Mining Contract: Passive Seismic Tomography for Monitoring Stress Redistribution at the Galena Mine

Contract #	200-2015-63456
Start Date	8/15/2015
Research Concept	As rock mass is perturbed by the excavation process, stress redistribution results in failure along new and/or previously-existing faults or joints. The seismic energy produced by these relatively small failures is termed “induced seismicity.” These events can be recorded by a microseismic monitoring system and analyzed by a method referred to as “passive seismic.” “Tomography” is the method whereby energy that has been transmitted through a body is analyzed to produce an image representing the interior of the body. This is frequently used in the medical industry, where a Computed Tomography (CT)-scan is used diagnostically. Combining these two technologies, passive seismic tomography can potentially be used to monitor small mining-induced seismic events and subsequent imaging of the surrounding rock mass.
Topic Area	Ground Monitoring

Contract Status & Impact

This contract is ongoing. For more information on this contract, send a request to mining@cdc.gov.

Excavation of underground openings is a difficult job in a challenging environment. The rigor of the process is compounded by the lack of a method that allows quantification of the changes within the rock mass as excavation and ore extraction progress. From an engineering perspective, the most important unknowns around underground excavations are the highly stressed areas, which can result in dynamic failure, and the extent of the damage. The highly stressed areas are identified with seismic tomography as high-velocity regions because the increased stress results in microfractures within the rock being closed, effectively increasing the elastic modulus of the rock, and resulting in elastic waves traversing the rock with greater velocity and less attenuation. Conversely, damaged areas are displayed as low-velocity zones. Knowledge of the location and extent of both areas is critically important so that safety and efficiency of the operation can be maximized.

The current practice for engineers tasked with monitoring and assessing the stability of underground openings is to use the evolution of field observations with time to calibrate the observation to output from a database of numerical models and seismic events. However, field observations may not be sufficient to detect problem areas before damage occurs. Numerical modeling is a useful tool for understanding rock mass behavior, but the results are frequently based on assumed and generalized physical properties within the rock mass. Seismic data may only give a partial picture of change in the rock mass. Geophysical measurements, as undertaken in this research, complement the field observation, seismic database, and numerical modeling efforts.

Under this contract, the Virginia Polytechnic Institute and State University will further investigate the geophysical method called passive seismic tomography. With this method, relatively small mining-induced seismic events can be monitored at an underground mine and subsequently used as sources for imaging the surrounding rock mass using the tomographic method. This technology is already used on a much larger scale to image the earth’s mantle and to understand plate tectonics, and there is a clear need to further develop this technology on the mine scale so that stress redistribution within the mined rock mass can be more clearly understood, thereby improving the safety and efficiency of the mine.

Ultimately, the goal of this research is to develop a publically available software package for passive seismic tomography and to also compare results to field observations so that hazardous ground conditions can be identified and mitigated. The resulting software tool will provide mines with an indicator of changing ground conditions (stress) using seismic data, which will allow mines to make an informed decision on the required ground support.

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