A second example of lateral velocity variation is Figure 22, also taken from Kjartansson's dissertation. The physical model shown on the inset in Figure 22 is three constant velocity wedges separated by broken line segments representing reflectors.
The bottom edge of the model also represents a reflector. The wavefield in Figure 22 was made using the exploding-reflector calculation, which Kjartansson regarded as a reasonable approximation to a zero-offset section. Notice that under the tip of the 4 km/sec wedge is a small diffraction on the bottom horizontal reflector. Because such a diffraction has nothing to do with the flat reflector on which it is seen, it is termed a ``phantom'' diffraction. Phantom diffractions are not easy to recognize, but they do occur. In reality, the ``bright spots'' in Kjartansson's section were probably phantom diffractions. It has been reported that phantom diffractions provide a means of prospecting for small, high-velocity, carbonate reefs.