Sonic log for rock mass properties evaluation ahead of the tunnel face — A case study in the Alpine region

Ahead of tunnel prediction of the rock mass properties and the detection of weak zones potentially related to failure zones encountered during excavation can be analyzed using acoustic logging in drilled ahead tunnel face. Indeed acoustic logging in a broad frequency band (1 kHz up to 30 kHz) may provide to estimate the compressional (P-wave), shear (S-wave), Stoneley wave velocity and amplitude. A systematic sonic logging survey was performed along a tunnel during the excavation in horizontal boreholes in Quartzite and Gypsum formations. The data processing involved the estimate of P-wave velocity, the analysis of the S-wave and the estimate of group and phase velocity of Stoneley wave. We used Stoneley wave to determine the S-wave velocity whenever the waveform did not allow the detection of converted S-wave. In soft rock the S-wave velocity can be determined using the Stoneley wave analysis from acquisitions taken at low frequency (2 kHz). We interpreted the results in terms of lithological changes between soft rock and hard rock and weak zones.Indeed, in this case study weak zones in Quartzite are well delineated by a sharply decrease of P and S-wave velocities. The reflection/diffraction effects of guided waves pointed out the presence of discontinuities and fractures. In the specific geological context the method was able to predict the lithological changes between Quartzite (higher Vp/Vs ratio) and Gypsum (lower Vp/Vs ratio).

► Acoustic logs in horizontal boreholes to estimate rock mass properties. ► Analysis of P, S and guided waves (Stoneley waves). ► Conversion of Stoneley phase velocity in shear wave velocity. ► Prediction of lithological changes and mechanical properties ahead the tunnel face.