Muography

Exploring Earth's Subsurface with Elementary Particles

László Oláh, Hiroyuki K. M. Tanaka, Dezsö Varga, László Oláh, Hiroyuki K. M. Tanaka, Dezsö Varga

• Editorial: American Geophysical Union
• Año: 2022
• ISBN:
  9781119723066
• Edición: 1
• Categoría: Scienze fisiche
• Categoría: Geofisica

A technique for visualizing Earth's subsurface at high resolution

Hidden out of sight in Earth's subsurface are a range of geophysical structures, processes, and material movements. Muography is a passive and non-destructive remote sensing technique that visualizes the internal structure of solid geological structures at high resolution, similar in process to X-ray radiography of human bodies.

Muography: Exploring Earth's Subsurface with Elementary Particles explores the application of this imaging technique in the geosciences and how it can complement conventional geophysical observations.

Volume highlights include:

• Principles of muography and pioneering works in the field
• Different approaches for muographic image processing
• Observing volcanic structures and activity with muography
• Using muography for geophysical exploration and mining engineering
• Potential environmental applications of muography
• Latest technological developments in muography

The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

1
Principles of Muography and Pioneering Works

Hiroyuki K. M. Tanaka

Earthquake Research Institute, and International Muography Research Organization (MUOGRAPHIX), The University of Tokyo, Tokyo, Japan; and International Virtual Muography Institute, Global

ABSTRACT

Visualization of the subsurface flow of geofluids with meter‐scale resolution is one of the essential components of current and future geophysical observation technologies. The principles and a critical account of key results on pioneering works in muography are presented. These are compared with other geophysical and geochemical experiments and observations for the study of volcanic dynamics, tectonics, and underground water behavior, which can help us to understand and possibly predict future volcanic eruption and underground water‐associated disasters.

1.1 INTRODUCTION

Many geodynamical activities on Earth are driven by movements of geofluids, i.e., any subsurface fluid, such as magmatic fluid, groundwater, petroleum, etc., that passes through subsurface porous media. In many cases in geophysical studies, it is important to identify such geofluid motions as well as the properties of these porous media, which serve as the pathways of these fluids. Muography, the technique of using high‐energy (relativistic) muons as a radiographic probe, can be used to image the internal structure of hectometric to kilometric‐scale objects. The term muography means “muon rendering” in ancient Greek. While photography utilizes photons (the word for “light” in ancient Greek), muography utilizes the characteristics of relativistic muons. Muography can be explained by using the analogy of medical imaging. One example is medical radiography, in which X‐ray transmission through the human body clarifies the shape and state of internal organs based on differences in thickness or density; for example, bones are easily differentiated from skin tissue. The muon, also indicated by the Greek letter μ, is a charged lepton, which is free from strong force interaction but sensitive to electromagnetic force. Therefore, high‐energy muons can be easily detected, but they have a stronger penetration power than X‐rays. Muons can therefore be used to create similar shadows inside objects with much larger proportions than the human body.

These high‐energy muons are produced continuously via the interactions of galactic cosmic‐ray (GCR) particles with the upper atmosphere of the Earth. Muography is perhaps the only technology that harnesses energy originated from outside of the solar system. Muography is a technique that doesn’t require active energy sources to transmit probing signals, and thus it is power‐efficient and almost maintenance free. Therefore, it is a cost‐effective technique, in particular for the purpose of long‐time‐range monitoring.

Since GCRs also arrive at other planets, comets, and asteroids, in principle, muography can be conducted on these celestial objects. Under the conditions of the extraterrestrial environments, muons are generated either in the atmosphere or within the planet itself, depending on the existence and thickness of the atmosphere. As a consequence, the resultant energy spectra vary among stellar bodies and thus, the size of the objects that can be imaged also depends on the type of stellar body to be studied. The purpose of this chapter is to provide a clear understanding of the advantages, disadvantages, and characteristic properties of muography. The layout of this chapter is as follows. In Section 1.2, we review the principles of muography. The general features of the muographic probe including the energy spectra on the Earth, Mars, and asteroids (Section 1.2.1), geomagnetic effect (Section 1.2.2), altitude effect (Section 1.2.3), time‐dependent variations (Section 1.2.4), the general concept of muography including energy‐range relationship in terms of muon flux reduction after passing through matter (Section 1.2.5), scattering by the target volume (Section 1.2.6), measurement background (Section 1.2.7), required measurement times (Section 1.2.8), muographically averaged thickness (Section 1.2.9), and limitations of muography and potential geological targets (Section 1.2.10) are discussed. In Section 1.3, after the brief introduction of early works in Section 1.3.1, we discuss the results of measurements in pioneering works by addressing two major topics: subsurface volcanism and tectonics (Sections 1.3.2, 1.3.3, and 1.3.4) and underground water (Section 1.3.5) and conclusions are given in Section 1.4. Various kinds of detectors used for muography will be introduced in the following chapters, and thus will not be described here.

1.2 PRINCIPLES OF MUOGRAPHY

1.2.1 Muon Energy Spectrum

Muons are produced during the interaction between primary GCRs and the nuclei in the planet’s atmosphere or within rocks if there is no atmosphere. These muons are called cosmic‐ray muons. An expression for the differential cosmic‐ray muon spectrum on Earth at various zenith angles has been derived by several authors in accordance with the hypothesis that the primary particles are isotropically incident at the top of the atmosphere, and that the mesons (pions and kaons) retain the directions of their producers (Bull et al., 1965; Bugaev et al., 1998; Gaisser & Stanev, 2008; Matsuno et al., 1984). These authors' analytical expressions have been validated and improved by comparison with the spectrum of muons taken from experiments performed at sea level (Achard et al., 2004; Allkofer et al., 1985; Haino et al., 2004; Jokisch et al., 1979; Matsuno et al., 1984), and the current expression is in good agreement with the observed muon flux. On Earth, the muon flux reaches its maximum (~200 1/m²/sr/s) at an atmospheric depth of approximately 300 g/cm², depending on the latitude, and then slowly decreases as the muons pass through the additional atmospheric depth (Particle Data Group, 2020). The muon flux is ~90 1/m²/sr/s at sea level. The horizontal muon rate on Earth is suppressed compared to the vertical muons mainly due to the increased atmospheric path length and correspondingly higher energy cutoff. Furthermore, the average charged pion multiplicity is different. There is an appreciable probability that (higher energy) pions will collide with atmospheric nuclei before decaying into muons, and (lower energy) muons decay before reaching the Earth’s surface. However, the average muon energy becomes increasingly high as the direction traveled is more horizontal because pions decay before they re‐interact any further. While the decay constant of muons is 660 m, those of pions and kaons are much shorter, and respectively 7.8 m and 3.7 m.

On Mars, the surface atmospheric pressure is only ~1/100 that of the Earth, and thus, ma...