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Short Range Lateral Variability of Seabed Properties (With Some Notes on Larger Scale Features) Near Port Hedland, W.A.,

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The spatial variability of seabed sediment properties over short ranges is investigated, and it is found that, at least for sands, sediment grain size varies within a factor of square root of 2 over distances of order 100 m. Evidence is then presented that this sediment variability, found off Port Hedland, is similar to that at many other locations around the world. Hence for acoustic backscatter and mine burial models the conventional categories very coarse, coarse, medium, fine and very fine, for sands are as precise as it is practical to be. This implies that survey methods, with, for example, acoustic sea floor classification systems, need only provide sediment grain size to this level of accuracy. It also means that, for mine-counter measures purposes, conventional survey methods can be relatively simple, and that many existing data bases are quite adequate. From underwater video footage it is clear that many important seabed features, such as shell beds, branching corals and seaweed clumps, can easily be overlooked in sea floor surveys, with either grabs or corers alone, and that this, at times, would lead to misleading conclusions concerning environmental factors relevant to mine warfare operations. A number of interesting seabed features have been observed near Port Hedland using a sub-bottom profiler and diver-operated underwater video cameras. Because so little is known in this area, it was thought these observations were worth recording, as an appendix to this report. In particular video-camera observations of some of the long, linear, underwater ridges off Port Hedland established them to be rocky reefs, rather than sand bars, as was previously thought. This changes previous perceptions of likely mine burial mechanisms off a number of Northwest Shelf ports.

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  • Geology, Geochemistry and Mineralogy
  • Undersea and Antisubmarine Warfare

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