There’s remote, and then there’s Antarctic remote.
Thwaites Glacier, located over 1,500 kilometers from McMurdo Station, falls squarely in the latter camp. An international collaboration is currently studying this notoriously unstable glacier and the significant sea level rise that would result from its collapse. And recent fieldwork in West Texas, a world away, is informing that research.
Last year, researchers working near the city of Tornillo, Texas, intentionally detonated a series of explosives mounted atop poles. The shock waves created by the detonations sent seismic waves into the ground. This technique, known as “active source seismic,” allows scientists to infer properties of the subsurface based on how those seismic waves propagate.
Later this year, researchers working in Antarctica will use this method to study Thwaites. Thwaites is currently responsible for about 4% of sea level rise worldwide. Some scientists believe that if Thwaites collapsed, the entire West Antarctic Ice Sheet might destabilize and break apart, boosting sea levels by more than a meter.
The Edges of a River of Ice
In November 2019, roughly 100 scientists and support staff departed for Antarctica as part of the International Thwaites Glacier Collaboration. This consortium, funded by the National Science Foundation and the United Kingdom’s Natural Environment Research Council, consists of eight different projects. One of those is Thwaites Interdisciplinary Margin Evolution (TIME), an endeavor to better understand the boundaries (the margins) of the glacier. The size of Thwaites dictates how much ice is flowing into the sea, said Slawek Tulaczyk, a glaciologist at the University of California, Santa Cruz, and lead principal investigator of the TIME project. But because Antarctica is blanketed in ice, glaciers are defined only as rivers of ice that flow within slower-moving ice masses, Tulaczyk said. “These boundaries can move. It’s not a very stable situation.”
That’s where active source seismic comes in. By detonating hundreds of explosives near the surface of Thwaites and mapping how the seismic signals propagate, it’s possible to “basically create X-ray images of the landscape that’s sitting beneath the ice,” said Tulaczyk. That’s important because the margins of Thwaites might be dictated by changes in the geology of the subsurface, researchers believe.
Hot Water in a Cold Place
The traditional way of doing active source seismic in Arctic or Antarctic environments involves drilling tens of meters into the ice, placing an explosive, and detonating it remotely. But all that drilling is a highly fuel-intensive process because a steady stream of hot water is necessary to melt the ice, said Steven Harder, an explosion seismologist at the University of Texas at El Paso. “The limitation becomes the amount of fuel you can transport to the field.”
In 2018, Harder and his colleagues experimented with another technique, one that didn’t require any drilling and had, in fact, been used in the 1930s in Antarctica. Known as “Poulter shooting,” after Thomas C. Poulter, the physicist who developed it, the method involves detonating explosives mounted on poles above the ground rather than sunk deep into the ice. Harder and his colleagues experimented with different types of explosives—ammonium nitrate/fuel oil, ammonium nitrate/nitromethane, dynamite, and pentolite, for instance—and metal and bamboo poles 1.2, 1.8, and 2.4 meters (4, 6, and 8 feet) long.
They worked in remote West Texas in the 849,840-hectare University Lands managed by the University of Texas and Texas A&M University systems. “This is a very small part of a big project,” said Harder. “It’s trying to figure out how we should do the bigger project.”
Not Too Tall, Not Too Short
The researchers discovered that ammonium nitrate–based explosives atop 1.8-meter-long metal poles yielded the strongest seismic signals. The 1.2-meter poles didn’t give the explosives enough time to fully detonate before the shock wave hit the ground, the team proposed. And the 2.4-meter poles were too tall: The gas pressure built up by the detonation was already dropping when the shock wave impacted the surface, Harder and his colleagues concluded. These results were presented in December at AGU’s Fall Meeting 2019 in San Francisco, Calif.
In the 2020–2021 austral summer, TIME researchers will begin detonating explosives above Thwaites. Hundreds of seismometers will pick up the signals, and Tulaczyk and his colleagues will begin assembling a picture of the glacier’s extent. There’s a certain irony to using this technique to study the retreat of Thwaites, said Tulaczyk, because it was originally developed by the petroleum industry for oil and gas exploration. “We’re stealing their ideas to help alleviate a problem they’ve created.”
Kornei, K. (2020), Controlled explosions pave the way for Thwaites Glacier research, Eos, 101, https://doi.org/10.1029/2020EO138299. Published on 10 January 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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