Seismic velocity distribution in the vicinity of a mine tunnel at Thabazimbi, South Africa

Abstract

Analysis of the refracted arrivals on a seismic reflection profile recorded along the wall of a tunnel at an iron mine near Thabazimbi, South Africa, shows variations in P-wave velocity in dolomite away from the de-stressed zone that vary between 4.4 and 7.2 km/s, though values greater than 5.8 km/s predominate along most of the profile. The seismic velocities at the tunnel wail, however, vary between 4.2 and 5.2 km/s. Time-depth terms are in the range from 0.1 to 0.9 ms, and yield thicknesses of the zone disturbed by the tunnel excavations of between 2 and 9 m. The very low seismic velocities away from the tunnel wall in two regions are associated with alcoves or 'cubbies' involving offsets in the wall of up to 10 m. The large variations in seismic velocity resolved over distances less than 15 m with signals of wavelength around 6-9 m are attributed to variations in the sizes and concentrations of fracture systems and cracks, and in the degree of groundwater saturation of the fracture systems. The results suggest that seismic velocity variations from reflection surveys may also assist modelling studies of the stress regime in deep mines, particularly if both P and S wave velocity variations can be determined. The seismic velocity variations inferred also show that application of refraction static corrections in the processing of 'in-mine' seismic reflection profiles is as important as in surface surveys, because of the higher frequencies of the seismic energy recorded in the deep mine environment.