Q_β estimates from spectral ratios and multiple lapse time window Analysis: Results from an underground research laboratory in granite

Abstract

An experiment to measure quality factor ( Q ) in‐situ was carried out at the Underground Research Laboratory in Manitoba, Canada using the Spectral Ratio technique and a form of the Mulitple Lapse Time Window Analysis technique. The volume of rock examined measures approximately 30 × 30 × 30 m at a depth of 420 m in Lac Du Bonnet Granite. The site encompases 16 triaxial accelerometers grouted in the bottom of boreholes drilled from access tunnel walls. The source for the seismic pulses was a mechanical hammer placed at the bottom of specially located boreholes. Results from the Spectral Ratio experiment gave S‐wave quality factor ( Q β ) of 114 ± 18. Based upon the short duration of the first arrival pulses and low scattered wave energy in the signal coda, we conclude that the Spectral Ratio technique measured the intrinsic attenuation of the rock mass. Further analysis using the more recently developed Multiple Lapse Time Window Analysis gave results for intrinsic Q β ( Q i ) of between 85 and 100. Conversely, values for scattering Q β ( Q s ) were greater than 800. The high correlation for Q i values between the two methods help to confirm the nature and absolute values of the attenuation, and also support the use of the later technique where spectral ratios cannot be applied.