Muography has already imaged several volcanoes throughout the world including Vesuvius, Etna, and Stromboli in Italy, Puy de Dôme in France, La Soufrière in the Caribbean, as well as Asama, Iojima, Usu, and Sakurajima in Japan. It is an ideal method to peer within otherwise inaccessible gigantic structures. In this article, Dr. László Oláh (UTokyo) describes one of the latest volcano muography experiments from the Hungarian-Japanese Sakurajima Muography Observatory (SMO), situated near the Japanese volcano Sakurajima, one of the most active volcanoes in the world.
He discusses the process by which he obtained these results, the unique properties that make muography useful to volcano research, how recent upgrades have increased the capabilities of the SMO detectors and also upgrades expected for muography in the near future.
Q. What did you find in the time-sequential muographic images from SMO regarding Sakurajima volcanic activities as described in your paper ”Plug Formation Imaged Beneath the Active Craters of Sakurajima Volcano With Muography” published recently in the AGU publication Geophysical Research Letters?
Learning about the interior of a volcano is of high interest, and at the same time, extremely challenging. Cosmic particles are a gift from nature, which enable us to look inside gigantic structures such as Sakurajima volcano: with time-sequential muography images (called muographs) we found evidence for the presence of increased, moving material (expected to be magma) which was present underneath the activated Minamidake crater and underneath the deactivated Showa crater of Sakurajima.
Q: What motivated the decision to conduct muography experiments of these particular craters?
The recent activation of Sakurajima volcano started in the middle of the 1950s. Thousands of small eruptions occur every year, throwing ash to heights of up to a few kilometers. Sakurajima volcano has also had powerful eruptions in the past that damaged the surrounding area (e.g. the eruption of 1914). Volcanologists expect a similarly powerful eruption within the next 30 years. The densely populated area of Kagoshima City is located just a few kilometers from the active craters. Volcano monitoring improvements are required to mitigate and protect against future volcano disasters. Sakurajima Muography Observatory (SMO) is an official contributor to the Integrated Program for the Next Generation Volcano Research and Human Resource Development of the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) for enhancing the sophistication of volcano observation.
Q: What has this measurement revealed about the normally unseen processes happening within Sakurajima volcano?
Indeed, muography allows us to observe the changes in the amount of material due to unseen volcanic phenomena occurring beneath the craters. Our observations revealed the formation of a volcanic plug laterally extended within a few 100s of meters beneath the two craters. This phenomena was not (yet) observed by other conventional geophysical observation techniques in Sakurajima volcano. This campaign was also a demonstration of the applicability of muography for monitoring magma movements inside active volcanoes after the pioneering experiment of University of Tokyo conducted at Satsuma-Iwojima volcano in 2013 (as described in the Nature Communications paper “Radiographic visualization of magma dynamics in an erupting volcano”).
Q: What are the advantages of using muography as a tool to better understand volcanoes?
Muography is based on the measurement of naturally occurring background radiation that is capable of penetrating across a few kilometers of material with relatively negligible deflection. These features allow this technique to produce the fluoroscopic images of these gigantic objects and reveal the density structure of deeper regions with an exceptional high precision of a few 10s of meters. The relatively high resolution capabilities of muography allow volcano observation to be conducted from a safe distance of a few kilometers from the active craters.
Q: Besides the obvious dangers of eruptions, what are the biggest challenges you face when making muography measurements of volcanoes?
Besides the volcanic hazard, there are few more challenges which depend on the type of targeted volcano. The gigantic size of the volcanoes is a challenge because the cosmic muons have finite yield (typically every second around 100 muons penetrate through the body of every human above ground on Earth) and the yield of cosmic muons is decreased by a few orders of magnitude after traversing a few hundred meters of rock.
Thus, muon monitoring of gigantic objects requires large surface area detectors. For example, the Hungarian-Japanese SMO has enough sensitivity to image approximately 10 square meters in surface area and can provide muographic images every few days. The large scale production of muographic observation technologies are still challenging nowadays, thus there are only few permanent muography observatories in the world: another one is operated by DIAPHANE collaboration around La Soufrière in the Caribbean and the newest ones are under development at Mt. Vesuvius and Mt. Etna by the MURAVES and MEV collaborations, respectively.
