Experimental measurements of seismic velocities on core samples and their dependence on mineralogy and stress; Witwatersrand Basin (South Africa)

详细信息    查看全文

• 作者：Nomqhele Z. Nkosi ; Musa S. D. Manzi ; Gillian R. Drennan…
• 关键词：Witwatersrand Basin ; seismic velocities ; 3D seismics ; stress ; mineralogy
• 刊名：Studia Geophysica et Geodaetica
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：61
• 期：1
• 页码：115-144
• 全文大小：
• 刊物类别：Earth and Environmental Science
• 刊物主题：Geophysics/Geodesy; Structural Geology; Atmospheric Sciences;
• 出版者：Springer Netherlands
• ISSN：1573-1626
• 卷排序：61

文摘

Physical property measurements were integrated with mineralogical analyses to better understand the nature of the seismic reflectivity of the deepest (>3.5 km depth) gold ore body (Carbon Leader Reef). The CLR lies at depths between 3.5 km and 4.5 km below the surface. Over 50 drill-core samples were selected for geochemical analyses, density and seismic velocity measurements. Ultrasonic measurements were conducted at ambient and elevated stresses, using transducers operating at 0.5 MHz. The study reveals that P-wave velocities generally increase with increasing bulk density. The CLR conglomerate, the gold-bearing reef, has slightly higher P-wave velocity (∼5070–5468 m/s) and density values (∼2.78 g/cm3) amongst the quartzitic units, possibly due to its massive pyrite content. The quartzite hangingwall and footwall rocks to the CLR exhibit similar P-wave velocity (∼5028–5480 and ∼4777–5211 m/s, respectively) and density values (∼2.68 and 2.66 g/cm3, respectively). The reflection coefficients calculated at the interface between the CLR conglomerate and its hangingwall and footwall units range between ∼0.02 and 0.05 which is below the required minimum reflection coefficient value of 0.06 to produce a strong reflection between two lithological boundaries. This suggests that seismic reflection methods might not be able to directly image the CLR, as observed from its poor reflectivity in the 3D seismic data. Samples were also subjected to stresses of up to 65 MPa to simulate in situ-like conditions and to investigate the dependence of seismic velocities on applied stresses. P-wave velocities increase with progressive loading, but at different rates in shale and quartzite rocks as a result of the presence of micro-defects.