Airguns – compressed air in a sealed tube that is suddenly released causing an explosive expansion
The airgun arrays on the aft deck of the ship were impressive.
Airgun arrays prior to being deployed.
They are now in the ocean trailing along behind the ship akin to tin cans tied to a car. We have to keep moving now that they are in the water or the airguns will crash into the back of us.
The air in your car tyres is about 2 atmospheres of pressure. Think of the bang when a tyre explodes. The air pressure in our array is 150 atmospheres of pressure. That is some boom. And it happens every 60 seconds for five days, night and day!
The airgun array produces explosive expansions at different sound frequencies. This broad bandwidth of frequencies gives the scientists a clearer picture than one frequency alone. The lower the frequency the further the wave of energy will travel. The size of the air chamber in the gun affects the frequency released – the bigger the chamber the lower the frequency.
The waves released are P-waves – sound waves are a type of P-wave – they are push waves (longitudinal/compression). The number of airguns used and their chamber sizes are chosen to maximise clarity of data received. In the array on the RRS James Cook there are chambers ranging from 100in3 to 500in3.
P-waves and can travel through solids, liquids and gases. Liquids attenuate (absorb) the amplitude of the P-wave signal. So this is how scientists determine if melt/magma is present beneath the ridge axis. Ironically, scientists are happy to have no/low amplitude (wave height) signals as this tells the scientists that there is magma – they actually get excited about a ‘no data’ return at their instruments! The lower the frequency the fewer oscillations (cycles of up and down) will occur therefore less energy is lost from lower frequency waves than the higher ones so they travel further. The signals can reach all the way to the Earth’s mantle – 15km below the sea surface at 13N. Think of a town 15km away from you and how far that is (there and back) – those waves travel a long way.
The lower frequency wave is red and the higher frequency wave is blue.
When the airguns are fired the waves move in all directions, including towards the sea surface. Below is a series of pictures, taken underwater yesterday, that show the air bubbles generated by the airguns.
The waves reflect off the surface of the ocean and back down towards the seabed. This causes interference with the data the scientist are trying to collect. This interference can be seen as a clearly defined notch is the data received and is called a ghost notch. The scientists arrange the airguns at such frequencies that they receive all their data before the ghost notch appears on the frequency spectrum graph or design the array to have a notch at a much higher frequency than the data they are interested in.
The scientists are interested in far left column only. The array is planned so that ghost notches arrive later in the frequency spectrum spectrum graph.
Here you can see the arrival of the air from the airguns at the sea surface
This is what is happening underneath the sea surface
The number of airguns used and the frequencies are chosen to maximise clarity of data received. The airgun array is also arranged to optimise the ability to record the signal back at the surface for waves that have travelled at least 60km below and along the deeper layers in the Earth. The largest airguns are most likely to need maintenance so these are placed on the outside of the array so that they can be easily brought back on board and fixed.
Picture of array schematic
For a more detailed explanation about how airguns are used on the ship please see obsatsea.wordpress.com – Professor Christine’s Peirce’s excellent blog.
Please do ask if you have any questions