Microseismic Monitoring of Highwall Mining Stability at Moura Mine, Australia

We present results obtained from a microseismic monitoring at Pit 20DU at Moura Mine in central Queensland. The objective of this study was to investigate the feasibility of using the microseismic method to map roof fracturing associated with highwall mining for highwall mining stability assessment. 14 triaxial geophones were installed in 7 boreholes across the highwall bench, covering an area of 300 m by 400 m. The experiment was earned out from Jun 12 to Jul 25, 1999. More than 7,000 events were recorded.

Generally, the recorded events were weak in seismic energy. Many of the events only triggered the nearest one or two geophones. The events were classified into three types on the basis of then’ frequency content and seismic duration. Each type appears to be associated with different fracturing mechanisms.

The first type consisted of two groupings of events. One was located in the main sandstone roof and constrained by existing faults. They may be caused by the release of a localised stress concentration. The second group was dominated by events located in the floor of the DU Seam near the highwall bench. They appeal to be controlled by the fault and floor stress concentration near the highwall face.

The second type of events occurred across the mining area and followed the general sequence of the mining. These are inferred to represent a general ground response to the mining. Events of the third group were located mainly in the immediate roof of the DU Seam. They were found to be associated with minor fracturing near the mine entries.

During the monitoring period, about 30 entries were mined and no significant entry or pit stability problems were encountered. Therefore, the fracturing giving rise to the microseismic activity observed at 20DU did not have an adverse impact on the highwall mining stability.