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.

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.

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.

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

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.

(October 21, 2021)