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Cosmic particles help scientists 'X-ray' volcanoes

The interior of Satsuma-Iojima volcano (Courtesy of professor Hiroyuki Tanaka)

TOKYO -- Even when there are warning signs, predicting the exact timing of a volcanic eruption is impossible, as much about the inner workings of volcanoes remains a mystery. But that could change now that volcanologists have the geological equivalent of a doctor's X-ray at their disposal.

     When cosmic radiation from supernova explosions and other events in deep space reaches Earth and collides with the atmosphere, large numbers of elementary particles called muons are generated. These so-called secondary cosmic rays account for 70% of the cosmic rays that reach the surface of the Earth. Hold out your hand, and one will pass through every second.

     Because they have such an extremely small mass, muon particles raining down from space pass through just about everything -- organic bodies like ourselves, concrete and even kilometers of rock. But some substances block more muons than others, similar to how bones interfere with X-ray particles. And, like doctors looking at an X-ray of a human body, a Japanese research team has figured out a way to use muons to "see" inside volcanoes.

A peek inside

For volcanologists, cosmic-ray muon radiography, or muography, is a relatively new and powerful tool that could eventually help unravel the mysteries surrounding volcanic activity.     

     The number of muon particles passing through a material depends on the material's density. This forms the basis of the visualization technique developed by the team of Hiroyuki Tanaka of the University of Tokyo Earthquake Research Institute, Mitsuhiro Nakamura of Nagoya University, and Hiroshi Shinohara of the National Institute of Advanced Industrial Science and Technology.

     Just like an X-ray plate captures radiation passing through the body, a special nuclear emulsion plate is used to capture muons passing through a volcano. Scientists then count the number of particles that reached the plate to measure the relative densities of the interior. This data is converted into a visualization showing the locations and shapes of conduits and magma reservoirs.

     To capture muons that traverse the volcano laterally, the nuclear emulsion plate is positioned on one side of the mountain. Magma containing water vapor and magma conduits are less dense than rock and pressurized soil, so more muons pass through these areas and reach the plate.

     Muography is not the only way to study the inside of a volcano, but with a resolution on the scale of tens of meters, it is an order of magnitude more precise than the conventional technique of measuring the reflection of seismic waves.

     In 2013, Tanaka and his colleagues successfully used their new muography technique to visualize the internal structure of the Satsuma-Iojima volcano in Kagoshima Prefecture. What they discovered was a low-density region that spread out from around 300 meters below the vent of the volcano. Scientists had already surmised that the volcano contained a magma reservoir, but muography revealed that the quantity of magma was far greater than predicted.

     The team also succeeded in creating a video from time-lapse images captured over the course of several days. They found that even after an eruption, magma remained in a reservoir situated some 200-300 meters below the vent.

     "This technique will help us clarify what takes place inside a volcano during an eruption," Tanaka explained.

Practical barriers

In a volcanically active country like Japan, scientists have long recognized the need to better understand eruption mechanisms. But the conventional toolset consists exclusively of equipment like seismometers, GPS sensors and volcanic gas detectors that measure and monitor volcanoes from the surface of the Earth. What goes on inside the volcano has been a matter for speculation only.

     The advent of muography thus presents a whole new opportunity for volcanologists to significantly advance their field of study.

     That is easier said than done, however.

     One big hurdle is cost. Muon detectors are specialized devices that cost hundreds of thousands of dollars apiece. At the same time, volcanology is not a well-funded field of research, largely because the past several decades have seen few eruptions causing destruction on the scale of other natural disasters, such as earthquakes and floods. High costs and budget constraints mean only there is a limit to the number of muon detectors that can be purchased and deployed.

     Another hurdle is the complexity of the science, which requires familiarity with both particle physics and volcanology. "You need an extremely large bag of specialized knowledge, from an understanding of elementary particles to the ability to read the detectors," Nakamura said, explaining why such a seemingly powerful and versatile technology has yet to see widespread use.

     While cost remains an obstacle, collaboration among scientists in different fields could help solve the second problem.

     To utilize muography for volcanic studies in Japan, the first order of business would be deciding which of the active volcanoes to target first. The selection process may not be that daunting. For reasons of geography, many of the country's volcanoes do not lend themselves to the technology.

     But even with only a limited data set, the information gained from muography should give volcanologists a wealth of data and suggest new avenues of study.

     Since its debut in the 1950s as a way to measure the distribution of densities, scientists have used muography to study the interior of such massive structures as the pyramids of Egypt and to explore for natural resources.

The muography technology was used to try to ascertain the location of nuclear fuel at the crippled Fukushima Daiichi nuclear power plant. (Courtesy of Tokyo Electric Power Co.)

     The sensitivity of muon detectors is no longer an issue, and technical innovation is broadening the applications of the technology in all directions. In a recent test, it was used to try to ascertain the location of nuclear fuel in the No. 1 reactor of the Fukushima Daiichi nuclear power plant, which was crippled by the earthquake that struck Japan in March 2011.

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