Seismic methods involve measuring the propagation of seismic waves through earth materials. In seismic surveys, seismic waves radiate outward from a sound source at the surface, which can be an explosive charge or a mechanical impact. In surface geophysics there are various surface seismic methods such as spectral analysis of surface waves (SASW), multi-channel analysis of surface waves (MASW), seismic refraction tomography and vibration monitoring. There are two major classes of seismic waves: body waves, which pass through the volume of a material, and surface waves, that exist only near a boundary.