On the limitation of the H/V spectral ratio using seismic noise as an exploration tool: Application to the Grenoble Valley (France), a small apex ratio basin

The H/V spectral ratio method based on seismic noise (HVSRN) was used in the Grenoble Basin (France), an Alpine valley characterized by a small apex ratio. The resonance frequencies obtained in the experiments were compared to the thickness of the sediments deduced from a microgravimetric survey and to the 1-D theoretical assessment of site responses. Given the abundance of data on the sediments and depth of the basin, the values of the theoretical resonance frequency f(o) can be determined quite accurately. However, it has been observed that the effects of basin geometry can disturb f(o) measurements using the HVSR method, in particular for a case like the Grenoble Basin, which has a small apex ratio (w/H) and strong suspected 2-D and/or 3-D effects. Interpretation of f(o) values in terms of bedrock depth gives rise to estimation errors of about 10% in certain cases, with the most significant errors (> 50%) occurring on the edges of the basin, where subsurface layers are characterised by larger heterogeneities and where the basin topography is accentuated. This study suggests that great care must be taken when using the HVSRN method as an exploration tool, at least in valleys with a small apex ratio.