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Bank Holiday Antics

It seemed like a small miracle that even when sailing in international waters, hundreds of miles offshore this ship has WiFi, email access and telephone access. Sadly on Friday the dream was over and the internet satellite gave up the ghost. Unable to email and instant message the outside world, or even desperately refresh BBC News for some more reading material, it looked set to be a bleak few days. Thankfully the Catering team stepped up and laid on a huge BBQ on the back deck for everyone. It wouldn’t be a Bank Holiday Weekend BBQ without some wind and rain (yes, it does rain here in the Caribbean), but we’re British and we persevered through it. This is also the last of a back log of blog posts since the internet crash, originally from Bank Holiday Monday 29th May.

Not fazed by the spitting rain and threatening skies, we celebrated the end of survey line one with a Bank Holiday weekend barbecue.

In my last post, the magnetometer got all the limelight, yet we have more tools at our disposal on the RRS James Cook to help understand the geology here in the Caribbean.

Magnetic properties may be particularly unique to certain rock types, but all sediments have a mass and a density and we can exploit this when measuring the local gravity field. Typically, on the surface of the Earth, if an object is dropped then the rate at which it will accelerate towards the Earth is 9.8m/s². Variations in the material underneath your feet, however, will vary ever so slightly what the acceleration due to gravity is.

We can therefore exploit this property to locate where there is surplus or missing mass, relative to an average mass and gravity pull in an area. Additional mass may suggest the presence of a very dense material and will leave us with a “gravity high”, whereas a mass deficit and possibly a lower density will show as a “gravity low”. One of the possible conduits for water into the system that we have discussed is these fracture zones from the Mid-Atlantic Ridge and the precarious triple junction where the North American, South American and African Plates meet. Serpentine group minerals that we might expect in these fracture zones, are less dense and so we are on the look out for diagnostic gravity lows. The Atlantic has also been squeezed from the North and South as well as the East and West and along these faults we see high topography, dense ridges, and so we’re looking for gravity highs along these features. Gravity measurements along with the magnetics become important considerations to tie in with the active source seismic data when we interpret it.

Where we may expect to see gravity variations in a subduction zone

Actually recording gravity is a very sensitive process and one raw measurement may need many corrections applied to account for exactly where the recording was taken. On Earth for example, as the planet spins on its axis, it becomes ever so slightly squashed at the north and south pole, so it’s not a perfect sphere but oblate. This means the radius of the Earth at the equator is ever so slightly more than at the poles. This also means if you were to stand at the equator you are further away from the centre of the Earth than at the poles. If we know gravity is “stronger” the closer you are to the Earth’s surface then gravity at the equator is “weaker” than we might expect. This difference is tiny, around 0.5%, but it’s measurable and we can correct for it with a “latitude correction”. The ship is also moving significantly whilst we record the data and so we must account for motion corrections too. These are called Eotvos corrections and accurately account for the balancing forces required due to the Earth’s spin.

The ship is fitted with a gravimeter on board, in its own special temperature controlled room. It is mounted in a frame surrounded on all sides by damping springs which act to keep the machine steady even as the ship pitches and rolls over the waves. The gravimeter is very sensitive so the room is kept at a constant temperature to ensure none of the springs or internal mechanisms expand or contract with temperature, and therefore stiffen or loosen.

Our gravimeter on board is located at the very centre of the ship in its own temperature controlled room.

Whenever the James Cook comes into port, the instrumentation team take an independent portable land gravimeter out to record the local reading. This is reading we refer to as a base station and we use it to tie in with our readings we have made at sea. It is normally measured at the dockside and the team carefully measure the height difference between the two gravimeters to ensure they are measuring from the same point. The local gravity in one place can vary from the beginning to the end of the day even by slight changes due to the moon and tidal forces, or temperature change of the equipment mechanisms – this is called drift. We account for drift and also try to limit the errors by recording 3 times and taking an average.

The seismic equipment joins the ship for limited periods and comes bundled with its own team of technicians, whereas the gravimeter is looked after by the Scientific Ship Systems team. These are technicians who work at NERC and are responsible for other instruments that are permanently fixed on board like the acoustic sounding and magnetometer. Headed up by Andy on this cruise, without his expertise we would have a lot less data to work with.

Update since internet crash: Andy is also the saviour who fixed the internet satellite on Monday evening along with Chris, so this blog post is an all-round appreciation post for the most popular guys on the ship at the moment.