Friday, 5 December 2008


Without air and any one of us would die in a matter of minutes. In my work, air can kill in a millisecond.

How can this be? Air is the stuff of life, a child’s plaything. Heat it and direct it into a vast bag, a hot air balloon. In the movies it is something to blow across the hair of an actress or up the skirts of Marilyn Monroe. Put it under pressure though, and our saviour and toy can rapidly turn nemesis.

It is a simple process. All a compressor is is a series of motorised bicycle pumps. The air enters the first cylinder at atmospheric pressure which as you know, is the weight of the atmosphere we are adapted to live at. As the air passes through the various stages of compression, it is compressed down and down in volume until it leaves the final stage at 300 BAR – that is three hundred times atmospheric pressure. From there it is passed into a series of gas bottles which acts as a reservoir.

What is the reservoir used for? The output is fed down to a regulator: basically a valve which takes the pressure down to 140BAR. From there another hose leads to a seismic gun. This isn’t the kind of gun that is hand-held. They vary in size but an average weight is about 50 kilograms and they look more like a heavy missile or a skinny beer keg rather than an Glock or a Colt. There purpose is to store and then release a set amount of high pressure air upon command and to do this instantaneously – certainly within one millisecond of the other guns in the array. Although it is possible to use a single gun, they are more often used in conjunction with others. But whether it is one or twenty in a cluster, the purpose is the same: to generate a seismic signal.

This is a complicated method to generate seismic energy. The most straightforward way would be to use explosive. An explosive is a substance that compresses the air (or water) around it by burning ferociously. High explosive burns at a rate that is quicker that the speed of sound through air, thus adding to the force of the shock wave generated. As a generator of a seismic signal, explosive has no peer: it generates every frequency simultaneously. But that is the reason why it is so deadly: both the direct blast from a bomb and a seismic gun will kill but another killer is high-frequency energy. An air gun is designed to generate most of its signal at low frequencies. Thus it is much safer and more environmentally friendly to use an air gun in a marine environment than compared to an explosive. Flipper and his friends may get upset but unless they are directly caught with the blast, they won’t be injured or killed. Of course, another benefit of an air gun over explosive is that it can be reloaded with compressed air. A five kilogram charge is one-use only!

I’ll leave you with a little horror story as to what high-pressure air can do. A sub-sea engineer once showed me a scar across his thumb and then told me how he got it. He was cleaning a needle valve, unaware there was 700 BAR pressure behind it. The release blew a hole through his glove and through his thumb leaving an entry and exit wound and out though the glove on the other side. He was flown to hospital where his thumb had to be cut open to flush the grease out of the wound. The wound had to remain open for five weeks.

Air. Maybe a little more respect in future?
(The picture above shows a four-gun seismic source. The guns are the metal tubes on the sides of the frame.)

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