Beamforming and imaging with acoustic lenses in small, high-frequency sonars

A high-resolution acoustic imaging system is an important aid in turbid water where optical systems fail. The three sonars described in this paper use acoustic lenses to form near-video-quality images. The first sonar, Limpet Mine Imaging Sonar (LIMIS), is diver-held, forms 64 beams, each with a beamwidth of 0.35° in the horizontal axis by 7° in the vertical axis. This sonar has a 20° field-of-view, operates at 2 MHz, has a practical range of 10 m, and forms between 5 and 12 images/second. The second sonar, Glendora Lake Acoustic Imaging System, (GLACIS), is used to monitor underwater tests and pans and tilts on a platform that can change depth. This sonar forms 64 beams, each with a beamwidth of 0.55° horizontal and 10° vertical. It has a 32° field of view, operates at 750 kHz, and forms 5 or 9 images/second at an operating range of 60 or 30 m, respectively. The third sonar, Acoustic Barnacle Imaging Sonar (ABIS), mounts on an ROV and forms 128 beams, each with a beamwidth of 0.25° horizontal by 10° vertical. This sonar has a 32° field-of-view, operates at 3 MHz, has a range between 1.8 m and 2.4 m, and forms 64 images/second. All three sonars use a set of thin, acoustic lenses made of polymethylpentene to focus sound on a 1-3 composite linear array. The acoustic lenses form beams at the speed of sound with no circuitry and thus eliminate the complexity and power consumption of conventional beamforming electronics. Two disadvantages are (1) the lenses and the spaces between the lenses add volume in front of the transducer array, and (2) multiple reflections between lens surfaces cause internal reverberation. The reverberation inside these sonars is about 40 dB down from the target echoes and scatters to form a slightly brighter background. No range-shifted “ghosts” of target images are seen