30 December 2016
By Dan Garisto
Small amounts of helium and argon gas released from rocks under stress could be used to predict rock breakage before it occurs, such as during an earthquake or in an underground mine, according to new research.
Rocks contain small amounts of gases, like radon and argon, that form when radioactive compounds decay underground. Over millions of years, these gas molecules can accumulate to levels thousands of times higher than the background, even in rocks as small as a soda can. When rocks are put under stress by events like a volcanic eruption or an earthquake, these gases can be released.
The new study finds that by monitoring the release of helium and argon, scientists can detect real-time deformation in a rock before the rock begins to break.
This kind of early-warning signal could be useful for keeping people safe in situations where rock is under high stress, like mining or construction operations, according to the study’s authors.
The new study, published in Geophysical Research Letters, a journal of the American Geophysical Union, is one of the first to observe argon and helium as a real-time signal of rock failure, according to the authors.
“We found a precursor of rocks breaking — this signal, we see it before the rock fails,” said Payton Gardner, a geologist at the University of Montana in Missoula and co-author of the new study. “That’s important for a whole bunch of different fields.”
“It provides a new tool to monitor changes in the Earth, be they engineering related or natural hazards related,” said Jacob Lowenstern, a geologist with the U.S. Geological Survey in Menlo Park, California, who was not involved in the new study. “[They] documented something that had been inferred in other studies, but never directly measured.”
Breaking things down
Attempts to predict earthquakes came to prominence in the 1980s, when scientists attempted a variety of methods to forecast the destructive events.
Radon, a heavy noble gas, is produced underground from other decaying radioactive compounds and is radioactive itself. Scientists showed that when rocks were under stress, which often happens before an earthquake, radon would escape, often into groundwater.
“Some folks—especially in Japan—noticed that there were these anomalies in radon emanation before earthquakes,” Gardner said. “It became a very hot idea that we might be able to monitor radon concentrations and tell something about the state of stress of the plates and predict earthquakes.”
But using radon as a signal for rock deformation has limitations: to measure radon, scientists have to wait for it to undergo radioactive decay. Radon’s half-life—the time it takes for half of its atoms to decay—is several days, making real-time measurements impossible.
To see if they could measure rock deformation in real-time, Gardner and his colleagues looked to other gases known to form in rocks from radioactive decay: helium and argon. Like radon, helium and argon are noble gases that don’t chemically react with their surroundings, making them easy to detect with a mass spectrometer, a device that measures gases using their molecular weights.
Gardner and his co-authors took measurements of rock deformation in the controlled environment of a lab so that they could carefully monitor how much stress the rock was under as it released helium and argon. The researchers applied compression along the length of small cylinders of various types of rock, such as granite and shale.
“What we do is slowly squeeze the rock,” Gardner said. “For the majority of that time the rock deforms elastically just like a spring. And then at some point we start to reach the yield strength and we start to crack and eventually form a large fracture in the rock.”
The researchers found that at one-third of a rock’s final breaking point, the mass spectrometer detected a strong signal of helium gas. Although the study’s authors are not entirely sure how the mechanical process to release helium and argon works, Gardner said the evidence points to small cracks known as microfractures.
“We can put little acoustic sensors on the rock and we can hear these small little cracks,” he said. “So we know [microfractures] are happening. But what’s amazing is that the helium signal picks up … well before we actually fail the rock.”
Although there are traditional methods for detecting stress on rocks, such as strain meters, helium and argon may be even more sensitive to deformation, Gardner said. However, putting the research into practice to save lives is a way off—Gardner and his team must first try to measure the helium and argon signal outside of the laboratory.
“We need to try and figure out some field experiments,” Gardner said. “It’s pretty hard to just wait around for an earthquake … some ideas we’ve had are actually going into an active mining operation where they’re constantly breaking rock.”
One thing is sure: Wherever the researchers go next, there is sure to be ground broken.
—Dan Garisto is a public information intern at AGU.