Today’s blog is about transect line R. The data has been received from instruments recovered today. So is very – hot off the press. The data requires more processing by scientists and this will take many months, even years.
Diagram 1 – Below: A map to indicate the position of transect line R, the approximate ridge axis, to the North this is not yet resolved, and also the transform fault. Transform faults occur because the Earth is round and the Ridge axis is a North/South tear.
Line R is 190km in length which is about the distance from Durham to Edinburgh.
Diagram 2 – Below: Line R from the 3km yellow streamer data – hydrophones being trailed behind the RRS James Cook (see picture 1).
Diagram 3 -Below: Line R annotated. Note the position of OBS 69.
Picture 1 – Below: The yellow streamer with hydrophones which record the image shown above.
Picture 2 – Below: An OBS (Ocean Bottom Seismic) Instrument that sits on the seafloor recording data.
See obsatsea.wordpress.com for more information.
Diagram 4 – Below: I thought you might find the image below useful to look at again. See if you can use this information to to interpret the data from OBS 69.
Diagram 5 – Below: OBS 69 with shots recorded over 55km distance in total and about 10km depth. That’s a long way subsurface – about the distance from Manchester to Sheffield.
Diagram 6 – Below: Here is a copy of OBS 69 that I have interpreted (with a little help) for you.
Amazingly the water waves can be recorded at a distance of over 100km!!
The scientists use more than one set of data for their results. The two ways for line R are the yellow streamer and the OBS (reflection and refraction seismics).
When information is received by scientists often data has to be altered to make sense. An angular rocky surface gives distinct hyperbolic shapes.
Diagram 7- The diagram below shows how hyperbolic shapes occur. When there is a point on the seabed that scatters sound waves, the scattered waves can be detected even when the ship isn’t directly above the scatter point. Since there is no way to know where the energy is coming from though, the incoming energy is plotted below this ship’s position. This means the energy is plotted in the wrong place, along curves known as hyperbolae.
Diagram 8 – Above: You can see these hyperbolic shapes in the diagram above. This data is also from Line R (see Diagram 3 so see precise location). Note the term TWO WAY TRAVEL TIME – so called because it is the time it takes for the acoustic signal to reach a surface and return to the receiver – there and back if you like.
Diagram 9 – Below: The data has been processed to remove hyperbolae.
You can see the image is much clearer and gives a more true depiction of the bathymetry.
This process is called MIGRATION.
Diagram 10 – Below: a close up of an unmigrated and migrated seismic image.
The horizontal lines are sediment.
Diagram 11 – Below: A simple interpretation of the seismic data in Diagram 9.
All the scientists have to work with great vertical exaggeration to be able to analyse their results.
Diagram 12: True vertical scale for diagram 11.
Pictures 13 and 14 – Below: Sediment that was on one of the OBS when it returned to the surface. There is little sediment here so this is unusual. The sediment comes from detritus that sinks to the bottom of the ocean. It mostly consists of small shells and foraminifera (single celled shell species).
How cool was that blog? I’m learning soooo much. I do hope you have enjoyed reading this post as much as I enjoyed writing it. A picture of Nigel for all those with Nigel fixation 🙂