1. 2015 Workplan for “VVAz” Analysis of Prestack Migrated Data
AASPI
2015 Workplan for “VVAz” Analysis of Prestack Migrated Data
Jie Qi and Kurt J. Marfurt
(The University of Oklahoma)

2. Outline Introduction Methodology Application Conclusion Motivation
AASPI
Introduction
Motivation
Challenge
Methodology
VVAz
AVAz
Azimuthal crosscorrelation
Application
Geology background
Azimuthal attributes
Conclusion

3. Motivation
Azimuthally limited or vector-tile gathers are part of the wide-azimuth processing workflow
Can we implement interpretation tools to provide residual AVAz analysis capabilities?
If the data were migrated using isotropic velocities, these residuals are a measure of VVAz
The application of such a tool would increase vertical resolution and precondition the data for subsequent AVAz analysis

4. Horizontal Transverse Isotropy (HTI) medium
(Left): No HTI anisotropy results in equal travel time paths in all azimuths;
(Right): In an HTI anisotropic media with aligned vertical fractures the travel time is azimuth dependent and is not equal in all directions.
(Courtesy of Close et. al., 2010)

5. Shear wave spitting in anisotropic medium
Receiver
VVAZ is based on azimuthal variations in stacking velocity(VNMO)
(Courtesy of Ed Garnero)

6. Fracture: (Photos courtesy of Brian Cardott) Woodford shale
Marcellus Shale
A key factor in the optimization of reservoir production
High natural fractures – high production
Helps to identify sweet spots
Anisotropic properties: intensity and orientation
Anisotropy analysis: amplitude and velocity
Left: fractured Woodford shale in the Arbuckle Mountains of southern oklahoma. Right: this is highly fracture shale from Marcellus.
Natural fractures enhance or create permeability in the reservoir.
(Photos courtesy of Brian Cardott)

7. Outline Introduction Methodology Application Conclusion Motivation
AASPI
Introduction
Motivation
Challenge
Methodology
VVAz
AVAz
Azimuthal crosscorrelation
Application
Geology background
Azimuthal attributes
Conclusion

8. Velocity vs. Azimuth (VVAz)
Advantages
Easy to generate azimuthally-binned data
Computation is fast and simple, providing a level of confidence
Requires phase- but not amplitude-preservation
Disadvantages
Suffers from vertical resolution problems associated with Dix’s equation
Amplitude vs. Azimuth (AVAz)
Advantages
Easy to generate azimuthally-binned data
Computation is fast and simple, providing a level of confidence
Computations are volumetric within the (properly registered) zone of interest
Disadvantages
Requires amplitude-preserving processing and migration (AVAz)
There are two ways to analyze anisotropy in seismic data. One is velocity vs azimuth; and one is amplitude vs azimuth. VVAz and AVAz are both easy to generate azimuthally-binned data

9. Velocity variation with angle and azimuth (VVAZ)
Vint(φ)=V0+εcos[2(φ- φsym)]
If ε is zero, it becomes interval velocity. θ N φ
φsym
VVAZ is based on azimuthal variations in stacking velocity(VNMO)
φsym

10. Amplitude vs. Azimuth (AVAz)
Amplitude vs. Offset (AVO)
Amplitude vs. Offset (AVO)
8. Attributes applied to offset- and azimuth-limited volumes
R(θ,φ)=A+{Biso }sin2θ
R(θ,φ)=A+{Biso }sin2θ
Amplitude vs. Azimuth (AVAz)
Amplitude vs. Azimuth (AVAz)
R(θ,φ)=A+{Biso+Banisocos[2(φ- φsym)]}sin2θ
R(θ,φ)=A+{Biso+Banisocos[2(φ- φsym)]}sin2θ θ N φ
φsym
The equation for amplitude vs. azimuth. Note that if Baniso=0 , we obtain the conventional AVO slope-intercept equations. (After Rueger, 1996).
(Rueger, 1996)

11. Workflows Conventional VVAz: AVAz:
Generate long-offset sectors or ’tiles’ at different azimuths φ (unmigrated)
At discrete picked horizons, compute VRMS as a function of azimuth, φ
Compute interval velocities Vint(φ) using Dix’s equation
Fit a sinusoidal curve to Vint(φ) to obtain the magnitude and azimuth of anisotropy
AVAz:
Generate long-offset sectors or ’tiles’ at different azimuths φ (migrated)
Pick discrete upper and lower horizons and generate either flattened or stratal slices throughout the volumetric zone of interest
At every time or depth sample, fit a sinusoid to the amplitude as a function of azimuth φ

