Mining Project: Coal Mine Safety Applications of Seismic Monitoring

Research Summary

Sudden, brittle failure of highly stressed ground around deep underground coal mines can drive ground falls, coal bursts, bumps, and even sudden collapse of large mine areas. The collapse at Crandall Canyon Mine in 2007 is a notable example. In many instances, failures occur well away from the perimeter of the mine opening and can involve geologic structures that may not be known to miners. Loading of these structures may cause cracking, failure, or caving, which can generate seismic signals that can be monitored, measured, and interpreted. The interpretation of these data combined with other tools (e.g., core testing), can be used to help identify potentially dangerous instabilities. This approach is routinely employed in deep metal mines in countries including South Africa, Canada, Australia, and the United States. It is also employed in some deep European longwall coal mines.

Despite the successful use of seismic monitoring in mines, technical obstacles have limited their application in deep coal mines in the United States to a small handful of experimental installations. Research under this project sought to overcome these obstacles and bring this hazard identification tool into the mainstream of coal mine safety systems. This objective was one of several recommendations made by NIOSH in a report to Congress in response to the Crandall Canyon coal mine collapse.

This project had four research aims:

1. Implement a seismic monitoring system at cooperating mines to evaluate sensor density and distribution requirements for resolving event locations on different structurally important features.
2. Characterize seismic response associated with key deformation processes (caving, bridging, pillar deformation, etc.) in the instrumented mines.
3. Quantify observed seismic response with stress and deformation predicted by numerical models.
4. Use seismic data to evaluate mine design performance and critical design assumptions.

A variety of approaches to implementing seismic monitoring in deep coal mines was demonstrated. These methods were tailored to specific monitoring objectives and range from simple and inexpensive to costly and complex.  In one collaborating fast-moving deep longwall coal mine, a microseismic monitoring system was adapted to  include both surface stationsand underground stations. The system successfully collected a wealth of data including seismic data sequences occurring in conjunction with three damaging rockbursts. Analyses of these data sets indicated the ability to track large-scale ground movements in ways currently unattainable by other methods. To establish sufficient confidence to use such observations as the basis for mitigation actions, independent confirmation of data interpretation with, for example, simultaneous rock mechanics measurements, and more field trials, was needed.

Limitations on event location accuracy were shown to restrict the ability to apply this data in certain applications. Standard location methods utilizing homogeneous layered velocity models appear satisfactory for accomplishing objectives where the demands on location accuracy are not stringent, including awareness of ground response, and documentation of seismic activity.  When the objective is to resolve microseismic activity on particular mining or geological structures, it is necessary to consider using a more realistic 3D heterogeneous model.  

Attempts to calibrate and constrain numerical models with ground deformation behavior observed visually with rock mechanics measurements and microseismic event distributions demonstrated the major role that geology plays. More complete information on geologic structure and strata material properties and constitutive laws are needed in current models to characterize deformation response in these environments.

With improvements in implementing 3D velocity models, real-time analysis and display of microseismic event locations has the potential to become a key element of a system for continuous mine-design performance assessment.

Data sets collected during both rockbursting and trouble-free mining conditions continue to be analyzed to find the best approaches to identifying and assessing ground failure risks in the Detecting and Managing Dynamic Failure of Near-Seam Features in Coal and Nonmetal Mines project.