The Art of Sound in the Ocean

Jim Gaherty
December 21, 2011

The NoMelt experiment aims to image the structure of an oceanic plate, including its deepest reaches up to 70 km beneath the seafloor.  One of our primary means to do so is to create sound (acoustic) waves in the ocean from the ship, and record those waves at receivers on the seafloor, after they have traveled 10’s-100’s of miles through the rocks that underlie the ocean basin.  The R/V Langseth is equipped with a large airgun array, which is capable of producing such sounds.

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Airgun (silver cylinder) hanging from its float (black) as it is being deployed by gun tech.

Each airgun consists of a steel cylinder that can hold a large volume of compressed air.  When fired, the gun forces the air into a bubble in the water, which quickly pops and collapses under the water pressure.  The “bang” associated with the collapsing bubble travels efficiently through the water, and when it reaches the seafloor, much of the sound energy converts to seismic waves that travel through the crust and mantle.  With a single airgun, the sound is loud enough to penetrate only into the upper layers of sediment beneath the ship.  We can crank up the volume by firing multiple guns at once, allowing the energy to travel deeper into the Earth and to greater distances.  During NoMelt, the Langseth tows an array of 36 airguns that produce sufficient energy to probe well into the oceanic plate and travel back up to receivers deployed several hundred km away on the seafloor (see my previous post).

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Float supporting a 9-gun array being deploy aft of ship. Guns hang below float, and a GPS positioning unit projects up. Yellow cables carry compressed air to three identical arrays previously deployed.

But just being loud is not sufficient.  Simply shooting all the guns at once will produce a loud but “ringy” sound, in much the same way that turning a stereo up to maximum volume (11!) will distort the music.  The Langseth’s airgun array is “tuned” to produce the ideal sound for our purpose.  In practice, this means firing the guns microseconds apart, such that they interfere to produce a sharp, clean sonic pulse, rich in the low frequencies (think bass, rather than treble) that penetrate most effectively into the Earth.  The array is also oriented to direct these pulses downward into the seafloor, rather than in all directions into the surrounding ocean.  Finally, we fire the guns only once every 4 to 5 minutes, much more slowly than most seismic surveys.  This allows the noisy echoes within the water column to die away, even out at the most distant instruments.  This is critical for detecting the subtle lower amplitude arrivals returning from deep in the plate.

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Gun array deployed 200 meters behind ship. Bubbles from previous shot just visible behind the gun floats.

Over 11 days, we traversed 1000 miles of the ocean floor, traveling at 4 mph, shooting every 4 to 5 minutes, 24 hours a day.  Student watchstanders and technicians continuously monitored the computers controlling the shooting.  Protected species observers also worked around the clock, searching for nearby marine mammals (whales, dolphins) and protected sea turtles that may be sensitive to the noise.   This is only a concern within  ~1000 meters of the ship, but to be safe we monitor and report any activity up to four times this distance.  In 11 days, we only encountered one small pod of sperm whales; when they meandered too close, we shut down our operation until they left the area, and then circled around and restarted the survey.

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PSO watch tower high above the ship.

If all goes well, the recordings of these shots on the ocean bottom seismometers (OBS) will provide a truly unique portrait of the deeper (mantle) portion of the oceanic plate.  This structure has not been comprehensively explored since the pre-airgun 1970’s, when large explosives tossed off the ship (essentially scientific depth charges) served as the sound source.  Instrumentation and analysis tools have improved immeasurably since then.  We now need to retrieve our OBS…