Because of rapid development, muography has been applied to many fields including mining engineering in recent years. In the first part of this section, the applications of muography that have already tested in mining or rock engineering are introduced, and in the second part, the potential future applications of muography in mining engineering are described.
State-of-the-art of Muography in Mining and Rock Engineering
Underground Cosmic-ray Experiment in the Pyhäsalmi Mine
The Pyhäsalmi mine is located at the Pyhäjärvi municipality in the central Finland, and is owned by First Quantum Minerals Ltd. The Pyhäsalmi deposit is a volcanogenic massive sulphide deposit of which main products include zinc, copper and pyrite. This type of deposits typically has a large density contrast relative to their surroundings. This mine is a typical hard rock underground mine. In the Pyhäsalmi mine, three muon tracking stations, being part of the cosmic-ray experiment EMMA, are located at the depth of 75 m (Kuusiniemi et al. 2018). One station (see Fig. 3) covers an area of 15 m2 and consists of position-sensitive detectors in three layers separated by 1.1 m and of one layer of fast timing detectors. Approximately 10 muon tracks per second are observed in one tracking station. However, EMMA is designed for the studies of origins of cosmic rays, not for muon imaging.
Muons Detected at Levels of 1390 m, 2000 m and 2400 m Underground
Prior the present work, a portable muon detector was placed at different levels in the Pyhäsalmi mine related to underground physics experiments for measuring the muon flux as a function of the depth (Enqvist et al. 2005). At the deepest level of 1390 m below the ground a total flux of 0.4 muons/m2/h was measured. The result from all levels is shown in Fig. 4 based on the data detected by Enqvist et al. (2005). In addition, in two other underground physics facilities muon data were measured at depths of 2000 m and 2400 m below the ground surface in Canada (Jillings 2016) and China (Wu et al. 2013) as shown in Fig. 4. This indicates that muons can penetrate into hard rock mass with at least a thickness of 2400 m, but the muon flux decreases with increasing depth. In all these three very deep sites muon flux measurements are conducted to understand background conditions, which is an essential parameter for all underground physics experiments.
Muon Telescopes Tested in Tunnel Boring Machine
Similar to borehole detectors, small-size muon non-cylindrical telescopes can be mounted in a boring machine or a drill rig. During boring or drilling, a nearest disorder such as a weak zone and a water reservoir in the area a telescope is facing can be found. As soon as the disorder is detected, a premeasure can be taken to handle the disorder. According to Sloowere et al. (2018), a muon telescope was mounted on a tunnel boring machine (TBM) to detect disorders in excavating the galleries in France and Switzerland, especially in the Croix-Rousse tunnel in Lyon. Similarly, Chevalier et al. (2019) reported that during excavating a new subway in the suburbs of Paris (line 15), a muon telescope was attached to a Tunnel Boring Machine (TBM) to collect a vast amount of data related to the density of the ground around the tunnel. The captured muon data were split into a sequence of short-period 2D photographs to reconstruct a 3D density model of the ground, aiming to predict changes in the density of the ground in front of the TBM and detect geological heterogeneities or man-made buried objects. One of their experiments involved the detection of a 1-m-wide sewer above and besides the path of the TBM. Their simulation results suggested that it was possible to detect and reconstruct the sewer.
Trials of Muography Techniques in Other Underground Mines
Muography has already been tried in rock and mining engineering as mentioned before. Some investigations have been conducted and reported, e.g., by Schouten (2019) who presented a few case studies in which three ore bodies in three mines were detected by muon tomography. In the Myra Falls study, British Columbia, Canada, a single muon detector was exposed for 15–20 days at seven locations inside a mine tunnel in a depth of approximately 70 m below the surface. Like Pyhäsalmi, this deposit falls into the VMS class of dense ore deposits. Muon flux data were inverted to a 3-D density image of the deposit. The inverted data were in good agreement with drill core data (Bryman et al. 2014). Schouten (2019) presented also results of a muon tomography test carried out in the Pend Oreille Zn–Pb mine, Washington, USA, at a depth of 540 m. This deposit belongs to the Mississippi Valley-type (MVT) polymetallic deposit class and like the VMS deposits provides an ideal high average density target for testing density detection power of muography. In this test, which was carried out as blind, four detectors operated for 68–153 days in their respective locations. As a result, the muon geotomography measurements were found to be consistent with a simulation of the expected muon tomography data (Bryman et al. 2015). The third case study described by Schouten (2019) was conducted in the McArthur River uranium mine, Saskatchewan, Canada. The McArthur River represents yet another deposit class, namely unconformity-related uranium deposit class. The survey was carried out at 500 m depth. The results indicated a good overall agreement between the muon tomography measurements and an expected anomaly of a simulated deposit with known density properties. It was concluded that differences between the simulated and measured data likely arose from the discrepancy between a simulated one-density ore model and the highly variable true densities in the ore deposit. Although muography has been tried successfully in a number of mines, as mentioned above, many potential and important applications have not been touched yet in mining and rock engineering.
