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Refraction seismic survey is a method of investigation based on the measurement of path timing that the elastic waves (P waves), generated in the ground at a point-source, take to reach the receivers (geophones) placed on the surface of the ground and aligned with the energization point. The distance between the geophones and that of the energization point are chosen according to the thickness and to the types of material that you want to investigate.
You can thus get to calculate both the propagation velocity of the compression waves P and the thickness of the lands that have been traversed by the seismic waves.
The principle is based on the assumption that the subsoil is constituted by a limited number layers, parallel to the topographic profile, each characterized by constant velocity both vertically and laterally. It 'also necessary that the velocity of each layer is significantly greater than that of the overlying layer.
The down-hole test is a seismic technique for the determination of the physico-mechanical properties of soils; in particular it allows to determine the velocity of propagation of the seismic waves (compression and shear) at different depths, by measuring the time that such waves employ in the path between the source (on the surface) and the receiver (placed inside a hole suitably prepared) and to derive the parameters of the ground.
The test is used for the geotechnical characterization of the subsoil in order to identify its properties (Poisson's ratio, the shear modulus, Young's modulus, the volumetric compressibility module) in order to obtain a seismo-stratigraphic reconstruction of the soil by use for different uses, such as those provided by the technical regulations on construction.