H/V ratio: a tool for site effects evaluation. Results from 1-D noise simulations

Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification facto

Local site effects in Ataköy, Istanbul, Turkey, due to a future large earthquake in the Marmara Sea

Since the 1999 Izmit and Düzce earthquakes in northwest Turkey, many seismic hazard studies have focused on the city of Istanbul. An important issue in this respect is local site effects: strong amplifications are expected at a number of locations due to the local geological conditions. In this study we estimate the local site effects in the Ataköy area (southwestern Istanbul) by applying several techniques using synthetic data (hybrid 3-D modelling and 1-D modelling) and comparing to empirical data. We apply a hybrid 3-D finite-difference method that combines a complex source and wave propagation for a regional 1-D velocity model with site effects calculated for a local 3-D velocity structure. The local velocity model is built from geological, geotechnical and geomorphological data. The results indicate that strongest spectral amplifications (SA) in the Ataköy area occur around 1 Hz and that amplification levels are largest for alluvial sites where SA reaching a factor of 1.5-2 can be expected in the case of a large earthquake. We also compare our results to H/V (horizontal to vertical component of the recorded signal) spectral ratios calculated for microtremor data recorded at 30 sites as well as to ambient noise synthetics simulated using a 1-D approach. Because the applied methods complement each other, they provide comprehensive and reliable information about the local site effects in Ataköy. Added to that, our results have significant implications for the southwestern parts of Istanbul built on similar geological formations, for which, therefore, similar SA levels are expecte

Sesame Project - Deliverable D08-02 - WP02 H/V technique : experimental conditions - Final report on Measurement Guidelines

In the following we report the final results for WP02-Measurement Guidelines. This work was conducted under the framework of the SESAME Project (Site Effects Assessment Using Ambient Excitations, EC-RGD, Project No. EVG1-CT-2000-00026 SESAME), Task A (H/V technique), Work Package 02 (WP02 – Measurement Guidelines).European Commission – Research General Directorate Project No. EVG1-CT-2000-00026 SESAMEPublished4.1. Metodologie sismologiche per l'ingegneria sismicaope

Nature du bruit de fond sismique: implications pour les études des effets de site

The H/V ambient vibration method has the potential to significantly contribute to effective seismic risk mitigation, in particular in urban areas. However, physical basis and actual relevancy for site effects estimates of this method have never reached a scientific overall agreement. Within the framework of the SESAME (Site EffetcS using AMbient Excitations) European project we investigate the nature of ambient seismic noise in order to define the reliability of the H/V technique. Through numerical and experimental approaches we will answer the following questions: what is the ambient noise? Which origin? Which nature? Which kind of waves composes the noise wavefield? Body waves or surface waves? We will point out that the answers of these questions are not univocal, the physical composition of the ambient noise wavefield, and thus the H/V peak origin, is influenced by: 1) the soil characteristic (especially the impedance contrast between sediment and bedrock); 2) the spatial localization of noise sources. One of the most important issue of this study is, whatever the H/V peak origin (Rayleigh waves ellipticity, Love waves Airy phase, or the S-wave resonance), the H/V peak frequency observed on H/V curves gives a good estimate of the fundamental resonance frequency (for a 1D sedimentary structure). On the other hand, we show the limits of this method to estimate the site amplification factor.Ce travail de thèse s'inscrit dans le cadre du programme de recherche européen SESAME (Site EffetcS using AMbient Excitations) dont l'objectif est de définir les limites et les possibilités de la méthode H/V basée sur le bruit de fond sismique. Située en amont de ce programme, cette thèse a pour objectif de déterminer la nature du bruit de fond sismique afin de mieux comprendre les principes et le domaine de validité de la méthode H/V. Par une double approche (numérique et expérimentale) nous apporterons des réponses aux questions suivantes : qu'est ce que le bruit de fond ? Quelle est son origine ? Quelle est sa nature ? De quelles ondes est-il composé ? Ondes de volume, ondes de surface ? Nous montrons que les réponses à ces questions ne sont pas univoques, la composition physique du champ d'ondes du bruit de fond sismique, et donc l'origine du pic H/V, est influencée 1) par les caractéristiques du sol (notamment par la valeur du contraste d'impédance entre les sédiments et le rocher) ; 2) par la localisation spatiale des sources de bruit. Un des résultats importants de cette thèse est que, quelle que soit l'origine du pic H/V (ellipticité des ondes de Rayleigh, phase d'Airy des ondes de Love, ou résonance des ondes S), la fréquence du pic observé sur les courbes H/V donne une estimation correcte de la fréquence de résonance d'une structure sédimentaire 1D. En revanche, nous montrons les limites de la méthode pour estimer la valeur d'amplification d'un site

