Ray versus full wave velocity model building: status and challenges

While full wave based velocity model building approaches have earned their stripes during the last decade, ray based approaches remain the working horse in industry. I review here first the successes and the limitations of both families of approaches. If it appears that ray based approaches suffer of limitations in complex media, others of their characteristics like the picking (often seen as a weakness) or the computation of Fréchet derivatives may also appear in practice as decisive advantages. I believe that these are points on which we should challenge and even inspire full wave approaches. By the end rather than opposing ray based and full wave approaches I review the various trends in terms of the combination of the tools and concepts which appear from my point of view as the most promising.