1. Velocity and anisotropy parameter scan
Prestack Multi-parameter Scanning for
Average Vp/Vs and h in Multicomponent
Seismic Data
Christopher Ogiesoba*
Jim Gaiser**
Robert Stewart*
* University of Calgary
** WesternGeco, Denver
Christopher Ogiesoba

2. Outline Introduction to anisotropic velocity analysis
Review of traveltime equations
Validity and sensitivity tests
Numerical model and real data results
Conclusions, Future work, Acknowledgments

3. Introduction
Average Vp/Vs plays a crucial role in multicomponent seismic analysis
Some VTI parameters depend on it; for example:
But how should we recover it?

4. Traveltime Equation (1) (Tsvankin and Thomsen, 1994; Thomsen, 1999)
(2)
(3)
(4)

5. Traveltime Equation where depending on the value of
(5)
where
depending on the value of
But if is 1, then the traveltime equation becomes,
(6)

6. Validity of traveltime equation
Model Parameters
Thickness = 533m
Vp = 1600 m/s
Vs = 400 m/s
r = 2.2gm/cc
h =
Thickness = 427m
Vp = 2700 m/s
Vs = 900 m/s
r = 2.3 gm/cc
e =
d =
h =
Thickness = 1300m
Vp = 4000 m/s
e =
d =
Vs = 1429 m/s
r = 2.5 gm/cc
h =
Thickness = 1000m
Vp = 5500 m/s
e =
d =
Vs = 2500 m/s
r = 2.75 gm/cc
h =
e = 0.137
d =

7. Black line is an offset-depth ratio of one.
Validity of traveltime equation
Anivec Synthetic: Four Layer Anisotropic Model
P-wave
PS-wave 1
PS-wave 3
PS-wave 2
PS-wave 4 E
Black line is an offset-depth ratio of one.

8. Validity of traveltime equation
Comparison between Anivec Synthetic and Equation (2)
Pwave
PS-wave 1
PS-wave 3
PS-wave 2
PS-wave 4
Shot record from anisotropic Anivec Plot from equation (2)

9. Validity of traveltime equation
Comparison between Anivec Synthetic and Equation (2)
P-wave
PS-wave 1
PS-wave 3
PS-wave 2
PS-wave 4
ANIVEC
Time
(s)
Offset (m)
Shot record from anisotropic Anivec Plot from equation (2)

10. Sensitivity Test Traveltime curve at constant Vps and constant h
Constant Vps = 1675 m/s
Constant h = 0.2
g0 = 4.0
g0 = 2.8
g0 = 3.0
g0 = 2.2

11. Sensitivity Test Traveltime curves at constant Vps and constant g0
Constant Vps = 1675 m/s
Constant g0 = 2.2
h = -0.2
h = -0.1
h = 0.1
h = 0.2
Traveltime curves at constant Vps and constant g0

12. Velocity Analysis .
Vps

13. Vp/Vs Analysis: Summed over all h
2D semblance obtained for vertical velocity ratio

14. . Dual Parameter Scan: h and g0 Layer 1
g0, h = 2.75,
Timeslice at secs after rescaling colorbar

15. . Dual Parameter Scan: h and g0 Layer 2
g0, h = 3.75,
Timeslice at secs after rescaling colorbar

16. . Dual Parameter Scan: h and g0 Layer 3
g0, h = 3.6,
Timeslice at secs after rescaling colorbar

17. . Dual Parameter Scan: h and g0 Layer 4
g0, h = 3.4,
Timeslice at secs after rescaling colorbar

18. Model vs Scanned Parameters: h-g0 scan
Error analysis from model 3 results
Table showing error analysis in g0
Table showing error analysis in h

19. Results from the Blackfoot Area, Western Canada
Scanned Vp/Vs values range from 1.8 to 2.3

20. Conclusions
We can find h and g0 values from moveout analysis.
The derived equation is adequate to describe converted wave but scans slightly higher vp/vs values.
It is inaccurate at shallow depths where the offset to depth ratio is greater than 1.5
Accuracy increases with depth
Post-critical angle events degrade analysis from shallow levels.
We need to modify traveltime equation to make use of the far offset data

21. Future work
Improve traveltime equation to target the far-offset events.
Improve the algorithm so that velocity ratio-time log will be displayed while picking velocities.

22. Acknowledgements CREWES sponsors WesternGeco for a summer internship
CREWES personnel: Kevin Hall
Richard Bale
Linping Dong
Carlos Nieto
Dr. Charles Ursenbach
Dr. Mehran Gharibi

23. Thank you for your attention

24. Traveltime Equation
(1)
(2)
(Li, 2001)
(3)
(4)