Methane, a greenhouse gas and also a source of renewable energy, comes from more than just flatulent cows: It also bubbles up from the seafloor. Thousands of so-called methane seeps have been spotted on the ocean bottom.
Scientists typically use visual cues to identify such seeps in action. These visuals include photographs and videos, taken at discrete locations and times, focusing on only a small field of view.
But now researchers have shown that the tiny gas bubbles can be detected using sensitive underwater microphones and that the sound data can be used to estimate the bubbles’ sizes. These acoustic detections allow scientists to monitor large swathes of the seafloor and estimate how much methane is escaping into ocean ecosystems.
“Instead of a snapshot of the activity, we can get a gauge of how things change over time,” said Bob Dziak, an oceanographer at the National Oceanic and Atmospheric Administration and the Pacific Marine Environmental Laboratory in Newport, Ore. Dziak and others are helping to develop the relatively new technique.
Methane Seeps: Tricky to Track
Methane, produced from bacterial activity and the decay of organic material, is found in the ocean as both a gas and an ice-encrusted solid. In places like the Oregon continental margin, where tectonic plates collide, gaseous methane can bubble up from the seafloor.
“Earthquakes potentially give new pathways for methane to rise up,” said Tamara Baumberger, a marine geochemist at Oregon State University and the National Oceanic and Atmospheric Administration in Newport, Ore. That gas can have positive and negative effects. Methane seeps are often sites of rich ecosystems that include microbes, crabs, and clams, but the escaping gas can also contribute to ocean acidification, said Baumberger.
And the number of known methane seeps is increasing exponentially, said H. Paul Johnson, a marine geophysicist at the University of Washington in Seattle who was not involved in the research. Therefore, quantifying how much methane is escaping from the seafloor is a “difficult but important scientific problem,” he said, noting that scientists are still working on accurately estimating the number of methane seeps and the rate at which gas is escaping from them.
Audio measurements could fundamentally help.
Listening to Bubbles
In 2016, Baumberger and her colleagues working aboard the E/V Nautilus deployed a sensitive underwater microphone—a hydrophone—on the seafloor roughly 80 kilometers off the coast of Oregon. They left the instrument for about 12 hours in an area of known methane bubble activity at a depth of roughly 1,200 meters.
Dziak and his team analyzed the data. They detected bubbles moving through the water, producing sound with frequencies between 1 and 45 kilohertz. For context, humans hear sounds with frequencies between 20 hertz and 20 kilohertz.
Here’s a snip of the raw sound file from one of their hydrophones. The crackling sound is a stream of hundreds, maybe thousands, of bubbles.
First, Dziak and his collaborators carefully examined the frequency spectrum of the sound waves they collected, mapping individual peaks to what they presumed were bubble streams. Then, using an equation that relates sound frequency to bubble size, the scientists calculated a range of bubble radii for each frequency range.
Overall, the bubbles ranged in radius from roughly 8 millimeters to 3.5 centimeters, the team reported this spring in Deep-Sea Research Part II. These sizes seem realistic: “The calculated estimates are consistent with that we’re seeing on video,” said Dziak.
What’s more, the data in the audio recording, which can be broken up into time chunks as small as a few milliseconds, give a rough idea of the rate at which bubbles escape. Louder signals imply more escaping bubbles.
This work is a proof of concept that bubble sizes can be accurately measured using sound, Dziak said. That’s useful because hydrophones can be left on the seafloor for days or even weeks to monitor methane seep activity, which is sometimes intermittent.
The next step will be to calculate the total volume of methane being released from the specific section of the seafloor under examination, Dziak explained. To do that, the researchers will need to add up the signals from all of the bubbles detected by the hydrophone.
New data that will be useful for these calculations were just collected earlier this summer. In June, Baumberger and her team completed another cruise on the E/V Nautilus to survey methane seeps between San Francisco, Calif., and Astoria, Ore. The researchers conducted 13 dives with a remotely operated vehicle and collected more than 150 samples of gas and sediments, along with video of bubbles escaping from the seabed. They also deployed another hydrophone on the Oregon continental margin, this time for a full day.
Dziak and his collaborators are eager to analyze the data, particularly to determine how everyday phenomena such as tides might affect the escape of methane. That signal is likely in the data, Dziak explained—we just need to have the “ears” to hear it.