cgs_logo auf_logo Using Ambient Vibration Array Techniques for Site Characterisation ujf_logo
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Saturday, July 21st to Thursday, July 26th 2007
Algiers, Algeria

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All aspects of ambient vibrations analysis are reviewed during a six-day course, read more ...

english fran�ais

In recent years, the number of research papers dealing with ambient vibration analysis methods has increased considerably. Clearly, the interest in these methods originates from both the economical attractive cost benefit ratio and the straightforward data acquisition. Being a non-destructive passive technique, these methods also complement geotechnical and/or active geophysical methods within highly populated regions.

Within the SESAME project (''Site EffectS assessment using AMbient Excitation'', EU-Project EVG1-CT-2000-00026) and Sismovalp project (Interreg III B) detailed research has been accomplished to identify the capabilities and limitations of ambient vibration analysis techniques. Within this context a set of software tools has been developed to ease the processing and interpretation of ambient vibration wave field recordings.

The main findings of the SESAME project however show, that ambient vibration array analysis techniques have to be applied with care. It requires not only a careful measurement, but especially the interpretation and the inversion of analysis results need a (self-)critical review. From these conclusions it has been agreed among SESAME-partners, that it is highly necessary to distribute the software tools together with adequate training.

The main purpose of this course is to achieve the necessary understanding among the course participants for the problems related to these techniques. By doing so, we try to enable a correct usage of the software tools and to avoid misuse of these methods (''black-box usage''). The course fees are intended to support current and future improvements of the software package, distributed under an open source and free license.

Contributions

organised by LGIT (Grenoble, France), IGUP (Potsdam, Germany) and CGS(Algiers, Algeria)
hosted by Agence Universitaire de la Francophonie (AUF) and Centre de G�nie Parasismique (CGS).
presented by C�cile Cornou (LGIT/IRD), Matthias Ohrnberger (UP Potsdam), Marc Wathelet (LGIT), Bertrand Guillier (LGIT/IRD), Jean-Luc Chatelain (LGIT/IRD)
supported by IRD, NERIES (EU-Project), SESAME (EU-Project EVG1-CT-2000-00026), Sismovalp (Interreg IIIB, Alpine Space)

Course outline

This is a program which may be subject to slight changes in the future.

Saturday
8:30-09:00 Reception and welcome  
 
  • Technical issues and program overview
09:00-10:00 Physical background of ambient vibrations Lecture
 
  • Basic assumptions and state of the art about the nature of the ambient vibration wave field.
  • The linkage between subsurface structure and wave field propagation properties: relationships between site conditions, Rayleigh ellipticity, Airy phase of Love waves and SH transfer function.
10:00-10:15 Coffee break
10:15-11:15 Single station measurement, H/V Lecture
 
  • Computation and interpretation of horizontal to vertical spectral ratio technique (H/V).
  • Effects of experimental conditions.
11:15-12:15 Ambient vibration for characterizing dynamical parameters of structures. Lecture
 
  • Fundamental frequency and harmonics, modal shape, damping and torsion analysis, transfer function.
  • Effects of experimental conditions.
12:15-14:00 Lunch break
14:00-15:30 Single station measurement, H/V Exercises
 
  • Geopsy tools for H/V.
  • Field data exemples.
 
15:30-15:45 Coffee break
15:45-17:15 Ambient vibration for characterizing dynamical parameters of structures. Exercises
 
  • Geopsy tools to evaluate dynamical parameters of structures.
  • Field data exemples.
Sunday
9:00-10:30 Basic array processing concepts (frequency wavenumber, f-k) Lecture
 
  • Overview array processing methods.
  • The intuitive shift-and-sum technique in time domain and its transposition into the frequency wavenumber (f-k) domain.
  • Discrete spatial sampling of the continuous seismic wave field (by small groups of seismic stations - arrays) : concepts of resolution and aliasing.
  • Resolution and aliasing of simple linear array layouts and generalisation to bi-dimensional array settings.
  • Relation between array geometry and resulting estimation capabilities: rules of thumb and the array response function.
10:30-10:45 Coffee break
10:45-12:15 Array geometry and f-k response Exercices
 
  • Computation of array response with build_array software: optimum usage and functionalities.
  • Building experience on how modifications in array layout affect the theoretical array performances.
12:15-14:00 Lunch break
14:00-15:30 Conventional f-k processing Exercises
 
  • Data management for array processing with Geopsy.
  • Application of the conventional f-k algorithm to estimate the dispersion curves from synthetic signals
15:30-15:45 Coffee break
15.45-17:15 Conventional f-k processing Exercises
 
  • Building your own experience with f-k input parameters (frequency sampling, time window length, ...)
  • From the discussion of the analysis results, new insight into dependencies between the observability of dispersion curves, array geometries and signal spectra (high pass filter effect and H/V).
Monday
9:00-10:30 High resolution f-k (Capon's method) Lecture and exercise
 
