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A World-First: Undersea Deployment of Muography Visualization Technology Is Changing the Face of Disaster Prevention and Natural Resource Development

A ground-breaking, world-first trial is underway at the Tokyo Bay. The University of Tokyo and NEC are collaborating to harness muography, a visualization technology that uses elementary particles to build radiographic images of the sea, allowing us to visualize the seafloor,waves, and more. What is the aim of this trial? What benefits do society and the world stand to gain from the deployment of this technology? We spoke to key people involved in the project from The University of Tokyo and NEC about muography's potential across a variety of applications, including sea current and seafloor research, disaster prevention, and natural resource development.

The Potential of Elementary Cosmic Ray Particles (Muons) That Can See Inside Volcanoes

Creating world-class Japanese innovations is an important aspect of Japan's growth strategy and in mapping out its future. Developing new research infrastructure that connects state-of-the-art research with governments and industries is essential to realizing this.

Professor Hiroyuki Tanaka
Director, International Muography Research Organization
The University of Tokyo

Of course, collaboration between industry, academia, and government has been underway for some time. However, what is needed now is a future-oriented research infrastructure that creates more effective innovations to solve increasingly complex and diverse societal issues. Professor Hiroyuki Tanaka of the International Muography Research Organization at The University of Tokyo explains in more detail:

"The university is a place to master your field of study, and the more you pursue this, the more highly specialized you become. This is the right approach to have in terms of progressing with your studies. However, in the future there will be a stronger public interest in how research can lead to resolving the issues that modern society faces. So that is to say, pursuing research while also applying it to resolving societal issues. Examples of this could also include the creation of new industries based on research results, and training young technical experts acting at the forefront of corporations and other organizations."

In addition to the experiment monitoring systems that are being deployed in the NEWCUT laboratory*1 as well as in volcanoes, research infrastructure capable of advancing muography will need to consist of high-performance computing (HPC) and large-scale storage in order to connect researchers from around the world with the data collected by these observation systems. Data obtained in the real world will be shared in cyberspace to promote international data-driven science. The Virtual Muography Institute (VMI) is making all of this possible. The VMI is a virtual laboratory in which 34 institutions (11 in the private sector) in 11 countries share relevant infrastructure to work toward the goals of standardizing sensor module analysis methods, and open science.

So what is muography, the key word in all this new research infrastructure? Simply put, it is a visualization technology that uses elementary particles (muons) that come from space. Muons are produced when particles accelerated by supernova explosions reach Earth and collide with atomic nuclei of molecules in the atmosphere. They have such high penetrative power that they can pass through even several kilometers of solid rock. In medical X-ray imaging, an image is able to be created due to the X-rays coming to a halt when they reach the bone. Muons similarly experience difficulty in passing through incredibly dense rock, meaning that internal structures can be reproduced as an image based on the difference in penetration rate.

Muography is a visualization technology that uses high-penetrating elementary particles called muons to image the interior of an object in the same way as an X-ray image. This remarkable new technology*2 has progressed to the point where it can successfully visualize large-scale structures such as volcanoes, and is ready to be applied in a variety of other fields.

Previously, muography was limited by various issues relating to resolution, detector size, cost, etc. However, technological innovation has progressed since the beginning of the 21st century, and in 2006, Tanaka and his research team succeeded in taking the world's first radiographic image of a volcano (Mount Asama). Ever since then, muography has been applied in various fields, including in surveys of the three main pyramids at Giza, Egypt, the inside of the Fukushima Daiichi Nuclear Power Plant that suffered a meltdown, to check dams, and inside ancient tombs. In recent years, it has also been utilized together with AI for purposes such as predicting volcanic eruptions.

"We have trained AI to predict eruptions based on radiographic images around the summit of the Sakurajima volcano. In collaboration with The University of Tokyo Hospital, we applied medical X-ray imaging technology to create radiographic images of the volcano. This is just one example of how research infrastructure can link different fields. We also plan to conduct a radiography trial of a volcano in Italy. By combining this with AI, we hope to improve eruption prediction technology and potentially provide information regarding disaster prevention in the future," says Tanaka.

