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  • Mae'r tîm @PrifysgolBangor a @PrifysgolAbertawe yn bwriadu cynnal rhagor o waith gyda'r sector morol Cymru mewn project newydd SEACAMS2.

How does a multi-beam work and what can it measure?

Multi-beam echo sounders are used to acquire water depth information in a survey area; to determine least water depths over critical items such as wrecks, obstructions, and dangers to navigation; and to detect objects in general. Multi-beam echo sounders emit sound waves in the shape of a fan from directly beneath a ship's hull. These systems measure and record the time it takes for the acoustic signal to travel from the transmitter to the seafloor (or object) and back to the receiver. In this way, multi-beam sonars produce a "swath" of soundings (i.e., depths) for broad coverage of a survey area. The coverage area on the seafloor depends on the depth of the water, typically four times the water depth.

Figure 1. Multi-beam bathymetry from the Menai Strait in the area of the Swellies between the two bridges. (colours represent depths with red representing the shallowest areas and blue the deeper areas) The multi-beam data is superimposed on Admiralty Chart Number 1464-3.

Multi-beam systems are also capable of recording data on acoustic backscatter -i.e. the intensity of the returned signal. This shows changes in the type of seabed and can be used for "Bottom Classification" to identify the type of material making up the seafloor provided the backscatter data is supplemented with "ground truth" information from grab samples or underwater photographs.

SEACAMS Multi-beam systems

SEACAMS operate two multi-beam echo sounders, a Reson Seabat 7125 SV2 dual frequency ultrahigh resolution system permanently mounted on the RV Prince Madog and a Reson Seabat 7101 system mounted in the moonpool of a Cheetah Marine 8m catamaran. Both systems utilise a shared Applanix POSMV Wavemaster Inertial Navigation system together with Reson's PDS2000 data acquisition and processing software. These are efficient tools for obtaining 3-D seabed data for determining seafloor conditions including habitat mapping and shipwreck identification.

Figure 2. Multi-beam image of the shipwreck of the SS Derbent compared to a photograph of the ship in operation (Note the masts in both images)

Figure 3. This image produced by Vizworx was created from the multi-beam data provided by SEACAMS and the known geometry of the ship from the photograph.

The map below displays the locations where SEACAMS have collected multi-beam data. The extents of the multi-beam surveys are show by polygons shaded in yellow. Clicking on the polygon will display the surveys metadata. If you are interested in viewing any of these data sets please contact Dr. Michael Roberts, Seacams R & D Manager, Bangor University.