A seismic investigation of the Kaapvaal craton

Abstract

Tte structure of the Kaapvaal Craton of southern Africa invesirgated by means of deep seismic sounding using mine tremors as the energy source, and by deep seismic reflection profiling. Seranoneru "9 ware deployed at 10 km intervals on two profiles stretching between the major mine tremor source regions, 15 stout 250 km in length. Record sections have ocnpiled for both P- and S-waves, and the travel-times and anplitudes interpreted using ray-tracing techniques. Synthetic seismograms have also been oenputed using the reflectivity method. A 16 s two-way-time seismic reflection profile, 112 km in length and traversing the northwestern portion of the Witwatersrand Basin, was surveyed during 1988 under the auspices of the National Geophysics Programme and the Geological Survey of South Africa. The reflection profile, which intersects both the refraction profiles, has been interpreted. The seianic models have been integrated with other geophysical and geological data. This study has shewn that the use of mine tremors for deep seismic sounding has particular advantages. Tremors are rich in shear energy, enabling joint interpretation of P- and S-waves and the estimation of Poisson's ratio. Tremors also have a wide bandwidth, with significant energy at frequencies as low as 3 Hz. Consequently reflections are produced from velocity gradient zones which are invisible to conventional reflection profiling using vibrators as the energy source. The seismic model for the Kaapvaal Craton has the following features: supracrustal strata 0-10 km thick; upper crystalline basement with P-velocities of 6.0-6.2 km/s; the boundary between upper and lower crust at a depth of ca. H km is either a discontinuity giving rise to reflections, or a gradient zone giving rise to a caustic; the lower crust has a uniform seismic velocity in the range 6.4-6.7 km/s; the crust mantle transition takes place over 1-3 km; and Moho at a depth of ca. 35 km. The lower crust was also found to be strongly attenuating, and to have a Poisson' s ratio of ca. 0.28. It is also known to be electrically conductive. These observations are in accord with the presence of hydrated mantle rock at the base of the crust.