The equivalent of the zero-offset data is to excite all the surface sources at the same time. In an actual field experiment this is difficult, because then all sources must be set off at exactly the same time. The recording is for a single experiment, and the signal enhancement benefit of multiple shots is lost.
The simulation of a normal plane wave, where all surface sources are excited at the same time and send energy in the form of a wave into the earth model, is illustrated in Figure 11. This plane wave moves down until a reflector is struck, and then the reflected wave travels the same path back to the receiver and the transmitted wave continues on. The wave front is distorted by the velocity field and bends and twists according to Snell's law, Equation 52,
where the angles are between the reflector normal and the wavefront normal. Above the reflector is the velocity and below is the velocity.
Figure 11: Plane Wave at the Surface.