Wednesday, February 18, 2009

Days 24-26

The past few days, we have broken a bit the monotony of the CTD casts by alternating them with VMP casts.
VMP stands for Vertical Microstructure Profiler : it is an instrument designed to measure the dissipation of energy in the ocean, which occurs on centimeter scale. Therefore, it must sample very fast water properties such as temperature, salinity and velocity shear. It must also not be contaminated by any spurious vibrations, and has therefore to be free-falling in the water, so that it is not subjected to any tension in the line linking it to the ship. But since it must be able to sample water properties with a resolution on the order of 1 cm, it must not be falling too rapidly in the water. Therefore, a big plastic fan is attached at the top of the instrument (see picture below, where I am in red, holding the VMP) to slow it down to about half a meter per second. The instrument sensors are mounted at the bottom, so that no structure protrude before them as the instrument is falling through the water.

The game then consists in unspooling enough cable out for the instrument to free fall, and stop it at some level above the bottom, and then respool it to prevent the instrument from hitting the bottom, which could break the fragile sensors ! Given that we do not know the depth precisely (the ship is always drifting a bit over a variable bathymetry), nor the speed at which the cable is unspooled, but only know the pressure at which the instrument is, it is quite of a gamble to approach the bottom as close as possible (turbulent dissipation is often increased near the bottom, so we like to measure it) without running into it. Below, Kjersti is anxiously watching the instrument pressure to give us the signal of spooling back ! She did a very good job : we never hit the bottom, while the people on the other watch hit it twice (without breaking the sensors, fortunately). But they were trying to get much closer to the bottom as we did, since the owner of the instrument was in their watch.

For deep casts, with a lot of cable out, the winch was sometimes not strong enough to respool the cable alone, so we had to help him a bit by pulling the cable in. A good warming exercise when it is cold.

Some winches have an automatic cable driver to help the cable respool properly, but the one we had was lacking such luxury. Therefore we had to do it ourselves with a wooden stick. I literaly became a living cable driver...
NB : the pictures where I appear were taken by Kjersti, who kindly gave them to me.

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