Shallow seismic velocity structure of the Karoo Basin, South Africa

Abstract

A Near Vertical Reflection (NVR) Seismic profile from Prince Albert to Slingersfontein across the Karoo Basin of South Africa was generated using 182 controlled source shots at ~500 m spacing and receiver points at 100 m nominal spacing in an 18 km spread. Travel time data of seismic phases are used as input data to derive shallow velocity models of compressional waves (P) and shear waves (S). Using First Arrival Seismic Tomography (FAST) software, we derive shallow tomographic P- and S-velocity models for the upper 1 to 1.5 km. Checkerboard tests indicate good resolution down to 1 km depth. The models of P- and S-velocity and Vp/Vs ratio show an abrupt change at ~50 km, dividing the model into two regions. A southernmost region consists mainly of steeply dipping in-situ sedimentary bedrock with an east-west strike, approximately perpendicular to the profile. The northern 50 km region comprises predominantly unconsolidated sediment and/or highly weathered rock, yielding a high Vp/Vs ratio. The tomographic models compare well with corresponding lateral variation in the surface geology of the Permian Karoo Supergroup sedimentary rocks (Dwyka, Ecca and Beaufort Groups) deformed at the Cape Fold Belt front. The correlation between the velocity models and outcrops is stronger for the southernmost 25 km of the NVR seismic profile. Although the surface geology is more uniform from kilometre 25 to 50, the velocity models and Vp/Vs ratio suggest continued systematic lateral variation. Based on this, we infer a sub-surface continuation of tight folding not seen in outcrops. The S-velocity model supports this theory, as velocity variations correlate well with the location of major fold axes in the regional scale gentle tight folding of the Ecca Group (Abrahamskraal Formation). In the northern part of the model, from 50 to 100 km, minimal change in the velocity models indicate more uniform and undisturbed lithologies.