Ambiant Vibration Test Sample

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Application of ambient vibration array techique for sitecharacterization: Angra do Heroísmo and S. Sebastião

Teves-Costa P.(1), Veludo I.(1), Almeida J.(1)

(1) University of Lisbon, Centro de Geofisica da Universidade de Lisboa-IDL,FFCUL & DEGGE-FCUL, Campo Grande,Edificio C8,1749-016 Lisboa,

[email protected], [email protected], [email protected]

Abstract

Within the joint research activity JRA4 - Geotechnical Site Characterization –

performed formed in the framework of Neries, a tool was developed using ambient

vibration measurements. One of the main objectives was to estimate Vs30 profiles,

usually used on EC8 for engineering purposes. Five sites were selected in Portugal to

test this tool. Preliminary results will be present here from data acquired in two

different sites in Ilha Terceira (Azores archipelago): one located at a High School, in

Angra do Heroísmo, and the other at S. Sebastião crater.

Key-words: array techniques, ambient vibrations, Neries, Geopsy, Dinver

1. Introduction

This work is being developed within Neries European Project (http://www.neries-eu.org/). One ofthe main objectives of the Joint research activity (JRA4) – Site Geotechnical Characterization - is todevelop a reliable low cost tool for geotechnical site characterization which means, estimate Vs 30profiles, usually used on EC8 for engineering purposes. The methodology was based on ambientvibration measurements using an array configuration. To process the data, the Sesarray package(http://www.geopsy.org/index.html), mainly developed by University Joseph Fourier (in Grenoble)and University of Potsdam, was used. This software includes Geopsy (for ambient vibrationanalysis) and Dinver (for data inversion) routines.To test the developed tool, five sites were selected, two in Portugal mainland and three in theAzores Archipelago (Terceira Island), to take into account different geological conditions. InPortugal mainland the sites were selected in the Lower Tagus Valley, which is mainly a largeplateau, slightly dipping to the river, composed by Miocene and Pliocene formations. This plateau,which is larger in the left margin than in the right margin, is covered by ceno-antropozoic soils.From top to bottom they are composed by recent alluvium, quaternary deposits of fluviatic terraces,Pliocene sandstone and conglomerates, and a thick layer of sandy clays and sandstone(Zbyszewski, 1953-1979). The Azores Islands have a volcanic origin. The main geologicalformations in Terceira Island are stiff and composed by trachyts and trachy-andesites. However,due to past eruptions, pumice and pyroclastic deposits may exist, with thickness up to 20 m.Preliminary results, of Vs 30 profiles, will be presented here, from data acquired in Ilha Terceira – Azores Archipelago, at an High School in Angra do Heroísmo (located over a stiff formation), and inS.Sebastião crater (filled with recent deposits).

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2. Materials and Methods

2.1. Field Measurements

The two experiments carried out consisted on measurements of ambient vibrations in circular arrayconfigurations. Eight stations were used, with one central station. The arrays were not perfectly

circular because of constrains at the field work. Measurements started with the smallest array andincreased the radius for the following arrays. The duration of the data acquisition in each arrayvaried from half an hour, for the smallest array, until two hours for the largest array (Cornou et al.,2006).We performed 4 arrays at the High School and 5 arrays in S.Sebastião. In figure1, it is possible tosee the position of the stations for the largest array performed in each site.

Figure 1 - Configuration of the largest arrays: a) at the High school; b) in S. Sebastião. In both sites theWA_WAUO1 is the central station

The equipment used was developed in previous European projects (SESAME, HADU) at theUniversity of Potsdam and made available for this experiment (Endrun & Renalier, 2008). Itconsists of 8 Lennartz 5 second three component seismometers, which are connected to EarthData digitizers. At each station, a Mesh Cube Linux System manages recording andcommunication with the other stations. Each station is powered with a 12Volts battery. Data aretransmitted in quasi-real time via Wi-Fi. The Wi-Fi connections need line-of-sight between anysingle station and at least one other station. However no central unit can contact all stationsdirectly, so each station has to act as a transmitter and at that same time as a recorder. Datapackage can consecutively be transferred between stations until they reach the Wi-Fi receiverwhich is connected to the field PC. One of the advantages of data transmission by Wi-Fi is thequick and comparatively easy (without laying cables) deployment of quite large arrays, even in anurban environment, as it happened at the high school in Angra do Heroísmo: we deployed thestations in two schoolyards that were separated by one building, which was impossible to do itwithout Wi-Fi system (figure 2).

