BobGeo 2009. Day one at sea

My first time at sea started today when we set sail this morning at 8.00AM.Wednesday 14th – Day 1 at sea

My first time at sea started today when we set sail this morning at 8.00AM. We were having breakfast at the time and I did not even realise we were moving until someone pointed it out. It was still dark at the time and the only indication we were on the go was seeing the harbour lights moving past the window.

As this is my first time at sea I was not sure how well I would cope with being sea-sick. However, it was a beautiful morning with a very light swell and while my stomach only gave the occasional impression of being on a roller coaster, the patches I was given by my Doctor seem to be working very well. I hope this continues if the weather changes and the sea gets rougher.

The first thing we did this morning was to get together for a cruise safety briefing from the Chief Officer of the ship. This was followed with an introduction from all of the BoBGeo team and a short talk from Jean-Francois.

Sophie Arnaud

I have to thank Sophie Arnaud for translating the meeting for Alessandra Savini and myself who are the only two non-French-speaking members of the team.

Guess who?

A short time after the meeting we had a safety drill to abandon the ship and had to go to our muster (gathering) points where we were shown where to get our survival suits, life jackets, lifeboat and the inflatable life rafts were in the event that the lifeboat could not be launched. After this we all headed for the bridge to try on a survival suit. Let us hope that we do not have to do it for real!

Sediment profiling using Chirp
Since we left harbour the subop acquisition (Chirp) has been in operation using high resolution seismic waves to record the profile of the sea floor, like slicing a cake to show the different layers. The beam goes straight down from under the boat and can only penetrate sedimentary surface of he seafloor but is unable to penetrate rock.

High resolution Seismic profiling
In the afternoon the other seismic system was launched. The image below show the buoy being deployed, at the bottom centre you can see the drum of cable which will be attached to the buoy a runs out to 600m behind the ship.

Deploying the seismic buoy

This system also penetrates the sea floor using a different frequency which is lower than chirp but does not produce the same detail as the Chirp. It can penetrate the rock interfaces in the seafloor and goes much deeper into the seabed, around 300m. The signal can penetrate further but loses resolution as it goes deeper.

Alexis

Thankyou to Alexis Rochat for helping me to explain some of how it works.

Multibeam
There is another system which looks at the seafloor called multibeam which can produce a bathymetric map, formed by using Isobaths (lines of the same depth). Not to be confused with Isobars on a weather map showing lines of the air pressure on land. From the multibeam we also get a signal which is called backscatter. This is like an echo from the sea bed and depending on the strength of the echo it should be possible to determine the density of the seafloor i.e. a strong echo might indicate rock that bounces back most of the signal while a weak echo indicates soft sediment which absorbs most of the signal. Like a golf ball which bounces high off concrete, not so high off grass or gets stuck in the sand bunker and does not bounce at all.

Multibeam Bathymetry and Backscatter images. Data from Medico Cruise, RV Pourquoi pas? (2007).

Alessandra

While we are sailing through the night these instruments will gather their data from several sweeps which when processed will help the scientists and geologists decide where the best place to send the carottier (corer) down to recover the sediment samples for the sedimentologists and biologists to examine.

As I have said earlier, it is a very steep learning curve and I would like to thank Alessandra Savini for her help and patience in preparing this blog. Alessandra is a geologist and we will hear more once the cores start coming on board.

Mapping the world ocean floor

Mary Tharp completed the first World Ocean Floor map. You can find out more about Marie Tharp and her colleague Bruce Heezen at the Columbia News web site.

I had hoped to have some video to place on our YouTube site but unfortunately the video editing software on my laptop is not capable of doing anything more than trimming a clip and cannot join two pieces of video together. I am hoping to download some suitable software but due to the limitations of our connection, it may have to be downloaded at night when the load on the connection is less.

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