1

Xuyao Zheng

Institute of Geophysics of CEA

2

Outline

1. Motivation

2. Model and synthetic data

3. Calculation of Green functions

4. Pre-stack depth migration in arbitrary

anisotropy media

5. Conclusions

3

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

4

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

5

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.

water

anisotropic

anisotropic

isotropic

anisotropic isotropic

2.0

4.0

3.0

1.0

0.0

5.00.0 2.0 4.0 6.0 8.0 10.0

0.0

2.0

4.0

6.0

8.0

10.0

6

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.

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.

7

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

Table 2. Origin of gradient and the gradient of velocities and Thomsen

parameters in each layer.

8

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.

9

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.

10

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).

11

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.

12

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.

13

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.

14

Figure 8. Migration result from isotropy model.

15

• 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,

Conclusions

10 nodes (20 CPUs)

Synthetic seismograms

Green functions Migration

13 hours 5 hours 21 hours

16