Abstract

Three high-resolution seismic lines over a Merensky Reef target were recently acquired using vibrators. The objective of this survey was to identify and map the depth of the reef in the area, and the structures displacing it. High-resolution frequency input was instrumental in imaging the reef horizons and faults. The Merensky Reef reflection can be followed with confidence along the seismic data as shallow as 50 ms ( approximately 130 m depth) on the section; with the aid of borehole control, it can be followed up to 30 ms ( approximately 65 m depth). The Merensky Reef and UG2 Chromitite are clearly imaged as long as they are approximately one wavelength apart, nominally 12 ms ( approximately 39 m). When they are closer, interference signals are noticeable. Possible potholes in the Merensky Reef have been identified using changes in the reflection character in the seismic data, notably amplitude and wavelength. These potholes have yet to be confirmed by drilling. Where these areas are sufficiently extensive, they represent significant mining targets; thus, one of the objectives is to locate such areas of regional pothole reef. Fault throws as small as approximately 10 m can be resolved in this data set. The lack of reliable interval sonic velocity data for the rocks of the Bushveld Complex has prevented a proper depth conversion of the horizon interpretations from time to depth. It also prevents wavelet modelling of the reef package via synthetic seismograms, in order to determine what part of the wavelet actually represents the Merensky Reef, and how the "reef zone" rocks (Bastard pyroxenite to UG2/1 layers) affect the seismic wavelet shape. This is important for proper interpretation of the seismic data and depth conversion of the interpreted horizons. The successful imaging of the Merensky and UG2 Reefs in this seismic survey allows for detailed interpretation of the relative depth and structure of the reefs in the seismic prospect. Use of the reflection seismic method to identify the position of the Merensky Reef and UG2 Chromitite horizons and associated structures in the seismic prospect was very successful. The seismic method is an asset to platinum exploration in the Bushveld Complex.

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