Experimental and In-Situ Physical Properties Measurements From a Seismogenic Zone in a Deep South African Mine

Abstract

Summary Physical properties of sixty-six cylindrical core specimens from three boreholes are presented and compared with downhole physical property data. To investigate the source of seismic reflectivity observed on the 2D legacy seismic data, we computed synthetic seismograms for adjacent rock units using downhole petrophysical data. The experimental measurements show that the metasediments exhibit lower bulk densities and seismic velocities than the metabasalts and intrusive specimens. The porosity was found to be less than 2% for all the samples. No clear trends emerge when the Poisson’s ratio is plotted against the P-wave velocities and porosities of the samples. The calculated reflection coefficients show that several strong reflectors can be identified using seismic reflection methods. The computed reflection coefficients and synthetic seismograms revealed strong reflections that were not identified using the calculated reflection coefficients. Successful correlations are carried out between the synthetic seismic data and lithological logs enabling us to correlate the stratigraphic sequence drilled in the boreholes to the seismic reflections observed on the legacy 2D reflection seismic data. Our experimental results show that the intrusives characterised by low velocities and high densities could potentially be the host structures of the ML5.5 Orkney earthquake.