Some authors develop many facets of wave-propagation theory.
Others begin from a simplified propagation model and develop many facets of information theory. These one-dimensional theories are often regarded as applicable only at zero offset. However, we will see that all other offsets can be brought into the domain of one-dimensional theory by means of slant stacking. The way to get the timing and amplitudes of multiples to work out like vertical incidence is to stop thinking of seismograms as time functions at constant offset, and start thinking of constant Snell parameter. In a layered earth the complete raypath is constructed by summing the path in each layer.