Hydrographic Society, Denmark

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The old Surveyor
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State-of-the-art Survey
Scour Protection Swathe Survey in Danish Water Svitzer Surveys is presently performing detailed bathymetric surveying for Sound and Belt A/S in areas around the Great Belt Bridge in Denmark. The main purpose of this year’s work is to perform a detailed survey of the protective islands surrounding the anchor blocks to the suspension bridge - the East Bridge - crossing the eastern part of he Great Belt. The surface and sub-sea topography of the protective islands are surveyed and monitored at regular intervals with the purpose of controlling the islands’ ability to protect the anchor blocks against collision from vessels off course. Surveying the protective islands around each of the anchor blocks is only part of the survey programme, undertaken at regular intervals at the Great Belt crossing.

The West Bridge, as for the East Bridge, is founded on caissons immersed on sand and stone layers on the sea floor. After the placement of the caissons a stone layer for scour protection was established around each caisson. It is essential for the stability of the bridges, that the scour protection is well maintained and any erosion near the caissons is avoided. With the purpose of monitoring possible changes to the scour protection and the nearby seabed topography, detailed bathymetric surveying is also regularly performed in these areas.

The work has for quite some years been a challenge for Svitzer Surveys. All specific survey areas are to be fully covered by swathe bathymetry in depths from 1 to 50 metres. The physical environment for surveying close to and under bridges is relatively hostile; mainly for following two reasons:

• Under the bridge - and to some extent close to the bridge - traditional RTK GPS positioning has to be sup-ported by alternative methods as line of sight to the GPS satellites is cut off by the bridge element
  
• The acoustic environment for swathe bathymetry is difficult as the caisson wall often represents a different acoustic reflector compared to the scour protection and the natural seabed
  

The positioning of the survey vessel is determined using RTK GPS in combination with Doppler log technique and a range bearing robot station operated from the top of pier shafts under the bridge.
As the nature of the seabed - natural seabed, scour protection and caisson wall - represents different reflection and thereby a difficult acoustic environment, the correct adjustment of the power output and gains, together with other variables, has become vital.
SeaBat 9001 and SeaBat 8101 multibeam echo sounders have been used as the swathe system. As before, the survey is currently being performed with the survey boat Svitzer Surveyor and the following list indicates the basic set-up in use:

• Dual RTK positioning with reference station established on Sprogoe. One RTK system at the bow and one at the stern
  
• Doppler log
  
• Leica robotic total station for positioning close to and nearby the L
  
• SeaBat multibeam echo sounder including heave compensation
  
• Atlas Deso 25 (210/30 KHz) echo sounder
  
• Gyro compass
  
• NaviPac, NaviLine and NaviBat data acquisition and processing software
  

The objective of the surveys has been to achieve a state-of-the-art survey, which determines the topography of the seabed to an accuracy in X,Y,Z of better than 10 cm. Further info: main@ svitzer-surveys.dk

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Contact
Hydrographic Society, Denmark.
Att. Hugo Meister
Agern Alle 11, DK-2970 Horsholm
Denmark
Tel: +45 45169458
Fax: +45 45169292
www.hydrosoc.dk