| Summary: | 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.
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