Identification of rock discontinuities by coda wave analysis while borehole drilling in deep buried tunnels

Abstract

Rock discontinuities are widely and randomly developed in deep buried tunnels and have a profound impact on construction safety, support design, and geological hazard prevention. Identifying these discontinuities while drilling is a critical but challenging task. This paper presents an innovative approach that leverages the coda wave analysis method to assess rock discontinuities during impact drilling in deep tunnels. Our method is facilitated by a drilling process detection (DPD) system, which records coda waves generated by impact drilling pit sources. We observe that coda waves, with dominant frequencies of 500 Hz to 1 kHz, can be detected in heterogeneous rock discontinuities featuring decimeter-scale discontinuities. Building upon this observation, we introduce a rock discontinuity index (RDI), which is a novel metric designed to highlight low-velocity anomalies associated with rock discontinuities along boreholes. To validate the efficacy of the approach, we conduct numerical simulations of checkerboard and random realistic rock models and perform field tests in deep buried tunnels. A borehole optical scanner is utilized to compare the identified rock discontinuity results. The results affirm that the coda wave-based method can reliably identify rock discontinuities by RDI peaks, even under highly heterogeneous geological conditions. The proposed method offers a practical means for identifying discontinuities and assessing geological conditions during impact borehole drilling.