Abstract:

Various techniques for the detailed classification of submerged shells insonified by short pulses from either an active sonar or a small explosion are discussed [Ultrasonics 33, 147--153 (1995)]. The returned shell echoes in several signal domains are examined. These included the frequency, the time, and particularly the joint time-frequency domain. The use of Wigner-type distributions was most informative in the latter case. Selected features in these echo-displays which provided information about a certain specific target characteristic were identified. The achieved ``in situ'' classification is rapid, unambiguous, and accurate. Examples dealing with short pulses simultaneously scattered by one, two, or more elastic shells will be shown. Theoretical predictions and measurements show good agreement. This novel analysis in the above domains determines the size, shape, wall thickness, material composition of the shell(s), and their possible filler substance(s), as well as the number of shells involved, and their ranges from the projector. The basis of this technique has been proposed as a possible explanation of how echolocating dolphins successfully identify submerged structures [J. Acoust. Soc. Am. 100, 2820 (1996)].