1. Time-reversal acoustics, virtual source imaging, and seismic interferometry Haijiang Zhang University of Science and Technology of China

2. Time-reversal acoustics Fink, 1997

4. Time-reversal and super resolution More heterogeneous and scattered the medium between the source and the TRM, the more focused the time- reversed signal.

5. Time-reversed seismic location –No picking required –Automatic (real-time) location –Using full waveforms

6. Virtual source method Bakulin and Calvert, 2006 Sources S k on the surface Receivers in the borehole The goal is to construct seismic record from virtual source to receivers.

7. Synthetic velocity models

8. Virtual source gather (at 900 m) Real source gather (at 900 m)

9. Constructing virtual source (VS) data Bakulin and Calvert, 2006

10. Problem: Overburden+statics defocus VSP migration Redatum sources below overburden Local VSP migration Solution: VSP -> SWP Transform (Calvert, Bakulin) ApplicationsVSPVSPSWP Shuster, SEG 09

11. VSP Geometry Offset (m) 0 1000 Depth (m) (m) 1500 3500 Time (s) 0 3 Reflectionwavefield (He, 2006)

12. VSP Geometry Offset (m) 0 1000 Depth (m) (m) 1500 3500 (He, 2006) Time (s) 0 3 Reflectionwavefield superresolution China

13. VSP Salt Flank Imaging (Hornby & Yu, 2006) ? 98 geophones 120 shots Overburden Poor image of flank by standard migration

14. Interferometric Migration Result

16. Seismic Interferometry Interferometry is a family of techniques in which waves, are superimposed in order to extract information about the waves (from wikipedia). Seismic Interferometry involves cross correlating and stacking diffusive waves to extract Greens functions.

17. Cross-correlation and stacking G sa (t): Greens function from source s to a. e(t): Source time function. Cross-correlation Stacking: source averaging

18. Homogeneous medium Circular sources a)τ= -3T 0 b)τ= -T 0 c)Τ=10T 0 Larose et al., 2005

19. Homogeneous medium 800 sources (partially covered) a)τ= -3T 0 b)τ= -T 0 c)Τ=10T 0

20. Heterogeous medium 800 sources (partially covered) a) first-part coda b) late coda

21. Time-reversal interpretation of the correlation process Treat a as the virtual source

22. (Weaver and Lobkis, Ultrasonics, 40, 435-439, 2002)