Dual-Frequency Identification Sonar (DIDSON)

With increasing frequency, underwater work is situated in rivers and coastal areas where visibility is a fraction of a meter. The Dual-Frequency Identification Sonar (DIDSON) has sufficiently high resolution and rapid refresh rate that it can substitute for optical systems in turbid water where optical systems fail. DIDSON operates at two frequencies, 1.8 MHz or 1.0 MHz, and forms 96 beams spaced 0.3/spl deg/ apart, or 48 beams spaced 0.6/spl deg/ apart respectively. It images out to 12 m at 1.8 MHz and 40 m at 1.0 MHz. It has an update rate between 5-27 frames/s. Identification of objects with this sonar eliminates the need to send a diver to identify them by tactile means. DIDSON provides biologists with a new technology for observing marine life including counting crustaceans and monitoring fish behavior in turbid water. The transmit and receive beams are formed with acoustic lenses with rectangular apertures and made of polymethylpentene plastic and FC-70 liquid. DIDSON can be commanded to focus on objects from 1 m to its maximum range. The sonar is approximately 30-cm long, 20-cm high, and 17-cm wide. It consumes 30 Watts, which is important to submersibles with a power budget. The paper shows images from DIDSON, and discusses the basic principles of beam formation with transducers and lens elements. The sonar forms images with "line focused" beams that provide good images in many but not all conditions. If objects were at the same range in the same beam but at different elevations, this type of imaging could not sort them out. An example would be to try to image an object embedded in a pile of debris on the ocean floor. True video, using "point-focused" optics could meaningfully image the object imbedded in the pile as long as it were not totally covered. Fortunately, for the great majority of imaging tasks, DIDSON provides unambiguous, near-photographic quality images.