Locating Trapped Miners Using Time Reversal Mirrors

1. Locating Trapped Miners Using Time Reversal Mirrors Sherif M. Hanafy Weiping Cao Kim McCarter Gerard T. Schuster November 12, 2008

2. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

3. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

4. Motivation Problem: Miners are lost in a mine collapse, death could happens Proposed Solution: Time Reversal Mirror (TRM) with super resolution and super stacking properties

5. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

6. 3 Step RTM Methodology Step # 1: before the collapse • Geophones are planted on the ground surface above the mine. • Select some communication points inside the mine • From each communication point a band-limited natural Green’s function is recorded Receiver Line Ground Surface Subsurface Mine …………………………………… G1 Gn

7. 3 Step RTM Methodology Step # 2: get the SOS call After a collapse occurs, trapped miners should go to the nearest communication point and hit the mine wall at this point This (SOS) call will be recorded by the geophones on the ground surface Receiver Line Ground Surface Subsurface Mine G1 G2 G3

8. 3 Step RTM Methodology Step # 3: where are the trapped miner(s)? Does the recorded SOS looks like one of our previously recorded band-limited calibration Green’s functions? G1 G2 G3 ………. Gn NO NO Yes NO The location of the trapped miners is the location of the calibration Green’s functions that best match the recorded SOS We can use a pattern matching approach between the recorded SOS and the calibration Green’s function gathers Recorded SOS

9. 3 Step RTM Methodology Mathematically, better match means higher d & g dot product value Refers to the location of the communication point Band-limited Green’s function Dot product results Recorded SOS call Time Reversal Mirror equation Post Stack Migration

10. 3 Step RTM Methodology Location of trapped miners Subsurface Mine ……....... ………....... ………… G1 G2 G3

11. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

12. Field Examples U of U Test Tucson, AZ Test • Number of receivers = 120 @ 1m interval • Number of communication points = 25 • Comm. point interval; • Points 1- 6 & 20 – 25 = 4 m • Points 6 – 20 = 0.5 m • Distance from receiver line to tunnel = 35 m • Number of receivers = 120 @ 0.5 m interval • Number of communication points = 25 @ 0.5 & 0.75 m intervals • Distances from receiver-line to tunnel are 30 & 45 m, respectively

13. U of U Test Generating both Green’s function and SOS call

14. Tucson, Arizona Test Generating both Green’s function and SOS call

15. Dot Product Results Sample results from U of U and Tucson Tests Normalized m(x,0) Normalized m(x,0) Normalized m(x,0) X (m) X (m) X (m)

16. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

17. Unknown SOS Excitation Time We use a simple time shift test Amplitude Time Shift Excitation Time

18. Unknown SOS Excitation Time U of U Test Tucson, AZ Test Excitation Time Location of Trapped Miner 1 Normalized Amplitude 45.0 45.0 0.0 0.0 -1 Mine Depth = 35 m Mine Depth = 45 m -0.25 45 Time Shift (ms) X (m) 0.25 0

19. Low S/N ratio of the SOS call • We generated a random noise CSG • This random-noise CSG is added to the recorded SOS • The results are then used in our calculations + =

20. Results with Random Noise Normalized m(x,0) Normalized m(x,0) X (m) X (m) Normalized m(x,0) Normalized m(x,0) X (m) X (m)

21. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

22. Rayleigh Spatial Resolution Spatial resolution is defined by Sheriff (1991) as the ability to separate two features that are very close together, i.e., the minimum separation of two bodies before their individual identities are lost. 2L Ground Surface Z

23. Expected Spatial Resolution Rayleigh resolution 3 m 5 m 7.5 m

24. Can Scatterers Beat the Resolution Limit? Recorded shot gathers (SOS & G) are divided into: • Full aperture & direct arrivals - Full aperture & scattered arrivals • Half aperture & direct arrivals - Half aperture & scattered arrivals

25. Super-Resolution Results using traces with only • Direct waves, full aperture width • Direct waves, half aperture width • Scattered waves, full aperture width • Scattered waves, half aperture width • Spatial resolution of correlating traces with scatterer-only events is much higher. • Spatial resolution of correlating traces with direct-only events depends on the aperture width. X (m)

26. Expected Spatial Resolution Our approach shows a resolution 6 – 10 times better than the expected Rayleigh resolution limit. Rayleigh resolution Scatterers resolution 0.5 m 0.5 m 0.75 m

27. Outline • Motivation • RTM Methodology • Field Examples • Practical Problems • Super-resolution Tests • Summary and Conclusions

28. Summary and Conclusions • We have successfully introduced a TRM method to locate trapped miners in a collapsed mine • Two field tests are made to validate the proposed TRM method • Field tests show that TRM can successfully locate trapped miners with signal-to-noise ratio as low as 0.0005

29. Summary and Conclusions • Super Resolution • Using traces with scatterer only improve data resolution 6-10 times • Aperture width does not change the scatterer only results, while direct only waves is highly affected by the aperture width

30. Summary and Conclusions For the first time in EM waves, Lerosey et al. (2007) succeeded to get a resolution of /30 To the best of our knowledge, our work is the first time super-stack and super-resolution properties are validated with field seismic data.

31. Implication • Hydro-Frac Monitoring • Time reversal mirrors (TRM) approach has super stack property • No velocity model is required • Small aperture width gives good results • If we have the exact velocity model • Reverse time migration (RTM) has both super-stack and super-resolution properties. Increasing the RTM resolution by 3-7 times deserves the effort of finding the exact velocity model.

32. Thank You

33. Outline • Motivation and Introduction • Methodology • Field Examples • University of Utah test • Tucson, Arizona test • Practical Problems • Time shift test • Super-stack results • Trapped between two communication points • Two groups are trapped • Complex example • Super-resolution Tests • Summary and Conclusions

34. CP CP SOS SOS CP CP SOS SOS Miners are trapped between two CP Example from U of U test

35. Outline • Motivation and Introduction • Methodology • Field Examples • University of Utah test • Tucson, Arizona test • Practical Problems • Time shift test • Super-stack results • Trapped between two communication points • Two groups are trapped • Complex example • Super-resolution Tests • Summary and Conclusions

36. CP SOS 1 SOS 2 Two groups of miners sending SOS call Example from U of U test

37. Outline • Motivation and Introduction • Methodology • Field Examples • University of Utah test • Tucson, Arizona test • Practical Problems • Time shift test • Super-stack results • Trapped between two communication points • Two groups are trapped • Complex example • Super-resolution Tests • Summary and Conclusions