At the 2021 annual meeting of the Seismological Society of America, researchers discussed how they use fiber-optic cable to detect small earthquakes that occur in the ice in Antarctica.
The results could be used to better understand the movement and deformation of ice under changing climatic conditions, as well as to improve future monitoring of carbon capture and storage projects, said Anna Stork, a geophysicist at Silixa Ltd.
Stork explained how she and her colleagues are perfecting their Distributed Acoustic Detection, or DAS, methods for microseismicity – earthquakes too small to be felt. DAS works by using the tiny internal defects of an optical fiber like thousands of seismic sensors. An instrument at one end sends laser pulses along the cable and measures the “echo” of each pulse as it is reflected from internal defects in the fiber.
When the fiber is disturbed by earthquakes or ice tremors, there are changes in the size, frequency and phase of the laser light returned to the DAS receiver which can be used to characterize the seismic event.
Michael Kendall of the University of Oxford said the Antarctic research demonstrates how DAS can be used to monitor underground carbon capture and storage at other sites around the world. For example, the configuration of the Antarctic network provides a good example of how a similar network could be configured to best detect microseismicity that could be triggered by carbon storage.
“Our work also demonstrates a method of using DAS fiber arrays to study microseismic earthquake source mechanisms in more detail than conventional geophones,” said Tom Hudson of the University of Oxford. “If we can analyze the source mechanism – how an earthquake fails or fractures – then perhaps we can attribute the earthquake to the movement of fluids like carbon dioxide in a reservoir.”
The Antarctic microseismic icequakes recorded by DAS “are approximately magnitude -1, approximately the size of a book falling from a table,” Hudson explained, “so they are very small earthquakes.”
The study by Hudson and colleagues is the first to use DAS to examine ice tremors in Antarctica. The fiber optic cable was deployed in a linear and triangular configuration over the ice surface at Rutford Ice Creek.
Kendall said there were a number of challenges in using fiber optic sensors in the harsh environment of Antarctica. The material was to travel in pieces by boat and several planes to the study site. The researchers had to bury the fiber to reduce wind noise contaminating the seismic signal, as well as to suppress the signal from a generator that powered the DAS instrument.
“We housed the instrument in a mountaineering tent, which basically served as a small office,” Stork explained. “Keeping temperatures within recommended operating limits was a challenge. Radiant heat from the sun warned the tent well in the 1930s [degrees Celsius], even though it was -10 degrees Celsius outside. “
The researchers are sharing their analyzes of the ice quake data with climatologists and other researchers studying glacier sliding and other ice movements in Antarctica, Kendall said.
“I hope that in the future we will interact more with the scientists who drill ice cores, because they use fibers as distributed temperature sensors, but these fibers that they dug could also be used for studies. earthquakes like ours, ”he noted.
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