Making Bubbles in the Ocean with an Airgun

September 14, 2008 · Filed Under Journey · Comment

Jessica Robertson, U.S. Geological Survey Public Affairs Specialist

We saw a polar bear and cub today! First we saw the tracks, and finally we spotted them. They were about two miles away, so I couldn’t get a close-up picture. Those on the bridge were peering through their binoculars trying to tell the crowd gathered together on the bow where to look. It took a while to find them, and when we did, it was truly spectacular!

Now, on to science. I learned today that Louis is using airguns, which create an acoustic sound signal under water, to image the geologic structure of the sub-seafloor. Even more interesting, scientists used to use dynamite to generate this sound source before airguns were developed! The airgun pulse is heard by everyone onboard Louis and shakes the fantail when set off. These airguns are also called air hammers or pneumatic sound sources.

So how do the airguns work? The gun has two chambers—a release chamber with vents at the top and a pressure chamber underneath. The pressure chamber contains compressed air at a pressure of 1,800 pounds per square inch. A piston is located between the two chambers and when triggered, it shoots into the release chamber, letting air out of the pressure chamber and into the ocean through the vents. The piston slams down and closes before water can come in. This whole process takes about 10 milliseconds. There are three airguns towed behind the ship at one time and they are fired about every 20 seconds.

The resulting air bubble emits energy waves into the seafloor. A signal bounces back, much like an echo sounder, helping image the underlying geologic structure. So, how are the resulting signals recorded? Louis has a streamer about 100 meters long trailing behind in the ocean. Within the streamer are 16 hydrophone channels, which listen for the waves’ return signals. Scientists also deploy sonobuoys behind the ship, each with an attached hydrophone. The hydrophone transmits the signal from the airguns back to the ship by radio. The sonobuoys drift freely behind the ship for several hours before they self-scuttle and sink to the bottom.

In a previous blog, I discussed how Healy is also imaging the geologic structure of the Arctic sub-seafloor. What’s the difference between the instruments on the two ships? The sub-bottom seismic reflection profiler, which is used on Healy, emits lower energy waves and can reach at most 100 meters into the sediment. The airguns, however, can penetrate 100 times further, reaching through sediment up to 10 kilometers thick.

At one point yesterday, Louis was stuck in the ice, and one would think they could just back up and ram forward to break through, right? In this situation, however, that would not be the most productive solution. Since the streamer is behind the ship, backward movement would tangle it with the propellers and disrupt data collection. In the end, Healy altered their track, made a circle around Louis, and set it free. This shows one of the many benefits of this joint expedition, as they will help us if put in a similar situation.

From the cold,

Jessica Robertson

: Polar bear tracks and remains from their last meal.

: Airguns are deployed by Louis earlier in the expedition. The airguns are a cylinder shape, constructed with heavy stainless steel, and range from 12 to 48 inches in height and 6 to 12 inches in diameter.