ALBERT

Description: A new implementation of indirect boundary element method allows simulating the elastic wave propagation in complex configurations made of embedded regions that are homogeneous with irregular boundaries or flat layers. In an older implementation, each layer of a flat layered region would have been treated as a separated homogeneous region without taking into account the flat boundary information. For both types of regions, the scattered field results from fictitious sources positioned along their boundaries. For the homogeneous regions, the fictitious sources emit as in a full-space and the wave field is given by analytical Green's functions. For flat layered regions, fictitious sources emit as in an unbounded flat layered region and the wave field is given by Green's functions obtained from the discrete wavenumber (DWN) method. The new implementation allows then reducing the length of the discretized boundaries but DWN Green's functions require much more computation time than the full-space Green's functions. Several optimization steps are then implemented and commented. Validations are presented for 2-D and 3-D problems. Higher efficiency is achieved in 3-D.

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: Among other methods, passive imaging technique is widely applied to obtain surface-wave velocities. This technique implies that the average cross correlations between diffuse wavefields recorded at two observers is proportional to the imaginary part of the Green’s function. For this purpose, most applications rely on both seismic ambient noise and the coda of earthquakes. Instead, we use a generalized diffuse field (GDF), defined as the waves produced by a multiplicity of distant seismic sources. These wavefields undergo multiple scatterings along their way and at the local surface geology. In this communication, we use GDF to extract the locally generated surface waves in a 2D alluvial valley model for both inplane and antiplane cases from the retrieved Green’s function. For the inplane case, an equipartitioned cocktail of plane P , SV , and Rayleigh waves is used, whereas for the antiplane case, the incidence is a set of plane SH waves. In addition to isotropic illumination, we explore the partial illumination from one side of the valley. In both cases, we obtain dispersion curves for the Rayleigh and Love waves’ group velocities from the retrieved Green’s functions and found good agreement with the exact result for the fundamental modes of both Love and Rayleigh waves in an infinite horizontal layer. This theoretical validation is a proof of concept within an ongoing project whose goal is to improve the characterization of Mexico City subsoil throughout tomography maps of surface-wave velocities using a collection of historical strong earthquakes recorded by the Mexico City Accelerometric Network.