| Seismic wave properties have been studied qualitatively to delineate the stress levels in rock masses for a long time. Wave velocity has been the property targeted in most of the studies to date, but theoretical evidence suggests that seismic attenuation is a more sensitive criterion. Laboratory studies on attenuation are readily available in the published literature but its field verification is uncommon. The attenuation of seismic waves of acoustic range has been studied at the Brunswick Mine No. 12 of Noranda Inc., Bathurst, New Brunswick, Canada in 1999. During this study, acoustic sensors and Ground Movement Monitors (GMM) were installed near the face of an advancing heading. Acoustic waveforms produced by rotary-percussion drilling in the heading were used to identify changes in the quality of the freshly exposed back near the face when supporting roof bolts were installed and when blast-holes were drilled into the face. Analysis of the recorded waveforms indicated that the rock mass becomes loose at the back during drilling at the face but subsequently becomes stiffer during the installation of supporting roof bolts. These observations were corroborated by the GMM roof displacement measurements and by independent numerical modeling of the field site. |
