The estimate of the velocity of shear waves (
) is essential in seismic engineering to characterize the dynamic response of soils. There are various direct methods to estimate the
. The authors report the results of site characterization in Macerata (Italy), where they measured the
using the seismic dilatometer in a 100 m deep borehole. The standard
estimation originates from the cross-correlation between the signals acquired by two geophones at increasing depths. This paper focuses on the estimate of the dependence of
on the wavenumber. The dispersion curves reveal an unexpected hyperbolic dispersion curve typical of Lamb waves. Interestingly, the contribution of Lamb waves may be notable up to 100 m depth. The amplitude of surface waves decrease rapidly with depth; still, their influence may be essential up to depths considered unusual for standard geotechnical investigations, where their effect is generally neglected. Accordingly, these waves may bias the outcomes of the standard
estimations, which ignore frequency-dependent phenomena. The paper proposes an enhancement of the accepted procedure to estimate
and addresses the importance of Lamb waves in soil characterization.