Far-Field Stress Determination Using Seismic Stress Inversion
The objective of this research is to develop tools and methodologies to achieve more reliable information about the stress field in deep mines, especially around mining excavations by using the method of seismic stress inversion. After investigating several stress inversion algorithms, one method, MOTSI, has been applied to the seismic data collected from Nickel Rim South Mine (NRS) located in Sudbury, Canada. Although NRS hosts relatively massive rock mass with high strength, it is structurally complicated. There are many faults located in the mine, which can both influence the stress field and be the source of seismic events. The seismic array has good coverage over the active volume of the mine and consists of a number of sensors with different types (uniaxial and triaxial geophones and accelerometers). This combination of sensors covers the seismic events with a range of moment magnitudes between -3 and 3. Extensive filtering has been applied to the seismic data and finally, for mapping the stress field, more than 1000 seismic events were manually processed and after removing noise and applying different filters, it was possible to determine the orientation of the principal stresses through seismic stress inversion from the middle level of the mine. The aggregate of the inversions inferred a north–south, northeast/southwest orientation of the maximum principal stress and near vertical minimum principal stress. In each inversion processed, several events were tagged as outliers according to the MOTSI software criteria. Several statistical analyses including mean, standard deviation, variance, t-test, box plot, Kruskal–Wallis test, and principal component analysis (PCA) were conducted over 10 seismic source parameters (i.e. source radius, seismic moment, etc.) to determine the relation between outliers and used data. Finally, a relation between principal stresses and relative stress ratio has been described and the application of this relation and the overcoring stress measurements for determining the far-field stresses are explained. Based on the results of this research, guidelines are prepared outlining how the stress inversion technique can be applied to mining seismicity to extract information about the stress field.