GCSE Seismic Science Today
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 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 possibly 10 – 13 km below the sea surface at 13N. Think of a town 13 km 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 frequencies 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
Collecting Seismic Data:
PROBLEM: For each shot gather (one shot, lots of receivers) the point on the geological boundary where the reflection occurs varies with distance between the shot and the receivers. If there is any lateral variation (e.g. folding, dip etc.) then image would smear out badly.
SOLUTION: using data from many different shot gathers (each with lots of receiver traces)re-sort the traces so they don’t have a common shot, but so they have a common mid point = CMP gathers – approximates common reflection point
· each trace is a shot-receiver pair with a common mid point (e.g. shots all on one side, receivers all on the other). See below:
· a – single ray path of energy.
· b – each shot samples several locations
· c- some rays are not needed by scientist and are discarded
· d- the acoustic records arranged not by a common shot but by a common mid point. CMP. This will give a trace line of zero at the CMP. Scientists can work out data from there.
· A trace is vertical black line as seen in the diagram below.
What happens next?
Collecting Seismic Data Continued
How does seismic reflection imaging work?
· The airgun makes an acoustic (sound) wave.
· This wave travels through the water in all directions.
· Some waves reflects off the sea floor. The angle of bounce in will be the same as the angle of bounce out.
· Other waves will travel through the sea floor and into the rock below.
· Some waves will reflect from rock and sediment layers below the sea floor. These waves tell scientists about the structure of the Earth.
· The scientists discard the waves they don’t need.
· Faults can be seen as breaks in the rock layers but can usually only be seen after a great deal of processing.
A multiple is a reflection of the original ‘boom’ on the recording of the data.
Symmetrical and Asymmetrical Spreading and Ocean Core Complexes
Symmetrical and Asymmetrical Spreading
About 10 years ago, scientists discovered that there is a new mode of seafloor spreading operating in some 50% of the Mid-Atlantic Ridge. Whereas the ‘classical’ mode of spreading is symmetric (see diagram), this new mode is highly asymmetric.
In the symmetric mode, as the plates separate, ductile asthenosphere wells up to fill the gap, and partially melts (about 10%). The melt rises and solidifies to form the gabbro plutons, doleritic dykes and basaltic lavas of the crust. The melt may accumulate in one or more small mid-crustal magma chambers on the way up. Once the crust has formed, the stresses pulling the plates apart produce normal faults, with displacements of ~100m, which add some 5-10% of strain. The structures of both plates are very similar, as are their accretion or spreading rates; new material is added equally to both sides.
The newly discovered asymmetric mode is quite different. Such faults may penetrate right through the lithosphere (detachment fault), and accumulate displacements of tens of kilometres over millions of years (see diagram). The hanging wall plate (right-hand one in the diagram) receives less melt than normal and so its crust is thinner. The footwall plate (on the left) is formed by pulling mantle material up. If all of the plate separation is taken up by slip on the detachment fault, then no new material will be added to the hanging wall plate, and spreading will be 100% asymmetric. The resulting seafloor landform of an Ocean Core Complex (OCC) is formed. The one studied by the JC132 at 13oN in the Atlantic Ocean is about the size of Ben Nevis.
(Diagrams created for the Teacher At Sea by Professor Roger Searle – the diagrams have extreme vertical exaggeration)