Changes in the Early Part of the Seismic Coda due to Localized Scatterers: The Estimation of Q in a Stope Environment

Abstract

A single scattering model was used to analyse the temporary changes in the mean density of scattered waves in a discrete random medium. The model of the mean energy density, originally proposed by Sato (1977) for spherical radiation and isotropic scattering, has been modified and applied to a medium in which the scatterers are confined to a specified volume. The time variation of the early part of the mean energy density function for the different source durations was investigated. The dominant effect on the theoretical mean energy density is caused by the specified volume containing scatterers. The duration of the source pulse influences the early part of the coda for t/t ₀ < 1.2, where t is the lapse time measured from the source origin time, and t ₀ is arrival time of the body wave.

The analysis of the coda signal of micro-events occurring immediately in front of the face enables us to estimate the size of the fracture zone induced by the stope. The model of the mean energy density of coda for a medium containing scatterers close to the seismic source was used to analyse a large number of events recorded close to an advancing mine face in a deep level gold mine in South Africa. The coda decay rate has two trends: the first, with a steep decay of coda, is produced by a larger deviation of rock parameters and/or larger size of the scatterers; the second trend, which decays more slowly, has the corresponding mean-free path ranging from 20 m to 200 m. The analysis indicates that the rock mass about 15–20 m from the stope contains a large proportion of fractured and blocked rock, which is the source of scattering. The scattering of the S-wave was much stronger and more stable, with the mean-free path varying from 11 m to 45 m. This is due to the shorter wavelength of the S wave in comparison with the P wave. The quality factor for the P coda wave varies from 30 to 100 in the fracture zone of stope and outside this zone it has a value of 300. The quality factor of the S wave varies from 20 to 78 in the equivalent volume. For rock surrounding the stope the ratio $Q_{sp}^{ - 1}/Q_{ss}^{ - 1}$ varied from 0.31 to 0.69. This suggests that the radii of scatterers are smaller than 3.5 m.