Illumination compensation for subsalt image-domain wavefield tomography

Wavefield tomography represents a family of velocity model building techniques based on seismic waveforms as the input and seismic wavefields as the information carrier. For wavefield tomography implemented in the image domain, the objective function is designed to optimize the coherency of reflections in extended common-image gathers. This function applies a penalty operator to the gathers, thus highlighting image inaccuracies due to the velocity model error. Uneven illumination is a common problem for complex geological regions, such as subsalt. Imbalanced illumination results in defocusing in common-image gathers regardless of the velocity model accuracy. This additional defocusing violates the wavefield tomography assumption stating that the migrated images are perfectly focused in the case of the correct model and degrades the model reconstruction. We address this problem by incorporating the illumination effects into the penalty operator such that only the defocusing due to model errors is used for model construction. This method improves the robustness and effectiveness of wavefield tomography applied in areas characterized by poor illumination. The Sigsbee synthetic example demonstrates that velocity models are more accurately reconstructed by our method using the illumination compensation, leading to more coherent and better focused subsurface images than those obtained by the conventional approach without illumination compensation.