Q-BETA ESTIMATES FROM SPECTRAL RATIOS AND MULTIPLE LAPSE TIME WINDOW ANALYSIS - RESULTS FROM AN UNDERGROUND RESEARCH LABORATORY IN GRANITE

An experiment to measure quality factor (Q) in-situ was carried out at the Underground Research Laboratory in Manitoba, Canada using the Spe ctral Ratio technique and a form of the Mulitple Lapse Time Window Ana lysis technique. The volume of rock examined measures approximately 30 x 30 x 30 m at a depth of 420 m in Lac Du Bonnet Granite. The site en compases 16 triaxial accelerometers grouted in the bottom of boreholes drilled from access tunnel walls. The source for the seismic pulses w as a mechanical hammer placed at the bottom of specially located boreh oles. Results from the Spectral Ratio experiment gave S-wave quality f actor (Q(beta)) of 114 +/- 18. Based upon the short duration of the fi rst arrival pulses and low scattered wave energy in the signal coda, w e conclude that the Spectral Ratio technique measured the intrinsic at tenuation of the rock mass. Further analysis using the more recently d eveloped Multiple Lapse Time Window Analysis gave results for intrinsi c Q(beta) (Q(i)) of between 85 and 100. Conversely, values for scatter ing Q(beta) (Q(s)) were greater than 800. The high correlation for Q(i ) values between the two methods help to confirm the nature and absolu te values of the attenuation, and also support the use of the later te chnique where spectral ratios cannot be applied.