Echoes from the Deep

Hi all, I am Thomas Mestdagh, a PhD student from the Department of Geology of Ghent University. For the second year in a row I am aboard of RV Belgica to join the team that sails out to the Porcupine Seabight. However, we are unfortunately not in the Porcupine Seabight at this moment, since we had to go sheltering for the windy weather again. Instead we are anchored in our beloved Bantry Bay, of which Lotte’s earlier post featured some nice pictures.

Although we are not on the study site, there is still enough work to keep us busy. After the successful dive last Monday, the ROV has done her job for this campaign and has been disconnected from the winch this morning. After a delicious lunch (fresh, local Irish mussels!), David and Lotte gave a preview of the results and scientific outcome of this and last year’s Porcupine campaigns (DynaMOD and GOLLUM) for the RV Belgica crew members. Obviously, they were also very eager to learn what we (the scientists) are doing with the data they help to acquire, and to see that their efforts pay off.

img-20190612-wa0005.jpg

David updating the crew on the scientific work.

Personally, I have various tasks on board, like keeping track of the different activities (and list them in the cruise report), managing this blog page, monitoring the seismic data acquisition etc. I also try to do some preliminary processing of the seismic lines and visualize them, which helps to identify interesting features and plan the locations (and priorities) of potential additional lines. As you can see in the example below (which was acquired last Sunday), the seismic reflection method can provide very nice images of how the subseafloor structures and strata look like. This technique relies on a seismic source (a ‘sparker’ in our case) and receiver (a ‘streamer’), which are towed behind the ship. Every two seconds the sparker emits an acoustic signal, which is reflected at the seabed and at interfaces between layers of different acoustic character in the subsurface. The reflected signal then travels back upwards, where the arrival time of the various reflections is registered by the hydrophones in the streamer (in fact you can compare this to hearing echoes when you shout in the mountains, where your voice is the source, the echo the reflected acoustic signal, and your ear the receiver). The arrival times can be converted to depth, which finally allows us to ‘look’ several hundreds of meters below the seabed (in the example below, this corresponds to going a few million years back in time).

seismisch profiel

One of the 32 seismic profiles acquired during this campaign. This section is 5.5 km long, and shows structures and strata up to 300 – 400 m below the seafloor.

Now we are looking forward to Friday, when hopefully the weather will allow us to spend one more day on the study site to pick up the mooring that we installed the first day of the campaign (see Eoghan’s blog), and to acquire some final seismic profiles. Stay tuned for more!

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