Scientists Tune in to the Earth’s Ambient Hum

Ambient sea noise
The background noise of the deep Earth resembles the random behavior of the sea surface. Advanced techniques can extract robust data from these whispers of information. (Andrew Alden)

In the last ten years, a lively field of research has arisen from the Earth's background noise—what you might call the sound of silence. New research has shown how to monitor offshore oil fields with this quiet, passive technique instead of whale-deafening airgun surveys.

Almost everything important that we do underground—mining, petroleum production, waste disposal, construction planning and deep-Earth research—relies on seismic technology.

Typically, we map the underground the same way a doctor examines your chest by tapping on your ribs, only scaled up. A source of energy is set off, a network of sensors records the arrival of the sound waves, and the resulting set of data is massaged to yield a picture of the ground's structure.

On land, surveying a building site can be done with microphones set in the ground and the energy supplied by a person hitting a steel plate with a sledgehammer.

In geological research, natural earthquakes supply the energy that helps us probe the deep Earth. And out at sea, large areas are surveyed using explosive charges or airguns. This practice is effective but expensive, and the extreme noises involved injure whales.

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The newer, gentler technique uses ambient noise as its energy source. It's analogous to musicians checking out the acoustics of a room. They might walk into the room and clap their hands, sing a few syllables or rap their knuckles on a table. But if they care to, they can do the same by sitting at the table and listening intently to the background noise.

Today, theorists have shown how to make use of the Earth's various background noises. The most useful of these is basically a steady ambient hum at frequencies well below audible sound, which is produced by ocean waves as they interact with the seafloor near the beach.

Ambient seismic noise
The ambient seismic field recorded at the Valhall field. Click here to watch the field shimmer, like waves in a pool. (Sjoerd de Ridder/Stanford)

Large networks of instruments are ideal for listening to this seismic noise and using it to map the ground inside the network. The enormous USArray seismic network has pioneered this method in mapping the deep crust and upper mantle of the United States. The Center for Imaging the Earth's Interior in Boulder, Colorado actually delves deeper into that science.

Stanford University scientist Biondo Biondi has been improving the ambient-noise technique with the cooperation of the Valhall and Ekofisk oilfields, in the North Sea off the coast of Norway. The operators of the fields have outfitted the seafloor there with wired sensor networks, and they do airgun surveys several times a year to monitor conditions.

With his graduate student Sjoerd de Ridder, now at the University of Edinburgh, Biondi has succeeded in using ambient noise data from these networks for the same purpose.

Until now, ambient seismic techniques have needed to save months of data to extract clean results. Using their stream of Norwegian data, Biondi and de Ridder have been perfecting ways to speed the process up. Their most recent papers, including one in the latest issue of Geophysical Research Letters, document their success in mapping the ground with a single day's data.

Ambient seismic mapping
Subsurface map of the Valhall oilfield from a typical airgun survey (left) compared to the map from a single day of ambient noise (right). The white tracks represent the wired network of seismic sensors buried several meters below the seafloor.  (Sjoerd de Ridder/AGU)

This is nearly good enough to monitor the oilfield every day, not just with snapshots a few times a year using ships and airguns. Not only would daily monitoring let operators respond quicker to unforeseen events, but it also would help them save money and whales by using fewer airgun surveys.

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The advances of cutting-edge researchers spread quickly in geoscience today. What one professor can do today, any grad student can do tomorrow, and soon enough it's routine industrial practice. Ambient seismic technology is showing up all over the literature today, from deep-mantle studies to the detection of mine collapses.

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