Subsalt Illumination Study through Seismic Modeling

We presented a subsalt illumination problem related to a steeply dipping subsalt target observed from the well and complex salt structure. Both raytracing and full wavefield modeling methods were employed to understand the subsalt imaging and illumination problems. The salt velocity model for PSDM and the target horizon consistent with the well were used as input to the studies. We started our study by applying normal incident (NI) raytracing, which leaded to the subsequent critical angle analysis at the base of salt. The results suggested that all signals reflected from steeply dipping target were blocked by the base of salt due to post critical angle reflection. Then we applied a Common Reflection Point (CRP) raytracing to compute the full azimuth illumination map. The resulting illumination map confirmed the results from NI raytracing and critical angle analysis. Finally, we applied a finite difference full wavefield modeling and followed by an RTM imaging of the synthetic data using the true velocity model. Comparison of images from real data and synthetic data further proved that this particular imaging problem was due to lack of illumination other than the migration algorithm.