The high‐resolution Weltevreden 3‐D seismic survey in the Klerksdorp goldfield, South Africa: A case study in the hard‐rock mining environment

Abstract

Summary A recent high-resolution, shallow 3-D Vibroseis survey over the Weltevreden area of the Klerksdorp goldfield in South Africa has successfully imaged the economic goldbearing target horizon, the Ventersdorp Contact Reef (VCR), over the depth range 200 - 1000 m. The Archean aged VCR is a thin (± 1 m thick) tabular conglomerate horizon that is located on the angular unconformity that exists between the sedimentary Central Rand Group (Witwatersrand Supergroup) and the overlying volcanic Klipriviersberg Group (Ventersdorp Supergroup). Seismically, the VCR is directly detectable and corresponds to a strong negative acoustic-impedance contrast. In order to image the shallow orebody successfully, acquisition parameters were designed to maximize near surface fold of coverage, wavelet frequency bandwidth, signal-to-noise ratio and lateral and vertical resolution. Critical in achieving these objectives were the following: small CDP bins (7.5 × 7.5 m), high maximum fold (50), short maximum shot-to-receiver offset (658 m), high fold for short offsets (20 for offsets < 330 m), high frequency vibrator sweep (30 - 160 Hz) using a single vibrator and a single sweep per vibration point. The VCR was successfully imaged over a large percentage of the survey area, with dips varying between 10 - 30° and fault throws varying between 8 - 70 m. The smallest fault throw of 8 m is estimated to be the resolution limit of the survey, and is only detectable using 3-D visualization tools. In the area where the VCR is relatively poorly imaged, the stratigraphy is disrupted by several steeply dipping dykes, the positions and orientations of which have been constrained by the modeling high resolution airborne magnetic data. A new shallow underground mine may be developed over the Weltevreden area, and the 3-D dataset represents an invaluable asset for future mine planning purposes, and for the future placement of development boreholes, throughout the anticipated life-of-mine. In terms of information economics, the survey has proved to be highly cost effective, with the total cost of the 3-D survey amounting to approximately 2% of the likely direct mining costs of the orebody.