the bottom

The development of a wide field of view, dual-frequency acoustic lens sonar system compatible with both diver-held operation and small vehicle installation is described. The lower frequency is for buried target detection while the higher frequency is to provide an imaging/classification capability of non-buried targets. The lower frequency subsystem was fabricated from thin solid disks of polymethylpentene with a focal length of 34.3 cm. The design of this subsystem was evaluated by comparing measured and theoretical beampatterns and by determining the levels of internal reflections from lens surfaces. A measurement was conducted in a very shallow water area to asses the acoustic performance of the lower frequency lens design and to determine the optimum combination of frequency and aperture sizes. This measurement utilized three partially populated lower frequency subsystems with aperture diameters of 20, 25, and 30 cm; partially buried and fully buried targets; and a buried transducer array to determine the coherence of the transmitted signal in the sediment. Results are presented. Future system developments which include the design of the higher frequency subsystem are described.