1. Joint 3D relocation of earthquakes by surface and borehole data
Aldo Vesnaver & Lara Lovisa
Trieste, 7 October 2008
Annual Meeting of the Italian EAGE-SEG Section

2. Main topics Microseismicity caused by hydrocarbon production
and storage, and by CO2 sequestration
Wadati’s method in wells ...
... and joint 3D tomography by active and passive seismic,
from well and surface receivers

3. Method Voxels in irregular grids Ray tracing for direct, reflected,
refracted, diving and converted
waves

4. Method 1. Time Zero by Wadati’s method (or Least Squares)
2. Initial model from 3D seismic, well data and geology
3. Hypocentre estimate by “Shrinking Grids”
4. 3D tomography
5. Iterations of steps 3 and 4, until convergence

5. Method 1. Time Zero by Wadati’s method (or Least Squares)
2. Initial model from 3D seismic, well data and geology
3. Hypocentre estimate by “Shrinking Grids”
4. 3D tomography
5. Iterations of steps 3 and 4, until convergence

6. Wadati in a well ?

7. Wadati in a well ?

8. Wadati in a well ?

9. Wadati in a well ? Rp, Rs = Recorded P or S time
Tp, Ts = Actual P or S traveltimes
Vp, Vs = Average P or S velocity
To = Time Zero

10. Method 1. Time Zero by Wadati’s method (or Least Squares)
2. Initial model from 3D seismic, well data and geology
3. Hypocentre estimate by “Shrinking Grids”
4. 3D tomography
5. Iterations of steps 3 and 4, until convergence

11. Method - “Shrinking grids”

12. 3D recording geometry 225 receivers at surface 28 receivers in 2 wells
(Jervis and Dasgupta, 2006)
(Dasgupta, 2006)

13. 3D recording geometry Vertical arrays Area : 3 x 3 km
Maximum depth : km

14. Relocation – 1.5D model (“true” traveltimes, without Wadati) NonLinLoc
Hypo3D

15. Relocation – 3D model Missing (“true” traveltimes, without Wadati)
NonLinLoc
Hypo3D

16. Method 1. Time Zero by Wadati’s method (or Least Squares)
2. Initial model from 3D seismic, well data and geology
3. Hypocentre estimate by “Shrinking Grids”
4. 3D tomography
5. Iterations of steps 3 and 4, until convergence

17. Synthetic model
P velocity km
km/s

18. Reflection tomography
P velocity km
km/s

19. Active + passive tomography
P velocity km
km/s

20. Synthetic model
S velocity km
km/s

21. Reflection tomography
S velocity km
km/s

22. Active + passive tomography
S velocity km
km/s

23. Conclusions Wadati’s Method does NOT work in wells
and if the Vp/Vs ratio changes a lot
along the ray-paths of recorded events
Thus, surface and well data should be
jointly processed for the Time Zero
The joint 3D tomography of active + passive
seismic can better image the reservoir
Gualtiero (Walter) B