The elastic wavefields of reflected seismic waves recorded at the surface of the Earth carry enormous information about the physical properties of the subsurface. However, those wavefields have traditionally been underutilized, both due to limits of computing power and due to limits of knowledge regarding imaging methodology. One of the recurrent themes in seismic imaging over the past decades was that of approximation, e.g. data approximation - acoustic vs. elastic, model approximation - v(z) vs. v(x,y,z) or isotropic vs. anisotropic, operator approximation - rayfield-based vs. wavefield-based, etc.
Those approximations were driven by practical considerations, since the initial targets of seismic imaging were simple enough to justify using crude approximations. However, as the targets of seismic imaging become more complex, the methodology required needs to increase in complexity and use more of the information carried by the recorded elastic wavefields. The past decades have shown tremendous progress in this respect, as illustrated by many accurate images of complicated targets. The pace of innovation is gaining speed, and we are likely to witness more progress in imaging methodology in the coming years. This thesis is a stepping stone along this path.