8th SAGA Biennial Technical Meeting and Exhibition

Radiometric data give information about the amounts of potassium, uranium and thorium that are present on the surface of the earth (Heiland 1963). This information is measured as a spectrum of the gamma radiation that is emitted as a result of decay in radioactive elements. Each element emits a unique amount of gamma radiation, and by counting the radiation at each energy level, a spectrum can be built up that can be related to the ground concentrations of potassium, uranium and thorium.

Dual gamma-ray computed tomography (or CAT - Computer Axial Tomography) is a very useful radiological imaging technique allowing direct three-dimensional investigation of textural features and petrophysical parameters of polyphase crystalline geomaterial.

A borehole radar survey undertaken on a Witwatersrand gold mine, in a highly resistive host rock (Alberton Porphyry Formation lavas) returned poor results characterized by a fast first arrival followed by a distinctive large, low frequency pulse, on which real reflected pulse information was superimposed at much smaller amplitudes (Figure 1a). The radar that was used to acquire the data, the CSIR Miningtek Aardwolf BR40 (Vogt, 2002), has limited instantaneous dynamic range, so it is not possible to resolve desired signals in the presence of the large low frequency pulse.

Thermal modelling of the crust and upper mantle below the Kaapvaal Craton shows that there are substantial variations in the equilibrium temperature structure of the crust, but these variations become smoothed out in the upper mantle. The range in temperature at depths of 100-200 km is not sufficient to cause the observed seismic tomographic anomaly below the Bushveld Complex. A more likely explanation for the anomaly is a compositional difference.

A palaeomagnetic study has been conducted on dykes that intrude the c. 2 Ga eastern Bushveld Igneous Complex (BIC). The dykes have previously been assumed to be of Karoo (Jurassic) age based on their NE-SW orientation. Palaeomagnetic data, however, demonstrate that these dykes are Precambrian in age (1800-2050 Ma) and perhaps close to the Early Proterozoic Bushveld age. Both normal (three dykes) and reverse (two dykes) polarity directions are identified, and a positive reversal test attests to a primary magnetisation where secular variation has been averaged out. 40Ar/39Ar dating is currently in progress to verify this age.

Standard procedures for the interpretation of transient EM data over a confined conductor in a very resistive host rock have been developed and published. These are based on the assumption that the measured response is due only to the induced currents in the conductor.

Despite the size of the Bushveld Complex and the wealth of its mineral deposits, it is perhaps surprising that its three dimensional structure, tectonic environment and emplacement mechanism are so poorly characterised and understood. Here we review the geophysical data that constrain the three dimensional shape of the Bushveld complex, the tectonic environments that have been proposed for the emplacement of the Bushveld Complex and a variety of proposed emplacement mechanisms.

Filtering and image processing of aeromagnetic data are essential tools in mineral exploration. The methods are most effective when dealing with shallow source depths and rock types with relatively large contrasts in magnetization.

Sunshading is a powerful tool for the enhancement of linear features in images. Given the azimuth and elevation of a source of illumination it calculates the reflectance from a surface (which is composed of the data to be interpreted). Linear features lying at 90o to the illumination azimuth are enhanced, while those that lie parallel to it become less apparent. It is a standard tool used in the interpretation of geophysical potential field data.

From an inspection of airborne electromagnetic data sets collected during the past decade by the BGR helicopter-borne geophysical system over various geological settings, it can be shown that such data sets not only indicate layered horizontal geological features but also vertically and laterally confined structures, such as buried valleys. In cases where the valleys are incised into magnetic basement and refilled with non-magnetic material, the magnetic data records can also be used for identification and thickness estimates of the infill.