Thin, acoustic lenses applied in 64-beam, 750-kHz diver-held sonar

Divers and small submersibles often need to find and identify objects and sense the terrain in turbid water. This paper discusses a prototype, diver-held imaging sonar that uses aspherical lenses instead of an electronic beamformer. The lenses and transducer array acoustically form 64 fan beams. The central 32 beams are each 0.5° in azimuth and 10° in elevation. The outer 32 beams are 0.75° in azimuth and 10° in elevation. Each beam ensonifies a narrow stripe on the bottom; together they form a sector display covering a 40° field of view. The lenses are made of polymethylpentene plastic and RTV60 rubber selected for their acoustic refraction and absorption qualities. The sidelobes of the beams are down 20 dB, and the internal reverberation among the lens elements is down 35 dB. Divers record images digitally in the sonar´s memory and with a video recorder patched to the sonar video output. The sonar, including batteries, weighs 16.4 kg in air and operates between 3 and 4 hours on a single charge. A color video display connected to the sonar and mounted on the diver´s mask provides the diver with high-resolution acoustic images of objects out to a range of 60 m. The images are updated between six and nine times per second. The sonar also provides auxiliary information such as the diver´s location, the locations of marked targets within a 1000 m by 1000 m grid, the water temperature, and the sonar´s depth, altitude, and heading. The acoustic lenses allow a tremendous amount of processing to be done acoustically. This allows a smaller, slower, less power-consuming processor and electronics cards