ANALYSIS OF THE MINING INDUCED SEISMICITY AT KIIRUNAVAARA MINE

LKAB Contact Christina Dahnér Lindqvist    LTU Contact Savka Dineva
Posted in Sustainable mining

A sub-level caving method is used in Kiirunavaara mine and the increased induced stresses with each mining level cause increased induced seismicity. The mine became seismically active in 2007 – 2008 at about 900 m depth when a number of large rock falls took place. Since then the mine has implemented a number of changes in order to minimize seismic risk. This project is initiated in order to maintain a high level of safety in the future and to increase the understanding of the rock mass behaviour and seismicity sources caused by the induced stresses. The final goal of the project will be to find better understanding of the interaction of mining with the geological environment and in particular how the resulting stress field causes fracturing and seismicity.

 One production area is chosen (Block 34) for this research (volume ~ 600 x 250 x 150 m). The present production level is approximately at the top of this volume and will progress gradually downwards. The research will attempt to define the stress changes in space and time using different types of measurements. In situ stress measurements will be made at three different levels in the early stage of the project.  The stress changes will be monitored at the same location for about two to three years. Stress changes will be monitored while at least one sublevel is mined out.

In the same areas there will be three high-frequency active seismic sources installed and the possible changes in seismic velocities will be monitored continuously for the same period of time (two to three years) in the whole volume.  Passive tomography with induced seismic events as sources will be performed in regular time intervals within the same volume to follow the changes of the seismic velocities. The result from the monitoring of the seismic wave velocity changes will be correlated with the result from the stress monitoring.

 Analysis of the focal mechanisms and the seismic moment tensors of the larger induced seismic events will be performed. The results from this analysis will be compared and evaluated together with a kinematic model of the structural geology which is under construction. Stress inversion of the focal mechanism/moment tensors of the seismic events with magnitudes larger than ~0.5 will be used as additional information for monitoring the stress changes within the study volume.

 The results from all measurements and analysis of seismic data together with the extended work on the structural geology will form important input into developing methods for numerical modelling of the rock mass behaviour related to the mining induced seismicity. The main focus of this modelling will be to determine how to incorporate faults from the structural model in a way that is compatible with the observed stress field characteristics and seismic response, and to develop guidelines for interpreting the modeling results.