Three vessels are available to the BSH for the re-survey: the survey, wreck-search and research vessels DENEB, ATAIR, and WEGA. They are equipped with multi-beam echo sounders (except ATAIR), single-beam echo sounders, and side-scan sonar systems. DENEB additionally has an object search sonar. Divers will be deployed as well.
The BSH will use two methods in its Order 1 survey. One method will be a survey using a multibeam echo sounder meeting the feature detection criteria (2 m cubic features), which presently only exists on board WEGA. The other method involves use of a single-beam echosounder in connection with a state-of-the-art side-scan sonar system. The former method is required to obtain a complete picture of depth conditions in an area with a highly mobile seabed and high relief energy. The latter is adequate when the seabed
is relatively uniform and there are hardly any major depth differences. Complete side-scan sonar coverage ensures that individual objects on the seabed are detected. On board DENEB, this method is supported by an object search sonar scanning the area in front of the vessel. The shallowest depth of features projecting from the seabed is then determined during a wreck investigation.
Section from a side-scan sonar image
Both of the above methods have already been tested in a survey of the Terschelling-German Bight Traffic Separation Scheme in the North Sea and in surveying a submarine training area in the Baltic Sea. The latter method allows clearly faster surveying and evaluation, and has therefore become the standard method used. The HELCOM re-survey is carried out primarily by the survey, wreck search and research vessel DENEB, which is supported temporarily by a second multi-purpose vessel. The ship’s master decides on
the number and spacing of survey lines, taking into account local conditions like, e.g., depth, relief energy, sea state. The ships alternate in two-week periods of deployment. If weather conditions in a particular area are unfavourable, the vessels are deployed in other, more sheltered waters.
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