General Description

The SESAME project deals with the impotant issue of site effect estimation for seismic risk mitigation, with a special attention to urban areas. It focuses on two low cost techniques using ambient seismic vibrations, and aims at clarifying their actual ability to provide useful, direct or indirect, information for local amplification estimates.

The first technique, a very simple one, is the so-called H/V technique advertised by Nakamura; the second, more advanced, one uses noise array measurements to derive the dispersion curves of surface waves, and from that the velocity profile. The proposed work includes a theoretical and numerical part to better understand the nature of seismic noise, and to develop validated numerical tools to simulate seismic noise in arbitrary environments. It also includes thorough experimental and data processing investigations to clearly assess the stability, robustness, reliability and physical meaning of these 2 techniques.

All the theoretical, experimental and numerical results will be used to propose user guidelines for the H/V technique (a drastic need at the European and world levels since this technique is very often misused), and practical recommendations for array measurements (not yet as developed), and corresponding software.
 

Keywords: Earthquake - Site effects - Noise - Urban Area
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Objectives

After recent earthquakes, a priori estimations of site effects became a major challenge for an efficient mitigation of seismic risk. Unfortunately, the few methods known as reliable systematically appear as far too expensive for local and national authorities, especially in moderate seismicity countries or in developing countries. There is therefore a drastic need for reliable, low cost techniques.

The objectives of the present proposal are to investigate the reliability of two techniques born in Japan using ambient noise recordings: the very simple H/V technique ("Nakamura"), and the more advanced array technique. They offer many advantages, especially in urban areas, and their use (perhaps misuse) is rapidly spreading world-wide; but their physical basis and actual relevancy for site effect estimates has never reached a scientific agreement.

This project gathers experts in seismology, engineering geology, surface geophysics, data processing,numerical modelling and earthquake engineering, to tackle these methods under different viewpoints, understand their physical basis, assess their actual meaning in view of site effect estimation, and propose user guidelines and processing software to ensure a correct use, and thus improve significantly the mitigation tools.
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Work Plan

The work will consist of 4 main, complementary tasks. On the upstream side, the project will try and fill the gap concerning the understanding of the real nature of noise, especially in urban areas. On the technical side, series of investigations will be carried out to clearly identify the key points in each of these techniques and their reliability, and to clearly assess the conditions under which they have to be performed: experimental conditions for the measurements, and processing techniques as well. Finally, on the downstream side, after - hopefully - having shown that these techniques do provide useful information when applied with care, we want to offer a framework for reliable measurements by proposing user guidelines that could form the basis for a quality label.

The scientific work will thus be separated into a total of 12 workpackages, three for each main task:

• Task A : H/V technique, experimental conditions, data processing, and empirical reliability assessment : experimental aspects for warranting the stability and reproducibility of measurements, investigations on the various data processing alternatives and choice of the most robust ones, and finally experimental assessment of the meaning of this ratio by a thorough comparison with instrumentally measured site effects, or damage distribution in several recent earthquakes.

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• Task B : Array measurements technique: experimental conditions, data processing, and inversion of velocity profile : experimental aspects for an optimal adaptation of the instrumental characteristics and layout to the site under study, analysis of several multitrace signal processingtechniques (f-k, spatial autocorrelation) and implementation of a robust software, and improvements in the inversion of velocity profiles with an optimum use of a priori information.

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• Task C : Physical background and noise simulation for cross-checks with observed data: data analysis at several sites to identify the composition of noise wavefield (nature, proportion and origin of surface and body waves) in urban areas, development and validation of numerical models (FD) with random surface sources, and numerical analysis of the H/V and array techniques on noise synthetics, and finally cross checking of observations, numerical simulations, and known structure and site effects for a few well-known test-sites.

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• Task D : Practical Implementation and Guidelines : organisation of the dissemination of the scientific knowledge and technical know-how through special workshops and special issue in an international journal, redaction of user guidelines and realisation of a CD-ROM with validated processing software for the H/V technique to be advertised in international committees, and recommendations concerning the array measurements technique.

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Expected Results and Milestones

The main outcome will be a clear, solid assessment of the meaning of these methods, and recommendations as to their practical implementation. This will materialize through user guidelines for each technique, to be discussed in specialized committees of international bodies (joint IAEE/IASPEI working group on effects of surface geology), and thus widely disseminated, in order to provide the basis for a quality label.

