This geophysical survey is based on measuring the “round trip” travel times of the seismic waves transmitted from the surface and reflected towards the surface of geological horizons with different characteristics.
The energy transmitted is only reflected when there is a contrast of acoustic impedance (the product of the velocity x the density of the material) between two superimposed layers. The entity of the contrast in the acoustic impedance between the two layers determines the amplitude of the reflected signal, which is measured on the surface thanks to a spreading of high frequency geophones (40 Hz, 100 Hz). As in the case of seismic refraction, the energy is produced by a “shot” or impact on the surface. A striking mass, a seismic energiser or (where possible) an explosive charge is normally preferred to a sledge hammer.