Surface Wave Methods (Detyre)

8/2/2019 Surface Wave Methods (Detyre)

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Surface Wave Methods for Soil Characterisation

Objective:

Develop a technique non invasive cost and time effective ,to retrieve the shear modulus of the subsurface

Method

Based on dispersive behaviour of surfacewaves

The technique is carried out in three steps:

1- Generating and recording surface waves

2- Imaging the surface waves dispersion curve

3- Retrieving the shear wave velocities with depth by inversionof dispersion curve


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Gimbals

Field Testing

2.0 m5.0 m

DispersionInversion

Seismograph:

Geometrics 24 Traces ;


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Summary of main properties of Rayleigh waves

- Easily generated and detected on the ground surface;

- 2/3 of the total energy released by a vertical harmonic point

source acting on the surface of a homogeneous halfspace;

- Reduced geometrical attenuation (1/(sqrt r)) compared toother waves;

- The propagation involves only a limited depth ( ~ 1wavelength);

- In homogeneous linear elastic media: velocity of propagation

is almost equal to Vs and is not frequency dependant;

- In vertically heterogeneous media: dispersive behaviour, i.e.phase velocity is function of frequency.


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Extraction of dispersion curve from a shot gather by a doubletransformation of the data

i

pxj

ei

j

eixA

N

iCpU

),()(1

1),( =

=

Where

)(1),1

()(

j

exAC =

The largest values in the U(p,) wave field is exactly the dispersioncurve


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Inversion scheme

min

2)()( xxyAxyAx TT +


The objective is to find the best soil parameters from measured phase velocities

Initial model parameters:

- P wave velocity

- S wave velocity- density- layers thickness

Steps are

Synthetic dispersion curves calculationPhase velocity residuals calculationFind parameters minimizing the residuals

M0

Calculateddispersion curve

Minimizingcalculated - observed

A- partial derivative matrix

- a priori information on the model


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Vp (m/s) Vs (m/s) (g/cm3) h (m)

1500 700 1.2 5

1700 900 1.3 4

2300 1200 1.5 10

2500 1300 1.8

Shot gather Dispersion Diagram Dispersion curve fitting

Stack of linear elastic layers



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Vp variation 10 to 50% Density variation 10 to 50 %



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Equipments for surface wave methods



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Site d Orange

Shot gather Dispersion diagram



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100 1000Vitesse (m/s)

0

5

10

15

20

25

30

Profondeur(m

)

10 100E (bar)

SITE: TGV SE ORANGE DISPOSITIF:OR2FORAGE: SC137 - CH1 - CH2

Mesures rfraction

Vp

Vs

Mesures cross-hole

Vp

Vs

Mesures SASW

Vs=f(/3)

Vs

Pressiomtre

E

marne

sable&gravier

argile

limon

Calculated/observed

curve fitting



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Site de Jarry

Shot gather Dispersion diagram



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Site de Jarry

100 1000Vitesse (m/s)

0

5

10

15

Profondeur(m

)

SITE: Z.I. de JARRY DISPOSITIF:JAR2

FORAGE: SC1-SC2

Mesures SASW

Vs

Mesures rfraction

Vp

Vs

Mesures cross-hole

Vp

Vs

Calculated/observed

curve fitting



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Site Meung-sur-Loire


S f W M th d f S il Ch t i ti


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Surface Wave Methods for Soil Characterization


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Calculated/observed

curve fitting



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Site Meung-sur-Loire




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Surface wave (QR ) and shear wave (Qs) are parameters that have an important influenceupon the amplitude and duration of earthquark ground motions. First, we determined

the observed group velocity values using multiple filter analysis.

Shot record Dispersion diagram

Second, we estimated the anelastic factor ( ) by regression analysis of thelogarithm of the amplitudes of the group envelopes versus distance. The theoretical( ) values where computed using a velocity model and assuming a starting valueof Qs=Qp. By a trial and error procedure Qs was modified to obtained the best fitbetween theoretical and experimental( ) factors.



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Shot record

Inversion for shearvelocity (Vs)

Inversion for shearquality factor (Qs)



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Finals Remarks

- Surface waves methods are cost and time effective and allow foraccurate characterisation

- Non - Invasive method (hard to sample soils)

- Multisatation methods show several advantages ( near field effect, lowfrequencies, automatisation, stability, noise influence) with respect toSASW two-station approach.

- Multisation acquisition gives a chance to get an estimate of damping ( Qfactor)

- Higher modes are important in some sites ( the interpretation of the testbecomes more complex)

- Non uniqueness and resolution in Surface Waves Methods



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Imaging with surface waves

I i i h f


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Acquisition - Edition

Dispersion Analysis

DLMO Correction

CRG Collection

Staking

Wavefield separation

Scattered field Incident field

Migration

Data processing Procedure

I i ith f


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Tim

e(s)

Frequency(Hz)

I i ith f


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Dynamic linear move out

),(),( 0

2

fxUefxU fc

fxj

c

=

),(0 fxU Fourier transform to time axis of a shot gather

),( fxUc Fourier transform of DLMO cerrected shot gather

where

X - Distance from source

Cf Phase velocity for frequency f



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Tim

e(s)

Tim

e(s)

Shot pointDLMO correction



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Tim

e(s)

Temps(s)

Stacked section




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Scattered section


Migration



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Karstic conduit detection in Parc Floral Orlans la Source


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Surface Wave Methods (Detyre)