  • *1:
    A research center established for the purpose of collecting detailed data on muon fluxes and creating international standards of such data. Jointly established by The University of Tokyo, NEC, the Central Research Institute of Electric Power Industry, and the Hungarian Academy of Sciences.
  • *2:

A World-First: What We Can See by Deploying Muography Under the Sea

Although there are several different geophysical sensing techniques, one of the advantages of muography is that it allows for passive monitoring. Since muons rain down all over the surface of the Earth, it is possible to monitor a certain area over a certain period of time anywhere on the Earth, simply by setting up detectors. That means that no energy source is needed for measurement, so the longer you monitor an area for, the more cost-effective this monitoring method becomes.

Another major plus factor is the ubiquity of the equipment and methods developed in Japan, which means they can be deployed anywhere around the world. In addition, the high-penetrating muons can be applied to large-scale objects, and the pathways they have taken can be clearly understood. This area of research is expected to be highly valuable, with muography also having been selected for the Master Plan formulated by the Science Council of Japan.

With advantages such as this, muography will next be deployed under the sea, a current hotspot. This is because Tanaka's research group together with NEC have set themselves the challenge of being the first in the world*3 to measure these elementary particles under the seafloor of Tokyo Bay.

Until now, muography's achievements have been limited to land-based measurements, such as in volcanoes and nuclear power plants. The aim of performing undersea measurements is to lead to the application of this technology to disaster prevention and natural resource development. But the question is, why has no one else in the world ever attempted this before?

"First of all, there are no examples anywhere in the world of elementary particles measurements having been conducted below the seafloor. There are two instances of particles having been measured on the seafloor, both of which were experiments dealing with larger elementary particles. Setting up an elementary particle detector on the seafloor is far more expensive than doing so on land due to factors such as having to put it in a pressure-resistant container. Placing it under the seafloor also requires digging a hole in the seafloor in which to put the detector. Meanwhile, the potential of marine muography has not yet been quantified, making it not possible to receive large-scale investments. We hope to quantify its potential by linking this research infrastructure with the VMI, broaden the diversity of muography researchers by increasing the number of researchers participating in muography research, and further promote the standardization of sensor module analysis methods," says Tanaka.

However, if muography can be deployed in the sea, it will certainly widen the scope of applications for society in the future. And so, Tanaka and his colleagues have set their sights on the Tokyo Bay Aqua-Line. A part of the undersea muography sensor array (hereinafter referred to as HKMSDD, Hyper KiloMetric Submarine Deep Detector) has been installed along 100 meters (approx. 328 feet) of the undersea tunnel between Kanagawa and Chiba. By measuring the number of muons that arrive over time, they have been able to successfully measure in real time the sea water depth and changes in sea level (astronomical tide level).

"Although conventional detectors can be as small as 400 mm (approx. 15.75 inches) in width, in many cases there was not enough space, so miniaturizing them was an urgent issue. In light of this, in cooperation with the Wigner Research Centre for Physics, of the Hungarian Academy of Sciences, they have been successfully miniaturized to a size of as little as 70 mm (approx. 2.75 inches) in width, and NEC is now carrying out measurement assessments on them. Due to the way it has been constructed, we are able to keep adding sensor modules to the HKMSDD, and by the end of 2021 we will have extended it to 1 km (approx. 0.6 miles) in length," says Tanaka.

Instruments installed in the Tokyo Bay Aqua-Line tunnel (Left: muon detector; Right: measuring device).
Muography has been trialed on land and is now beginning to be used under the sea, with the International Muography Research Organization at The University of Tokyo having installed observation equipment in the undersea Tokyo Bay Aqua-Line tunnel. In addition to disaster prevention, such as predicting tsunamis caused by earthquakes, muography is also attracting a great deal of attention for its potential application to energy development, such as for surveying gas fields.

So what is deploying muography under the sea likely to reveal? One example is the monitoring of things such as high waves, seawater density distribution, and the structure of the seafloor through detecting the muons that have permeated the surface of the sea in Tokyo Bay. The ability to image tsunamis and high tides caused by earthquakes, and choppy sea conditions caused by typhoons before they reach the shores of the Tokyo Bay can be used to help enact rapid countermeasures and subsequent disaster prevention plans. It can also be used to explore the untapped natural gas deposits located at the bottom of the sea in Tokyo Bay.

