Xuyao Zheng
Institute of Geophysics of CEA
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Outline
1. Motivation
2. Model and synthetic data
3. Calculation of Green functions
4. Pre-stack depth migration in arbitrary
anisotropy media
5. Conclusions
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Motivation
• It is well-known that much of the Earth’s crust
appears to have some degree of seismic anisotropy
• Seismic anisotropy arises from partial alignment
of minerals, grains, macrocracks and regular
sequences of thin layers
• It would be better to describe the crustal material
by anisotropy than isotropy, but it needs much
more parameters
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Motivation
Goal: To get image of subsurface structures in 2-D
or 3-D arbitrary anisotropy media
Data: qP wave from 2-D or 3-D reflected profiles
Tool: Parallel computer is absolutely necessary
4
0.0
water
1.0
anisotropic
2.0
anisotropic
3.0
isotropic
anisotropic
isotropic
4.0
5.0
0.0
2.0
4.0
6.0
8.0
10.0
Figure 1. 2.5-D model with a size of 10×10×5 km consists
of 5 reflectors and 6 isotropic and anisotropic layers. The
anisotropic layers are laterally inhomogeneous VTI media.
Both shot and receiver intervals are 25 m. The maximum
offset is 2750 m.
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Table 1. Vertical velocities of P and S waves and Thomsen parameters
at the origin (Table 4) of each layer for VTI and isotropic media.
layer
Vp (m/s)
Vs (m/s)
epsilon
Delta
1
1500
0
0.
0.
2
1800
894
0.117
0.161
3
2800
1086
0.061
-0.087
4
4000
1673
0.
0.
5
4500
2683
0.056
0.120
6
5000
2915
0.
0.
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Table 2. Origin of gradient and the gradient of velocities and Thomsen
parameters in each layer.
layer
origin of gradient
(m)
gradient of Vp
(x/s/km)
gradient of Vs
(x/s/km)
gradient of ε
(1/km)
gradient of δ
(1/km)
xo
Zo
x-axis
z-axis
x-axis
z-axis
x-axis
z-axis
x-axis
z-axis
1
4000
100
0
0
0
0
0
0
0
0
2
4000
1500
0
360
0
360
0
0.002
0
0.003
3
500
2200
100
300
100
300
0.0003
0.001
0.0007
0.002
4
6000
3200
-50
500
-50
500
0
0
0
0
5
1000
3500
-100
200
-100
200
-0.0002
0.0004
-0.004
0.0008
6
1000
4900
0
0
0
0
0
0
0
0
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Figure 2. Distribution of vertical velocity (m/s) of P wave in six layers. The
first layer is water, the fourth and the sixth layers are isotropic media.
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Figure 3. Distribution of vertical velocity (m/s) of S wave in six layers. The
first layer is water, the fourth and the sixth layers are isotropic media.
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Figure 4. Distribution of Thomsen parameter ε in six layers. The first layer is
water, the fourth and the sixth layers are isotropic media in which the values
of ε are zero (red areas).
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Figure 5. Distribution of Thomsen parameter δ in six layers, the first layer is
water, the fourth and the sixth layers are isotropic media in which δ is zero
(green areas). The values of δ in the third layer are negative.
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Figure 6. Common shot gather seismograms obtained by finite differential
method. For each shot there are 110 receivers. The interval of two adjacent
receivers is 25 m and the maximum offset is 2750 m.
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Figure 7. Migration result from anisotropy model. The image of the fourth
reflector is not clear since the contrast of the medium above and below the
reflector is small. Image from multiple waves can be observed around the
second reflector.
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Figure 8. Migration result from isotropy model.
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Conclusions
• Made seismic data interpretation easier and more reliable
• High quality image of subsurface structures
• Parallel calculation is absolutely necessary for carrying
out pre-stack depth migration in 3-D anisotropic media,
10 nodes (20 CPUs)
Synthetic
seismograms
13 hours
Green functions
Migration
5 hours
21 hours
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