Cosmic-ray based density-scanning of large geological objects in mineral exploration and mining

Abstract

Cosmic-ray geophysics is a subdiscipline of geophysics that multidisciplinary combines various geosciences with high-energy physics and relies on several types of instruments specifically designed for elementary particle detection. The cosmic-ray geophysical methods can be further divided into a couple of subfields, such as cosmogenic isotope geochronology and cosmic-ray muography. In the case of muography, the elementary particle of interest is the muon. Muons exploited in muography are generated in the upper atmosphere via a continuous process in which atmospheric molecules constantly interact with primary cosmic-ray particle radiation. Muons made this way are highly energetic and almost light-speed-fast (relativistic), due to which they have a high potential to penetrate geological matter far more than any other type of radiation, excluding neutrinos. Although most muons are stopped by the first few meters of terrestrial material, the more energetic muons can reach hundreds of meters in depth. Muography applied to geology is an imaging technique of soil and rock densities. It harnesses the ever-present flux of muons to collect information on the distribution of density contrasts within the volume of interest. Imaging is made possible because muons advance in straight paths and lose energy (and hence speed) according to the average density of the material they pass through. When a single muon has lost its speed, it instantly breaks down into other particles. In brief, muon flux attenuation allows imaging of density variations in geological materials, just like X-ray imaging allows transillumination of the human body. The number of muons detected in any given direction reveals average densities in those directions. Muographic imaging is carried out radiographically or tomographically, i.e., the produced images are either two-dimensional or three-dimensional, respectively. Considering physical appearance, two detector types stand out: borehole probes and larger box-like "telescopes." The two disadvantages of muography are that (a) the detector(s) must be installed underground, and (b) collecting robust statistics is a relatively slow process (from days to months) due to the low flux of muons. However, as demonstrated by the steadily growing number of applications and publications (Holma et al., 2022), the benefits of muography surpass its shortcomings. First, muography is a totally independent density-characterization method from those already existing (gravity, seismics, petrophysics). Second, muographic data is not hampered by magnetic or electric fields. Third, if need be, the image resolutions can be improved relatively easily by technical means, such as adding more observation points or measurement time. Potential applications of muography in geoscience are numerous and span from surface to deep applications. These include, for example, volcanology, groundwater exploration, weathering studies, glaciology, sedimentology, speleology and structural geology. The current work focuses on mineral exploration and mining. REFERENCES Holma, M., Joutsenvaara, J., Kuusiniemi, P. 2022. Trends in publishing muography related research.

Full text

XXII International Congress of the CBGA, Plovdiv, Bulgaria, 7–11 September 2022, Abstracts
Cosmic-ray based density-scanning of large geological objects in mineral
exploration and mining
Marko Holma1,2,3,4
1 Muon Solutions Oy, Finland; marko.holma@muon-solutions.com
2 Kerttu Saalasti Institute, University of Oulu, Finland; marko.holma@oulu.fi
3 International Virtual Muography Institute, Tokyo, Japan
4Arctic Planetary Science Institute, Finland
Cosmic-ray geophysics is a subdiscipline of geophysics that multidisciplinary combines various
geosciences with high-energy physics and relies on several types of instruments specifically designed
for elementary particle detection. The cosmic-ray geophysical methods can be further divided into a
couple of subfields, such as cosmogenic isotope geochronology and cosmic-ray muography. In the
case of muography, the elementary particle of interest is the muon. Muons exploited in muography are
generated in the upper atmosphere via a continuous process in which atmospheric molecules
constantly interact with primary cosmic-ray particle radiation. Muons made this way are highly
energetic and almost light-speed-fast (relativistic), due to which they have a high potential to penetrate
geological matter far more than any other type of radiation, excluding neutrinos. Although most
muons are stopped by the first few meters of terrestrial material, the more energetic muons can reach
hundreds of meters in depth.
Muography applied to geology is an imaging technique of soil and rock densities. It harnesses the
ever-present flux of muons to collect information on the distribution of density contrasts within the
volume of interest. Imaging is made possible because muons advance in straight paths and lose energy
(and hence speed) according to the average density of the material they pass through. When a single
muon has lost its speed, it instantly breaks down into other particles. In brief, muon flux attenuation
allows imaging of density variations in geological materials, just like X-ray imaging allows
transillumination of the human body. The number of muons detected in any given direction reveals
average densities in those directions. Muographic imaging is carried out radiographically or
tomographically, i.e., the produced images are either two-dimensional or three-dimensional,
respectively.
Considering physical appearance, two detector types stand out: borehole probes and larger box-like
“telescopes.” The two disadvantages of muography are that (a) the detector(s) must be installed
underground, and (b) collecting robust statistics is a relatively slow process (from days to months) due
to the low flux of muons. However, as demonstrated by the steadily growing number of applications
and publications (Holma et al., 2022), the benefits of muography surpass its shortcomings. First,
muography is a totally independent density-characterization method from those already existing
(gravity, seismics, petrophysics). Second, muographic data is not hampered by magnetic or electric
fields. Third, if need be, the image resolutions can be improved relatively easily by technical means,
such as adding more observation points or measurement time.
Potential applications of muography in geoscience are numerous and span from surface to deep
applications. These include, for example, volcanology, groundwater exploration, weathering studies,
glaciology, sedimentology, speleology and structural geology. The current work focuses on mineral
exploration and mining.
REFERENCES
Holma, M., Joutsenvaara, J., Kuusiniemi, P. 2022. Trends in publishing muography related research.
The situation at the end of 2020. Journal for Advanced Instrumentation in Science Vol. 2022(1).