Borehole Deformation Preceding Induced Seismic Events-Examples from Coal Mines in Upper Silesian Coal Basin in Poland

Underground exploitation of coal in Upper Silesian Coal Bassin in Poland is accompanied by intense induced seismic activity. It has been well recognised, that presence and deformation of hard, rigid strata overlaying the productive layer is an important factor conducive to rockbursts occurrence. Therefore observation of the rock deformation process can be useful to monitor the process preceding major rockmass crashes. Borehole deformation measurement method is applied in this purpose in "Myslowice" and "Slask" coal mines. The deformations are measured in axial and radial directions in boreholes located in vicinity of mine openings. Results of measurements obtained in different geological and mining conditions are presented. The distance between measuring point and the longwall appears to be main factor influencing the long term results of measurements. It is possible to build a simple linear regression model exemplifying the dependence of deformation and distance between sensors and longwall. Anomalies and disturbances in this relation indicate that the unstable deformation start to appear which can be terminate with a strong seismic event. Many examples confirm that method of borehole deformation measurements can be used to monitor the state of rockmass fracturing process and premonitory changes before a main crash. A retrospective analysis of the deformation and seismic data collected from the stopes at a depth of 500-650 m in "Myslowice" and 900-1050 m in "Slask" coal mines shows the effectiveness of prediction based on cyclic borehole deformation measurements.