The nature of noise wavefield and its applications for site effects studies A literature review

International audienceThe aim of this paper is to discuss the existing scientific literature in order to gather all the available information dealing with the origin and the nature of the ambient seismic noise wavefield. This issue is essential as the use of seismic noise is more and more popular for seismic hazard purposes with a growing number of processing techniques based on the assumption that the noise wavefield is predominantly consisting of fundamental mode Rayleigh waves. This survey reveals an overall agreement about the origin of seismic noise and its frequency dependence. At frequencies higher than 1 Hz, seismic noise systematically exhibits daily and weekly variations linked to human activities, whereas at lower frequencies (between 0.005 and 0.3 Hz) the variation of seismic noise is correlated to natural activities (oceanic, meteorological...). Such a surface origin clearly supports the interpretation of seismic noise wavefield consisting primarily of surface waves. However, the further, very common (though hidden) assumption according which almost all the noise energy would be carried by fundamental mode Rayleigh waves is not supported by the few available data: no “average” number can though be given concerning the actual proportion between surface and body waves, Love and Rayleigh waves (horizontal components), fundamental and higher modes (vertical components), since the few available investigations report a significant variability, which might be related with site conditions and noise source properties

Nature du bruit de fond sismique (implications pour les études des effets de site)

CE TRAVAIL DE THESE S'INSCRIT DANS LE CADRE DU PROGRAMME DE RECHERCHE EUROPEEN SESAME (SITE EFFECTS USING AMBIENT EXCITATIONS) DONT L'OBJECTIF EST DE DEFINIR LES LIMITES ET LES POSSIBILITES DE LA METHODE HN BASEE SUR LE BRUIT DE FOND SISMIQUE. SITUEE EN AMONT DE CE PROGRAMME, CETTE THESE A POUR OBJECTIF DE DETERMINER LA NATURE DU BRUIT DE FOND SISMIQUE AFIN DE MIEUX COMPRENDRE LES PRINCIPES ET LE DOMAINE DE VALIDITE DE LA METHODE HN. PAR UNE DOUBLE APPROCHE (NUMERIQUE ET EXPERIMENTALE) NOUS APPORTERONS DES REPONSES AUX QUESTIONS SUIVANTES: QU'EST CE QUE LE BRUIT DE FOND? QUELLE EST SON ORIGINE? QUELLE EST SA NATURE? DE QUELLES ONDES EST-IL COMPOSE? ONDES DE VOLUME, ONDES DE SURFACE? NOUS MONTRONS QUE LES REPONSES A CES QUESTIONS NE SONT PAS UNIVOQUES, LA COMPOSITION PHYSIQUE DU CHAMP D'ONDES DU BRUIT DE FOND SISMIQUE, ET DONC L'ORIGINE DU PIC HN, EST INFLUENCEE 1) PAR LES CARACTERISTIQUES DU SOL (NOTAMMENT PAR LA VALEUR DU CONTRASTE D'IMPEDANCE ENTRE LES SEDIMENTS ET LE ROCHER); 2) PAR LA LOCALISATION SPATIALE DES SOURCES DE BRUIT. UN DES RESULTATS IMPORTANT DE CETTE THESE EST QUE, QUELLE QUE SOIT L'ORIGINE DU PIC HN (ELLIPTICITE DES ONDES DE RAYLEIGH, PHASE D'AIRY DES ONDES DE LOVE, OU RESONANCE DES ONDES S), LA FREQUENCE DU PIC OBSERVE SUR LES COURBES HN DONNE UNE ESTIMATION CORRECTE DE LA FREQUENCE DE RESONANCE D'UNE STRUCTURE SEDIMENTAIRE 10. EN REVANCHE, NOUS MONTRONS LES LIMITES DE LA METHODE POUR ESTIMER LA VALEUR D'AMPLIFICATION D'UN SITE.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Array performances for ambient vibrations on a shallow structure and consequences over Vs inversion

