Shallow structure in the Karoo Basin, South Africa, inferred from a near vertical reflection seismic profile

A Near Vertical Reflection seismic profile from Beaufort West to Klaarstroom was carried out using controlled source vibroseis sweeps and a rolling spread of recording geophones. Arrivals of refracted P- and S-waves generated in the sweeps were manually picked. 2-D travel time tomography was used to compute P-wave and S-wave velocity variations for depths down to ~300 m beneath the profile. Checkerboard tests illustrate the resolution of the velocity models. A probabilistic classification approach was used in an attempt to identify lithological classes based on observed P- and S-wave velocities. Two classes were identified, but they do not correspond to different geological formations. Thus the variations of seismic velocities due to pressure and compaction are greater inside a given formation than between different formations. The subsurface down to a depth of 50 to 100 metres shows very little variation in S-wave velocity, 2.2 ± 0.1 km/s, irrespective of the underlying geological formation. Directly below this zone variations in P- and S-wave velocities are observed, where the Vp/Vs ratio is approximately 2. Care must thus be taken when attempting to classify lithologies using statistical distribution of geophysical parameters – systematic variations inside a single geological unit can be more significant than between separate geological formations