Seismic Moment Method
Seismic moment method is a new method to predict rockbursts as well as microseismicity in deep longwall coal mining. The method is able to predict fractures in the coal seam and in the rock mass by using 3-D elastic numerical stress analysis. Fracturing intensity and its variation with respect to face advance are represented by maximum shear seismic moment release rate.
Example of the case studies are as follows.
1. Panels W8-5U and W8-5 in Horonai Coal Mine at 1055 to 1125m deep. Even though no rockburst occurred, areas where microseismic events concentrated were well predicted. It was also clear by comparing calculated maximum shear seismic moment release rate to observed data that the magnitude of the seismicity and its variation with respect to the face advance were also successfully predicted.
2. Panel W6-3 in the same mine at 915 to 985m deep. Microseismic monitoring had not been introduced yet, however, the rockburst in this panel was successfully predicted as fracturing of coal seam elements on the coal face.
3. The last case considered panels E3 to E6 in Miike Coal Mine at 600 to 650m deep. Location and intensity of microseismicity were well predicted as in the case for W8-5U in Horonai Coal Mine. Toppling as well as rockburst were also predicted.
Example for D Colliery. Red: compressive failure of rock mass, blue: tensile failure of coal seam, yellow: compressive failure of coal seam and green: tensile failure of coal seam. Top: a plan view in which the retreat direction is left to right. The yellow line denotes the face position. Face width is 210 m. Bottom left: a sectional view in which the retreat direction is the same as the plan view. The length of the yellow line showing face position is +100 m. Bottom right: another sectional view in which the tailgate and maingate are shown right and left. The radiuses of the circles are proportional to the logarithm of the maximum shear seismic moment. The centers of the circles are biased by using random numbers up to 7.5 m to avoid overlapping. Only compressive failure occurred for this case.