64PE376 - Dagboek 4 (in het Engels)

The Acoustic Eye

When you walk down the Pelagia hallway at 2 o‘clock in the morning, things are quiet. Lights in the floors are constantly burning and in some labs there seem to be some brave souls, wandering around, analyzing some samples, or checking if their machines are still working.

(You can’t let your machines be running without supervision when there is only the slightest possibility that the machine could break down and loose data of hours. Well... and this slightest possibility actually does always exist).

However less people run around because at night we are not deploying any sampling machinery or ROVs from board. So if we don‘t, what is the ship actually doing? Throwing out the anchor scratching through our study site, wasting lots of our valuable time? Of course not.

The reason why we don’t deploy any machinery is not because we are to lazy to work. It is because we scheduled the night-times to multibeaming time.

While most of us are gathering their sleep, the ship is running tracks that allow us to map the area with a multibeam system. Multibeam systems use acoustic waves to see what is happening below the ship. Just like a bat on land. They are our acoustic eye that can look far further through the water than our real eyes can.

Because gas bubbles are responding very strong to acoustic waves, they can be seen really nicely in the Multibeam system (see picture above). Excluding the noise as well as possible we can do a 3D-reconstruction of the methane seep area (indicated by bubble streams, i.e. flares). Now don’t get your hopes up. You won’t see single bubbles. As the resolution is only roughly 1 meter, the 3D-Picture is merging a lot of flares to one blurry mess.

However we can see the development of the bubbles flares through the water column and create a map by slicing through pictures at different heights. Because the bubble density is linked to the back scatter amplitude we can make out in this map where the most active spots are.

But our multibeam surveys go way farther. Close to our main seep area there are a lot single flairs but also other seepage areas that are mapped since 3 years so that we can investigate how stable they are and how they behave throughout the years. Nice about this cruise is, that we have other instruments that do real time measurements during our surveys, making interpretation much better.

Amazing how much data we collect during the night. (And during this whole short cruise anyways) And I want to close today's block thanking the Bridge for checking for us during their shift, that the multibeam is working. Thus allowing me to sleep from time to time.

Well there is more processing to be done to keep the acoustic eye open.

See you...

Peter Urban

This is the last log of the NIOZ Monitoring cruise 2013 Leg 1 (alias the Methane Bubble cruise).

After several days of around the clock collecting data, we have encountered a major misfortune…. a cable of the FLY equipment got entangled in the ship’s propeller. Luckily the crew managed to get the FLY back on board in time. And good we had the ROV to inspect the situation under the ship – however, unfortunately the ROV images confirmed the fear that the cable had entangled itself around the ship’s propeller indeed. This means the end of our short scientific expedition. We are now heading back to Texel using only the bow thrusters, which is more noisy and will take a while….

Despite the bad news, I am very glad we had the opportunity to go with this highly motivated scientific crew, covering all disciplines, to this unique site in the North Sea! This cruise was partly a pilot study and we already got highly interesting data, undertaken collaborative research and experiments while on board, and discussed how to move forward. Let’s hope we can do continue this research next year!

Corina Brussaard, chief scientist