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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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Surface Wave Methods for Soil Characterisation
Gimbals
Field Testing
2.0 m5.0 m
DispersionInversion
Seismograph:
Geometrics 24 Traces ;
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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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 +
Surface Wave Methods for Soil Characterisation
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
Surface Wave Methods for Soil Characterisation
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Vp variation 10 to 50% Density variation 10 to 50 %
Surface Wave Methods for Soil Characterisation
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Equipments for surface wave methods
Surface Wave Methods for Soil Characterisation
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Site d Orange
Shot gather Dispersion diagram
Surface Wave Methods for Soil Characterisation
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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
Surface Wave Methods for Soil Characterisation
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Site de Jarry
Shot gather Dispersion diagram
Surface Wave Methods for Soil Characterisation
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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
Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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Surface Wave Methods for Soil Characterisation
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Site Meung-sur-Loire
Surface Wave Methods for Soil Characterisation
S f W M th d f S il Ch t i ti
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Surface Wave Methods for Soil Characterisation
Surface Wave Methods for Soil Characterization
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Surface Wave Methods for Soil Characterization
Calculated/observed
curve fitting
Surface Wave Methods for Soil Characterization
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Site Meung-sur-Loire
Surface Wave Methods for Soil Characterization
S f W M th d f S il Ch t i ti
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Surface Wave Methods for Soil Characterisation
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.
S f W M th d f S il Ch t i ti
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Surface Wave Methods for Soil Characterisation
Shot record
Inversion for shearvelocity (Vs)
Inversion for shearquality factor (Qs)
Surface Wave Methods for Soil Characterisation
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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
Surface Wave Methods for Soil Characterisation
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Imaging with surface waves
I i i h f
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Imaging with surface waves
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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Imaging with surface waves
Data processing Procedure
Tim
e(s)
Frequency(Hz)
I i ith f
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Imaging with surface waves
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
Imaging with surface waves
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Imaging with surface waves
Data processing Procedure
Tim
e(s)
Tim
e(s)
Shot pointDLMO correction
Imaging with surface waves
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Tim
e(s)
Temps(s)
Stacked section
Imaging with surface waves
Imaging with surface waves
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Scattered section
Imaging with surface waves
Migration
Imaging with surface waves
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Karstic conduit detection in Parc Floral Orlans la Source
Imaging with surface waves