An application of tapered, PZT composite lenses in an acoustic imaging sonar with 1-cm resolution

This paper describes an experimental sonar with a resolution of 1 cm and a maximum range of 2.4 m. It was built to inspect hulls for fouling and damage in turbid water where optical systems fail. The system specifications called for a forward-looking sonar that could ensonify an area 0.6 m×1.5 m with 1-cm resolution. The sonar would mount on a remotely operated vehicle (ROV) that crawled systematically over the hull to assess fouling or to specific points to assess damage. The image needed to be refreshed rapidly enough that the sonar could also serve as a navigation aid for the ROV. Our solution was a mechanically scanned system with four transducers on a single shaft. The operating range and resolution allowed a refresh rate of two scans per second. The maximum range of 2.4 m allowed an operating frequency of 3 MHz, and thus a transducer aperture of only 12 cm was needed to obtain the 0.2° beamwidth. Each transducer was cut from a PZT composite. The cut transducers were given a tapered, diamond shape that significantly reduced the beampattern sidelobes in azimuth and elevation. Unfortunately, the calculated far field of the transducers was 22.6 m! The far field was shortened to 2 m by placing a plano-concave lens in front of each tapered PZT composite. The in-water electronics were connected to the surface electronics by a 150-m fiber optic cable. The system provided images with enough detail to allow the user to note individual barnacles or colonies of barnacles, peeling paint, and details on fixtures mounted on the hulls