Potential Applications of Muography in Mining and Rock Engineering
Optimistically, there are many more potential applications than those reported in mining engineering and rock construction. The following is a summary of these applications.
Imaging of Geological Structures and Isolated Bodies in Rock Mass
A planar geological structure may be a fault in which filling materials often have different densities from its surrounding mass. An isolated body may be an ore body with different density from its surrounding rock mass, a cavity in a rock mass, a water or fluid reservoir within a rock mass, or hard-rock boulders in the unconsolidated sedimentary cover above the crystalline bedrock or in a consolidated sedimentary rock sequence made up of originally loose sedimentary materials of similar nature, as shown in Fig. 5. In theory, all these structures and isolated bodies can be detected by muography as long as their densities differ from the average density of their background. Taking the boulders (or erratics) as an example, they are often located in some sedimentary rock masses where boulders have much higher strengths than their surrounding masses. If a tunnel must go through such a rock mass and the tunnel is excavated by a TBM, the TBM machine often faces problems such as faster-wearing in cutters, frequently damages to the cutters, lower excavation speed, and higher costs of tunnelling. To find such boulders, geological core drilling is often used. However, the core drilling is very expensive and time consuming. In this case, muography can be a good option for detecting the boulders before the TBM excavation reaches them. For example, a muon telescope can be mounted on the TBM machine and the boulders close to the machine can be found before the machine approaches them. Alternatively, a borehole muon detector can be placed in a much fewer number of holes than in ordinary geological drilling method mentioned above. If so, the cost of geological drilling can be markedly reduced using muography. As soon as the boulders are found, they can be preconditioned by special blasting in the field (Zhang et al. 2019).
Rock Excavation and Support
Rock mass may contain water reservoirs and cavities with various sizes. For example, the low-density body in Fig. 5 may be a water reservoir, for example a particularly tectonic zone in a crystalline bedrock or a previously unknown cave in a limestone terrain. During excavation or tunnelling, an undetected reservoir may suddenly be touched and a lot of water may spout to the tunnel. This may result in an accident and economic loss. If muography imaging is used during the excavation, such an accident can be avoided since the reservoir can be detected before the excavation reaches it. In addition, using muography, a weak zone located in the immediate vicinity of an excavated tunnel or an underground space can be found during excavation. In this case, a particular rock support design can be made on the basis of the detected information of the zone.
Monitoring of Fracturing and Deforming Rock Mass
By developing simulation algorithms it is possible that muography can be used to monitor a rock mass vulnerable to fracturing and deformation. When the technology is mature, many challenging problems in underground mining and tunnelling can be handled. For example, since an ore mass in a certain region may always be in a process of fracturing and deforming due to mining activity and/or increasing mining depth, many blastholes in deep mines are prone to damage, fracture and deform (Ghosh et al. 2015; Zhang 2016). The displacement of the rock in the walls of production blastholes with a diameter of 115 mm was found to be up to 70–80 mm (Ghosh et al. 2015). A severely damaged or deformed hole makes it difficult or even impossible to load with explosives. As a result, rock fragmentation will be worse and even ore recovery ratio be lower for some mining methods, such as sublevel caving. In addition, fracture and deformation in a rock mass may be caused by a moving fault. For instance, as mining production was approaching an inclined fault, it was found that the ore mass in and near the fault was broken and the ore mass above the fault slid down (Zhang 2014), indicating that rock fracture had happened within the rock mass. This type of rock fracture in an ore mass to be mined often causes a problem in ore recovery and increases mining cost. By means of muography, it is possible to find such a fractured or damaged region at an early stage after the fracturing process starts. If so, some measures can be taken to prevent poor fragmentation and low ore recovery.
