Underground » Coal Burst
This project investigated the feasibility and performance of pre-installing fibre optic cables in exploration boreholes for Distributed Acoustic Sensing (DAS) monitoring of longwall caving behaviour and potential coal burst hazards.
Unlike conventional microseismic systems which require large numbers of triaxial geophones and extensive cabling, DAS offers a cost-effective alternative by transforming standard fibre cables into dense, continuous arrays of virtual sensors spanning several kilometres. A standard methodology for deploying fibre optic cables in multiple boreholes has been developed in this project. A 3D fibre optic sensing network was deployed in multiple vertical boreholes and in surface trenches at an underground longwall coal mine. The DAS network captured intense seismicity, with daily event counts ranging from ~380 to ~4,000. Event occurrence strongly correlated with longwall retreat rate. The dense DAS measurements enabled intuitive identification of fracture depths from the curvature of first arrivals, while surface arrays provided reliable planar location constraints.
Analysis of 2,370 manually selected seismic events revealed three distinct fracturing zones.
A comparative study between DAS and geophones showed that while geophones retain higher signal-to-noise ratios on individual channels, DAS recorded many more weak seismic events, particularly shallow fractures, and deep activity beyond the sensitivity range of the geophones. For strong events, DAS and geophones produced broadly consistent waveforms and spectra. However, due to the sparse distribution of geophones, the fracture depths can be more reliably determined by the fibre optic cables installed in vertical boreholes.
DAS wavefields also revealed seismic reflections and travel-time discontinuities corresponding to known geological boundaries, demonstrating the potential of borehole DAS for subsurface characterisation beyond traditional microseismic monitoring capabilities.
The results confirm that pre-installed fibre optic cables in exploration boreholes provide a highly effective and economical approach for longwall seismic monitoring. The network achieved reliable fracture detection, caving pattern identification, and geological boundary mapping, even under challenging conditions where boreholes progressively collapsed. The research also identified optimal installation sequencing, splicing strategies, and network design considerations that minimise maintenance and maximise monitoring continuity.
The project demonstrates that DAS offers substantial advantages for longwall hazard assessment, including dense spatial coverage, low installation cost, robustness in dynamic mining environments, and enhanced capability for understanding caving mechanisms. The findings support further integration of DAS for geohazard assessment into mine routine seismic monitoring systems at mines, using fibre optic cables pre-installed in boreholes.