All the moorings in the water!

This week you’ll get a letter from my colleague Svein Osterhus who is onboard another research ship on the other side of the continent… but I thought I’d send you a short update from the Amundsen Sea and a new problem for you to solve!

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Foto: Povl Abrahamsen


All of my moorings are now in the water – and the nervous butterflies that have flown around in my belly the last couple of weeks are finally gone! There are so many things to prepare, so many things to think about when putting the moorings together – so many things that can go wrong. When I’ve deployed moorings before I’ve always had a technician with me, a technician who has full control over what screw goes where and where those shackles and rings are supposed to be… but this time it was me who was out on deck with a spanner in my hand trying to get everything ready in time. Finally Araon got into position and we could start!

Mr Ham, the Korean mooring technician, gave sign to the winch driver and he elegantly lifts my first orange buoy into the water. Araon is slowly advancing as we feed the mooring line out behind us. Every fifty meter or so mr Ham stops, and Karen and I attach an instrument to the line. Everything goes quick and smoothly – we all know what to do. The buoy gets smaller and smaller in the distance, and soon there is 375 m of mooring line out in the water. Only the anchor (3 old railway wheels) is left on deck, and finally it too disappears into the water with a splash. The orange buoys are pulled through the water as the anchors sinks, but eventually gravity wins and they too are gone. The anchors sinks down and lands on the bottom, normally somewhere between the location it was released and the place where the buoys are last seen. To find out exactly where the mooring is, we use the acoustic release to “triangulate”. At three positions we send down signals to the acoustic release, and based on the time it takes from the signal to come back we can calculate the distance between the hydrophone that we lower into the water to talk to the acoustic releaser and the releaser itself. Since we know where the boat is, we can determine where the releaser (and thus the mooring) is located… maybe you’d want to help out?

When the anchor to the first of my moorings were released we moved 443 m towards -40.2 degrees (see explanation of the directions in the end) and the first triangulation point. The distance to the releaser was then 802 m. Thereafter we moved 862 m towards 172 degrees and got a distance of 754 m. At the last point, the distance to the releaser was 754 m. Then we’d moved 867 m in towards 53 degrees from point number two. The depth is 600 m and the releaser is locate 25 m above the anchor. The hydrophone was lowered 10 m into the water.
Where is the anchor? How far had the anchor drifted? How exact is the triangulation? Can you estimate the error?

 

The deck unit talking to the acoustic release assumes a sound speed of 1497 m/s when calculating the distances. Using data from our CTD cast we calculated the true sound speed (which is a function of salinity, temperature and pressure) and found the mean speed of sound to be 1447 m/s. Does that matter for the calculations above? Can you correct for it?

The directions are given as degrees counterclockwise from the x-axis. The X-axis is pointing eastward, so if we go towards the east, the direction is 0 degrees, if we go north, it is 90 degrees and if we go south, it is -90 degrees.

 

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Foto: Elin Darelius Chiche

 

Do the Math! Straight lines and melting ice

Experiment: How cold can water get?

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