f Hard and smooth models for seismic imaging based on raytracing

Understanding of wave propagation is essential in geophysical imaging techniques. Most of these techniques require an a priori model of the subsurface. Conventional imaging techniques usually employ a highfrequency approximation: only the wave front is considered. In the limit of infinite frequency, only discontinuous variations of the velocity produce reflections. It therefore seems an obvious choice to require the velocity model to be smooth: all reflection data are caused by discontinuities that can be considered as perturbations on the smooth background model.