Shear wave structure of a transect of the Los Angeles basin from multimode surface waves and H/V spectral ratio analysis

We use broad-band stations of the ‘Los Angeles Syncline Seismic Interferometry Experiment’ (LASSIE) to perform a joint inversion of the Horizontal to Vertical spectral ratios (H/V) and multimode dispersion curves (phase and group velocity) for both Rayleigh and Love waves at each station of a dense line of sensors. The H/V of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the Green’s function. The presence of low-frequency peaks (∼0.2 Hz) in H/V allows us to constrain the structure of the basin with high confidence to a depth of 6 km. The velocity models we obtain are broadly consistent with the SCEC CVM-H community model and agree well with known geological features. Because our approach differs substantially from previous modelling of crustal velocities in southern California, this research validates both the utility of the diffuse field H/V measurements for deep structural characterization and the predictive value of the CVM-H community velocity model in the Los Angeles region. We also analyse a lower frequency peak (∼0.03 Hz) in H/V and suggest it could be the signature of the Moho. Finally, we show that the independent comparison of the H and V components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field

The inversion of spectral ratio H/V in a layered system using the Diffuse Field Assumption (DFA)

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 modeling perspectives. At the experimental end, the idea is to define general criteria that make the data processing closely supported by theory. On the modeling 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

A computer code for forward calculation and inversion of the H/V spectral ratio under the diffuse field assumption

During a quarter of a century, the main characteristics of the horizontal-to-vertical spectral ratio of ambient noise HVSRN have been extensively used for site effect assessment. In spite of the uncertainties about the optimum theoretical model to describe these observations, several schemes for inversion of the full HVSRN curve for near surface surveying have been developed over the last decade. In this work, a computer code for forward calculation of H/V spectra based on the diffuse field assumption (DFA) is presented and tested.It takes advantage of the recently stated connection between the HVSRN and the elastodynamic Green's function which arises from the ambient noise interferometry theory. The algorithm allows for (1) a natural calculation of the Green's functions imaginary parts by using suitable contour integrals in the complex wavenumber plane, and (2) separate calculation of the contributions of Rayleigh, Love, P-SV and SH waves as well. The stability of the algorithm at high frequencies is preserved by means of an adaptation of the Wang's orthonormalization method to the calculation of dispersion curves, surface-waves medium responses and contributions of body waves. This code has been combined with a variety of inversion methods to make up a powerful tool for passive seismic surveying.Comment: Published in Computers & Geosciences 97, 67-7

Urban Seismic Site Characterization by Fiber‐Optic Seismology

Accurate ground motion prediction requires detailed site effect assessment, but in urban areas where such assessments are most important, geotechnical surveys are difficult to perform, limiting their availability. Distributed acoustic sensing (DAS) offers an appealing alternative by repurposing existing fiber‐optic cables, normally employed for telecommunication, as an array of seismic sensors. We present a proof‐of‐concept demonstration by using DAS to produce high‐resolution maps of the shallow subsurface with the Stanford DAS array, California. We describe new methods and their assumptions to assess H/V spectral ratio—a technique widely used to estimate the natural frequency of the soil—and to extract Rayleigh wave dispersion curves from ambient seismic field. These measurements are jointly inverted to provide models of shallow seismic velocities and sediment thicknesses above bedrock in central campus. The good agreement with an independent survey validates the methodology and demonstrates the power of DAS for microzonation.Key PointsWe demonstrate the potential of DAS for site effect analysisDAS recordings are used to compute dispersion curves and horizontal‐to‐vertical spectral ratio (HVSR)Joint inversions suggest that the crystalline bedrock lies 115 m beneath Stanford University central campusPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154310/1/jgrb54043.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154310/2/jgrb54043-sup-0001-Text_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154310/3/jgrb54043_am.pd

Ultrasons rayonnés par une source laser ponctuelle dans des milieux isotropes transverses et applications à la mesure du tenseur d'élasticité de cylindres et de couches minces

The subject of this thesis is the study of the radiation of ultrasounds generated by laser sources focalised along a line or a point in transverse isotropic samples. The geometries considered are one-layer or double-layer planes, or cylindrical. The application to the stiffness tensor measurement is also described. A classical laser ultrasonic setup, using a YAG laser and an interferometric probe allow studying the response of cylinders of millimetric diameter to several laser/material interactions. A picosecond acoustic setup is also used in order to investigate the acoustic waves diffraction in micrometric films. Experimental signals were confronted to results obtained by numerical models. The influence of the optical reflections on the interfaces is also analyzed. The formulation of the directivity functions in anisotropic media for bulk sources and the study of the corresponding directivity diagrams give qualitative informations which are necessary to interpret the signals.Le travail présenté dans ce mémoire porte sur le rayonnement d'ultrasons générés par des sources laser linéiques ou ponctuelles dans des milieux anisotropes, de géométries planes monocouches ou bicouches ou de géométries cylindriques, et sur son application à la mesure du tenseur d'élasticité. Un dispositif classique d'ultrasons laser, constitué d'un laser YAG et d'une sonde interférométrique, permet d'étudier la réponse de cylindres de diamètres millimétriques à différentes interactions laser-matière. Un dispositif d'acoustique picoseconde est aussi utilisé afin d'étudier la diffraction d'ondes acoustiques dans des films plans d'épaisseurs micrométriques. Des modèles numériques ont été développés pour rendre compte des signaux obtenus expérimentalement. L'influence des réflexions optiques sur les interfaces est notamment analysée. L'écriture des fonctions de directivité en milieux anisotropes pour des sources volumiques et l'étude des diagrammes de directivité correspondant apportent par ailleurs des renseignements qualitatifs nécessaire à l'interprétation des signaux

Ultrasons rayonnés par une source laser ponctuelle dans des milieux isotropes transverses et applications à la mesure du tenseur d'élasticité de cylindres et de couches minces

Le travail présenté dans ce mémoire porte sur le rayonnement d ultrasons générés par des sources laser linéiques ou ponctuelles dans des milieux anisotropes, de géométries planes monocouches ou bicouches ou de géométries cylindriques, et sur son application à la mesure du tenseur d élasticité. Un dispositif classique d ultrasons laser, constitué d un laser YAG et d une sonde interférométrique, permet d étudier la réponse de cylindres de diamètres millimétriques à diferentes interactions laser-matière. Un dispositif d acoustique picoseconde est aussi utilisé afin d étudier la diffraction d ondes acoustiques dans des films plans d épaisseurs micrométriques. Des modèles numériques ont été développés pour rendre compte des signaux obtenus expérimentalement. L influence des réflexions optiques sur les interfaces est notamment analysée. L écriture des fonctions de directivité en milieux anisotropes pour des sources volumiques et l étude des diagrammes de directivité correspondant apportent par ailleurs des renseignements qualitatifs nécessaire à l interprétation des signaux.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF