Description of seismic sources in underground mines: Dynamic stress fracturing around tunnels and strainbursting

TL;DR

This paper models seismic sources in underground mines caused by dynamic rock fracturing around tunnels, revealing that seismic radiation is mainly due to elastic convergence rather than fracturing itself, and proposes an analytical approximation for source mechanisms.

Contribution

It introduces a new analytical model for seismic source mechanisms in underground mines based on mechanical and geometric properties, validated with real seismic data.

Findings

Seismic radiation is controlled by elastic convergence of rockmass.

The model's approximations align with observed seismic waveforms.

The approach aids forensic analysis and monitoring of fractured zones.

Abstract

This paper considers dynamic fracturing of the rockmass surrounding a tunnel statically loaded by compressional stress as a possible source of seismic events in underground mines. This begins with two-dimensional dynamic modelling of failure for six plausible scenarios. In each case, the seismic source derived from these models has significant negative isotropic and negative compensated linear vector dipole components as well as a P-axis approximately aligned with the direction of maximum compressional principal stress. These features indicate that at wavelengths larger than the diameter of the tunnel and the extent of damage along it, seismic radiation is controlled by the elastic convergence of the surrounding rockmass rather than by rock fracturing. An analytical approximation of the source mechanism is suggested that is based solely on mechanical and geometric properties: the magnitudes $\sigma_{\mathrm{max}}$ and $\sigma_{\mathrm{min}}$ of the maximum and minimum principal stresses orthogonal to the tunnel's axis, the Poisson's ratio $\nu$ of the rockmass, the length $L_{3}$ of dynamic fracturing along the tunnel, the effective tunnel dimension $\bar{L_{A}}$, and the increase in depth of failure $\triangle d_{f}^{A}$ in the direction of $\sigma_{\mathrm{min}}$. Furthermore, it is shown that the scalar seismic moment can be approximated as $|\mathbf{M}|\approx2[(1-\nu)/(1-2\nu)]|\sigma_{max}|L_{3}\bar{L_{A}}\triangle d_{f}^{A}$. The suggested approximations are considered in the context of seismic data from a real underground mine. It is shown that many mechanisms inverted from observed waveforms are consistent with the suggested model and that the proposed source mechanism approximation can be used for the forensic analysis of damaging seismic events and quantitative monitoring of the evolution of fractured zones around tunnels.