Analysis of multiples in onshore Niger Delta: a prelude to fault shadow phenomenon

Abstract

In this study, multiples have been analyzed as a prelude to fault shadow phenomenon using 3D seismic data from onshore Niger Delta. This involved detailed velocity analysis of a 3D seismic data using semblance plots, from which depth/interval velocity models were generated for two velocity scenarios, the primaries and lower-than-normal velocities in the shadow zone. These velocity models were subsequently used as inputs in prestack depth migration. The result of migration with the lower-than-normal velocities shows that reflections within the shadow zone are coherent, continuous and better aligned than with the primary velocities. This is an indication that these lower-than-normal velocity events have been well accounted for in the shadow zone using these velocities. This result establishes the presence of lower-than-normal velocities in the shadow zone considered as the primary cause of the reflection distortions and misalignment of events observed in the zone, due to multiple reflections. This occurs beyond 3 s two-way time (3800 m), especially at locations corresponding to the footwall of main boundary faults on seismic. This assertion is made based on the fact that all sources of low-velocity events at this depth such as overpressure and anisotropy have been accounted for by previous researchers in the study area and as such, multiple effects are the only plausible low-velocity candidate for the observed reflection distortions in the shadow zone.