A Few Weeks with the MVP

By Charles Brouard and Sarah Dyck

Now that the ice has started to break up the CCGS Amundsen has become an open water vessel and we can start taking advantage of the MVP. During our months of winter hibernation, the MVP is stored away, and the rosette is used via the moonpool every time we need to take a vertical profile of the water column. MVP stands for moving vessel profiler. It is a hydrodynamic CTD attached by a kevlar electro-mechanical cable to a really fast winch at the aft of the ship. Rather than stop the ship at a station and lower the heavy rosette and its CTD on a relatively slow winch, we can lower the MVP and take the measurements we need quickly while the ship is in motion. Another advantage of the MVP is the fact that, in theory, its operation is totally automated: the fish goes down to a predetermined depth off of the bottom (automatically determined by sonar), winds up automatically, waits for a specified amount of time, then descends again to take another profile a few nautical miles farther along the ship track. All of this happens while we are enjoying ourselves at the crew bar or sleeping!!!! Just joking! In reality, this complicated system needs constant supervision. During transects we need to constantly monitor MVP behavior to avoid problems as we watch for steep slopes in the bathymetry

(Fig. 1 Charles attending the MVP. Photo by Eva Alou Font).

Another crewman needs to be “on watch” at the aft of the ship, near the winch to intervene at a moments notice to take manual control in case of an emergency (the MVP is a really expensive piece of equipment and nobody wants to take the risk of losing it at the bottom of the sea , not that it ever happens of course....).

Now let's talk about the numbers! The MVP is a really fast profiler, analyzing samples of water 25 times a second. The winch itself is so fast that the fish is going to the bottom virtually in freefall (which means that the winch is supplying about 7 meters of cable per second!!!) The maximum speed that the winch is able to wind is about 3 meters per second, but we usually use a slower speed to lessen stress on the cable and avoid problems. The spool of cable is 1700 meters long, which allows us to take profiles as deep as 300 meters while moving at the speed of 12 knots. As the Admundsen Gulf is at most about 600 meters deep, and as we strive for a good safety margin, we usually need to limit our profiling speed to 5 to 8 knots, taking a profile every few nautical miles.

We now need to talk about the Fish!

The fish is what is attached to the business end of the cable. It is so named because it quite frankly looks like a fish (two eyes, an open mouth and a tail)!!! During ascent and descent, water travels through the open mouth of the fish, to the “guts”, where the instrumentation is located. The instrumentation measures conductivity, temperature and pressure which are used to calculate the density of sea water in the water column. Some sensors also allow us to measure fluorescence and transmissivity, which are parameters important to many biologists, physicists and chemists alike.

During the last three weeks the MVP has been used to profile McLure Strait, a northern passageway that has rarely been studied, either scientifically or otherwise due to ice cover. We have also completed a transect from the MacKenzie River across the Amundsen Gulf. When we began the transect from Cape Bathurst we ran into serious sea ice conditions, providing many obstacles during transit and limiting the usefulness of the MVP. We were therefore forced to use the fish as a stationary CTD when the ice conditions permitted, stopping the ship for a few minutes every time we lowered it and taking it out of the water between each cast. That is when the MVP becomes a pain to use!!! Nonetheless, even in the icy conditions of the Canadian Arctic, the MVP is a wonderful instrument allowing us to efficiently take home an incredible amount of much needed spatially distributed physical data concerning the waters of the Amundsen Gulf and its surroundings.