| Abstrakt: |
For 3 years, the Institute of Oceanographic Sciences has operated a long-range side-scan sonar for viewing the texture and topography of the ocean floor (project G.L.O.R.I.A., Geological Long Range Inclined Asdic). It was recommended that an experiment should be arranged to discover if such a device could detect commercial pelagic fish on the Continental Shelf at long ranges and, if so, to determine any research on commercial applications. This paper describes such a trial, carried out on a Scottish inshore herring fishery in the Sea of the Hebrides, during late September,1971. A survey was first made in the area by echosounder and short-range side-scan sonar to confirm the bathymetry, to locate false targets, and to observe the distribution and diurnal vertical movement of the herring. During the subsequent G.L.O.R.I.A. runs, up to 9kW of acoustic power was transmitted at 6.4 kHz, with the signal returns processed by a linear correlator with a time-bandwidth product of 400, so that signal/noise ratios obtained were equivalent to a short-pulse sonar with peak powers in excess of 1 mW. Due to the summer water-conditions, it was found that the propagation depended critically on the position of the source in the water column. Under the best conditions, with the source towed at 33 m in an isothermal surface layer, herring were detected to a range of 15 km in a water depth of 120 to 170 m. For a period of 3 days, a fishery area of 170 km was kept under surveillance by steaming up and down 13 km-long base lines at a speed of 13 km/h facing SE towards Hawes Bank and the islands of Tiree and Coll. As a result, a plan view of the area, including both fish and the geological features of the bank, was generated every 1 1/4 h. It was found possible to track certain aggregations of herring using these records for periods up to 5 h. On the final might, remotely directed catches were made on 3 aggregations by guiding a purse-seine vessel to within 1 km of each target. The internal composition of the aggregations detected by G.L.O.R.I.A. is discussed in terms of both the echosounding data and expanded A-scan excerpts from the recorded G.L.O.R.I.A. target signals. Possible research and commercial applications for the long-range detection of pelagic fish by low-frequency towed or fixed sonar systems are briefly discussed. [ABSTRACT FROM AUTHOR] |
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