7146 shots later ….

….. and we have made it to the end of the first phase of the seismic survey with the seismic source array intact.

 Looking at our track chart you can see that we have spent the last week apparently going round and round in circles. This might seem odd given that one thing we are trying to do is see (or image) into the crust vertically as if we were cutting through it with a knife.

OBS_shooting_nav_web

The reason we do this lattice of shooting profiles is because we also want to generate signals, that our ocean-bottom seismographs on the seabed record, that are also fired from every direction surrounding each instrument – this direction is called the azimuth.

In this way not only can the individual profiles – 2D lines – be analysed as though we had cut through the crust vertically, we can also look at the structure and rocks of the crust in 3D – or volume-space imaging or seismic tomography as it is known. That way we can see how things change not only across the Mid-Atlantic Ridge, but along it as well – in our case, how the target oceanic core complexes of deep crust and upper mantle rocks may or may not be connected at depth.

It is a similar process to how body scanners work in hospitals, just that what we are trying to image is much bigger in scale and covered by 3000 m plus of sea water – so it’s quite a challenge.

So over the last 7 days we have fired 7146 shots of high pressure air at 2000 psi. Each shot had an air volume was 4800 cubic inches at that pressure, which equates to a total volume of high pressure compressed air vented into the ocean of:

34.3 million cubic inches

or

562104 litres

or

562 cubic metres.

When we recovered the magnetometer that we towed through this phase of the seismic survey, it appears to have had an encounter with something with very sharp teeth which has sliced though the outer waterproof covering of the cable, and left puncture and scrape marks.

P1290035_web

P1290036_trim

So we will be replacing this cable while we recover four of the OBSs currently on the seabed and redeploy those with four more in addition, into a different part of the survey grid before we undertake some more seismic surveying, this time towing the streamer cable as well, whose far end will be more than 3 km behind the vessel.

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