ALBERT

Description: The H/V spectral ratio has emerged as a single station method within the seismic ambient noise analysis field by its capability to quickly estimate the frequency of resonance at a site and through inversion the average profile information. Although it is easy to compute from experimental data, its counter theoretical part is not obvious when building a forward model which can help in reconstructing the derived H/V spectrum. This has led to the simplified assumption that the noise wavefield is mainly composed of Rayleigh waves and the derived H/V often used without further correction. Furthermore, only the right (and left) flank around the H/V peak frequency is considered in the inversion for the subsurface 1-D shear wave velocity profile. A new theoretical approach for the interpretation of the H/V spectral ratio has been presented by Sánchez-Sesma et al. In this paper, the fundamental idea behind their theory is presented as it applies to receivers at depth. A smooth H/V( z , f ) spectral curve on a broad frequency range is obtained by considering a fine integration step which is in turn time consuming. We show that for practical purposes and in the context of inversion, this can be considerably optimized by using a coarse integration step combined with the smoothing of the corresponding directional energy density (DED) spectrum. Further analysis shows that the obtained H/V( z , f ) spectrum computed by the mean of the imaginary part of Green's function method could also be recovered using the reflectivity method for a medium well illuminated by seismic sources. Inversion of synthetic H/V( z , f ) spectral curve is performed for a single layer over a half space. The striking results allow to potentially use the new theory as a forward computation of the H/V( z , f ) to fully invert the experimental H/V spectral ratio at the corresponding depth for the shear velocity profile ( Vs ) and additionally the compressional velocity profile ( Vp ) using receivers both at the surface and in depth. We use seismic ambient noise data in the frequency range of 0.2–50 Hz recorded at two selected sites in Germany where borehole information is also available. The obtained 1-D Vs and Vp profiles are correlated with geological log information. Results from shallow geophysical experiment are also used for comparison.

Description: In order to evaluate the site effects on seismic ground motion and establish preventive measures to mitigate these effects, the dynamic characterization of sites is mandatory. Among the various geophysical tools aimed to this end, the horizontal to vertical spectral ratio (H/V) is a simple way to assess the dominant frequency of a site from seismic ambient noise. The aim of this communication is contributing to enhance the potential of this measurement with a novel method that allows extracting from the H/V the elastic properties of the subsoil, assumed here as a multilayer medium. For that purpose, we adopt the diffuse field assumption from both the experimental and the modelling perspectives. At the experimental end, the idea is to define general criteria that make the data processing closely supported by theory. On the modelling front, the challenge is to compute efficiently the imaginary part of Green's function. The Cauchy's residue theory in the horizontal wavenumber complex plane is the selected approach. This method allows both identifying the contributions of body and surface waves and computing them separately. This permits exploring the theoretical properties of the H/V under different compositions of the seismic ambient noise. This answers some questions that historically aroused and gives new insights into the H/V method. The efficient forward calculation is the prime ingredient of an inversion scheme based on both gradient and heuristic searches. The availability of efficient forward calculation of H/V allows exploring some relevant relationships between the H/V curves and the parameters. This allows generating useful criteria to speed up inversion. As in many inverse problems, the non-uniqueness issues also emerge here. A joint inversion method that considers also the dispersion curves of surface waves extracted from seismic ambient noise is presented and applied to experimental data. This joint scheme mitigates effectively the non-uniqueness.

Description: 〈span〉〈div〉Summary〈/div〉Advances in the field of seismic interferometry have provided a basic theoretical interpretation to the full spectrum of the microtremor horizontal-to-vertical spectral ratio [〈span〉H〈/span〉/〈span〉V〈/span〉(〈span〉f〈/span〉)]. The interpretation has been applied to ambient seismic noise data recorded both at the surface and at depth. The new algorithm, based on the diffuse wavefield assumption, has been used in inversion schemes to estimate seismic wave velocity profiles that are useful input information for engineering and exploration seismology both for earthquake hazard estimation and to characterize surficial sediments. However, until now, the developed algorithms are only suitable for on land environments with no offshore consideration. Here, the microtremor 〈span〉H〈/span〉/〈span〉V〈/span〉(〈span〉z, f〈/span〉) modeling is extended for applications to marine sedimentary environments for a 1D layered medium. The layer propagator matrix formulation is used for the computation of the required Greenâs functions. Therefore, in the presence of a water layer on top, the propagator matrix for the uppermost layer is defined to account for the properties of the water column. As an application example we analyze eight simple canonical layered earth models. Frequencies ranging from 0.2 to 50 Hz are considered as they cover a broad wavelength interval and aid in practice to investigate subsurface structures in the depth range from a few meters to a few hundreds of meters. Results show a marginal variation of 8 percent at most for the fundamental frequency when a water layer is present. The water layer leads to variations in 〈span〉H〈/span〉/〈span〉V〈/span〉 peak amplitude of up to 50 percent atop the solid layers.〈/span〉