The consequences will be two-fold: on one side, their wide dissemination will hopefully prevent misuses, wrong microzonation maps and misleading earthquake safety feelings. On the other side, for countries which till now have been reluctant to use them, it will offer a validated, simple, low-cost toolto contribute in systematic, first-level evaluations of seismic risk in urban areas.
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Task A : H/V technique, experimental conditions, data processing, and empirical reliability assessment

• WP02 - Experimental Conditions - is dedicated to investigations on the required experimental conditions (instrumental characteristics, data acquisition environment) for warranting the stability and reproducibility of measurements.

 
• WP03 - Data Processing - is devoted to investigations on the various data processing alternatives and choice of the most robust ones to produce a standard processing software.

 
• WP04 - Technique, Empirical Evaluation - will use the software developed in WP02 to perform a systematic and homogeneous comparison of H/V ratios with more reliable, instrumental estimates of amplification functions (site to reference spectral ratios) for a wide collection of European sites. It will also include a comparison of H/V ratios with damage distribution observed in several recent earthquakes. This will allow a comprehensive, statistically significant, purely empirical assessment of the meaning of H/V ratio with respect to site transfer functions.

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Task B : Array measurements technique: experimental conditions, data processing, and inversion of velocity profil

• WP05 - Instrumental Layout for Array Measurements - is focusing on the purely instrumental and experimental aspects of the array measurements. It is intended to assess the dependence of the array performance on the following experimental conditions: array gepmetry, aperture, number of sensors, sensor types, timing accuracy. The objective is an optimal adaptation of the instrumental characteristics and layout to the site under study, the criterion for the performance being to maximize the reliability on the phase velocity determination.

 
• WP06 - Array Measurements: Derivation of Dispersion Curves - aims at developing a semi-automatic processing system for array analysis of ambient vibrations, based on frequency-wavenumber and spatial autocorrelation methods. Besides providing all the necessary facilities to obtain dispersion curves, the system should allow for rapid in-situ quality control of the array performance.

 
• WP07 - Array Measurements: Inversion of Velocity Profile - concentrates on the development of a flexible software allowing to retrieve the Vp and Vs velocity profiles in an easy and reliable way. A particular attention will be paid to the introduction of a priori information which can greatly help to constrain the model during the inversion process.

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Task C : Physical background and noise simulation

• WP08 - Nature of Noise Wavefield - is intended to increase our knowledge on the actual consistency of the noise wavefield, a key point in the deep understanding of the H/V and array techniques. It will be based on the analysis of noise data recorded with arrays at several sites, in order to identify the composition (nature, proportion and origin of surface and body waves) of noise wavefield in urban areas.

 
• WP09 - Numerical Simulation of Seismic Noise - focuses on the development and validation of numerical models producing realistic noise synthetics. It will mainly use Finite-Difference techniques (FD) with spatially and temporally random surface sources, and include parameter studies to investigate the ability of H/V and array techniques, applied on synthetics, to recover the information on the structure.

 
• WP10 - Simulation for Real Sites - will consist in a final cross-checking between actual noise observations, noise synthetics from numerical simulations, and the known geological structure and local site effects for a few well-known test-sites.

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Task D : Practical Implementation and Guidelines

• WP11 - Scientific Outcomes - has the objective of summing up and disseminating all the scientific learnings obtained throughout the project. This will be reached through the organization of a specific workshop, and the publication of a special issue of an international journal, or a book.

 
• WP12 - User Guidelines for H/V Technique - will elaborate on results of tasks A and C to build practical user guidelines for single station ambient vibration measurements using the H/V technique. Minimum quality requirements for its practical use will be fixed, a standardized validated processing software will be provided on a CD-ROM, and - as far as possible - short scientific explanations will be given. These user guidelines will be submitted to international working groups and committees.

 
• WP13 - Recommendations for Quality Array Measurements and Processing - will similarly issue, on the basis of the results of Tasks B and C, recommendations for a sound use of the array technique. These recommendations will go beyond the problem of seismic amplification, and will address the more general issue of site characterization.

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