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 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 isotropic anisotropic isotropic

6 layerVp (m/s)Vs (m/s)epsilonDelta 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) xoZox-axisz-axisx-axisz-axisx-axisz-axisx-axisz-axis 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 functionsMigration 13 hours5 hours21 hours

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