For me as one the few geologists on board, this cruise is a great opportunity to learn more about how seismic data is gathered and processed – as I have to admit I did not really focus on geophysics during my studies. Now, having completed the first profile and on our way to the second and third, I think I already learned a couple of things so far:
At first I was really surprised to see how many different sizes and shapes the devices have, we – in need of a better word – just throw overboard to sit on the seafloor to collect our data. There are yellow and orange drum shaped floating bodies made of wood we call “lobsters” or simply just “drums”. Then, we have some really large devices looking like giant Easter eggs, some of them mounted on three legs, others have just one. There are glass spheres and block-shaped devices, but basically all devices share the same purpose to listen to and to record the sound waves we later produce to let us see beneath the seafloor.
Getting all the devices to their designated position though is not as trivial as it may sound. Weights have to be constructed and attached to the OBHs and OBSs. There is also a lot of screwing, heaving, maneuvering – not only the ship itself, but also the devices on board – and also programming involved during the preparation of what may appear a simple operation (shooting a profile). It also seems to prove useful to always have a considerable amount of duct tape and zip ties with you when going at sea.
Once all listening devices are deployed the second part of the operation is executed by first bringing out the airguns, a device which basically consists of a long string of tubes and cables and is towed behind the vessel. I was lucky to be part in both deploying and recovering the device so I can tell you that there are not only the actual “shooting” devices, that are connected by quite heavy tubes and cables and are later fed by them, but also the cable string as thick as my thigh that is not easy to carry or even to pull. Thus, this procedure always requires a lot of teamwork and good communication between all involved persons, crew as well as technicians and other helpers, such as me. Then, the “shooting” begins. The airgun canisters are filled with air at a maximum pressure around two hundred bars by a compressor located on the backdeck. Once every minute the air is released as a shot under water by the 12 airguns we use, simultaneously. This, on one hand generates a muffled sound like a canon heard from a long distance but on the other hand also generates a tremor that you can feel regardless where you are on the ship. These waves or more precisely the reflections of these waves from penetrating the different subsurface layers can later be used to do our geologic research. This however is done later during the processing and interpretation of the data.
After the profile is finished, the ship turns again and with the airguns back on deck, the recovery operation starts. Each device has its unique code that can be sent on a specific frequency and when the so-called “releaser” receives this code, it lets go of the weight that is holding it and begins its ascent. It seems pretty obvious when you think about it, but I was quite surprised at first when I heard that it can take up to one hour or even longer for one device to come back up again from sometimes more than over five or six thousand metres water depth here in the study area. There is of course always the risk that one of the devices does not come back – which can have a number of reasons – and when you look in the faces of everyone, there is always a smile when finally the red colour of the flag during daytime or the flashing light of the beacon during nighttime is spotted from the monkey island above the bridge. Sometimes, you can see an even wider smile when the device and the stored data with it are safely back on board again. And I have to say it did not take very long and I am now also thrilled every time we are waiting for our equipment to come back to us to the surface.
If you are still reading now I would like to thank you and hope I did not use too many technical terms describing our operations. As for life on board here on the R/V METEOR I can only say it is absolutely fantastic. I have not really said before but here on board, there are three different working groups (one American, one British and one German group), all contributing to this project concerning the seismic investigation of the Cayman trough and the slow-spreading ridge. The first few days were more characterized by intensive preparations and maybe to some point even nervousness, with everyone hoping that everything would go as planned. But now, after a couple of days and after the first successful operation I think everyone has more or less settled in living and working here. As I already said before I am not a geophysicist, but everyone here on board is very friendly and kind explaining the things to me that I yet do not know about and also apart from that I think we are all working together very good. That also includes the crew here on board, which are very experienced with these kind of operations and are always willing to assist in any way they can, so that at the end of the day everyone is happy and we can all sit in the lounge having a beer together and get to know each other a little better. I, for myself, enjoy this cruise M115 very much so far and I am looking forward to the coming three weeks to work and live here with this amazing team of people on this interesting and sometimes challenging project.
Cheers from the R/V METEOR,
Florian Gausepohl, geologist, GEOMAR, Kiel (Germany)