Underground » Strata Control and Windblasts
The objectives of the project were to develop new microseismic methods to map tensile microseismic events round operating longwalls, and apply the new methods to the data set collected from microseismic monitoring projects already undertaken by CSIRO.
Methods of noise filtering in the frequency domain and noise suppression in the time domain have been developed in order to remove non-periodic and periodic noise from seismic records. A novel method, the noise wavelet method, has been developed in this study particularly for the suppression of non-smooth periodic noise. The application of the methods greatly improved the signal-to-noise ratio and allowed seismic arrival times, polarities and amplitudes to be accurately measured. This improvement is necessary for the determination of the locations and failure mechanisms of weak seismic events in which many of them are associated with tensile failures.
The moment tensor inversion method for the determination of failure mechanisms has been developed and tested successfully through both modeling and inversion of real events recorded by a large number of geophone stations. The method allows the characterization of rock failures associated with mining using seismic waveforms. However, this inversion method appears to be very unstable for weak events recorded by a small number of geophones with a poor spatial coverage.
An alternative approach based on the particle motion directions of seismic waves has been developed to estimate the failure mechanisms of the weak events. This approach is efficient in the discrimination of shear and non-shear type events when the moment tensor inversion is not applicable. Although this method can only give approximate information about the source mechanisms, for many cases this is sufficient for mining geomechanics interpretation.
An event location method using ray paths has been developed particularly for locating weak events. This method allows a weak event to be located using seismic ray paths and arrival times observed by as few as two triaxial geophone stations.
The accurate determination of released seismic energy proved to be difficult for weak events because their source mechanisms cannot reliably be determined. A method has been developed in this study to approximately estimate the relative source intensity of the events.
These newly developed methods allowed many weak seismic events which could not be analysed using previous procedures to be processed in this study.
The results of this study have significantly improved our understanding of the main failure modes associated with longwall mining caving processes. In previous studies, only strong events had been analysed and it had been assumed that this shear failure was the dominant failure mechanism. Shear failure is still the dominant mechanism under the stiff and confined conditions ahead of the face but it is evident from this study that a greater number of non-shear type events occur as the caving process proceeds.
This project has demonstrated that tensile events can be detected and located using a dense seismic network and newly developed methods of analysis. Undoubtedly, knowledge of the tensile activity, its location and relative intensity will greatly benefit the mining industry in managing longwall geomechanics and associated issues in face and caving control.