Three Dimensional Interpretation of Sonar Image for Fisheries Research
Three-dimensional interpretation of fish images from a sector scanning sonar to determine the shape and the distribution of fish schools was investigated. The sonar drove a half-circular cylindrical array transducer, which was mechanically rotatable on two axes. It emitted 2ms acoustic pulses of 160kHz in a half-circular plane omnidirectionally, then received echoes by scanning the insonified plane. In general use, the scanning plane is tilted a few degree downward from the surface, and the fish school echoes are indicated on a display as a PPI image around the ship. Therefore we can only observe fish echoes as a two-dimensional image, even if the scanning plane is tilted largely from the surface.
The new idea we discuss here is to reconstruct a three-dimensional image from hundreds of sectional sonar images using a computer image processing technique. In this method, the scanning plane is set vertically downward and perpendicular to the ship’s course. Two hundred successive sectional images were digitized and the three-dimensional information was displayed as a top view projection and a side view projection along the ship,s course, this method of data processing allows the shape and the distribution of fish schools and bottom features to be easily understood.
A fisheries survey using sonar was conducted near artificial fish reefs located at about one hundred meters depth. Many fish-school echoes were observed and the sonar images were analyzed. While the general fishery echo sounder provides information only for fish schools directly below a ship, this new method is a promising technique to obtain three-dimensional information from fisheries surveys.
KeywordsSwimming Behavior Fish School Sonar Image Scanning Plane View Projection
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