This is another breakdown of the exploding-reflector concept. The problem is worst with land data. This energy is called ground roll. This data is not a zero-offset section. The shot is on the left, and the traces to the right are from geophones at increasing distances from the shot.
Steeper slopes are slower surface waves.
Geological events have lesser slopes. Like the surface of the earth, which varies greatly from place to place, the immediate subsurface which controls the ground roll varies substantially. So although Figure 16 is a nice example, no example can really be typical. In this figure there are two types of ground roll, one at about half of water velocity, and a stronger one at about a quarter of water velocity. The later and stronger one shows an interesting feature known as frequency dispersion.