[S02-P] Poster

[S02-P-08] Experimental measurements of seismic velocities on core samples and their dependence on mineralogy and stress, Witwatersrand Basin (South Africa)

^○Nomqhele Nkosi, Musa Manzi (University of the Witwatersand, Johannesburg, South Africa)

The measurement of seismic velocities, especially at great depths (~2 km-3 km) under high stress conditions (~65 MPa-70 MPa), are crucial for proper interpretation of reflection seismic data, and therefore, for understanding the seismic reflective nature of lithological boundaries. The integration of geological and geophysical concepts has enabled us to better understand the nature of the seismic reflectivity of the world's deepest gold-bearing reef, the Carbon Leader Reef (CLR). This was done by measuring the ultrasonic velocities and bulk densities as well as conducting mineralogical analyses on drill-core samples. The results show that the quartzite samples overlying and underlying the CLR exhibit similar velocities (~ 5028 m/s-5480 m/s and ~ 4777 m/s-5211 m/s, respectively) and bulk densities (~ 2.68 g/cm3 and 2.66 g/cm3). This is due to similar mineralogy and chemical compositions observed within the units. However, the CLR has slightly higher velocity (~ 5070 m/s-5468 m/s) and bulk density (~ 2.78 g/cm3) than the surrounding quartzite units probably due to higher pyrite content in the reef. In the data set it is found that seismic velocities increase with (1) decreasing silica content, (2) increasing iron and pyrite content and (3) decreasing grain size. Reflection coefficients calculated using the seismic velocities and densities at the boundaries between the CLR and its hangingwall and footwall units range between ~0.02 and 0.05, which is below the suggested minimum of 0.06 required to produce a strong reflection between two lithological units. This suggests that reflection seismic methods might not be able to directly image the CLR as a prominent reflector, as observed from the seismic data.Samples were also subjected to stresses of up to 65 MPa to investigate the dependence of ultrasonic velocities on stresses. Velocities increase with progressive loading, but at different rates in shale and quartzite rocks as a result of the presence of micro-defects.