Using a new geophysical tool for improving underground safety in mining and civil engineering: time-sequential muography

Tunnelling and underground mining face many risks threatening underground operations. Such hazards include sudden incidents of dangerous and violent rock bursts and cave-ins. The likelihood of these disastrous events increases as operations go deeper and the in-situ stresses increase. Triggers leading to such accidents can be regional seismic events related to faults and tectonically active contacts between rock types (e.g., dyke contacts). Therefore, it is paramount to know the locations of such pre-existing brittle rock structures, understand their 3D extent, and monitor their changes in time. This allows proactive measures to be taken and stresses to be mitigated before disastrous events occur.

Muography is a novel and passive method for imaging rock densities. Muographical techniques can image and distinguish faults and dykes as long as their densities differ from the surrounding rock. Such anomalies are identified by collecting data and statistics on muons - elementary particles which form in the atmosphere and, at near lightspeed, penetrate all matter. The most energetic ones travel over 1 km in rocks. The number of muons coming from each direction reveals the density of the rock column the muons traversed through.

Muography is conceptually akin to X-ray imaging: In both, the formed image relates to the density profile of the target, i.e., a higher-density medium stops more X-rays and muons than a lower-density medium. Images are reconstructed based on the attenuation of natural background radiation flux. Muography can yield both 2D radiography and 3D tomography density images based on the number of survey locations. A third option, time-sequential (time-lapse) muography, allows long-term monitoring of the target rocks and can detect if any changes occur within it as a function of time. This type of imaging works in both radiographical and tomographical modes.

The flux of muons is high at ground level and decreases with depth as bedrock attenuates muons. This means that muon detectors located at shallow observation depths will be faster to record a statistically sound dataset and, as such, quicker in pinpointing any time-varying changes within the target density.

We propose that stationary muography arrays in underground settings could map potentially risky bedrock structures and monitor their density-affecting changes over time. E.g., hidden faults may become visible due to the passing of seasons or after the passing of substantial rainfall as the excess water percolates through the mechanically broken fractures. Another advantage of the time-sequential approach is that it reveals if the studied structure is stable and time-invariant, i.e., no ongoing processes affect its density. Therefore, we propose that applying muography in underground spaces improves understanding of the conditions of the rock body and, hence, increases safety.

We aim to conduct pilots for this application soon.