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The technique uses off-the-shelf components mounted on an unmanned aerial system (UAS, a.k.a. drone). In September 2017, USGS scientists collected data along a mile and a quarter of coastal bluffs on Barter Island, Alaska, to test the system. The results revealed spots where methane release was particularly high—methane “hotspots”—that appear to be associated with zones of accelerated erosion, the focus of the USGS project that developed the new technique.

Project scientists describe the drone setup and the Barter Island test in “Towards determining spatial methane distribution on Arctic permafrost bluffs with an unmanned aerial system,” published February 2019 in Springer Nature Applied Sciences.

Methane is a powerful greenhouse gas, so many groups have been trying to determine how much escapes from the land and the sea into the atmosphere. They typically rely on satellite imagery, data from sensors on manned aircraft, and samples collected on the ground.

“Methane measurements, up until now, have been reserved to large research institutions, government agencies, and so on, because they’re extremely costly—hundreds of thousands of dollars,” says Ferdinand Oberle, USGS Mendenhall Research Fellow and designer of the new methane-measurement technique. The USGS system costs much less—approximately \$2,500 for a month of use or \$14,000 for a permanent purchase.

“It opens the door,” says Oberle, allowing “any organization, any person really, to do repeated studies at low cost, each year, to see how [the pattern of methane release] changes in coastal environments.”

In addition to its low cost, the new system collects unusually detailed data. As the drone flies along the coast, it takes measurements every tenth of a second, or about every 10 inches at typical flight speeds. “There is no study out there, so far, that has identified methane at that spatial scale in a permafrost environment,” says Oberle.

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Prior to field tests, Oberle examined and calibrated the methane laser in controlled lab experiments at the USGS Pacific Coastal and Marine Science Center. He shot the laser through gas-fillable spectrophotometric cells that were flooded with known quantities of methane. These quantities were then compared to the laser’s readings. The experiments proved that the methane sensor had the necessary accuracy and sensitivity to changing methane levels to warrant the field test on thawing Arctic permafrost.

Oberle and his co-authors from the USGS Pacific Coastal and Marine Science Center study erosion along the Arctic permafrost coast, one of the most dramatically changing environments in the world. Some of their previous work, led by Ann Gibbs, Li Erikson, and Bruce Richmond, shows that erosion is generally increasing along Alaska’s north coast, with the shoreline retreating an average of about 4.5 feet per year and, in some stretches, more than 65 feet per year. The researchers—along with collaborators at the USGS Alaska Science Center—aim to determine the dominant forces behind this erosion, which threatens villages, wildlife habitats, and oil- and gas-production facilities. One of the factors they are examining closely is the permafrost itself.

“Most of the Arctic coastline is held together by permafrost,” says Oberle. “That’s what makes the coasts stable, essentially.”

Erosion of permafrost coasts is driven not just by the impact of waves but also by the thermal energy—or warmth—of the seawater. Erosion produced by the combined action of the sea’s mechanical and thermal energy is called “thermal abrasion.”

“Thermal abrasion usually can only be detected after the fact,” says Oberle. “Once things have started to erode, you can see that permafrost has melted and has collapsed. What I’m trying to get with this methane study is, can we identify areas of thermal abrasion through methane release—can we develop an early detection system?"

The results look promising so far. Data collected in the September 2017 test, at the end of the thawing season, show hotspots of escaping methane that were closely associated with meltwater run-off channels (see map). The scientists plan to return to Barter Island in 2019, at the onset of thawing, to further study permafrost degradation and thermal abrasion. Oberle is optimistic, though he cautions: “It will probably take multiple years of data and multiple collection methods to show that there really is a solid overlap between the two phenomena.”

The full citation for the new paper is:

Oberle, F.K.J., Gibbs, A.E., Richmond, B.M. Erikson, L.H., Waldrop, M.P, and Swarzenski, P.W., 2019, Towards determining spatial methane distribution on Arctic permafrost bluffs with an unmanned aerial system: SN Applied Sciences, vol. 1, no. 3, article 236, https://doi.org/10.1007/s42452-019-0242-9.