In an underground mine, no matter which mining method is used, the rock and ore mass in and close to a mining area are continuously deforming or moving as long as the mine is in operation. With an increasing mining depth, extensive deformation may occur inside of the overburden rock. A worse or the worst result is that large-scale deformation may trigger the formation of multiple large fractures or reactivation of the pre-existing ones. Such a process progresses gradually unless the worsening situation is recognised and taken care of. If the situation is not handled, the risk for a cave in increases, for example, in the roof of a stope or in the overburden of a production level, as indicated in Fig. 5. Such large fractures inside a rock mass are usually difficult to find unless there is sufficient and effective deformation (or displacement) monitoring in the locations of the fractures. However, it is possible to detect such fractures using muography since the density of a fractured mass must be different from its initial density.
Imaging of Caving Body
In sublevel caving, a mass mining method used in many underground mines, the ore loss in a mining operation can be up to 20%. If a mine using this mining method has, for example, an annual crude ore production of 20 Mt, the crude ore loss in mining per year can be up to 5 Mt. This lost or remained ore is mixed in the caved waste rocks, as shown in Fig. 6. These caved rocks always move or flow, as long as the blasted ores are extracted. The lost crude ores within the caved waste rocks will move together with the surrounding waste rocks. However, it has not been clear how the lost ores move in the caved rocks and where the remained ores are. If the locations of the remained ores are known, it is possible to extract them out of the caved rocks through the finished production drifts in the footwall. To find the locations, muography can be a suitable technique to use.
Non-destructive Evaluation and Classification of Rock Masses
Up to now, it has been a challenging issue to evaluate and classify rock masses, although many classification methods have been developed. These methods include rock quality designation (RQD) (Deere and Miller 1966), tunnelling quality index (Q) (Barton et al. 1974), rock mass rating system (RMR) (Bieniawski 1973), geological strength index (GSI) (Hoek et al. 1995), rock mass index (RMI) (Palmstrøm 1996), etc. In addition, it has been tried to classify rocks using sonic velocity (e.g. Rawlings and Barton 1995; Zhao and Wu 2000; Nourani et al. 2017; Chawre 2018). However, most of the methods such as RQD, Q, RMI and RMR require specimen collection, tests of intact rock properties and extensive field work for identifying the frequency and nature of the discontinuities, and the methods using sonic velocity have significant variability in measured values of properties of rock for a given velocity, even though they are convenient to implement (Butel et al. 2014; Karakus et al. 2005). Therefore, a classification method that is simple, reliable and easy to use is needed.
Zhang (2016) proposed to use characteristic impedance (product of sonic velocity and density) to evaluate the quality of a rock mass and classify rocks since the characteristic impedance of rock can to a large extent represent the actual state of the rock mass. For example, sonic velocity must be dependent on geological structures such as joints, faults, bedding, etc., while density relies on mineral composition, etc. Thus, the characteristic impedance is a more reasonable parameter than the sonic velocity in evaluating and classifying rock masses. Since the sonic velocity of a rock mass can be determined by non-destructive methods such as a seismic system or vibration monitors in a mine or a rock construction site, we only need to determine the density of the rock mass. If a muography monitoring system exists in the mine or the construction site, the densities of different rock masses can be determined by the non-destructive method.
Exploration in Operating Underground and Open Pit Mines
In operating underground mines, some small ore bodies either close to or isolated from a large ore body may not be found in initial or earlier geological exploration due to sparse exploration. For example, in the Malmberget iron mine, Northern Sweden, a small ore body close to a large ore body was occasionally found after the planned ore was completely mined out in the large ore body. To prevent ore resource in such small ore bodies from loss due to insufficient exploration, different muography techniques can be used to detect such ore bodies in an operating mine. In addition, muography can be used to largely reduce the quantity of exploration holes in an operating mine because a muon detector in the underground can cover a large area, as shown in Fig. 2.
Compared to other types of detectors, borehole muon detectors can be used in most standard-size drill holes. This makes it possible for muography to be applied to open-pit mining. For example, by drilling a few deep holes in an open-pit mine, borehole detectors can be placed in the holes to search weak zones and structures in the slope areas and the ores in the surrounding areas of the mine.
In both open pit and underground mining both muography and conventional geological drilling can be used together. For example, muography can be used to image a large area that has the potential to contain ores or geological structures. If an ore body or a structure is detected by muography, geological drilling can be used to drill some necessary holes in the ore body or the structure to determine its boundaries in more detail. In this way, the quantity of geological drill holes, and the cost and the time of the exploration can be reduced, compared with that only conventional geological drilling is used in the exploration.