SUMMARY
We have used both single‐station and array methods to determine shallow shear velocity site profiles in the vicinity of the city of Cologne, Germany from ambient vibration records. Based on fk‐analysis we assume that fundamental‐mode Rayleigh waves dominate the analysed wavefield in the frequency range of 0.7–2.2 Hz. According to this view a close relation exists between H/V spectral ratios and the ellipticity of the contributing Rayleigh waves. The inversion of the shape of H/V spectral ratios then provides quantitative information concerning the local shear wave velocity structure. However, based on tests with synthetic data believed to represent a typical situation in the Lower Rhine Embayment, dispersion curves were found to provide stronger constraints on the absolute values of the velocity–depth model than the ellipticities. The shape of the ellipticities was found to be subject to a strong trade‐off between the layer thickness and the average layer velocity. We have made use of this observation by combining the inversion schemes for dispersion curves and ellipticities such that the velocity–depth dependence is essentially constrained by the dispersion curves while the layer thickness is constrained by the ellipticities. In order to test this method in practice, we have used array recordings of ambient vibrations from three sites where the subsurface geology is fairly well known and geotechnical information is at least partially available. In order to keep the parameter space as simple as possible we attempted to fit only a single layer over a half‐space model. However, owing to earlier studies from the region, we assume a power‐law depth dependence for sediment velocities. For all three sites investigated, the inversion resulted in models for which the shear wave velocity within the sediment layer both in absolute value at the surface and in depth dependence are found to be remarkably similar to the results obtained by Budny from downhole measurements. This is strong support for the interpretation of H/V spectral ratios as Rayleigh wave ellipticities. For all three sites the predicted SH‐wave site amplification factors at the fundamental frequency are of the order of 5–6 with a slightly smaller value south of Cologne.
Passive recordings of seismic noise are increasingly used in earthquake engineering to measure in situ the shear-wave velocity profile at a given site. Ambient vibrations, which are assumed to be mainly composed of surface waves, can be used to determine the Rayleigh-wave dispersion curve, with the advantage of not requiring artificial sources. Due to the data uncertainties and the non-linearity of the problem itself, the solution of the dispersion-curve inversion is generally non-unique. Stochastic search methods such as the neighbourhood algorithm allow searches for minima of the misfit function by investigating the whole parameter space. Due to the limited number of parameters in surface-wave inversion, they constitute an attractive alternative to linearized methods. An efficient tool using the neighbourhood algorithm was developed to invert the one-dimensional V s profile from passive or active source experiments. As the number of generated models is usually high in stochastic techniques, special attention was paid to the optimization of the forward computations. Also, the possibility of inserting a priori information into the parametrization was introduced in the code.This new numerical tool was successfully tested on synthetic data, with and without a priori information. We also present an application to real-array data measured at a site in Brussels (Belgium), the geology of which consists of about 115 m of sand and clay layers overlying a Palaeozoic basement. On this site, active and passive source data proved to be complementary and the method allowed the retrieval of a V s profile consistent with borehole data available at the same location.is of major importance in earthquake engineering, and ambient vibrations measured by an array of vertical sensors are increasingly applied for determining V s profiles (e.g. Horike 1985;
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
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