Why a ship needs an airgun

The cruise is nearly over. Everyone now is busy packing, cleaning and writing the cruise report. Everyone? No! One group is actually working for the first time in weeks: the seismic team. Last time they worked in the middle of the night. But this week end they started soon after dinner, which allowed me to go and observe them.

First, let’s be clear about their intention. They are from Danish universities, we are in Greenland waters, but they are not here for oil prospection. They simply do not have the right equipment for it. And I’m not sure why they are called “seismic” actually, as for me that means “people who study earthquakes” (but I may be lost in translation on that one), which they don’t either.

So what do they do? They deploy an airgun and a series of hydrophones (i.e. microphones to listen underwater) behind the ship, and they shoot the airgun every 5s. From the ship, all that we can see is a large bubble, surfacing regularly. Sometimes we can also hear the “bang” of the airgun.

Trail of airgun bubbles on wavy Friday night

Trail of airgun bubbles on wavy Friday night

The airgun creates a shockwave that will travel to the sea floor, even penetrate it for about 200 m, be reflected and deflected by the various sediments and rocks that compose the sea floor, and eventually come back and be heard by the hydrophones. By measuring the time between the airgun shot and the reception of the reflected signal by the hydrophones, we learn what the sea floor is made of. It’s a bit like a sonar, but at a lower frequency: 100 Hz for the airgun, several thousand for the multibeam aboard. And thanks to this lower frequency, the seismic team can look much deeper than the multibeam, or the coring. Seismic studies actually are the usual complement of coring studies: the coring sees a very small portion of the sea floor at a high resolution, and the low resolution seismic sees how far (and deep) the area around the coring site, with similar characteristics, extends. Plus, seismic studies are non-destructive: you do not need to collect a sample of the sea floor to see through it.

Now, for various reasons, I am not allowed to show you what they see on their screens as they work, or what the sea floor looks like according to the series of hydrophones. And I am expected to join a mud sculpture competition now-ish. So I am leaving you with a picture of the most sophisticated and precious bit of equipment that can be found in their container:

The seismic coffee machine!

The seismic coffee machine!

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