SEISMIC ATTENUATION IN UNDERGROUND MINES - A COMPARATIVE-EVALUATION OF METHODS AND RESULTS

The goal of this study is to evaluate methods of estimating seismic at tenuation in underground mining environments, and to relate the attenu ation information to rack mass characteristics as determined from geom echanical data. Therefore, four methods - Spectral Ratios, Spectral De cay, Coda-Q, and Multiple Lapse Time Window Analysis (MLTWA)- of measu ring attenuation are examined at three underground excavation sites. E ach site utilizes microseismic monitoring systems 'tuned' to different frequency bandwidths. Considering the MLTWA at the dominant corner fr equencies for each site (Site 1, 2.1 to 4.3 kHz; Site 2 and Site 3a, 0 .5 to 1.0 kHz; Site 3b, 0.1 to 0.2 kHz), the results indicate that ove r the entire frequency range examined at all study sites, the intrinsi c S-wave Q (Q(beta)) is nearly constant at approximately 180, and the scattering Q(beta) follows a frequency dependence of Q(beta) = Q(o)f(1 .4) (where Q(o) = 120). Interestingly, at Site 3 there is a crossover in the dominance of scattering to intrinsic attenuation, and this tren d continues through to the Site 1 data. In order for the frequency-ind ependent intrinsic Q(beta) to be substantiated, it is speculated that a fractal distribution of heterogeneities in the rock mass is responsi ble for intrinsic attenuation (e.g., velocity anomalies, cracks, inclu sions). Conversely, the increasing scattering Q(beta) may be attribute d to minimum characteristic underground opening dimensions of 3-5 m, c ausing decreasing wavelengths to encounter lower scattering due to the se openings, and vice versa. (C) 1998 Elsevier Science B.V. All rights reserved.