Modern mining operations rely on dependable, real-time knowledge of the area being mined during operations, and seismic tomography is an effective way to acquire that information.
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The typical mining operation causes significant trauma to the surrounding rock strata. The ability to track the seismic responses of surrounding rocks is essential to maintaining both safety of the mining crew and economic viability.
Mining operators are particularly interested in highly stressed areas of rock, as these regions can be the source of dynamic failure. When the level of stress exceeds the strength of the rock structure, it can lead to different kinds of collapses and bursts of rock. A rock burst often involves flying rock and is therefore inherently dangerous. It can also cause air blasts that can damage ventilation, releasing dangerous gases, or creating explosive dust.
Before modern technology, physical methods were used to assess the structural integrity of mining operations. However, physical methods used to analyze and monitor the area around an underground or open-pit mine tend to be demanding and time-consuming, even with the use of high-powered equipment. For instance, the stripping away of rock or investigative drilling procedures can only provide a limited amount of data and can cause production delays.
Mining and the Use of Seismic Tomography
A better approach than using physical methods is to use seismic tomography. Meaning “writing by slices” – tomography is a non-invasive method that creates cross-sections and three-dimensional digital models of the surrounding rock. The technology was originally developed in the medical field, and in both medical and mining applications, it uses a network of sensors to detect energy passing through a physical body.
In mining, geophones placed into boreholes are often used to detect seismic activity in the surrounding rock. Activity detected by the
sensor array is sent to a central computer system for processing. The system can identify and monitor the status of various geologic features, particularly telltale concentrations of stress. In some systems, tomography data will be averaged to increase the clarity of geographic features within the rock. However, averaging of tomography data decreases the visibility of stress-related features.
The application of seismic tomography in mining is based on differences in the speed of seismic waves passing through various geologic features. To put it simply, seismic waves travel at a slower pace through rock that is hot, less dense, partly melted, or containing hydrated minerals. Conversely, seismic waves will travel faster through rock that is cold, dense, and solid.
In modern operations, seismic tomography is particularly valued for its ability to detect stress and instability in geologic formations. Regions of rock containing high levels of stress tend to contain large amounts of micro-fractures that increase elasticity. Seismic waves traveling through these regions, therefore, move with greater velocity.
Meanwhile, seismic waves travel slower through geologic regions with high levels of damage. Through training and experience, users can read tomographs to identify critical areas so that any operational changes or safety measures can be applied.
A seismic tomography system is considered either “active” or “passive” based on the source of seismic waves being measured. During mining operations, considerable amounts of seismic waves are produced, and an active tomography system can detect how the waves are affected as they pass through geologic regions.
Mining operators do not need to rely on drilling or blasting operations for seismic tomography. The ground beneath our feet is constantly abuzz with low-level vibrations. For example, the crashing of ocean waves against the shore can create a low-level “seismic hum” that a tomography system can use to detect geologic features. Because the strength of these seismic waves is so small, it takes longer for a tomography system to detect features based on seismic hum.
Active Tomography in the Mining Industry
Active tomography is far more common in mining operations. Excavation, drilling, and explosions all send very strong seismic reverberations through the ground.
An installed tomography system makes use of these waves to create detailed cross-sections or three-dimensional models.
Over time, these cross-sections and models are analyzed to demonstrate a progression of the surrounding geology. The vertical and lateral information gathered by these systems has been shown to be representative of actual conditions found within the rock being studied.
In addition to being placed in boreholes, geophones for seismic tomography can also be placed on mining equipment. For example, geophones can be mounted on roof bolts in the front and rear of a longwall shearer. The shearer acts as the source of seismic waves while the geophones act as the receivers. In this arrangement, the roof bolts would provide data on the geologic conditions above the coal being mined by the shearer.
References and Further Reading
Perkins, S. et al. (2019) Core Concept: Seismic tomography uses earthquake waves to probe the inner Earth. Proceedings of the National Academy of Sciences. 116 (33) 16159-16161 https://doi.org/10.1073/pnas.1909777116
Institute of Mining Services. Mining. [Online] Available at: https://www.imseismology.org/mining/