• Top of page
• Contributions
• Course outline
• Saturday
• Sunday
• Monday
• Tuesday
• Wednesday
• Thursday
• Suggested readings
• Registration and fees
• Contacts
• Photo gallery

		Using Ambient Vibration Array Techniques for Site Characterisation
		Saturday, July 21st to Thursday, July 26th 2007 Algiers, Algeria
		All aspects of ambient vibrations analysis are reviewed during a six-day course, read more ...

 

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

• Aki, K. (1957). Space and time spectra of stationary stochastic waves, with special reference to microtremors, Bull. Earthq. Res. Inst. 35, 415-456.
• Bard, P.-Y. (1999). Microtremor measurements: a tool for site effect estimation?, State-of-the-art paper, 2nd International Symposium on the Efects of Surface Geology on Seismic Motion, Yokohama, Balkema, 3, 1251--1279.
• Bettig, B., P.-Y. Bard, F. Scherbaum, J. Riepl, F. Cotton, C. Cornou and D. Hatzfeld (2001). Analysis of dense array noise measurements using the modified spatial auto-correlation method (SPAC). Application to the Grenoble area., Bolletino di Geofisica Teorica ed Applicata 42, 281-304.
• Capon, J. (1969). High-resolution frequency-wavenumber spectrum analysis, Proc. IEEE 57, 1408-1418.
• Sambridge, M. (1999a). Geophysical inversion with a neighbourhood algorithm: I. Searching a parameter space, Geophys. J. Int. 138, 479-494.
• Sambridge, M. (1999b). Geophysical inversion with a neighbourhood algorithm: II. Appraising the ensemble, Geophys. J. Int. 138, 727-746.
• Tokimatsu, K. (1997). Geotechnical site characterization using surface waves, In Proc. 1st Intl. Conf. Earthquake Geotechnical Engineering, Ishihara (ed), Balkema, 1333-1368.

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 30^th 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:

• Grenoble, November 2005(thanks to Sylvette Bonnefoy, Mehmet Ergin, Fabrizio Cara and Carlo Cauzzi).
• Berlin, April 2006 (still missing).
• Caracas, October 2006 (still missing).