Figure 2 - One of the schoolyards at Angra do Heroísmo.

a b

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Recordings were done with a sampling rate of 100Hz and a sensitivity of 2.5 µV/bit. GPS receiverswere included in each station to synchronise the clock of the recorders. Precise positiondetermination, however, as demanded for array analysis methods, needs the use of different GPSmeasurements, carried out with a Leica GPS 1200 system as reference station and another LeicaGPS 1200 as a Rover.This combinations leads to a relative accuracy in the centimetre-range, for

stations locations.On the field PC, all arriving data are archived via seed link and can be processed in near-real time,allowing for first evaluation of the data (Wathelet, 2005; Konno & Ohmachi, 1998; Ohrnberger etal., 2004).

2.2. Methodology

The ideal array would contain an infinite number of sensors perfectly sampling the whole space.Unfortunately, for economical and practical reasons we use discrete sampling. As the number ofsensors decrease, resolution and aliasing restrict the ability of the array to correctly measure thevelocity.At both sites 8 stations per array were used. The quality of the velocity profile estimation is afunction of the sampling in space. In Angra do Heroísmo, High school data was acquired using four

arrays. For the smallest array the distance between stations was around 5m and for the largestarray was around 100m. In S. Sebastião, the distances between stations was, for the smallestarray, around 5m and, for the largest array, around 200m.The first step consisted on the verification of the nature of the ambient vibration, to check if it wasnatural or artificial, and on the estimation of H/V. The H/V peak obtained gives the resonancefrequency of the soil layers beneath the sensors (figure 3).For estimating the array validity range, we did our approach using theoretical array response fromwhich the resolution and aliasing ranges were calculated in the wave number space (figure 4).Three methods can be used to extract the dispersion curves from ambient vibration signals(Wathelet et al., 2004; Wathelet, 2005; 2006):

- Frequency – Wave-number (FK)- High Resolution Frequency (HRFK)

- Spatial autocorrelation method (SPAC)

All of them are dedicated to vertical component according to their current implementation inGeopsy (http://www.geopsy.org).High Resolution FK is sometimes less affected and allows extending a bit the valid frequency rangetowards low frequencies. Some times in both, FK and HRFK, it is possible to see that there is morethan one mode. It was used a probability function, that estimates over the raw output of FK orHRFK the fundamental mode (Capon, 1969).In the absence of other means to confirm the velocity at low frequency and to avoid the inclusion ofbiased information into the inversion, the dispersion should be cut close to the lower limit of thevalid wave number range of the array. For this reason we used HRFK (figure 5).The autocorrelation computation was performed with SPAC routine (Kudo et al., 2001). The resultsof are given versus frequency. A dispersion curve can be clearly delineated for each array. They

are directly compared to the dispersion curve obtained with FK and HRFK methods. SPAC methodallows to successfully checking the dispersion curve obtained with FK methods, even at lowfrequency. SPAC results carry almost the same information as the dispersion curve obtained withFK methods (figure 6).Both results, from HRFK and SPAC, can be directly included in the inversion, using the Dinverroutine (Wathelet, 2008). It allows the inversion of FK and SPAC results either alone or together. Abi-modal dispersion curves determined by FK methods and crosschecked with SPAC methods isinverted to retrieve the shear wave velocity profile (Vs). A simple ground model is usually used inthe first approach (Figures 7 and 8).


