The refraction seismic method is aimed at the direct determination along the vertical scanner of the speed of propagation of seismic waves, with continuity, within the medium examined. From the analysis of the speed values, it is possible to reconstruct the subsurface geometries and obtain information about the mechanical condition of the crossed materials (thickening, stiffness, state of fracturing, alteration, etc.). The methodology considers the phenomenon of refraction of elastic waves, artificially generated on the surface, in correspondence of interfaces that separate soils at different conditions of density, assuming that the ground explored has the behavior of a linear elastic stress-strain.
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.
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