Tanaka explains, "The Minami Kanto gas field, the development of which had started prior to WWII, accounts for over 90% of Japan's recoverable natural gas reserves. However, the Tokyo Bay area has yet to be surveyed, and details such as the location and condition of the gas remains unknown. If the HKMSDD can be deployed over a wider area, it should be able to provide us with information that will be useful for Japan's energy planning. We just started to develop a new type of detector for underwater muography with the Wigner Research Centre for Physics in Hungary. In addition, we can gain even more extensive knowledge by deploying the HKMSDD not only in Japan but also in seafloors abroad, such as in the North Sea. We intend to work together on this with The University of Sheffield and the UK Science and Technology Facilities Council's Boulby Underground Laboratory. Furthermore, we are planning to deploy HKMSDD at the Callio Lab in collaboration with Oulu University, Finland in order to evaluate if HKMSDD can be used for monitoring the accumulation and flow of surface waters into the mine through the cracks in the bedrock. The accumulation could indicate the extent of the damages both in the area and in volume. This could be an important topic for other underground mines and open pits as well”

The Need for a Research Infrastructure That Functions as an Ecosystem

Osamu Kamoshida
First Public Solutions Division

As mentioned above, realizing this vision will still require a research infrastructure that links different fields. At The University of Tokyo, its researchers and institutions in fields including atmosphere and ocean research, marine sensing, energy resources, seismology and volcanology are getting involved in muography. This is where NEC comes in, helping to create a research infrastructure that takes PoC (Proof of Concept) and seamlessly leads them toward implementation in society.

"Now that we can train our young technical experts using the HKMSDD infrastructure, the plan is to deploy the system in the undersea tunnel in the UK's North Sea, based on the Tokyo Bay trial, and monitor the oil and gas fields there. I believe that by building a network that links industry, academia, and government not only in Japan but also on a global level will help us to further expand what we are capable of achieving," says NEC's Osamu Kamoshida

High Expectations of NEC in Terms of Both Hardware and Software

So why did The University of Tokyo choose NEC out of so many other companies for this collaboration? There are two main reasons.

"Well, first of all is the fact that NEC is a solutions vendor that provides ways of solving societal issues in a variety of settings. In the future, standardizing detectors will be essential to the widespread use of muography, and we hoped that NEC's manufacturing experience would be of use in this regard. The other reason is its world-class technologies such as AI. This is necessary for disaster prevention and improving image precision based on muography measurements," explains Tanaka.

These sentiments are also shared by NEC. "The societal implementation of cutting-edge technology is something we have done countless times before, and is an area in which we have been able to leverage our expertise. For muography, we are developing two detectors (scintillator and gas), and software-wise we are working on improving automatic diagnostic technology for radiographic muography images using AI. We will continue to work with Professor Tanaka and his team to improve operability and promote standardization," says Kamoshida.

The basic principle behind muography is to measure the average density of an object by counting the number of muons that pass through it, and visualize the object based on that data. There are three types of instrument used for observation: nuclear emulsion plates, scintillators, and gases.

In terms of software, too, NEC is building its expertise in the operation of muography. One example of this is our initiative to visualize underground structures based on the measurement of muons.

Shinichi Miyamoto
Safer Cities Solutions Division

"Measuring underground doesn't always provide ideal or adequate measurement results. CT scans of the human body are capable of 360-degree measurements, but this is physically difficult to achieve underground, and whether or not we are able to reconstruct images from this limited data depends on what kind of constraints we place on machine learning and deep learning. For example, if we have prior information about the strata, we can reduce the number of parameters. Together with recommendations in this regard from universities and other organizations, we are working on improving muography by combining the image recognition technology and expertise that we have been working on for the past 40 years together with AI," explains NEC's Shinichi Miyamoto.

"Creating visualizations from limited observation data is important to the evolution of muography. Tanaka has said that he looks forward to working together with us as part of a collaborative relationship to develop new technologies based on observation results," he adds.

With a focus on muography, going forward The University of Tokyo and NEC plan to develop an ecosystem that enables the creation of new value and technology anywhere in the world, based on research infrastructure that transcends borders. "In addition to imaging that uses AI, we intend to follow up in areas where we can put our experience and expertise to good use, such as integrating electrical circuits into hardware, and creating highly pressure-resistant casings," explains Miyamoto.

A lot more data gathering and trial results are needed in order to eventually implement muography into society. "In addition to continuing with our research, we will also actively share information so that the world knows what to expect from us," enthuses Tanaka. Will we be able to build an ecosystem that connects other research fields, Japan-based and international educational and research institutions, private companies and governments? This will undoubtedly be the key point for finding solutions to societal issues using muography.

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(October 21, 2021)