image: Drone view of crevasses on Gornergletscher in Switzerland.
Credit: Tom Hudson
Fiber optic cable deployed on a Swiss glacier detected the seismic signals of crevasses opening in the ice, confirming that the technology could be useful in monitoring such icequakes, according to a report at the Seismological Society of America’s Annual Meeting.
Crevassing is important to the stability of glaciers, especially as they offer a pathway for meltwater to reach the rocky glacier bed to speed up the glacier’s movement and enhance melting. The harsh environment of a crevassed glacier, however, makes it difficult to place traditional seismic instruments to measure icequakes.
The source of seismic signals in an icequake differ from the shear forces of a tectonic earthquake or the explosive source of a chemical or nuclear detonation, explained Tom Hudson of ETH Zürich. A crevasse is an “crack source, where you have pure opening of a fracture just in one direction,” he said.
The new study by Hudson and ETH Zürich’s Andreas Fichtner, who presented the research at the meeting, and colleagues “is a real-world test case of detecting this opening crack fracture type of seismicity in the subsurface using fiber optics,” said Hudson. “This is pretty much as close as we can get to a seismic source. Our crevasse quakes are within ten meters of the fiber optic cable, which is quite rare.”
The success suggests fiber optic detection could prove useful in monitoring similar cracks that might open in the rock of a carbon storage reservoir or a geothermal energy system, he noted.
“Because ice is a seismically simpler medium than rock, it’s got a well-known velocity structure and we can really interrogate the source physics,” Hudson said. “So if we can do that in this simpler environment, then the hope is that maybe we can start to think about doing that in a more complex environment.”
The researchers deployed the dense 2D grid of fiber within a crevasse field on Gornergletscher, the second-largest glacier in Switzerland. Hudson said the team got lucky with the conditions during the deployment. They positioned the cable just as the seasons were turning from summer to winter, so there was no snow and the researchers could avoid the dangers of stumbling into a covered crevasse.
One of the main challenges of collecting seismic data with fiber optic cable is ensuring that the cable has good contact or is “coupled” with the ground it lays on. “It was still warm enough during the day that the fiber would get hot and melt into the glacier a little, because the fiber is black compared to the ice. And then when the fiber melts in, it was cold enough that it froze in place overnight,” Hudson explained.
“So we actually got the best coupling you could probably hope for in terms of the fiber melting and then freezing,” he added.
The team detected and located 951 icequakes, with their seismic waveforms containing strong oscillations or coda after the arrival of the seismic surface waves. These oscillations can occur when there is water inside a crevasse, with the movement of the water during the quake creating a resonant signal. But Hudson and colleagues’ analysis suggests the oscillations are more likely due to resonance created by seismic waves “as they bounce back and forth between the fractures” of several crevasses in the crevasse field, Hudson said.
The researchers also compared data from the fiber optic grid to data from a more traditional deployment of seismic nodes. Fiber optic cable offers almost 20 times the data volume of the node array. “You have some data processing challenges, but you have far more data, and that allows you to basically see the full wavefield in the data itself, which is quite unusual,” Hudson said.
Another advantage of using fiber optic cables is that they are sensitive to a greater range of signal frequencies, including low frequency signals that last for hours or even days, which allows seismologists to measure the flexure of the ice through time, he noted.
Hudson would like to use fiber optics to measure the velocity structure of the ice and develop a 3D image of its subsurface, he said.
“I really want to quantify the fracture extent, the fracture density, and see how damaged the ice actually is in this area,” he explained. “So we can see where the icequakes are generated by the fractures. We haven’t yet quantified how many fractures there are and how big they are, so that’s the hope going forward.”