Seismic in the Arctic: Sound can help you see below the ice and the seabed

This post was originally written while I was on the Arctic Ocean 2016 research cruise, but was never published. But hey – it’s Thursday, so why not throw a little #ThrowbackThursday post, and learn something new about seismics?

An important part of the Arctic Ocean 2016 research cruise have been to collect seismic data (more about the reason behind the cruise here). Seismic collection reveals the velocities in the sediments and the structure of the Earth’s crust. The velocities in the sediments are used to document the thickness of and types of sediments. Thus, these results can give new knowledge about the tectonic history of the Arctic!

Seismic surveys are principally made by making a loud bang underwater and recording the sound that comes back. We tow all seismic equipment behind the ship with what is called an umbilical cord (see photos below from a deployment). The air gun that makes the sound is closest to the ship and is towed between 12 to 20 metres below the surface. A streamer is attached behind the air gun, which floats 3 to 10 metres below the surface. The streamer contains hydrophones (underwater microphones) that listen to the reflected and refracted sound from the air gun.

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Deployment of the seismic equipment from the aft deck

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The air gun, before going in the water

When seismic surveys are done in open water, you usually have kilometres of streamers towing behind the ship. Longer streamers gives a larger picture of the sound image, both in time and space. This gives more information to the scientists about the deeper and older layers. When sailing in the ice, it is not possible to have long streamers because of the ice. Therefore we only have 200 metres of streamer, which is really short in a seismic setting. In order to get a better sound picture, buoys with hydrophones are deployed far away to listen to the sound. This serves the function as an artificially prolonged streamer:

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A schematic of seismic refraction (courtesy: Thomas Funck)

Some buoys are deployed directly in the water. They sink, and cannot be recovered. Other listening stations are deployed on the ice with helicopter, and are retrieved after some time, like in the photo below.

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Deploying a listening bouy on the ice (photo courtesy: Thomas Funck)

As I wrote in the post about icebreaking, we were sailing in straight lines due to the seismic lines (but sailing in straight lines is not easy while breaking ice!). The sound signals are recorded in the time domain, which means that in stead of seeing where the signal came from you only know when it came back. To convert this to the spatial domain (to be able to place it on a map), you have to calculate with the speed of sound and do some other calculations. Therefore, the calculations are easier if the signals are recorded along a straight line. Thus, for the most part of the research cruise, we have been breaking a nice, straight line through the ice for the LSSL, while they have been shooting seismic in our wake. However, after we split ways we have done several seismic lines by ourselves. Due to ice conditions, we actually had to first sail along the line and break the ice, then sail back, deploy the seismic equipment and sail the line again. It was a lot of back and forth, but it makes a really nice “broken highway”:

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Broken, Arctic highway; sailing in our own tracks when doing seismics

The sub-surface air gun fires around every 12 seconds. The bang is loud, and you can hear it well all over the ship. It is worst for the people with cabins in the annex, furthest to the aft of the ship. In our cabin on the second deck, the bang is not very prominent. On the bridge, however, you can actually feel the vibrations in your body, in addition to hearing the bang. But you get used to it after a while, and then, when it stops, it suddenly feels like something is wrong.

To minimize the impact of the loud noise on animals, we have a dedicated “mammal observer” on board. When doing seismic, Dale is on the lookout for whales and polar bears in the water. If one is observed, we stop the shooting immediately. Fortunately, we have not seen any mammals while shooting seismic this time. Or maybe I should say unfortunately? After 41 days in the Arctic we still have not seen a single polar bear!

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Seismic results, with some markings on it (courtesy: Thomas Funck)

(Thanks to Thomas Funck and Anders Dahlin for fact checking, providing photos and helping me learn about seismics!)

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Science and identity: Your name is everything

In scientific writing, standard convention is referring to a scientist by last name and/or initial of their first name. So let’s say you want to cite a paper I’ve written, you cite it in the text as Skarbø (year) (given that the paper has author-year referencing style, my favourite), or in the references as Skarbø, R.A. + year, name of paper, publication, etc…

As you get to know a topic, you start recognizing names and citations of authorities in that field. After a while, you many times don’t need to look in the reference list to know which paper the author is referring to, you know the article only by the citation.

Thus, as a scientist, your name is your identify and your everything. However, this can yield some problems.

What if your name is very common? Imagine searching for the author “J Smith“, or “J Wang“, which both gives 100+ hits on Google Scholar author search. In a big research field, how will you know which one is the one you want? In nanoscience alone, there are more than 1,200 “J Wang”‘s!

What if you change your name? Back in the days, most scientists were men, and women were the ones changing their names when marrying, so this was not really a big problem. But these days, scientists are both men and women, and both may change their name when marrying. What if author “H Olsen” goes to being “H Olsen-Solvang” or “H Solvang” after marrying — how will you know it’s the same person?

Fortunately, some clever people have thought of this. Just like research papers can have unique digital identifiers, known as a DOIs, researchers can have the same. An ORCID is a permanent digital identifier for researchers.  By associating it with all you publish, you avoid all the name troubles above, and more. If you want to know what it looks like, here is mine.

The people over at Impactstory has written a great summary of more of the benefits of using ORCID. You should check out the post, and of course register to get an ORCID if you’re a researcher! (It literally takes 30 seconds — easier than signing up for Facebook).

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My face when problems are solved and the weather is nice

First sea trials in ice completed

After three days of sailing, we arrived in the Gulf of Bothnia on Saturday afternoon. This northernmost sea between Sweden and Finland freeze over in winter, and this is where we will conduct the sea trials.

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Ice in the Bothnian Bay.

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Sunset meeting on the bridge.

We are performing sea trials in ice with two offshore anchor handling tug supply (AHTS) vessels. Magne Viking, the vessel I am on, has ice class but is not an icebreaker. This means it can sail and break ice in light ice conditions, due to its reinforced hull. The other vessel, Tor Viking, is an actual icebreaking AHTS. So we send that one in front when we are sailing to break the ice.

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New offshore adventures: SKT2017

Hey guys! I am writing to you from a ship again! The next three weeks I will spend offshore on vessel Magne Viking, an anchor handler with ice class. We are part of what is going to be sea trials in ice with two ships in the Bothnian Bay, north in the Baltic Sea. Joining us is the vessel Tor Viking, an anchor handling vessel that is also an icebreaker.

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Tor Viking and Magne Viking at berth in Landskrona during mobilization.

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Time zones on the top of the world

Illustration of the time zones of the world, where the earth is seen from the top (source).

The last week or so we have been doing bathymetric mapping surveys of the seabed in an area more west, close to Canada. We transited from the pole further south, to around 82°N 142°W, for these surveys. Transiting 7° south may not sound like much, but it is approximately 780 km (I wrote about the conversion between degrees and distances here). I was surprised by the noticeable difference in light.

I want to talk a bit about time zones and longitudes, which are actually a bit tricky up North. When you travel east and west in the world, depending on which longitude (breddegrad) you are on, there are different time zones. This is because night and day occur at different times in different places on Earth, due to Earths rotation. I assume most people are familiar with this. The first figure below illustrate the time zones of the world on a map of polar stereographic projection (the North Pole is in the middle, and the equator on the outer edge. I have another post coming up on maps can be tricky business in the Arctic.) You can see that the closer you get to the Pole, the closer are the lines/longitudes and therefore the time zones. Since the longitudes define the time zones, and the longitudes come together at the poles, the time zones in reality change a lot with small distances up here.

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Halfway through!

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The LSSL following in our wake

Local time: 2016-08-30 21:19
Position: 82° 34.80’N 138° 19.34’W
Heading: 160°
Speed: 2.5 knots
Water depth: 3337 m
Wind speed: 11.3 m/s
Air temperature: -0.75°C
Feels like (wind chill): -7.96°C
Sea temperature: -1.5°C

Monday was day 22 out of 44, so now we are over half way in the expedition. Time flies!

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