Stress field characterisation in Nickel Rim South Mine using seismic stress inversion

Abstract

Knowledge of stresses is important for many aspects of mine design, but conventional methods of measuring stresses produce estimates at only a limited number of points in space and time. Furthermore, stresses are known to be affected by geological structures, particularly faults, but mapping of how the stress field is affected by such structures is not currently possible. Therefore, there is a compelling reason to consider the use of techniques that can map such local stress field variations. The method of seismic stress inversion is utilised to address this limitation and its application is illustrated using seismic data collected from Nickel Rim South Mine (NRS) located in Sudbury, Ontario, Canada. NRS is a modern mine using blasthole open stoping with backfill as a means of bulk mining. The ellipsoidally shaped orebody is located between 1,160 and 1,710 m below ground surface, strikes east-west and is steeply dipping. Although the NRS host rock and orebody are relatively massive with high strength, the mine is structurally complicated. Many faults appear to influence the stress field, in addition to being the sources of seismic events. The seismic monitoring array in NRS has good coverage over the active volume of the mine and consists of uniaxial and triaxial geophones and accelerometers. This combination of sensors results in a large catalogue of events with good focal sphere coverage that permits source mechanism analyses to be performed. Extensive filtering has been applied to the seismic data to improve the quality, and for the stress inversion process the first motion stress inversion (MOTSI) software is used. MOTSI only uses the first motion polarities and estimates the stress tensor components with more complete uncertainties compared to other nonlinear methods. To facilitate development and refinement of the seismic stress inversion process, numerous clusters of seismic events over a period of seven months during the early stages of mining were initially analysed so as to minimise perturbations caused by the interaction between mining and geological structures. More than 500 manually processed events throughout the mine are utilised for the stress inversions. Results show that the clusters in the earlier stages of mining and further away from excavation boundaries demonstrate reasonable agreement with pre-mining stress estimates based on overcoring and breakouts.

Statistical analysis of the outlier events from seismic stress inversion

“…For various types of statistical analyses, it is necessary to have a sufficient number of samples. The time interval used for the stress inversions by Abolfazlzadeh and McKinnon (2017) was therefore extended by three months, resulting in an expanded time interval from February to mid-December 2011. Additional events were predominantly located in the upper right region of the middle level (Figure 4) and the stress inversion result was identical to the previous months.…”

“…Two shafts allow access to three main stoping levels, located at the 1,280 m level for the nickel deposit, 1,480 m level for nickel and copper transition zone and 1,660 m level for copper ( Figure 1). 1.2 Stress field characterisation using seismic stress inversion Abolfazlzadeh and McKinnon (2017) used MOTSI (Abers & Gephart 2001) to analyse first motion data in the stress inversion process, and to estimate regional in situ stresses for the Nickel Rim South Mine. The MOTSI code generates a stress tensor by taking in the first motion polarity of seismic events.…”

“…Data clusters were selected from the early stages of mining, well-contained within the system of uniaxial and triaxial sensors of the monitoring system. The clusters were also selected away from geological structures to avoid potential influence in stress change from mining or fault slip movement (Abolfazlzadeh & McKinnon 2017). More than 500 events over a period of seven months (February to September 2011) were analysed in several clusters using MOTSI.…”