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

摘要

A high-resolution acoustic imaging system is an important aid inturbid water where optical systems fail. The three sonars described inthis paper use acoustic lenses to form near-video-quality images. Thefirst sonar, Limpet Mine Imaging Sonar (LIMIS), is diver-held, forms 64beams, each with a beamwidth of 0.35° in the horizontal axis by7° 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 5and 12 images/second. The second sonar, Glendora Lake Acoustic ImagingSystem, (GLACIS), is used to monitor underwater tests and pans and tiltson a platform that can change depth. This sonar forms 64 beams, eachwith a beamwidth of 0.55° horizontal and 10° vertical. It has a32° field of view, operates at 750 kHz, and forms 5 or 9images/second at an operating range of 60 or 30 m, respectively. Thethird sonar, Acoustic Barnacle Imaging Sonar (ABIS), mounts on an ROVand forms 128 beams, each with a beamwidth of 0.25° horizontal by10° vertical. This sonar has a 32° field-of-view, operates at 3MHz, 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 ofpolymethylpentene to focus sound on a 1-3 composite linear array. Theacoustic lenses form beams at the speed of sound with no circuitry andthus eliminate the complexity and power consumption of conventionalbeamforming electronics. Two disadvantages are (1) the lenses and thespaces between the lenses add volume in front of the transducer array,and (2) multiple reflections between lens surfaces cause internalreverberation. The reverberation inside these sonars is about 40 dB downfrom the target echoes and scatters to form a slightly brighterbackground. No range-shifted “ghosts” of target images areseen