Automatic full wave-form based monitoring at the deep Garpenberg metal mine
Résumé
Several recent studies demonstrate the high value of array and full wave-form based automatic detection and location approaches for monitoring purposes of natural and induced seismicity. The main benefit of these approaches is generally the significant improvement in detection capacity and relative event location accuracy which improves significance of statistical analysis aiming at identifying changes of the seismic rate in space and time and nucleation phases of potential larger dynamic ruptures. In mines, the implementation of such approaches remains challenging and is today by far nonstandard but would probably significantly help to anticipate destructive rockburst events. Main challenges for usage of these methods are related to the presence of a wide range of seismic noises related to mining activities with often similar signatures as microseismic events. In addition, high sampling frequencies of seismic data (several kHz) used in these environments pose problems for real-time data transfer and processing. Here, we propose an adapted, full-waveform based automatic processing workflow for a local Ineris seismic network located at the deepest levels (> 1 km depth) of the Lappberget district of the Garpenberg metal mine (Sweden). To deal with high frequency sampling (8 kHz) we designed a pre-processing step based on a multi-frequency detection scheme and first-order amplitudebased location. Final source location is then obtained by applying an array coherency based backprojection approach (BacktrackBB) on the preselected and reduced data set. We estimate that detection capacity compared to a usual triggered monitoring system is increased by at least a factor 100. The approach is currently implemented and tested on the local monitoring system and is continuously improved to assure reliable automatic event classification. A second main field of current ongoing investigations focuses on the design and implementation of an automatic “wave form matching” based approach of seismic repeater families. The approach aims at measuring indirectly aseismic slip in the mining area and providing criteria for seismic hazard as asperity density and larger dynamic rupture potential. Indeed, recent results from source mechanism and parameter analysis, relocation and spatio-temporal statistics have shown that seismicity at Lappberget district seems to be dominated by seismic repeater occurrences that can be interpreted as seismic asperities that repetitively rupture (over periods of weeks to years) as a result of loading of the surrounding creeping weak rockmass (e.g. occurrences of talc) initiated from specific production blasts.