In the broadest sense, seismic waves can be categorized into two major groups: body
and surface waves (
Fig. 1).  Body waves propagate through the entire body, whereas
surface waves travel along the surface of the medium.  Seismic body waves include two
different types according to the relative direction of disturbance with respect to direction
of propagation: P- and S-waves (
Fig. 2).  Seismic surface waves also include several
different types, the Rayleigh wave being one (
Fig. 2).  Velocities of P- and S-waves (Vp
and Vs) are determined by several aspects of a material called elastic constants (or
moduli).  If a material’s shear modulus vanishes as with fluid, then only P-waves, not S-
wave, can exist and this special type of elastic wave is called acoustic waves.  Velocities
of surface waves are governed mainly by shear modulus of materials.  Seismic waves
used for the survey can be generated in two ways:
actively or passively.  They can be
generated actively by using an impact source like a sledgehammer or passively by
natural (for example, tidal motion and thunder) and cultural (for example, traffic)
activities.  A seismic survey of the former type is called the active method, whereas the
latter is called the passive method.  Most seismic surveys historically implemented
have been the active type.  
What are seismic waves?
Fig. 1.
Fig. 2.  Two different types of body waves (P- and
S-wave) and one type of surface (Rayleigh) wave.