The seismic refraction analysis consists in receiving, to varying horizontal distances from the source, the seismic waves refracted by the geological discontinuities when the waves correspond to changes in the speed of propagation: the measurement of the arrival times of refracted waves (P) at various distances allows determining, for each layer, the speed of propagation.
The speed depends on the elastic properties of the material and so it can date back to the geological nature of the rock layer.
The seismic waves source is generally an explosive material or blowback. For what concerns stringing, 6 or 12 or 24 aligned geophones are regularly used, recording devices are set on trucks or can be moving too.
Dromocrone or distance-time curve performance can give depth and slope discontinuities.
Applicability of the seismic reflection method
Only the interpretation of a few superimposed discontinuities is possible, not more than 3 or 4 (for deeper and more precise interpretations you may use the reflection technique).
The method requires an improvement of the mechanical characteristics of the layers studied by their depth, i.e. the speed of seismic waves must increase with depth. Otherwise, a total reflection is not possible, and so the refracted waves, for example in the second layer, never return to the surface.