Limpet mine imaging sonar (LIMIS)

When divers search for limpet mines on ship hulls in turbid or dark water, they must resort to tactile examination. Acoustic systems that detect objects in turbid water typically suffer from low resolution, a low image refresh rate, a large size, and/or high power consumption. This paper discusses the design, fabrication, and testing of a small, prototype diver-held sonar that generates near- photographic quality images at a fast frame rate. Its weight in air is 7.7 kg, and it is 100 g buoyant in seawater. It is 18 cm wide, 20 cm high, and 35 cm long, including a 10-cm handle. The sonar sues acoustic lenses made from polymethylpentene to form 64 beams, each of which has a beamwidth of 0.3 degrees yielding a 1.6 cm cross-range resolution at 3-m range. The sector display has a 19.2 degree field of view. The frame rate varies with range, going from 5.5 frame/s at 15 m to 12.5 frames/s at ranges less than 4 m. The sonar consumes 25 W. The internal batteries provide 3 hours of operation between charges. External packs and cabled power provide additional power options. The images are seen on a mask-mounted video display and can also be cabled topside to a video monitor. The sonar operates at 2 MHz and has a maximum range of 15 m. This sonar allows divers to sweep hulls more efficiency and with greater safety than possible with current methods.