Tomography algorithms using gridded model description and ray tracing have made continuous progress in terms of resolution and efficiency. However one strong limitation is the difficulty to recover strong velocity contrasts encountered in presence of salt bodies, chalk, basalt, carbonates… The conventional solution for velocity model building in such a context is to proceed in a top-down manner from one velocity contrast to the next one. In such a layer stripping approach velocities and horizons are updated layer after layer recursively from top to bottom. Such a workflow is time consuming and prone to velocity errors being propagated into deeper layers as the model building progresses. We present here a solution to remedy these drawbacks. Our solution involves a non-linear tomographic approach combining dense dip and residual move-out picks with horizons describing the main velocity contrasts. While dip and RMO picks are used to update 3D velocity grids inside each layer by non-linear slope tomography, the picked horizons describing layer boundaries are kinematically de-migrated and re-migrated recursively from top to bottom to reposition the major velocity contrasts after each velocity update. We present applications of the method to a marine North Sea dataset and to a land dataset with salt structures and compare the results with the layer stripping approach.

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