SEISMIC REFRACTION

The energy is introduced into the subsurface from a power source, whether it is a shot with an hammer or a weight in free fallor a seismic gun. The acoustic waves are propagated in the subsoil with a speed which is related to the elastic properties of the material that pass through. When the waves get an interface that separates soil with a different density or significantly different speeds, a part of the energy is reflected back to the surface and the rest is transmitted in the lower layer. If, and only if, the speed of the lower layer is greater than the one of the upper layer, a part of the energy is also significantlyrefracted along the interface itself. The significantly refracted waves traveling on the interface of a lower speed layer continuously refract energy to the surface, where there are special sensors (accelerometer or velocimeter) able to receive the disturbance propagated in the ground as a result of the generation of an elastic wave. Such sensors translate the stress into an electrical signal which, through a multicore cable ,is transferred to a recording instrument (multichannel seismograph). The seismograph digitizes the signals received by the geophones in the shape of “seismograms” where, for each channel,  the time domain is displayed on the vertical axis and the amplitude of the electrical signal (± mV) on the horizontal axis.