Friday, 21 August 2009

Day 5 - 11th August

As I mentioned in yesterday’s update, we spent the majority of today recovering moorings deployed in Kennedy Sound during the last cruise in 2007. There are four types of moorings that have been deployed. The first are the ADCP moorings. These, as I may have mentioned earlier, are used to primarily measure currents throughout the entire water column as well as collecting information on phytoplankton and suspended particulate matter concentrations via backscatter data. The second type of moorings contain ice profiling sonar instruments which as the name suggests are used to measure the thickness of ice floes passing over the instruments footprint. The third type of mooring are T/C moorings which are used to measure temperature and conductivity at specific depths depending on where the instrument has been attached to the mooring line. The final type of moorings are the pressure moorings. These basically measure the height of the sea surface and therefore their data can be used to calculate surface currents resulting from slopes in the sea surface (the sea is not flat!!!!).
All the moorings have basically the same structure. They are anchored to the seafloor using old chains or railway wheels depending on their weight and floats are attached to the mooring lines to keep them vertical. Between the weight and the floats/mooring lines there is an acoustic release which is used during recovery. However there are also some key differences. The ADCP moorings cannot rotate to ensure the current direction is always accurate. Usually a flux gate compass would be used to compensate for any rotation but because we are so near to the magnetic north pole these become useless. The T/C moorings are particularly long and therefore can be hit by the deep draft of icebergs (9/10s of the iceberg is below the water!). Therefore very few floats are used on these moorings enabling them to bend more easily as the ice berg pushes it over before hopefully popping up undamaged the other side.
The moorings are recovered by triggering one of the acoustic releases (each mooring contains two for back up). The process starts by us sending a coded sound pulse to the release to wake it up. This can take time as to save batteries, the release only listens for a minute out of every three. Once it has hopefully woken up we ping it to get a range. Then once the boat has manoeuvred to within a suitable distance and there is no ice around for it to get trapped under, we tell the release to trigger and allow the floats to bring the mooring to the surface. Its an incredibly tense time waiting for the release to wake up, but its awesome when you see a yellow buoy pop out the water just off the bow!
As is becoming the general pattern for the cruise the evening was spent completing another 11 profile CTD section along the mooring line. Fortunately I was only given the 8 till 10pm shift after working constantly for the last few nights. The rest of the evening was spent chilling with the other scientists and the crew before getting a relatively early bed!

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