Most seismic codes adopt the average shear-wave velocity as a key parameter the first 30 m of subsoil (VS30). Estimates of VS30 are therefore required for large- and small-scale seismic microzonation. We propose a technique to measure VS30 based on the horizontal to vertical spectral ratio (H/V) of microtremor at a single station. The H/V is fitted with a synthetic curve using the independently thickness of a superficial layer of the subsoil as a constraint. The procedure consists of three steps: (1) identify the depth of a shallow stratigraphic from independent geotechnical data, (2) identify its corresponding /V marker, and (3) use it as a constraint to fit the experimental H/V curve with theoretical one. The synthetic H/V curve is calculated by assuming a stratified -dimensional (1D) soil model and a tremor wave field from distant random sources are composed of Rayleigh and Love waves. This technique has been validated on geological settings following a triple check procedure: (1) the theoretical -wave phase velocity dispersion curves calculated for the model derived from H/V fit were compared to the experimental curves measured with the 2D extended autocorrelation (ESAC) method and the 1D refraction microtremor (ReMi) array ; (2) the VS profiles and (3) the VS30 estimates obtained by the proposed were compared to those obtained by using the latter array techniques. Finally, inferred stratigraphy was compared to the geological model. The proposed is not meant to provide accurate VS profiles but has the potential to provide 30 estimates that are coherent with those measured through ESAC and ReMi and to law requirements. Furthermore, the H/V is found to be capable in detecting from 1D subsoil geometry over the length of a few meters, the correctness which was confirmed by direct drilling.
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Castellaro et al. (2009) studied this question.