All wave phenomena, the most familiar being visible light, are subject to refraction, or a change in propagation direction, at interfaces between materials of contrasting propagation velocities. The bigger the difference in velocity, the more the energy is refracted or "bent". As the name implies, seismic refraction uses the travel times of refracted seismic energy to determine the seismic velocity of the earth. A short practical discussion of seismic refraction can be found here.
The most important thing to keep in mind when learning how the seismic refraction method works is this:
When doing seismic refraction, we are only interested in first-arrival energy at each geophone. The rest of the wave train -- reflected energy, surface waves, etc. are discarded and ignored. Except in the exceedingly rare case where the near-surface is slower than the speed of sound in air, the first-arrival energy will always be either direct or critically-refracted energy.
Common applications of seismic refraction include:
- Estimating rippability prior to excavation
- Mapping depth to bedrock/bedrock topography
- Mapping depth to ground water
- Measuring the thickness of the weathering zone
- Calculation of elastic moduli/assessment of rock quality
- Mapping thickness of landslides
- Identification and mapping of faults
Seismic refraction uses body waves, most commonly, p-waves. Shear-wave refraction can also be done, but it has become largely supplanted by MASW.