….. yet again
We are currently in the last stages of recovering the final ocean-bottom seismographs (OBSs) that we have on the seabed. We recovered four out of the final 14 deployed for the last seismic profile overnight while Autosub was conducting its current mission, and once Autosub is recovered this evening, we recommence the recoveries and hopefully collect six more before Autosub is ready to be deployed for what might well be its final mission.
So what did we record during the last profile – Profile R – during which we also towed the multichannel streamer as well as shooting into the OBS on the seabed.
Here is a data plot from OBS 69 which shows it heard our shots, with signals travelling through the subsurface for at least 55 km away from its position, and it also heard the signal travelling through the water column direct to the instruments for over 100 km. How far is 55 km and 110 km away from where you are? In the case of 55 km, that’s something like Manchester to Sheffield (in the UK for our overseas readers).
As mentioned above we also towed the streamer and measured the same singals bouncing off the interfaces between layers of different density and imaged, for the first time this trip, accumulated sediment in the troughs between the lumps in the surface of the oceanic crust. Our latest profile was oriented north-south and also crossed a fracture zone which is a lateral “tear” between two parts of the was plate to accommodate plate motion on a spherical Earth. There is even sediment in the base of that.
So on the left hand side of the plot above is ~4 million year old oceanic crust with its sediment ponds, the fracture zone lies in the steep gully between the large lumps in the basement, and the very young crust of our work area lies to the right hand side.
And yes we really did deploy OBSs onto the tops of these lumps – the vertical stretching (known as vertical exaggeration) of these kinds of plots makes it look far worse that it actually is – really!
Here are some zoom-ins showing the fracture zone, with an interesting lump in the bottom.
During the trials period before this experiment we also acquired a profile over old oceanic crust near the Canaries. This appeared on an earlier blog, but to remind you what the profile here will look like in about 120 million years time, after which a lot of sediment will have had a chance to accumulate, here is that again.
So for us its four instruments in, 10 still to go, and then we are off to Port of Spain in Trinidad and the experiment will be over. All that will be left to do then is pack up our equipment on the vessel during the transit to port, and then stow it into our 20’ sea freight containers for the long journey back to the UK, once we can get those off one of the upper decks on the RRS James Cook where they have been stored for the duration of the experiment. We can only do that with the ship’s crane in port!