Another challenge can be the topography of the surrounding area, for example the steep sides of Stromboli volcano provide limited areas for observation sites and those can be accessed only by helicopter which limits the size and weight of applicable instrumentation. Operation in harsh and varying environments are also challenges for muographic observation systems. For example, during the first 2 years of its operation, SMO had to be maintained more often during the typhoon seasons. The development and application of muography instrumentation for volcano monitoring is still a challenge to the cutting-edge experts in both the academic and industrial sectors.
Q: What is the basic design of the detectors used at SMO? How are the detectors maintained throughout the measurements and how automated is the system? Can it be operated remotely?
The SMO is a modular muography observatory assembled from ten two-meter-length, square meter sized muon tracking systems (these papers have more information on the detector designs: “High-definition and low-noise muography of the Sakurajima volcano with gaseous tracking detectors” (Nature Scientific Reports) and “Detector developments for high performance Muography applications” (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment).
Each tracking system consists of eight tracking detectors which are based on Multi-wire Proportional Chambers (MWPC). This Noble Prize winning (by Georges Charpak in 1992) innovation has been widely used in high-energy physics experiments since the 1970s, however it was optimised successfully for permanent operation in outdoor conditions by the REGARD group of Wigner RCP, Hungary.
These detectors are based on two perpendicular wire planes fixed inside a closed volume, flushed with an environmentally friendly gas mixture which is explained in more depth in the Advances in High Energy Physics paper “High Efficiency Gaseous Tracking Detector for Cosmic Muon Radiography”. In MWPCs, the localisation of penetrated muons is based on the ionization of gas atoms and the collection of secondary electrons on the wires. Data readout and detector control is based on custom designed electronics developed by REGARD. The system can be operated remotely and the data can be transferred to remote server computers via an internet connection. Thereafter, automated softwares are running on the data server for data quality assurance and for the processing of muographs (images created from muography).
Q: What is the process that must be undertaken to convert muon counts in the detector into muographs (images created from muography)? How long does it take to create a muograph once the data has been collected?
The muon imaging process is based on automated computer algorithms. It is initiated by the reconstruction of positions of muons in the detectors. A combinatorial algorithm combines these positions into 3D trajectories. Thereafter, after taking into account of the detector parameters, the trajectories are ”mapped” and the yield (flux) of muons is calculated. Finally, the muograms (a grid of muon counts from the detector that is the last step before the creation of the muograph image) are created via comparisons of the measured fluxes to the modeled fluxes for each pixel of the muon image. For more information on this aspect, please check this Nature Scientific Reports paper: “High-definition and low-noise muography of the Sakurajima volcano with gaseous tracking detectors”. When the data is transferred to the remote computer this imaging procedure takes from few seconds up to few minutes that depends on the size of data.
Q: What are some experiments and/or upgrades that are being considered in the near and distant future at SMO? How is the Multi-Aspect GeoMuographic Array project involved with this?
Actually, we are working on the integration of the muon imaging system of SMO into the volcano monitoring system of the National Research Institute for Earth Science and Disaster Resilience (NIED). Furthermore, the utilization of Machine Learning is ongoing for time series analysis of data collected by SMO as described in the EOS Science News by AGU article “Are Cosmic Rays A Key to Forecasting Volcanic Eruptions?”.
The Multi-Aspect GeoMuographic Array project is involved with future Sakurajima Muography Observatory upgrades. SMO will be upgraded with a few hundred tracking system units to reduce the imaging time of Sakurajima volcano down to a few hours. This action is required to support the industrialisation of muon detectors, as mentioned in the NEC article “Muography Reveals the Inner Secrets of Volcanoes” which is an ongoing initiative happening with the cooperation of Wigner RCP, Hungary, the University of Tokyo and NEC Corporation.
Q: What is it like to work on Sakurajima island at the foot of one of the most active volcanoes in the world?
The work is very exciting, but it is quite hard because the environment is very humid and hot. I recommend to all researchers interested in these topics — especially early career scientists — to consider visiting SMO to experience the work there for a few days. 🙂