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Good Vibrations: a closer look at the Ocean Bottom Seismometer (OBS)

Updated: May 19, 2021

An OBS is a high-tech instrument that is used to measure what is going on under our feet from metres to kilometres beneath the seafloor. The instrument consists of several parts:

  • Seismometer

  • Hydrophone

  • Data Logger

  • Releaser

  • Flag

The seismometer, or geophone, measures tiny ground vibrations. These can be due to earthquakes, underwater volcanoes, waves in the ocean, storms (and even marine vessels!). These events may cause very, very small ground vibrations that we may not sense unaided.

Orange OBS's lined up on the deck of the RV Celtic Explorer. The orange colour helps us identify it when we come back to recover them.

The hydrophone is a microphone designed to work underwater to listen to and record underwater sound. This way, you can hear how dolphins communicate with each other later on!

The Data-logger stores all the recordings taken by the OBS. When the OBS is back on the vessel we can download the data and start working with it. The OBS also contain batteries inside it so that it can operate while we leave it alone on the ocean floor.

One of the OBS's being lowered into the water. The OBS will sink down to the seafloor, which can be as deep as 3500 metres (and even deeper in some places!)

The last important component is the Releaser. It enables us to send a signal from our vessel to wake the OBS up and bring it to the sea surface, where we can take it back to the deck. The releaser plays a vital role, which is why we tested all of them before deploying the OBS (See the previous blog post on CTD).

The retrieval process can be tricky as the OBS will drift away from the location where we released it and we need to keep looking for it when it comes close to the sea surface. That's why the body of the instrument is painted orange. An orange flag is also attached to the instrument to help us see it when it resurfaces. There's also a flashing beacon attached to the OBS to help retrieval during the night time, as the flag is useful only during daytime.

A happy crew just after deploying the last OBS. Thankfully we have had calm water and fair weather throughout our deployment.

When depolying an OBS the vessel has to come to a halt and turn to shelter the OBS, to ensure the water isn't too choppy. We lower the OBS into the water then release it and it sinks to the seabed. We have now deployed 27 OBS's equally spaced along a 400 kilometre track in North Atlantic. We aim to retrieve these OBS's in two days time. Stay tuned.

As always, follow us on our social media accounts: Twitter, Instagram and Facebook.

We'd love to hear from you, if you have any questions for us or any aspect of the PORO CLIM Expedition that you'd like to see, let us know by dropping us a message or comment.

Written by Haleh Karbala Ali with additions from Niamh Faulkner and Ben Couvin

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