Seismic Tomography Velocity Modelling of Seaward Dipping Reflectors in the Orange Basin, Off Namibia Field, South Africa

Abstract

The Orange Basin is located off the western margin of Southern Africa, formed during the Late Jurassic to Early Cretaceous by the break-up of Gondwana and the resultant separation of the African and South American. Such rift events are associated with volcanism that, in this case, presents as packages of so-called ‘seaward dipping reflectors’ (SDRs) formed of stacked basaltic flows. We use first arrival traveltimes recorded in a long-offset acquisition (10km) to develop a 2-D regional tomographic P-wave velocity model of the basin, and to constrain the seismic velocity structure of the SDRs across the continent-ocean transition (COT); for efficiency, these models are differently downsampled from the original acquisition. Both regional and local tomographic models show significant variation of typical velocity structure of a passive continental margin. First, we attempt to image the whole margin through a one-time-inversion. High velocities (3750-5000 m/s) in the final inverted model suggest the existence of basaltic flows, correctly positioned geographically with respect to their appearance in the seismic profile. Our method shows large potential for analyzing the internal architecture of passive continental margins to validate the existence of SDR packages and further advances are anticipated where it is integrated with a full-waveform inversion approach.