Background
A microseismic monitoring system, consisting of uni- and tri-axial geophones, in an underground stoping mine in Finland, recorded high quality seismic data, including seismic source mechanisms of small events (consistently down to mW=-2.0). Seismic events around the stoping area were mostly crush-type mechanisms, associated with the failure of the rockmass in the pillars between primary stopes. The seismic events around the decline in the footwall were generally very small and showed slip-type source mechanisms. The activity of these footwall events was closely linked to production blasts of some stopes.
Section view of the microseismic events, visualized as beachballs.
Results
The structural model at the mine did not contain any existing geological features that matched the location and orientation of the observed footwall events. The location and source mechanism of these events were used to identify and parameterize the likely location and orientation of these geological features. Microseismic events could then be associated with specific weak geological structures, and their response to blasting of specific stopes could be analyzed. It was found that the activity of each structure increased dramatically after certain stopes were mined, allowing the stress changes induced by mining out the stopes to be understood and acting as a valuable data point for calibrating numerical stress models.
Left: Section view of the 4 identified structures. Coloured boxes identify 3 specific stopes that were blasted. Right: Time histories of activity on each structure. Dashed vertical lines indicate when significant increases in the activity rate on each structure were identified and related to blasting of the identified stopes with corresponding colour (coloured boxes on left).
Ground truth verification of the inferred structures was done with a diamond drill hole. Drilling towards the structure inferred from seismic data yielded a thin (<0.1 m) quartz/graphite vein within 1 m of the expected location. These thin veins were not picked up during initial structural mapping, and although the initial seismic events on these structures were small, they were observed to increase over time as extraction increased.
Core box photo of the drill core that targeted the structure. Quartz veining found near the expected depth highlighted in the red box.
Added value & benefits
This analysis was made possible due to the high quality seismic signals and routine calculation of seismic source mechanisms. Results of the study provided the following benefits:
- An improved structural model for understanding rockmass behaviour to mining, especially important as the mining progresses deeper
- A reliable data point for verification of numerical stress models
- Help in identifying priority areas for the installation of dynamic ground support as the seismically active structures are identified and become more hazardous
Further reading
Py, A., Meyer, S., & Falmagne, V. (2022). Early Experiences of Microseismic Monitoring in Kittilä Mine. Proceedings of the 9th International Symposium on Rockbursts and Seismicity in Mines.
