24 June - How we collect the data

Tuesday, June 24th 2009, Scottish Shelf
By Jane Read, physical oceanographer at NOCS

I’ve been skimming through the blogs and I think there is one area of work at sea which hasn’t been mentioned. All the scientists have come on board with grand plans for their research, whether its finding out about obscure elements or processes, or looking for the underlying mechanisms of global change and its effects – but where would we be without the equipment to provide us with the material we analyse?

Although some of the scientists have brought their own data collection equipment with them (Chris has his aerosol collectors, Dave has his plankton nets and Sebastian has a special “fish” to pump “clean” water in over the side of the ship), the rest of us depend to greater or lesser extent on the equipment provided for us from the national marine equipment pool. On this cruise we have depended on the shipboard instruments, such as the underway measurements of sea and air temperatures, fluorescence and wind, navigation, vessel mounted acoustic Doppler current metres, and most importantly, the over-the-side CTD (conductivity / temperature / depth).

The CTD is a more than a single instrument. The frame (as tall as a man with a diameter wider than he can reach) contains a plethora of instruments that measure conductivity (which we convert to salinity), temperature, pressure (depth), fluorescence (chlorophyll), transmittance and currents. It has an altimeter to measure height off the bottom, but that’s just to stop us bouncing the package on the seabed. It also holds a carousel of Niskin bottles, which are cleverly designed to collect water from whatever depth we require. The package is connected to a conducting cable so that as it is lowered over the side, it returns the signals from the instruments and we can see what it is measuring and choose where to sample. The CTD console, 3 separate computer monitors between two instrument racks is the hub of acitivity on board the ship. The variety and arrangement of lab space on Discovery can lead to isolation of scientists, but we all meet around the CTD. The crew look in to check what is happening and even the occasional officer has been known to make an appearance. Notices are posted and information exchanged around the CTD console. (There is also plenty of chat, backchat and laughter, It’s an important meeting place).

Once the Niskin bottles have been fired and the CTD is back on deck there is a complex procedure of sampling the water from the bottles for the analyses described in preceding blogs.

The whole process is time consuming and getting good results depends on our marine technicians and crew. The crew have the job of driving the winch (see Ian’s blog), and those who saw the CTD deployed and recovered in the rough seas we experienced mid-cruise, will appreciate just how tricky that task can be. The length of time a single CTD can take depends on the depth of the ocean where we are working. We estimate the time for a cast as 1 hour per 1000m depth (or 1km of wire paid out), so in the 3000 m water we had in the Iceland Basin it took 3 hours to complete a cast. On the Wyville Thomson Ridge, where we’ve been working the last few days, depths varied from 800 m to 1800 m. Our next stop is the continental shelf where the depths are about 100 m – CTDs every 20 minutes – eek!

(Just in case you wondered, I’m processing the data from the CTD, and it is just as time consuming, cleaning up, calibrating and making the data accessible, as it is collecting it).

The CTD package (and all the shipboard instruments) is looked after by our two technicians, Jon and Chris. Since we make CTD measurements around the clock, they have to work round the clock too, each working a 12 hour shift every day. They are responsible for maintaining the instruments, preparing the rosette before each cast “cocking the bottles”, supervising deploment, monitoring the measurements throughout the cast, bringing the package back on board and making it available for the scientists.

Throughout the cruise the package has worked flawlessly, and our only problems were with the frayed cable on the winch. These wires get worn with constant winding in and out and exposure to the elements, so it is quite normal to have to cut away lengths. It loses us time, but better that than losing the package (which has happened). So, a huge thanks to our technicians for the fantastic job they have done, and also to the crew – and has anyone mentioned the officers on the bridge, who have the task of maintaining the ship in exactly the same spot for hours on end?

Thanks guys!