ANCHORAGE, Alaska — A tool used by mining companies to find mineral ores has been adapted to map frozen soils below the ground in Alaska and could be used to track the effects of global warming, according to the U.S. Geological Survey.
The agency announced Monday that an airborne survey conducted in Alaska’s Yukon River drainage collected unprecedented images of the presence and absence of permafrost down to 328 feet. The study used an electromagnetic survey tool flown beneath a helicopter.
“We really think we’ve got the story nailed down from these data,” research geophysicist Burke Minsley said by phone from the Crustal Geophysics and Geochemistry Science Center in Denver.
Minsley is lead author of the study published Friday in the journal Geophysical Research Letters (http://www.agu.org/pubs/crossref/2012/2011GL050079.shtml).
Mapping permafrost extents has been done by satellite, Minsley said.
“It’s hard to get any information at depth, and that’s what’s unique here,” he said.
Permafrost is below about 24 percent of the land area in North America. The research will be important for climate scientists looking at the thawing of permafrost as a greenhouse gas contributor, studying ecology in lake systems, or looking at the effects of thawing ground on river chemistry, Minsley said. Thawing also will bring important impacts for northern infrastructure such as buildings and roads.
The Yukon River begins in Yukon Territory, Canada, and spans Alaska from east to west.
The research team surveyed more than 116 square miles of the Yukon Flats in an area centered 140 miles northeast of Fairbanks. The area was picked in part because it’s between continuous permafrost to the north and discontinuous permafrost to the south, according to the agency.
Electromagnetic surveys have been used in mining for years, Minsley said. Scientists recently have used it to define the geometry of aquifers in Nebraska.
“What’s new about it is that it’s being used for more discrimination of more subtle features related to things like groundwater and permafrost,” Minsley said.
The tool is torpedo-shaped and about 33 feet long. In front are coils oscillating at different frequencies, Minsley said. Towed by a helicopter and flown just under 100 feet off the ground, the tool sends electromagnetic pulses into the ground and determines what’s below by measuring how well the pulse is conducted. The ground itself has conductors and resistors. Permafrost is not as conductive as solid ground.
“It induces currents in the ground. Those currents induct a signal, a magnetic field, that’s picked up by another set of coils that’s in the back of that thing that we’re towing under the helicopter,” Minsley said.
The tool collected data that showed a lack of permafrost below the Yukon River and other water bodies that don’t completely freeze in the harsh interior Alaska winter.
“That’s consistent with a lot of conceptual models that people have developed, but they haven’t really had the solid measurements that we have to see that,” Minsley said.
The mapping also showed “thermal relics” where the Yukon River had been centuries ago. As the river migrated to a new location, ground slowly refroze. The farther away from the new location, the more it had frozen in a downward slope. Minsley said it was the biggest surprise of the study.
“It was not something we expected to capture in this data set, to actually see that thermal legacy over a thousand-year history. That’s pretty rare for this kind of data,” he said.
The tool collected data over only one week in June 2010. Minsley said he spent four to five months analyzing and processing data to make images, and the research team spent many more hours interpreting the results.
Drilling boreholes to acquire the same information would have taken a much greater effort, he said.