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The SDMT is the combination of the flat dilatometer with an add-on seismic module for the measurement of the shear wave velocity. The seismic module (Fig. 1a) is a tubular element placed above the DMT blade, equipped with two receivers located at 0.5 m distance. When a shear wave is generated at surface, it reaches first the upper receiver, then, after a delay, the lower receiver.

The seismograms acquired by the two receivers, amplified and digitized at depth, are transmitted to a PC at the surface, that automatically calculates the delay using the Cross Correlation algorithm. 

VS is obtained (Fig. 1b) as the ratio between the difference in distance between the source and the two receivers (S2 – S1) and the delay ΔT from the first to the second receiver.

The true-interval test configuration with two receivers avoids possible inaccuracy of the “zero time” at the hammer impact, sometimes observed in the pseudo-interval one-receiver configuration.