CALSENS team has a broad experience in geotechnical engineering. Ground characterization by GDS is one of our services.The tests performed consist in the emission of a surface wave of computerized frequency and in its reception to three low frequency geophones (2Hz) aligned with the vibrator and placed 0,50, 1,00 and 1,50 metres from the emisor. The analog signal of these geophones becomes digitalized and registered regarding time. On these data a Fourier transform is performed in order to determine the stage of the signal received by each geophone with respect to the vibrator. Thus, basing on the surface wave transmission theory through a flexible solid, the “Gmax” shear modulus is determined in different depths, defining a Modulus G vs Depth for the observation point.