02 januari 2019

Sugar Cubes And Seismic Measurements

Let’s start with some good news: the lost luggage we were all so worried about arrived on time. So complete in terms of scientists and material we left the harbour of Suva on the 11th of December in broad day light with lots of sunshine. There was no time really to get used to the movement of the ship, because within 36 hours of leaving port, we were to deploy the first Ocean Bottom Seismometer (OBS) station, which meant all hands on deck. 

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Leaving the safe harbour in Suva, Fiji. © Philipp Brandl

It was pouring, like somebody pulled the plug from a sink and we were standing right below it

The sunny weather with which we left port unfortunately did not last very long, the weather gods like to tease us. It started raining and I’m not talking about a light drizzle, no, it was pouring, like somebody pulled the plug from a sink and we were standing right below it. In no-time I was completely soaked. Luckily the outside temperature was still over 25 oC so it was not that cold, just very wet. When you have to go outside in very rainy weather, we have a saying in The Netherlands: ‘we are not made of sugar’. It means that you won’t melt in the rain (like a sugar cube would) and so you would just go outside and work, and that’s exactly what we did.

We continued working even though the weather was more like at home than expected. For almost two days after the deployment of the first station we continued assembling equipment, attaching it to a buoyant frame and letting it go into the Pacific Ocean. In between the deployment of the OBS stations, we also set out some Magnetotellurics (MT) stations and we did some heat flow measurements. 

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A Ocean Bottom MagnetoTelluric (OBMT) station is being deployed. © Philipp Brandl

I will briefly explain what kind of data every method acquires. The OBS stations have geophones that record shock waves that are produced by air guns, which are towed behind the ship. The shock waves travel through the layers of the Earth’s crust and sediments and get refracted and reflected. The OBS stations record the refracted waves which can be used to produce images of the subsurface. This will provide us with information about the crustal structure of the Lau Basin. 

Once the heat flow probe wiggles its way into the sediment it will measure the temperature and the conductivity of the rock

The MT stations have a similar appearance as the OBS stations, but instead of geophones the MT stations have four 5-meter long tubes attached on every corner of the station. Inside the tubes there are electrolytes that consist of a membrane adapted to salty waters. The electrolytes measure the geoelectric and geomagnetic field at the Earth’s surface to capture the electrical conductivity of the rocks at the bottom of the ocean. 

Last but not least we also took heat flow measurements along the same profile. The heat flow probe looks like a very big needle which is attached to a cable and lowered into the water in a vertical position. It is designed to penetrate into the sediments that cover the ocean floor. Once the heat flow probe wiggles its way into the sediment it will measure the temperature and the conductivity of the rock for some minutes after which it is recovered again.

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Deployment of the heatflow probe. © Philipp Brandl

Thanks to our efficient work in these first few days, we managed to complete the deployment of all OBS and MT stations and do all the heat flow measurements ahead of the planned schedule. The air guns are towed behind the ship as I am writing this. For the next few days we will be acquiring seismic data, first the refraction seismic data after which we will also deploy a streamer. The streamer will be towed behind the ship and record all the reflected shock waves. Once that’s done, we will start with the most exciting part, if you ask me: the recovery of the OBS and MT instruments. If all goes well, we should recover 61 stations in total. This is quite a challenge, but I am convinced  that we will be able to do so, but…. more on this later.

Warm greetings from the RV Sonne!

This blog post was published as A running start on the Archimedes-I Cruise at the cruise's website on 18 December 2018.

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