High definition subsurface imaging with cosmic ray muons

Voice of editors is a blog from the AGU Publications Department.

Many structures, processes, and movements of geophysical materials are hidden in plain sight, so imaging them can be difficult for scientists. A new technique, muography, allows scientists to visualize the internal composition of solid geological structures at high resolution. A book just published in the AGU Geophysical Monograph Series, Muography: Exploring the Earth’s subsoil with elementary particles, explores the application of this technique and how it can be used in conjunction with traditional observation methods. We asked the book editors a few questions about muography, its applications in geophysics, and what readers can expect from their book.

What are muons and what is muography?

Muons are elementary particles similar to electrons, but the mass of a muon is about two hundred times heavier than the mass of an electron. The main source of muons comes from nature. They are continually created as by-products of the showers of particles induced by the collisions of cosmic rays with atmospheric nuclei. Muons travel at a speed close to the speed of light, which makes them able to reach the Earth’s surface due to relativistic effects, despite the short decay time.

Muons are observed all over our planet (about one muon penetrates through a hand-sized surface every second). Muons are more penetrating than electrons due to their greater mass, and the most energetic muons can reach even a few kilometers into the Earth’s subsoil.

Muography is an imaging technique for visualizing the internal density composition of geological structures and man-made objects.

Muography is an imaging technique for visualizing the internal density composition of geological structures and man-made objects. It’s similar to X-ray radiography in the human body: muons lose energy in all sorts of media and are absorbed if the energy runs out. The energy loss of the muons depends on the integrated mass of the medium; thus, the yield of muons having penetrated a given medium will provide information on the density of the object crossed.

An example of subsurface muographic surveying from underground tunnels: the difference between geodetically and muographically measured rock thicknesses locating a region of low density (red spot) above the tunnel. Credit: Gergely Suranyi

How can muography be used in earth science research?

Muography produces “x-ray” images of the earth’s subsoil to reveal the density structure of deeper regions with an accuracy of a few meters. This technique can be used to monitor the movement of materials in geological structures and hydrological systems, as well as to improve our understanding of geophysical phenomena.

One of the main applications of muography to date has been the study of various phenomena inside volcanoes (such as the degassing process, the dynamics of eruptions, the formation of magmatic plugs and hydrothermal activities) and monitoring of volcanic edifices (eg deposition of volcanic ejecta and tectonic changes).

Besides volcanology, shallow subsurface muography has the potential to improve our understanding of climate change-induced processes in the cryosphere, reveal evidence of past natural disasters, enable sustainable exploration of hidden subsurface resources , improve geological engineering techniques, and much more. .

Cross-sectional views of the bedrock topography of the Eiger glacier reconstructed using muography. Blue lines and light blue bands are best-fit lines and 1 sigma errors, respectively. 1 credit

Besides stand-alone muography experiments, muography can also be used as a complementary monitoring technique; for example, it is possible to perform a joint analysis of muographic and gravity data for high-resolution three-dimensional density imaging of the subsoil.

What are the advantages of using muography over other imaging techniques?

Modular large-area muon tracker that can be used for earth muography of kilometer buildings. Credit: Laszlo Olah (DC BY 4.0)

Because it uses natural background radiation, muography offers passive, non-destructive imaging of the earth’s subsoil.

The high penetrating power of muons and the predictability of their trajectories in matter allow the imaging of kilometric objects with a spatial resolution of a few meters. With conventional geophysical techniques, large-scale and dense sensor networks are required to achieve this level of resolution.

The high-resolution imaging capability also has practical advantages such as observing inaccessible or dangerous geological phenomena from a safe distance.

What are the limits of muography?

Portable muographic observation device used in underground site to explore shallow subsoil. Credit: Richard Kovacs

Since cosmic muons only come from the sky, the angle of view for muographic imaging is limited. Users should install observation instruments at lower elevations than the targeted structures (such as in tunnels or boreholes) to explore the overburden structure.

Another limitation is that natural muons have a finite yield. Muographic scanning of thick objects requires careful design of the measurement setup (eg, optimization of detector size, imaging resolution, exposure time) to compensate.

How have muographic observation instruments evolved over the last decade and how have these advances impacted muographic practices?

Over the past decade, we have seen substantial advances in instrumentation. Different groups have successfully developed application-oriented instruments and have carried out joint muographic surveys at various sites to verify different technologies and demonstrate their applicability, such as in the Egyptian pyramids, in nuclear reactors and around volcanic edifices.

Recently, compact detectors based on innovative technologies have provided fair image resolution, operational reliability and efficiency in harsh and varied environments. Advances in technology development have made instrumentation more user-friendly and have expanded the applicability of this technique. For example, muography has recently been used for hydrosphere monitoring.

What are some of the unresolved questions in this area that require further research, data collection or modeling?

High-definition muography should contribute significantly to the efficient exploration of the earth’s subsoil resources and the permanent monitoring of infrastructures. Muography also has the potential to contribute to the assessment of direct and indirect natural hazards. Joint measures with other techniques are strongly desired for a successful social implementation of muography.

Strengthening cooperation between developers and potential users from academia (such as researchers, teachers and students) and the private sector (such as engineers and project managers) will be necessary to make muography a standardized technique – such as x-rays. radiography – with optimized and compact devices running user-friendly interfaces for various applications.

The aim of our book is to provide topical information to improve cooperation between developers and users as it is the key to finding an ever-widening range of applications for muography and to solving important problems in years to come.

Muography: Exploring the Earth’s subsoil with elementary particles, 2022. ISBN: 978-1-119-72302-8. List price: $199.95 (hardcover), $160 (e-book)

—László Oláh ([email protected], 0000-0002-4300-8331), University of Tokyo, Japan; Hiroyuki KM Tanaka ( 0000-0002-3816-1630), University of Tokyo, Japan; and Dezso Varga ( 0000-0002-2450-1331), Wigner Research Center for Physics, Hungary

Editor’s Note: It is AGU Publications’ policy to invite authors or editors of newly published books to write an abstract for Eos Editors’ Vox.

Quote: Oláh, L., HKM Tanaka and D. Varga (2022), High-definition subsurface imaging with cosmic-ray muons, Eos, 103, https://doi.org/10.1029/2022EO225009. Published February 14, 2022.
This article does not represent the opinion of AGU, Eos, or any of its affiliates. This is the author’s opinion only.

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