Abstract

Recently, imaging capabilities have allowed a major step forward in subsalt exploration. In particular, new PSDM algorithms have appeared on the market that account for complex wave propagation through salt canopies to produce better images. These improvements have allowed the drilling of numerous subsalt exploration wells, but at the time of appraisal and development, questions arise on the reliability of these seismic cubes to accurately predict geometries. This issue is crucial in the offshore deepwater blocks of Angola, where reflectors often have high dips and subsalt seismic visibility is not optimal. Furthermore, velocity models used for subsalt imaging are seldom as accurate as one could want, and significant discrepancies can often be observed between seismic and well depth of geological markers. These discrepancies can have several origins, such as neglecting to account for anisotropy, as discussed by Shumaker et al. (2007). The lack of accuracy of velocity models can also be due to the limitations of velocity estimation techniques in the subsalt domain given the available data. This results in a difference between the depth of markers observed in the well (true depth) and on seismic images (seismic depth). It is to be noted that the seismic depth is dependent on the PSDM velocity model to the first order, but also on the migration algorithm because of different ways to model the wave propagation (ray tracing or wavefield extrapolation).

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