Nonlinear mesoscopic elasticity revealed by passive seismic monitoring of a rock column during drilling operations

We report the repeated in situ observation of nonlinear mesoscopic elasticity in an ∼760 m3 unstable rock column undergoing drilling for bolting reinforcement. Analysis of the anomalous nonlinear fast dynamics at the onset of drilling showed a clear correlation between top-of-column acceleration and small (a few percent) drops in resonance frequency. Slow dynamics were also observed, appearing as reproducible recovery of the fundamental resonance frequency after both drilling stopped and earthquake shaking occurred. Characteristic relaxation times ranged from tens to several hundred seconds, with recovery in log10(t) spanning about 1-2 orders of magnitude. Significant nonlinear effects affecting the column's fundamental mode arose for strains as low as 10-9-10-8, with the proportion of the affected column volume depending on drilling location. The magnitude of the observed nonlinearities exceeded that found in laboratory tests on intact rock, revealing extensive, multi-scale cracking within the column. These results demonstrate that coincidental passive seismic surveys can effectively probe nonlinearity at the geophysical scale, with potential applications in geotechnical and civil engineering and in monitoring internal fracturing of rock structures.