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3. Results and Discussion

The results are presented in the following figures for the High school in Angra do Heroísmo and forS. Sebastião crater:

(i) H/V curve for the central station WA_WAUO1 (figure 3);

(ii) Theoretical array response for the smallest array – High school (figure 4);

(iii) Dispersion curve HRFK (figure 5);

(iv) Spatial autocorrelation curve plus HRFK dispersion curve (figure 6);

(v) Vs profile (figures 7 and 8).

Figure 3 – Average H/V curve with standard deviation. The grey area indicates the uncertainty of theresonance frequency. A)High school at Angra do Heroismo; B) S.Sebatião crater.

Figure 4 – Results from the smallest array at the High school: A - Theoretical array response in wavenumberspace. The small and large black circles are located at Kmin/2 and Kmax, respectively. B - Azimuthal cross

section of figure (A). C - Resolution and aliasing limits.

Kmin/2 =0.2507 Kmax =1.5161

AB C

A B


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Figure 5 - Dispersion curve for HRFK: A - High school; B - S. Sebastião crater.

Figure 6 - Spatial autocorrelation curve plus HRFK dispersion curve : A - High school; B - S. Sebastião crater.

Figure 7 - Dispersion Curve fit: A - High school; B - S. Sebastião crater.

BA


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Figure 8 – Vs profiles: A - High school; B - S. Sebastião crater.

For the High School (figure 8A) the ground profile shows a lack of resolution for the first 5m and

below 30m. Velocities start around 200m/s at the surface, and reach about 450m/s at 30m. The

model shows a LVZ below 50m, where velocities are close to 200m/s, but we are not sure about

the validity of these values because we are bellow of the “resolution range”. However, if these

values are true, this could be explained by the heterogeneity of the volcanic formations (that could

include holes or pyroclastic deposits, at higher depths, due to ancient volcanic eruptions).

For S. Sebastião the results we obtained are not satisfactory. It is possible to observe (figure 7B)

the bad fit between the dispersion curves obtained from the recording data and from the outputmodels. However, former studies (for instance, Lopes, 2005) present velocities below 200 m/s in

the upper 20m. More work is needed in order to understand this behavior.

4. Conclusions

The presented results are still preliminary. Crosscheck with available information and tests to the

consistency of the output, need to be performed. According to previous studies (Endrun & Renalier,

2008), a good agreement was found between the results of the ambient vibration array

measurements and the results obtained by other seismic methods (for instance, active seismic

measurements) in the frequency range covered. However, it is necessary to start the inversion with

a simple model of the site but close to the real structure. From our results it seems that the

structure beneath S. Sebastião is very complex and it is needed complementary information tobuild the starter model. In Angra do Heroísmo, the structure is simpler and the results seem

satisfactory.

Acknowledgements

The authors would like to thank to the Azores Civil Protection (SRPCBA – Protecção Civil dos

Açores) and Câmara Municipal de Angra do Heroísmo. Thanks are also due to Brigitte Endrun and

Daniel Vollmer, from University of Potsdam, for their valuable help. This work was financed by the

NERIES European project, I3, FP6, Contract No RII3-CT-2006-026130.


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References

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Endrun B. & Renalier F. (2008). Report on in-situ measurements at the 20 selected sites. JRA4 – D2, http://www.neries-eu.org/

Konno, K. & Ohmachi, T. (1998). Ground-motion characteristics estimated from spectral ratiobetween horizontal and vertical components of microtremors, Bull. Seism. Soc. Am., 88, 288-241.

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Wathelet M. (2006) Noise Blind test: Retrieving dispersion curves and inversion with the conditionalneighbourhood Algorithm, ESG 2006, Grenoble Sptember2006.

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Wathelet, M., Jongmans, D. & Ohrnberger, M. (2004). Surface-wave inversion using a directsearch algorithm and its application to ambient vibration measurements, Near Surf. Geophys., 2,211-221.

Zbyszewski, G., (1953-1979). Explanation notes of the geological maps at the scale 1:50 000 (in Portuguese ). Serviços Geológicos de Portugal, Lisbon.


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