Researchers used a fiber optic cable on the ice cap of an Icelandic subglacial volcano to detect low-frequency volcanic tremor, suggesting this know-how might be helpful in monitoring different ice-covered volcano techniques.
Their analysis printed in The Seismic Report signifies that the floating ice cap, a part of the Vatnajökull glacier, acted as a pure amplifier of the tremor indicators generated by the Grímsvötn volcano, one in every of Iceland’s most hazardous.
This seems to be the primary statement of a floating ice sheet appearing as an amplifier of tremor, stated Andreas Fichtner, a professor of seismology and wave physics at ETH Zürich. “Oscillations of ice cabinets in Antarctica or Greenland have been identified for a very long time,” he defined, “however they’re largely excited by ocean waves.”
Though the precise mechanisms behind volcanic tremor can differ, it may be an indicator of deep volcanic or geothermal exercise, Fichter stated. “Along with offering details about the underlying processes, tremor may additionally function a precursor of volcanic eruptions that ought to be monitored intently.”
Grímsvötn is one in every of Iceland’s largest and most lively volcanoes, with main eruptions happening on common each ten years. Geothermal heating melts the ice cap, making a subglacial lake on the volcano that sometimes bursts forth and floods the coastal plains. Its explosive eruptions create towering ash plumes that have an effect on agriculture, human well being and aviation. Ash from the final main eruption in Might 2011 closed Iceland’s foremost airport and led to the cancellation of 900 flights.
Researchers wish to study extra concerning the seismic setting of Grímsvötn, however putting in a conventional seismic community is dear and troublesome within the distant and harsh circumstances on the subglacial volcano. As an alternative, Fichtner and colleagues turned to Distributed Acoustic Sensing.
Distributed Acoustic Sensing, or DAS, makes use of the tiny inner flaws in an extended optical fiber as hundreds of seismic sensors. An instrument known as an interrogator at one finish of the fiber sends laser pulses down the cable which might be mirrored off the fiber flaws and bounced again to the instrument. When the fiber is disturbed by seismic exercise, researchers can look at adjustments within the timing of the mirrored pulses to study extra concerning the ensuing seismic waves.
The researchers deployed a 12.5 kilometer-long fiber-optic cable on Grímsvötn in Might 2021 and picked up knowledge from the DAS system for 3 weeks.
“We needed to know if a big DAS experiment in such a difficult and distant setting would truly be possible in any respect and if it’d train us one thing new,” stated Fichtner. “Now, having analyzed the info intimately, we all know that the discoveries we made wouldn’t have been potential with standard stations. This consists of not solely the tremor-related ice sheet oscillations but additionally the practically 3000 native earthquakes that we detected throughout the three weeks of the experiment.”
After analyzing the densely sampled DAS knowledge, the researchers realized that the floating ice sheet was appearing as a pure resonator of seismic indicators, permitting them to detect the volcanic tremor that may have in any other case been overwhelmed by different ambient or instrument “noise” in a conventional seismic community.
The analysis workforce acquired fortunate with unusually good climate — together with analysis huts geared up with a geothermally heated sauna on the very best level of the Grimsvötn caldera — in the course of the fiber optic deployment. A trenching sled developed by researchers on the Icelandic Meteorological Workplace, which ploughed and positioned the cable on the similar time, additionally helped.
“The actual problem was splicing within the subject,” Fichtner recalled. “Since we had three cable drums with 4 kilometers of cable on every of them, we needed to join the fibers, which known as splicing. An optical fiber is thinner than a human hair, and due to this fact it’s troublesome to deal with on a glacier.”