In an infinite homogeneous isotropic medium, only P and S waves exist. However, when the medium does not extend to infinity in all directions, other types of waves can be generated. These waves are called surface waves because they are confined to the vicinity of one of the surfaces that bound the medium.
In exploration seismology, the main type of surface wave of importance is the Rayleigh wave, often called ground roll. This wave travels along the surface of the earth and involves a combination of longitudinal and transverse motion with a definite phase relation to each other. The amplitude of this wave motion decreases exponentially with depth. The particle motion is confined to the vertical plane, which includes the direction of propagation of the wave. During the passage of the wave, a particle traverses an elliptical path and the major axis of the ellipse is vertical (near the surface). The direction of particle motion around the ellipse is called retrograde because it is opposite to the more familiar direction of motion of particles in waves on the surface of water.
The surface waves are of big importance in investigating the homogeneity of hard surfaces (concrete platforms, asphalt layers, etc.) because they have a higher energy than the other type of waves.