Summary
We describe two different approaches to estimate angledomain illumination: model-based and data-driven. The estimated illumination fields are then applied to image angle gathers as weights to correct seismic images for poor and uneven illumination. By comparing the resulting images we conclude that the best illumination-compensated subsalt image produced in our tests is produced through a combination of the model-based and data-driven illuminations. The new technique can increase S/N and provide better image amplitude fidelity. Our study indicates that the conventional model-based approach may not always predict the illumination with sufficient accuracy due to the imperfect velocity model of the subsurface used for the prediction. A modified approach, such as the one proposed here, should thus be adopted to mitigate the model inaccuracies while correcting seismic images for the illumination effects.
Introduction
Some degradation of the image in subsalt areas is always to be expected, even with the application of advanced modern imaging algorithms such as reverse-time migration, because, realistically, imaging cannot completely remove all the propagation effects due to a complex overburden. Illumination imprints in images, such as shadow zones, have always been a challenging problem especially in subsalt environments. Illumination analysis is thus often used as an aid in subsalt exploration to guide survey design and to potentially correct distorted images for the effects of uneven illumination. Various methods to correct for illumination-induced image distortions have been previously discussed (Schneider and Winbow, 1999; Muerdter and Ratcliff, 2001; Askim et al, 2010). Gherasim et al (2010) designed a one-way wave-equation based (Etgen, 2008) demigration-remigration workflow to generate illumination weights, which can be applied to migrated angle gathers to improve structural imaging in subsalt areas. Although one-way methods are efficient, they are dip-limited. Li et al (2012) proposed to use RTM 3D dip gathers of point diffractors for subsalt illumination analysis. Gherasim et al (2012) developed a two-way waveequation based "modeling-migration" workflow, during which they built a reflecting velocity model by combining the background velocity with reflectors following interpreted events across the field. The illumination weights generated from this workflow can be applied to angle-gathers to improve subsalt stacked images. Consequently, the resulting images can produce better well ties and provide a basis for more accurate amplitude analysis. Sufficient accuracy of the reflectivity model is, however, critical.
