Reliable location of microseismic events is an important objective for routine mine seismic monitoring. Reflections from the underground excavations result in complex recorded seismograms, and typically only the direct P- and S-wave arrivals are used to locate the source. We have developed an algorithm that uses the full waveform, including reflected arrivals, to locate a seismic event. The method uses a library of precomputed Green's functions based on a 3D model of the mine tunnels and excavations. The library is used to quickly compute trial waveforms, which are cross-correlated with the recorded seismograms to produce an objective function. This function is maximized by a differential evolution algorithm to simultaneously yield the source location and six moment-tensor components. If the mine has many tunnels and excavations, which is typically the case, then this fully automatic method can use even a single triaxial or biaxial seismogram to reliably determine the source origin and mechanism. This result has useful consequences for mine seismic monitoring, and is demonstrated with recorded data from an Australian underground mine.

INTRODUCTION

Passive microseismic monitoring has been routinely used for more than 30 years by many mines around the world to manage the rock mass response to mining (Gay and Wainwright, 1984). Reliable location of seismic events is important, and so distributed sensor arrays are used to record seismograms and quantify the position and source parameters (Mendecki, 1997).

Many tunnels, stopes, and other excavations exist in hard rock underground mines. Often stopes are backfilled by waste rock or cementitious paste, but in any case the stopes are filled with much weaker material than the surrounding rock and so there is a high contrast in acoustic impedance at the stope surface. Body waves from the seismic source are consequently reflected off the excavations and add to the complexity of recorded waveforms in underground mines. This complexity is typically ignored in standard processing, and only the picked arrivals of the direct P- and S-waves are used to locate the source with a calibrated velocity model and a method that minimizes the arrival time residuals.

This content is only available via PDF.
You can access this article if you purchase or spend a download.