  • Background of a widely used method, the high resolution f-k method after Capon (1969).
  • Application of the method to the synthetic data set, previously mentioned.
  • Comparison with the conventional f-k method.
10:30-10:45 Coffee break
10:45-12:15 Spatial autocorrelation method (SPAC) Lecture and exercise
 
  • Background of the spatial autocorrelation technique (SPAC) introduced by Aki (1957).
  • Ongoing developments around Aki's original work.
  • Application of the method to the synthetic data set, previously mentioned.
12:15-14:00 Lunch break
14:00-15:30 Synthetic data sets, high resolution f--k and SPAC Exercises
 
  • Application of the methods to the synthetic data set (continued).
  • Discussion of differences between SPAC and f-k techniques.
15:30-15:45 Coffee break
15:45-17:15 Integrating methods Exercices
 
  • Comparison of the three methods used for array processing, advantages and limitations of each approach.
  • Integrating results of all approaches.
20:00-... Workshop Dinner
Tuesday
9:00-10:30 Dispersion curve inversion Lecture
 
  • Short introduction to the fundamentals of inversion theory.
  • Direct search methods based on a random sampling, and particularly the Neighbourhood algorithm.
  • Specific issues solved for the inversion dispersion curves: robustness of the forward computation, non-uniqueness of the final solution.
10:30-10:45 Coffee break
10:45-12:15 Dispersion curve inversion Exercises
 
  • Introduction to dinver software
  • Inversion of theoretical dispersion curve in order to identify critical issues.
  • Model parameterisation and parameter range (vP, vS), a natural way to introduce prior information and to reduce the non-uniqueness.
12:15-14:00 Lunch break
14:00-15:30 Dispersion curve inversion Exercises
 
  • Inversion the analysis results which have been obtained from f-k (standard and high-resolution) and SPAC computations the day before.
  • Combining the results of arrays with distinct apertures and geometries: decision on the usable frequency range for the inversion target.
15:30-15:45 Coffee break
15:45-17:15 ... Discussion of inversion results Moderated discussion
 
  • Comparison of the inversion results obtained by the course participants with the true underlying model.
  • Discussion of the causes and the significance of apparent discrepancies between obtained inversion results.
Wednesday
9:00-10:30 Blind test experiment Exercises
 
  • Blind test experiment performed among the course attendees on a synthetic case.
  • Choice of the array layouts, analyse the selected records with f-k and SPAC techniques.
10:30-10:45 Coffee break
10:45-12:15 Blind test experiment Exercices
 
  • Analysis of the selected records with f-k and SPAC techniques.
12:15-14:00 Lunch break
14:00-15:30 Blind test experiment Exercises
 
  • Deriving the underlying velocity model.
15:30-15:45 Coffee break
15:45-17:15 ... Blind test experiment Exercices
 
  • Deriving the underlying velocity model.
Thursday
9:00-10:30 Discussion of results from data sets Moderated discussion
 
  • Comparison of the individual results obtained during the blind test.
  • General discussion of experience gained during this complete test.
10:30-10:45 Coffee break
10:45-12:15 Course summary Lecture
 
  • Summary of the various steps involved for the task of ambient vibration array analysis and site characterisation by the inversion of one-dimensional earth models. Recall of the advantages and limitations of using ambient vibrations to site characterisation.
  • Ongoing developments around ambient vibration techniques and currently open questions.
12:15-14:00 Lunch break
14:00-... Departure of Participants Open discussion

Suggested readings

All deliverables and reports of the SESAME project are available here (especially, D18.06, D19.06, D24.13 may be of interest).

Registration and Fees

To offer the best attention to each of the course attendees, the maximum number of participants is fixed to 15. A minimum of 8 registrations is required for the organisation of this cession. The official language is english. However, support in french langage will be provided.

Online registration is open until friday March 30th 2007. If the number of applicants is greater than 15, the organisers reserve their rights to select the participants.

The price for this course is fixed to 300 euros per participant.

The fees include lectures, practical exercises, workshop dinner, transportation from hotel to airport and coffee breaks. One computer per participant will be made available during the exercises. Profits will be devoted to the development and improvement of the SESARRAY softwares.

Contacts

C�cile Cornou cecile dot cornou at obs dot ujf-grenoble dot fr
Bertrand Guillier guillier at bondy dot ird dot fr
Mohamed Farsi mnfarsi at cgs-dz dot org
Marc Wathelet marc dot wathelet at ujf-grenoble dot fr
marc at geopsy dot org
Matthias Ohrnberger mao at geo dot uni-potsdam dot de
Geopsy http://www.geopsy.org
SESAME sesame-fp5.obs.ujf-grenoble.fr
Sismovalp www-lgit.obs.ujf-grenoble.fr/sismovalp

Photo gallery

Here you can find all photos taken during last editions: