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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#MusicMonday: I’ll be your icebreaker

I don’t know of many songs about ice management, but Norway’s ESC contribution from 2016, Icebreaker, comes pretty close. Ok, so maybe the lyrics are not meant to be taken that literally, but for me it’s almost hard not to!

Do you know the song? Or any other songs about ice management?

Lyrics

Every night
Before I sleep a shiver down my spine
Thoughts align
What can I do to make you listen

Like a northern light
You’re dancing over every border line
Passing every sign
Between reality and fiction

Every single promise that you ever made
Spinning in my mind like a hurricane
Baby, yes, I hear your mayday

I’ll be your icebreaker 
When you’re stuck in frozen water 
(When you’re stuck in) 
Frozen water 

You go astray
Just like a Polaroid
You fade away
I’ll be you partner
and liberate you from your prison

Baby, yes, I hear your mayday

We’re way too young to say goodbye
Whenever you loose faith just hold the line
It takes a lot of nerve to save a life

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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Photo of the day: Seaspray and bits of ice

Local time: 2016-09-04 15:52
Position: 84° 42.895’N 171° 59.988’W
Heading: 307°
Speed: 10.3 knots
Water depth: 2010 m
Wind speed: 19.2 m/s
Air temperature: -1.9°C
Sea temperature: -1.4°C

Ice conditions and concentration vary greatly in the Arctic Ocean . The last days we have been in transit in open water. The only ice we see are these bits of ice, bobbing up and down in the waves.

The contrast between the motions of the ship in ice and open water is huge. There are no waves in the ice, as they are damped by the ice. As we entered open water, the ship started rolling and pitching more due to the swell. It has been a long time since anyone has been sea sick on the ship, but today there has been a couple of grey faces. Luckily, we are entering more ice soon!

 

Two ships meet: Oden and LSSL

Two bows and a sunset (Even though it’s morning!? More on that later.)

Local time: 2016-02-09 16:21
Position: 82° 23.40’N 141° 13.14’W
Heading: 65°
Speed: 0.7 knots
Water depth: 2858 m
Wind speed: 6.3 m/s
Air temperature: -2.3°C
Feels like (wind chill): -8.19°C
Sea temperature: -1.4°C

Today we met up with the other ship who is part of the expedition Arctic Ocean 2016, the Louis S. St-Laurent (LSSL). This was the last day where we would sail together, after the meeting we were heading different ways. Therefore, today’s plan was to load over rocks collected from dredging from Oden to LSSL and to visit each other’s ships. Continue reading

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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