1 Local Reverse Time Migration: Salt Flank Imaging by PS Waves Xiang Xiao and Scott Leaney 1 1 Schlumberger UTAM, Univ. of Utah Feb. 8, 2008

2 Outline Motivation Theory Numerical Tests Schlumberger VSP Data Set GOM VSP Data Set Conclusions Motivation TheoryNumerical Tests Conclusions

3 Outline Motivation TheoryNumerical Tests Conclusions Motivation Theory Numerical Tests Schlumberger VSP Data Set GOM VSP Data Set Conclusions

4 Standard P-to-S Migration x s m(x) ~ s ~ ds G(x|s) Forward source P P S g G(x|g)* D(g|s)dg Backward data S g * Converted wave VSP D(g|s) Motivation TheoryNumerical Tests Conclusions

5 Interferometric P-to-S Migration x s P P S D(g|g’) ~ s ~ ds * g’ g D(g’|s) D(g|s) m(x) ~ g’ ~ dg’dg g D(g|g’)G(x|g)G(x|g’) ** Virtual source gather Motivation TheoryNumerical Tests Conclusions

6 Outline Motivation TheoryNumerical Tests Motivation Theory Numerical Tests Schlumberger VSP Data Set GOM VSP Data Set Conclusions

7 x s P P S m(x) ~ s ~ ds g’ G(x|g’)* D(g’,s) dg’ Backward P G(x|g)* D(g,s)dg Backward S g g’ g * Local Reverse Time Migration Theory Motivation TheoryNumerical Tests Conclusions

8 Benefits Target oriented! Introduction Numerical Tests –Only a local velocity model near the well is needed. –Salt and overburden is avoided. –Fast and easy to perform. Source statics are automatically accounted for. Immune to salt-related interbed cross- talk. Theory Conclusions

9 Outline Introduction Numerical Tests Conclusion Motivation Theory Numerical Tests Schlumberger VSP Data Set GOM VSP Data Set Conclusions Theory

10 Depth (km) Offset (km) Schlumberger 2D Isotropic Elastic Model 0 Introduction Numerical TestsTheory Conclusions 291 shots 287 receivers

11 Depth (km) 10 0 Offset (km) (a) Ray tracing direct P (c) PPS events (d) Pp events (b) PSS events Depth (km) 10 0 Offset (km) Aperture by Ray Tracing Introduction Numerical TestsTheory Conclusions

12 Direct P PPS PSS Depth (km) Time (s) VSP CSG X-component VSP CSG Z-component 4 Depth (km) 8 4 Two-component VSP Synthetic Data Set Introduction Numerical Tests Conclusion Theory

13 Depth (km) Offset (km) (a) Standard Kirchhoff (c) Interferometric migration (IM)(d) Local RTM (b) Reduced-time migration (RM) Depth (km) Offset (km) Introduction Numerical Tests Conclusion Theory Comparison with Migration Methods

14 Outline Introduction Numerical Tests Conclusion Motivation Theory Numerical Tests Schlumberger VSP Data Set GOM VSP Data Set Conclusions Theory

15 Depth (m) Offset (m) GOM VSP Well and Source Location m offset Introduction TheoryNumerical Tests Conclusions 2800 m 3200 m Salt 82 receivers

16 P-to-S ratio = 2.7 Velocity Profile S Wave P Wave Depth (m) m 3200 m Salt Incorrect velocity model P-to-S ratio = 1.6 Introduction TheoryNumerical Tests Conclusions Velocity (m/s)

17 Z-Component VSP Data Depth (m) Traveltime (s) Salt Direct P Reflected P Reverberations Introduction TheoryNumerical Tests Conclusions

18 X-Component VSP Data Depth (m) Traveltime (s) Salt Direct P Reflected P ReverberationsDirect S Introduction TheoryNumerical Tests Conclusions

19 Processing Workflow Original Data Rotate components Pick desired events Median filtering Migration (KM, RM, IM, RTM) Introduction TheoryNumerical Tests Conclusions

20 Raypath Coverage Depth (m) Migration of PPS Salt Offset (m) Introduction TheoryNumerical Tests Conclusions 39 receivers

21 Migration of PPS Salt RMIM KM Depth (m) Offset (m) Introduction Numerical Tests Conclusion Theory Offset (m)

22 Migration of PPS Salt IM, sediment floodLocal RTM RM Depth (m) Offset (m) Introduction Numerical Tests Conclusion Theory Offset (m)

23 Conclusions Local RTM improves salt flank imaging. Introduction TheoryNumerical Tests Conclusions Imaging improvement is attained with a 1D velocity model for GOM data. Local RTM doesn’t suffer from the source statics and incorrect overburden and salt velocity model.

24 Separation While Imaging Step 1: Elastic backward propagation of the whole wavefield; Step 2: P- and S- wavefield separation; Devaney and Oristaglio (1986) Dellinger and Etgen (1990) Step 3: Crosscorrelate the P- and S- waves;

25 Thank you! Thank the sponsors of the 2005 UTAM consortium for their support.

26 Depth (m) Offset (m) Local RTM Image Introduction Numerical Tests Conclusion Theory IM 0200 Offset (m)

27 Future works Motivation TheoryNumerical Tests Conclusions

28 Depth (m) Offset (m) Local RTM Image Introduction Numerical Tests Conclusion Theory