International audienceValuable information about one-dimensional soil structures can be obtained by recording ambient vibrations at the surface, in which the energy contribution of surface waves predominates over the one of other types of waves. The dispersion characteristics of surface waves allow the retrieval of the shear-wave velocity as a function of depth. Microtremor studies are usually divided in two stages: deriving the dispersion (or autocorrelation) curve from the recorded signals and inverting it to obtain the site velocity profile. The possibility to determine the dispersion curve over the adequate frequency range at one site depends on the array aperture and on the wavefield spectra amplitude which can be altered by filtering eects due to the ground structure. Microtremors are usually recorded with several arrays of various apertures in order to get the spectral curves over a wide frequency band, and dierent methods also exist for processing the raw signals. With the objective of defining a strategy to achieve reliable results for microtremor on a shallow structure, we analyse synthetic ambient vibrations (vertical component) simulated with 333 broad-band sources for a 25 m deep soil layer overlying a bedrock. The first part of our study is focused on the determination of the reliable frequency range of the spectral curves (dispersion or autocorrelation) for a given array geometry. We find that the wave number limits deduced from the theoretical array response are good estimates of the valid spectral curve range. In the second part, the spectral curves are calculated with the three most popular noise-processing techniques (frequency-wavenumber, high resolution frequency-wavenumber and spatial auto-correlation methods) and inverted individually in each case. The inversions are performed with a tool based on the neighbourhood algorithm which oers a better estimation of the global uncertainties than classical linearized methods, especially if the solution is not unique. Several array apertures are necessary to construct the dispersion (autocorrelation) curves in the appropriate frequency range. Considering the final velocity profiles, the three tested methods are almost equivalent and no significant advantage was found for one particular method. With the chosen model, all methods exhibit a penetration limited to the bedrock depth, as a consequence of the filtering eect of the ground structure on the vertical component, which was observed in numerous shallow sites

Use of Ambient Noise: From Spectral Amplitude Variability to H/V Stability

International audienceThe study of the variation over time of both spectral amplitudes and H/V curves, has been performed on three different sites, two close to cities and one in the countryside, during periods varying from week to over a month. It demonstrates the robustness of the H/V technique to give consistent peak frequency values. In particular, H/V peak frequencies, either fundamental (f0) or natural (fx, x•1), are not affected by weather nor the level of human activity. However, while fundamental H/V peak amplitudes are stable, they proved rather unstable for natural (secondary) peak. Spectral amplitude curves are very variable but follow human activity cycles from week-week end and day-night variations down to a very small scale, such as lunch breaks. Finally, the frequency limit between anthropic noise and natural noise, commonly taken at 1 Hz, is not straightforward and is varying from site to site from 0.7-0.8 Hz up to 2-3 Hz

Effects of Love Waves on Microtremor H/V Ratio

International audienceThe horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the European project, site effects assessment using ambient excitations (SESAME), we investigated the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulated ambient noise for a set of various horizontally stratified structures by computing efficiently the displacement and stress of dynamic Green's functions for a viscoelastic-layered half-space. We performed array analysis using the conventional semblance-based frequence-wavenumber method and the three-component modified spatial autocorrelation method on both vertical and horizontal components and estimated the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption that almost all the ambient noise energy would be carried by fundamental-mode Rayleigh waves is not justified. The relative proportion of different wave types depends on site conditions, and especially on the impedance contrast. For the 1D horizontally layered structures presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh wave ellipticity, Airy phase of Love waves, S-wave resonance). We also infer that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak

Third International Symposium on the Effects of Surface Geology on Seismic Motion ROBUSTNESS OF THE H/V RATIO PEAK FREQUENCY TO ESTIMATE 1D RESONANCE FREQUENCY

ABSTRACT - The H/V method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the SESAME European project (Site EffectS assessment using AMbient Excitations) we investigate the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulate the ambient noise for a set of various horizontally stratified structures. We perform array analysis (f-k) for both vertical and horizontal synthetics and estimate the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption according which almost all the noise energy would be carried by fundamental mode Rayleigh waves is not justified. The proportion of different waves is dependent on site conditions, especially the impedance contrast. However, for the 1D horizontally layered media presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh waves ellipticity, Airy phase of Love waves, S-wave resonance). We also show that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak