An introduction to this special section: Full-waveform inversion and the way forward

The practice of updating an Earth model and generating synthetic data from it that one can compare to the field data is an appealing concept. If the two data sets match, using some measure of misfit, one might have found a good Earth model. Radiation patterns show the amplitude of the partial derivative wavefield with scattering angle for a model-parameter perturbation, keeping the other parameters fixed. The second theme presents research directions where the authors try to combine the robustness of existing wave-equation-migration velocity analysis (WEMVA) techniques with FWI. This combination makes sense: unlike FWI, WEMVA methods do not suffer from the local minima issue and, thus, can start from a relatively poor initial model, while yielding improved low-resolution models needed for FWI.