H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations

Sylvette Bonnefoy-Claudet (1) , Cécile Cornou (1, 2) , Pierre-Yves Bard (1, 3) , Fabrice Cotton (1) , P. Moczo (4, 5) , J. Kristek (4, 5) , D. Fah (2)

1 LGIT - Laboratoire de Géophysique Interne et Tectonophysique
2 ETH Hoenggerberg
3 LCPC - Laboratoire Central des Ponts et Chaussées
4 Department of Physics of the Earth and Planets
5 Geophysical Institute

Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification factor

Mots clés: microtremor; seismic array; seismic modelling; seismic noise; site effects; H/V

Domaines: Planète et Univers [physics]

Sylvette Bonnefoy-Claudet, Cécile Cornou, Pierre-Yves Bard, Fabrice Cotton, P. Moczo, et al.. H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations. Geophysical Journal International, 2006, 167 (2), pp.827-837. ⟨10.1111/j.1365-246X.2006.03154.x⟩. ⟨insu-00265520⟩