Abstract

The presence of multiples has been investigated in Onshore Niger Delta using 3D seismic data. The aim of the study was to investigate the characteristics of reflection events beyond 3s two way time on seismic data behind the boundary faults associated with the shadow zone.  This involves detailed velocity analysis on semblance plot panel and accounting for moveouts due to reflections away and within the shadow zone. Interval velocity-depth models were generated from the velocity analysis and analyzed for shadow effect in the data. Results of the study revealed the presence of two velocity scenarios Onshore Niger delta. These are the primary and lower than normal velocities away and within the shadow zone, respectively. The interval velocity-depth models and their overlays on the seismic show a constant increase of velocity with depth for the primary model which seems normal, but this is contrary to the lower than normal velocity model where low seismic velocities predominate beyond 3s two way time (3.8 km), especially at the footwall of the boundary fault. These variations are likely due to the fact that sediments at the footwall of the boundary fault are thicker, compacted and thus yield stronger reflectors than the corresponding sediments away from the faults. The lower than normal velocity reflections in the absence of overpressure and anisotropy, which are also causes of low velocity reflections, are attributed to interbed multiple reflections in the data.

Key words: Seismic velocity, primary reflections, multiple reflections, boundary fault, multiple generators.