12. Residual “VVAz” Workflow
Shot gathers
Prestack
Time migration
Azim 1 gathers
Azim 8 gathers ……
Azim 2 gathers
AVAz anisotropy
Baniso, ψaniso
Migrated gathers
Dynamic alignment
VVAz Anisotropy
Structure oriented filter
Dynamic alignment

13. …… Dynamic alignment Correlate adjacent azimuths
Find ∆τ and value of highest correlation coefficient
Autocorrelate & crosscorrelate
Azim 1
Azim 2
Azim 8
Time ……

14. Vint(φ)=V0+εcos[2(φ- φsym)]
Dynamic alignment
Least-squares fit ∆τ to find εaniso and φsym
Vint(φ)=V0+εcos[2(φ- φsym)]
Azimuth, φ
V0+εcos[2(φ- φsym)]
ε iso
ε aniso
φsym V0
V0 + ε
V0 - ε
Example of ellipse fitting on a single bin and time sample. Vertical axis is intensity and horizontal azimuth. (After Roende et al., 2008).
(Modified from Roende et al., 2008)

15. Dynamic alignment Stretch and squeeze data to provide flattened events
εaniso and φsym
Isotropic Layer 1
Anisotropic Layer 2
High anisotropy
Azimuthal data
Dynamic
alignment
Isotropic Layer 1
Anisotropic Layer 2
Isotropic Layer 1
Anisotropic Layer 2
Aligned data

16. Outline Introduction Methodology Application Conclusion Motivation
AASPI
Introduction
Motivation
Challenge
Methodology
VVAz
AVAz
Azimuthal crosscorrelation
Application
Geology background
Azimuthal attributes
Conclusion

17. Stratigraphic Cross Section
Ellenburger
Marble Falls
Stratigraphic cross section of the Fort Worth Basin. In the “Core” area of Wise and Denton Counties to the East, the Barnett Shale is subdivided into Upper and Lower units by the intervening Forestburg Lime. The calcite-rich geomechanically ductile Marble Falls and Viola Limestones from hydraulic fracture barriers. The Viola fracture barrier pinches out to the west, such that the Barnett Shale lies unconformably on top of the more brittle, dolomitic Ellenburger Group. The survey in the following figures is on strike with the area of Young County in this image. (After Pollastro et al., 2009)
Unconformity
(Modified from Pollastro et al., 2009)

18. Stacked azimuth sector gathers
Anisotropy indicators
CMP no.
CMP
398
CMP
399
CMP
400
0.5
0.6
Time (s)
After such migration, note the six azimuthally limited volumes align at the shallow (probably Caddo Lime) horizon but are misaligned at the deeper Barnett Shale target. (After Roende et al., 2008).
0.7
0.8
Data aligned
Data Misaligned
(Roende et al., 2008)

19. Amplitude as a function of azimuth (AVAz)
~3500 ft
0.6
0.8
~8 ms
Time (s)
0.7
stronger
weaker
Sectors from migration with a single velocity field, note the far field move out that contribute to azimuthal velocity variations of around 6.5 %. (After Roende et al., 2008).
(Roende et al., 2008)

20. AVAz products Intercept, A Isotropic gradient, Biso
Time (s)
0.4
1.0
Intercept, A
Top Marble Fall
Top Ellenburger
Isotropic gradient, Biso
Time (s)
0.4
1.0
Products of the AVAz workflow. (After Roende et al., 2008).
Anisotropic gradient, Baniso
Time (s)
0.4
1.0
(Modified from Roende et al., 2008)
Low
High
1 mile

21. Outline Introduction Motivation Challenge Methodology VVAz AVAz
AASPI
Outline
Introduction
Motivation
Challenge
Methodology
VVAz
AVAz
Azimuthal crosscorrelation
Conclusion

22. Anticipated Challenges
Will there be a clear correlation between AVAz and VVAz?
Can the residuals be computed gather by gather, or will layer-stripping become important?

23. Acknowledgements Marathon Oil Co. for a license to their survey
AASPI
Marathon Oil Co. for a license to their survey
Sponsors of the AASPI consortium for financial support and technical encouragement