The seismographs that we will be using to image the sub-surface at 13N in the Mid-Atlantic will be deployed under free-fall onto the seabed. In the work area the seabed is, on average, 3500m deep. So any instrument we deploy not only has to survive the journey to and from the seabed and the landing onto the hard bare rock of the seabed, it also has to survive the ambient pressure conditions for the duration of the experiment. We will deploy the instruments on arrival at the work area and recover them just before departure around 32 days later.
At the surface we experience 1 atmosphere of pressure, or ~1 bar. This is equivalent to ~14.7 pounds per square inch – or psi. To put this into context a car tyre is inflated to a pressure of around 30 psi, or about 2 atmosphere.
Pressure increases with depth below sea surface at a rate of about 1 atmosphere every 10m. So by the time our instruments arrive at the seabed – taking about an hour to descend – they will experience 350 atmosphere or around 5200 psi of pressure.
To be sure that our instruments will not implode at these sorts of seabed depths we design them to be able to withstand pressures up to 8000m equivalent, but we will only deploy them to a maximum depth of 6000m. As much of the world’s oceans are shallower than 6000m, this covers most targets of geological interest.
Any new instrument that we build will undergo thorough testing before it is used for data acquisition. The actual data recorders are housed in watertight metal tubes called pressure vessels. We test these pressure vessels in a pressure tank and pump the pressure up to the equivalent of the pressure experienced at 6000m. This checks that they do not implode, and also that they do not leak.
Today we have been testing some newly manufactured pressure vessels including that of the main data recorder, the housing of the main ground motion sensing geophones, and the pressure housing of our newly developed broadband seismometer that we will also be deploying onto the seabed at 13N in the Atlantic for an under real conditions test.
The image below shows a collection of these sensor pressure housings being loaded into the test basket and lowered into the pressure test tank.
This image below shows the top of the tank being lowered into position for sealing before the pressure inside is raised to that equivalent of 6000m depth. Once at that pressure, the pressure vessels inside will be left for an hour, before being depressurised again back to atmospheric pressure. On completion of the test, the pressure housings are opened to see whether there is any fluid inside.
For the tests we have completed today, all of our newly fabricated pressure housings passed with flying colours and can now be used during the 13N research cruise.