Seismic reflection profiling: Going “underground”

We left Petermann fjord with a bang, literally. While shooting big compressed air bubbles into the water with the injector air gun, the Oden sailed into Hall Basin.
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Earth science is mostly a very tangible science. You can touch a rock, run your fingers through sand, study a fossil with the naked eye, but seismic reflection profiling seems to be a bit out of place with its weird lines and wiggles. Jokingly, it is often called a geo-fantasy.

 

What Earth scientists essentially do is reconstruct what the world looked like at certain moments in the past. But this is a world we have never seen and a world we will never be able to see, unless time travel becomes a reality. Sometimes, then, there must be room for imagination.

 

The seismic reflection method makes use of the properties of the velocity of sound. When we create a sound at or near the surface of the Earth, some energy will be reflected back (bounces back). These reflections can be characterized as echoes; from these echoes we can determine the structure of the seafloor and subsurface as each rock or sediment has a different density and sound velocity. We can also find out the depths where these echoes came from. In a way, we are talking to the Earth.

 

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To get these images we tow an acoustic source, an air gun which sends the sound waves, as well as a long array of hydrophones, which register the echoes. Every 5 seconds we blast a compressed air bubble into the water. This blast creates a sound wave into the water column which will travel in all directions, penetrating into the seafloor below. You can compare the creation of the bubble with throwing a really big rock in a pond which causes the water to ripple.

 

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In medicine we use a whole array of analyses to determine what is happening in our body. Doctors and nurses will scan and probe, touch and measure. Earth science is not that different. The seafloor and the Earth´s crust below could be considered our patient. We determine its exact shape, length, width, and height with multi-beam mapping. Seismic reflection profiling, on the other hand, could be compared to taking an X-ray. Like an X-ray,seismic reflection profiling takes an imag slice of the Earth’s interior. It is only an image in shades of grey, but without having to start digging you get a quick overview of what is going on below the surface, hidden below meters of water and seafloor.

 

By taking several slices at different angles, cross cutting each other, we can determine if the bumpy landscape we see on the multi-beam is made out of bedrock or of soft sediments deposited by the glaciers, or if there are any faults in the area. Once we know what material is where, we can better decide where we would like to take some sediment core samples to reveal even more details.

 

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Seismic reflection profiling is a rather quick and non-destructive method that gives a good overview of the underground. It is a very useful technique to tie the whole evolutionary story of this area together –  we can see how the different sediment basins are separated by all kinds of sills and were affected by different geological processes, an how they interacted with each other through time.

 

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Written by: Katrien Heirman

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