Seismic travel-time and attenuation tomography to characterize the excavation damaged zone and the surrounding rock mass of a newly excavated ramp and chamber

• 4D seismic tomography is able to resolve rock-mass changes due to excavation work.
• Attenuation tomography is more sensitive to changes than travel-time tomography.
• Changes of rock-mass are not restricted to the new excavation.

Seismic travel-time and attenuation tomography were applied to characterize the excavation damaged zone and the adjacent rock mass in the GFZ-Underground-Lab within the research and education mine Reiche Zeche of the Technical University Bergakademie Freiberg (Germany). The lab is situated in gneiss rocks at 150 m depth and comprises three galleries which enclose an area of approximately 50 m×100 m. Along these galleries two seismic surveys were performed before and after the excavation of a new ramp and chamber. For both measurements, travel-time and attenuation tomographies for P-waves were performed with the ray-based inversion algorithm SIMULPS14. The seismic velocities were calculated from first-arrival travel times whereas a logarithmic-spectral-ratio approach was used to calculate the corresponding quality factors (Q) for attenuation tomography. A comparison of the tomograms reveals a decrease of average P-wave velocity values from 5.64 km/s to 5.54 km/s and of average Q-values from 29.8 to 26.5 in the whole area after the excavation of the new cavities. The maximum changes are located at already weakened zones either at the conjunction of two galleries or of a major fracture zone with a gallery. The attenuation tomography shows a higher sensitivity to rock mass changes than the travel-time tomography. However, the calculation of the Q-values demands a higher signal quality than the determination of the seismic travel times.