We left Donegal at 06.00 this morning for the short steam to St John’s Point. When we arrived at St. John’s Point we anchored the ship to keep her stable while we performed the calibration procedure. St John’s Point is the closest deep, yet sheltered, water suitable for our acoustic calibration. The calibration involves lowering a target reference sphere [essentially a copper sphere about the size of an orange] under the boat and “pinging” it with sound waves specific to the frequency we are calibrating. This is repeated one by one for each of the 4 acoustic frequencies we use onboard [18,38,120 and 200 kHz]. Comparing expected and measured values ensures that all the equipment is working properly before we leave.
Each frequency is configured to pick up different target groups such as fish and plankton. The 38 kHz frequency is perfect for blue whiting due to their gas filled swimbladders. These reflected sound waves are how we detect the fish under the vessel as we progress along our survey transects.
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| One of the copper spheres used in calibration |
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| The reel system used to raise and lower the spheres |
Lowering the spheres can be tricky and involves three poles mounted on the sides of the ship. Reels of fishing line are attached to the poles and these lower and raise the spheres through controls in the drylab. Setup was quick this morning, with one minor electrical problem sorted by giving a motor a good shake [precision engineering at its best].
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| Chief Scientist Ciaran O’Donnell calibrating the acoustic equipment in the drylab. |
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All four frequencies need to be checked, so calibration is a slow process. However all was done by 13.00 and after a quick check to make sure all the equipment was secure, we started the long steam. Next stop Rockall
Hopefully there will be some interesting birds and mammals on the way.
Blog by Graham Johnston & Ciaran O'Donnell
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