It carries more meaning than could have been anticipated by the earlier geometrical derivation. The dispersion relation was originally regarded merely as where .There was no meaning in exceeding unity, in other words, in exceeding .Now there is. There was a hidden ambiguity in two of the previous migration methods. In practice then, there is always energy with an angle sine greater than one. This is depicted in Figure 15 .
Ground roll is energy that is propagating at the surface, but evanescent at depth.
This says that the depth-dependence of the physical solution is a growing or a damped exponential. These solutions are termed evanescent waves. Only if the airplane can move faster than the speed of sound in the earth will a wave be radiated into the earth. If the airplane moves at a subsonic speed the deformation is said to be quasi-static. Perhaps a better physical description is a thought experiment with a sinusoidally corrugated sheet.