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Summary An incorrect anisotropy in the Full Wave Inversion (FWI) velocity model leads to imperfect Common Image Gather (CIG) flatness. The main difficulty in the anisotropy estimation through FWI is the strong coupling with velocity. While FWI jointly updating velocity and anisotropy has been proposed, there is evidence that the long wavelength components of the velocity and anisotropic parameters cannot be reasonably decoupled inverting surface data only. The reason is that the long wavelength components of the velocity model inverted by FWI are mainly recovered from the kinematics of diving waves, while decoupling can only be done considering in addition the kinematics of reflected waves. To solve this challenge, we propose an innovative workflow involving joint reflection and diving wave tomography. To overcome the difficulty of first break picking, we propose a robust estimation of first arrival traveltime using the inverted FWI model of the first pass. With the application of this novelty on deep-water data from offshore Africa, we elaborate further with a sequence of first arrival modeling after tuned repositioning of sources and receivers at the sea floor.

Summary In the last decade, Full Waveform Inversion has gained popularity as a very powerful tool to estimate velocity models of the subsurface with unrivalled resolution. Interpreters can now use high-resolution models obtained by FWI directly, in addition to migrated images. However, the FWI velocity models do not necessarily ensure optimal flattening of common image gathers. A fundamental reason for this is that mid to high wavenumbers are brought into the model by the reflection amplitudes, which are only partly explained by changes in velocity. A residual low wavenumber update of the model is then frequently done with the consequence that the positioning of small-scale events in the model may no longer be consistent with the migrated image. In this paper, we first illustrate the above-mentioned pitfalls, and propose an innovative workflow based on FWI-guided tomography. It ensures the optimal focus of the migrated image and it also ensures a better match between the fine-scale velocity structures and the migrated image. The approach is validated on a 2D marine broadband dataset using 32 Hz High-Resolution FWI.

With the new broadband acquisitions, allowing to record frequencies down to 2.5 Hz, and the new tomographic tools, allowing to resolve for vertical velocity components up to 6 Hz, we have moved velocity model building from the situation of a “mid-frequency gap” to the situation of an overlapping area in terms of the resolution we can expect from tomography and migration-inversion approaches. Full waveform inversion-guided migration velocity analysis has been proposed to take advantage of this new situation. We show here a first real data application of this innovative approach.