Sonar is a powerful method for peering into the ocean’s depths. By sending out a pulse of sound and detecting echoes that scatter off the seafloor or other objects in the water column, scientists can locate lost shipwrecks, estimate the numbers of fish in the sea, and even find tiny bubbles of natural gas escaping from the deep ocean. Quantifying that gas is a more difficult task, however, and until recently, required the use of expensive underwater cameras to verify the sonar data.
In a new paper  in Continental Shelf Research, DCO member Larry Mayer (University of New Hampshire), Elizabeth Weidner (University of New Hampshire, USA, and Stockholm University, Sweden) and colleagues describe the use of a broadband sonar system that uses a wide range of sound to detect and differentiate individual bubbles of methane, without the need for camera verification. The researchers deployed the system on an expedition in the East Siberian Artic Sea to quantify gas escaping from methane seeps and calculated that the region emits far less methane than some other studies had estimated. If other research vessels adopted similar sonar systems, then scientists could gather invaluable data on the locations of methane seeps and arrive at a better estimate of the total methane flux from seeps in oceans worldwide.
“The sonars are our eyes in the deep ocean,” said Mayer. “As we develop our sonar technology, we see better.”
Mayer has a long history of developing sonar technology as director of the Center for Coastal and Ocean Mapping/Joint Hydrographic Center at the University of New Hampshire. Multibeam sonar techniques that he first used to map the seafloor 25 years ago evolved into methods for detecting fish for fisheries research, and then into ways to detect plumes of natural gas. When Deepwater Horizon, an ultra-deep drilling rig, exploded in the Gulf of Mexico in April 2010, the U.S. government called in Mayer, along with other experts, to find the underwater plume of oil. The resulting months of surveys allowed Mayer’s team to refine the technology further.
In previous studies of methane seeps, scientists have used narrow-band sonar systems, sending out a pulse at a single frequency, like a stroke of a piano key. In the broadband sonar system used in the new study, the pulse was more like running a finger down the entire keyboard. The broad frequency content of the acoustic data they got back allowed the researchers to identify individual bubbles that were just millimeters in size.
“It gave us really high resolution,” said Weidner. “We were able to look at the acoustic data and extract the size of the bubbles just by applying some modeling techniques. We didn’t need to take photos and put a diver or an ROV (remotely operated vehicle) down there.” The researchers also used existing models to calculate how much methane each bubble contained based on its size. During the trip to the surface, some methane leaves the bubble and other gases enter it, so only the largest bubbles will still contain a significant amount methane by the time they hit the air.
The researchers deployed their broadband system on the icebreaker Oden during a 2014 expedition through the East Siberian Arctic Sea. The continental shelf beneath this sea contains ancient deposits of permafrost and holds a large reservoir of carbon that is escaping as methane. Seeps in this region have the potential to contribute to accelerating rates of climate change, but scientists don’t yet agree on how much methane is escaping, or what risk the seeps pose as Earth warms.
The broadband system collected underwater sound data throughout the expedition, but picked up evidence of methane bubbling out from seeps only in the Herald Canyon region at the eastern edge of the sea. The researchers concluded that, contrary to some previous studies, the continental shelf currently was not a significant source of methane to the atmosphere.
This type of sonar system is commercially available, and so many research vessels could be equipped to collect methane seep data during expeditions. “It would be really cool to see this method utilized by other groups so we could start collecting more information about seep activity,” said Weidner.
Such data collection efforts could be transformative in our understanding of how much, and how fast, methane is escaping from the subsurface into the water column. “It’s a whole part of the methane story that is not yet understood in terms of fluxes,” said Mayer. “I think we’re only starting to scratch the surface of what those fluxes are and their impact on ocean chemistry.”
Main image: The Swedish government initially built the icebreaker Oden in 1988 to clear passages for cargo ships but later converted it into a research vessel for expeditions in the Arctic and Antarctic. (Public domain)