Monday, August 23, 2010

Anatomy of an Airgun

So far most of the talks have been an introduction to what we do, but little respect has been paid as to how we do it. Therefor, today I will discuss the not so humble air gun.The gun pictured at right is not of our guns but of a single gun to give you an idea. We use an air gun array composed of 40 guns (similar to the one at right), 36 of which fire in tandem while 4 are left on standby. The total capacity of the array when operating at maximum is 6600 cubic inches of air. The remaining four guns are used in situations where we lose power to any of the other guns. However, there is a catch. The system cannot exceed 6600 cubic inches and each of the four standby guns weigh in at a hefty 180 cubic inches each. The largest guns are 360 cubic inches and the smallest are 60. So if a 60 goes out, we stop sending air to it, but if a 360 goes out we turn on two of the standby's to bring the volume back to 6600.

If you need a rough approximation of what 6600 cubic inches pressurized to 2000 psi of air exploding is like, imagine that a standard SCUBA tank is pressurized to 3000 psi and is roughly 80 cubic feet at one atmosphere of pressure 80 cubic feet = 138240 cubic inches. 6600 cu in * 2000 psi = 1.1*10^6 ft lbs of force. The SCUBA tank if it explodes would be 138240 * 3000 = 3.456*10^7 ft lbs of force. The SCUBA tank is an order of magnitude larger, however this does not take away from the power of the air guns. Think of the tank as a bomb whereas the air guns are a controlled source. Regardless of the reference the air guns are still dangerous and they are kept at 2000 psi almost at all times. This requires one powerful air compressor. We refill the air guns every 20s (or 50m whichever comes first), so we need a large volume compressor to basically instantaneously fill the guns to be ready to fire for the next shot. If we were using the volume of the SCUBA tank we would need a compressor that was an order of magnitude greater in volume output and 1.5 times more powerful. The point is, is this is not JAWS and we do not condone exploding SCUBA tanks as our source material.

The air guns are relatively deep penetration sources, operating at 100 to about 1200 Hz, to identify subsurfacegeologic layers and define the subsurface structure. In studies that require less resolution but substantial penetration, the air gun is usually preferable as compared to a water gun, because it is far more efficient at producing low frequency energy. It can be used in fresh or brackish (less saline) water found in lacustrine and estuarine environments. Both air guns (and water guns) can be used in shallow w
ater surveys and relatively deeper water environments, achieving resolution on the order of 10 to 15 meters and up to 2000 meters penetration. With proper tuning, the air guns work well in a wide variety of bottom types. Minimum operating water depths of about 10 meters are possible in acoustically “soft” bottoms. In areas with acoustically hard bottoms, deeper water depths of operation are required. The harder bottoms produce multiples, unwanted reflective energy that travels repeatedly between the sea surface and the sea floor or shallow-subsurface and obscures the desired primary reflected energy arrivals.

The air gun requires an air compressor on board the ship. For maximum resolution, the smallest chamber size is used. If maximum penetration is the goal, a larger chamber is configured, but resolution is lessened. Both guns have a stable and repeatable pulse in terms of frequency composition and amplitude and can be tuned to optimize the source signature.Air guns generate more signal strength than boomer, and sparker, and chirp systems. The air gun is towed astern. The return signals are received by a towed hydrophone array.

This post has been updated. The volume for the guns was misunderstood. The current post reflects the changes.

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