Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the V S,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical Electronic supplementary material The online version of this article
S U M M A R YProceeding on the assumption that the seismic noise wavefield mainly consists of surface waves generated by a random distribution of independent, point-like and harmonic sources, we modelled a set of relationships that allows us to interpret empirical H/V and apparent dispersion curves in terms of the dynamic properties of a viscoelastic layered Earth. This formalization can be used to determine the effect of material damping on the expected noise wavefield in cases where the sources are relatively distant from the receivers and where the frequencies taken into account are higher than the fundamental resonance frequency of the local subsoil. Numerical experiments indicate that such effects are significant on the H/V ratio curve, whereas the apparent dispersion curve appears to be less strongly influenced by damping. This finding opens new perspectives for the use of noise measurements to infer the in situ anelastic properties of subsoil from passive seismic measurements made at the surface.
S U M M A R YAmbient vibrations generated by anthropic activity in the range of frequencies of engineering interest (0.5-20 Hz) were modelled as the wavefield generated by a continuous distribution of random, independent point-like sources acting at the surface of a weakly dissipative layered Earth. A full wavefield solution was deduced analytically and used to evaluate major properties of Horizontal to Vertical Spectral Ratios (HVSR) in a representative set of selected cases of observed subsoil configurations. The results obtained confirmed-on a more coherent analytical basis-several statements deduced from empirical observations and numerical simulations, which are of great importance for practical applications. It was confirmed that HVSR cannot be considered representative of the S-wave response function but as concerns the possibility of detecting the presence of 1-D resonance phenomena and of identifying the resonance frequency associated with the shallowest strong impedance contrast in the subsoil. The model enables evaluations of the reliability of HVSR interpretations provided in the surface wave approximation that can be considered valid in the frequency range above the resonance frequency. At the resonance frequency, HVSR values prove sensitive to the strength of sources in the near proximity of the receiver (within a few tens of metres) and this suggests caution in the interpretation of HVSR peaks in terms of subsoil properties only.
In spite of the Horizontal-to-Vertical Spectral Ratio (HVSR or H/V) technique obtained by the ambient vibrations is a very popular tool, a full theoretical explanation of it has been not reached yet. A short excursus is here presented on the theoretical models explaining the H/V spectral ratio that have been development in last decades. It leads to the present two main research lines: one aims at describing the H/V curve by taking in account the whole ambient-vibration wavefield, and another just studies the Rayleigh ellipticity. For the first theoretical branch, a comparison between the most recent two models of the ambient-vibration wavefield is presented, which are the Distributed Surface Sources (DSS) one and the Diffuse Field Approach (DFA). A mention is done of the current developments of these models and of the use of the DSS for comparing the H/V spectral ratio definitions present in literature. For the second research branch, some insights about the connection between the so-called osculation points of the Rayleigh dispersion curves and the behaviour of the H/V curve are discussed.
Different positions exist about the physical interpretation of horizontal to vertical spectral ratios (HVSR) deduced from ambient vibrations. Two of them are considered here: one is based on the hypothesis that HVSR are mainly conditioned by body waves approaching vertically the free surface, the other one assumes that they are determined by surface waves (Rayleigh and Love, with relevant upper modes) only. These interpretations can be seen as useful approximations of the actual physical process, whose reliability should be checked case-by-case. To this purpose, a general model has been here developed where ambient vibrations are assumed to be the complete wave field generated by a random distribution of independent harmonic point sources acting at the surface of a flat stratified visco-elastic Earth. Performances of the approximate interpretations and complete wave field models have been evaluated by considering a simple theoretical subsoil configuration and an experimental setting where measured HVSR values were available. These analyses indicate that, at least as concerns the subsoil configurations here considered, the surface-waves approximation seems to produce reliable results for frequencies larger than the fundamental resonance frequency of the sedimentary layer. On the other hand, the body waves interpretation provides better results around the resonance frequency. It has been also demonstrated that the HVSR curve is sensitive to the presence of a source-free area around the receiver and that most energetic contribution of the body waves component comes from such local sources. This dependence from the sources distribution implies that, due to possible variations in human activities in the area where ambient vibrations are carried on, significant variations are expected to affect the experimental HVSR curve. Such variations, anyway, only weakly affect the location of HVSR maximum that confirms to be a robust indicator (in the range of 10%) of the local fundamental resonance frequency.
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