Abstract

To provide guidelines for future seismic surveys in the Bushveld Complex, we have conducted laboratory experiments to measure P-wave and S-wave velocities at ultrasonic frequencies on drill-core samples as a function of stress and bulk density. The rock samples include gabbroic, anorthositic, troctolitic and magnetite-olivine rich horizons, as well as granitic sills. Ultrasonic measurements of P- and S-wave velocities were conducted at both atmospheric pressure, and at elevated uniaxial stresses up to 65 MPa (equivalent to depths of 2 km), using 0.5 MHz P- and S-wave transducers. The results show that the seismic velocities tend to increase with increasing olivine or pyroxene content (e.g., Vp ~6.00 km/s for olivine ferrodiorite), and decrease with increasing plagioclase content (e.g., Vp ~4.70 km/s for troctolite). The bulk density and ultrasonic measurements suggest that significant reflection coefficients (RC) occur at some stratigraphic contacts that are readily identifiable in surface exposures (e.g., RC~0.09 for granite-olivine ferrodiorite contact), implying that these horizons, at least in terms of acoustic impedance contrasts, are favourable targets for surface reflection seismic surveys. Our results also suggest that with increasing stress (0 to 20 MPa), seismic wave velocities increase slightly for both anorthosite (Vp~5.5 to 6.0 km/s) and troctolite (Vp~4.80 to 7.20 km/s). The velocities are constant at higher stresses (20 to 65 MPa), which can be attributed to closure of macro- and microcracks at depths of 1.0 to 1.5 km. The observed relationship between seismic velocities, modal mineralogy and stress has particularly important implications (e.g., velocity analysis for seismic processing and time-to-depth conversion of the seismic data) for the design of future long, deep seismic profiles that can be used to constrain the deep structure and connectivity of the western and eastern limbs of the Bushveld Complex at depths.

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