Description of Seismic Sources in Underground Mines: Theory

Abstract

ABSTRACTIn underground mines, episodes of sudden inelastic deformation in the rockmass are often induced by mining and are therefore localized near the excavations. The seismic radiation associated with such deformation can be described by a point seismic source using a volume integral of the stress-free strain (or incremental plastic strain)—expression (1) in the Theory section. This conventional description is valid if seismic waveforms are modeled or inverted using the elastodynamic Green’s function, which takes the presence of nearby excavation(s) into account. If the adopted Green’s function does not model these excavations, then the conventional expressions for a point source need to be adjusted by adding a term that depends on the displacement at the surface of the excavations (equations 2). Alternatively, a Kirchhoff-type representation can be used, in which the parameters of the point source are expressed using increments of displacement and traction over the surface covering both the volume of inelastic deformation and nearby excavation(s) (equations 3). Numerical simulations demonstrate that the suggested expressions provide a very different result to the conventional expressions for the case of inelastic deformation in a volume adjacent to an excavation. The utilization of the suggested expressions results in a change in the type of mechanisms from explosive to implosive and significantly affects other characteristics of the modeled sources (moment magnitude and orientation of principal axes). Typically, seismic waveforms recorded by monitoring systems in underground mines are processed using an elastodynamic Green’s function that does not take medium-sized excavations (e.g., tunnels and stopes) into account. Therefore, the results of such processing need to be interpreted using the suggested expressions: adjusted conventional (equations 2) or Kirchhoff-type (equations 3).