Monitoring mining-induced rock mass deformation using distributed strain monitoring based on fiber optics

Triggered seismicity remains one of the main geomechanics hazards that affect safety in deep mining. Mobilization or propagation of existing faults can potentially release large amounts of seismic energy that in turn may trigger rockbursts and falls of ground, threatening worker safety and generating production delays. Cur-rent modeling approaches, typically based on stress analyses, do not fully succeed in capturing such seismically triggered mechanisms as the main seismic event location may be beyond the volume of rock affected by the mining induced stress perturbation. However, the displacement field generated by the excavation process may explain the triggering of far-field seismicity. Deeper understanding of the mining-induced defor-mation field is the motivation for the development of mine-scale deformation monitor-ing techniques. Innovative deformation sensors developed for structural monitoring based on fibre optic technology allows distributed measurement of strain at high spa-tial sampling rates over large distances.
This paper presents the results of the testing of one of these systems in an active mining context. The system selected is based on Brillouin Optical Time Domain Ana-lyses (BOTDA) and allows for a spatial resolution of 10 cm. It has been tested in the laboratory and installed in five boreholes piercing through an actively mined, 25 m thick, 1000 m deep, sill pillar. Benchmarking of the system against extensometer re-sults was successful in a qualitative manner. The high spatial resolution of the fibre optic system brings valuable additional insights to rock mass deformation processes.

, 2012