1 Local Reverse Time Migration: Salt Flank Imaging by PS Waves Xiang Xiao and Scott Leaney 1 1...

1

Local Reverse Time Migration: Salt Flank Imaging by PS

Waves

Xiang Xiao and Scott Leaney1

1Schlumberger

UTAM, Univ. of Utah

Feb. 8, 2008

2

Outline

Motivation Theory Numerical Tests

Schlumberger VSP Data Set GOM VSP Data Set

Conclusions

Motivation Theory Numerical Tests Conclusions

3

Outline

Motivation Theory Numerical Tests Conclusions

Motivation Theory Numerical Tests

Schlumberger VSP Data Set GOM VSP Data Set

Conclusions

4

Standard P-to-S Migration

x

s

m(x) ~ s

~ ds G(x|s)

Forward source P

P

S

g

G(x|g)* D(g|s)dg

Backward data S

g

*

Converted wave VSP

D(g|s)

Motivation Theory Numerical Tests Conclusions

5

Interferometric P-to-S Migration

x

s

P

P

S

D(g|g’) ~

s

~ ds*

g’

g

D(g’|s) D(g|s)

m(x) ~ g’

~ dg’dgg

D(g|g’)G(x|g) G(x|g’)* *

Virtual source gather

Motivation Theory Numerical Tests Conclusions

6

Outline

Motivation Theory Numerical Tests

Motivation Theory Numerical Tests

Schlumberger VSP Data Set GOM VSP Data Set

Conclusions

Conclusions

7

x

s

P

P

S

m(x) ~ s

~ dsg’

G(x|g’)* D(g’,s) dg’

Backward P

G(x|g)* D(g,s)dg

Backward S

g

g’

g

*

Local Reverse Time Migration Theory

Motivation Theory Numerical Tests Conclusions

8

Benefits

• Target oriented!

Introduction Numerical Tests

– Only a local velocity model near the well is needed.

– Salt and overburden is avoided.

– Fast and easy to perform.

• Source statics are automatically accounted for.

• Immune to salt-related interbed cross-talk.

Theory Conclusions

9

Outline

Introduction Numerical Tests Conclusion

Motivation Theory Numerical Tests

Schlumberger VSP Data Set GOM VSP Data Set

Conclusions

Theory

10

Dep

th

(km

)

Offset (km)

10-12 12

0

Schlumberger 2D Isotropic Elastic Model

0

Introduction Numerical TestsTheory Conclusions

291 shots

287 receivers

11

Dep

th

(km

)

10

0

Offset (km)-12 120

(a) Ray tracing direct P

(c) PPS events (d) Pp events

(b) PSS events

Dep

th

(km

)

10

0

Offset (km)-12 120

Aperture by Ray Tracing

Introduction Numerical TestsTheory Conclusions

12

Direct PPPS

PSS

Dep

th

(km

)

Time (s)

8

0 6

VSP CSG X-component

VSP CSG Z-component4

Dep

th

(km

)

8

4

Two-component VSP Synthetic Data Set

Introduction Numerical Tests ConclusionTheory

13

Dep

th

(km

)

8.5

6

Offset (km)0 1.8

(a) Standard Kirchhoff

(c) Interferometric migration (IM) (d) Local RTM

(b) Reduced-time migration (RM)

Dep

th

(km

)

8.5

6

Offset (km) 1.80

Introduction Numerical Tests ConclusionTheory

Comparison with Migration Methods

14

Outline

Introduction Numerical Tests Conclusion

Motivation Theory Numerical Tests

Schlumberger VSP Data Set GOM VSP Data Set

Conclusions

Theory

15

Dep

th

(m)

Offset (m)4878

0 1829

0

GOM VSP Well and Source LocationSource @150 m offset

Introduction Theory Numerical Tests Conclusions

2800 m

3200 m

Salt

82 receivers

16

P-to-S ratio = 2.7

Velocity ProfileS WaveP Wave

Dep

th

(m)

0

45000 5000 0 5000

2800 m

3200 m

Salt

Incorrect velocity model

P-to-S ratio = 1.6

Introduction Theory Numerical Tests Conclusions

Velocity (m/s) Velocity (m/s)

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Z-Component VSP DataD

epth

(m

)

Traveltime (s)

2652

3887

1.2 3.0

Salt

Direct P

Reflected P

Reverberations

Introduction Theory Numerical Tests Conclusions

18

X-Component VSP DataD

epth

(m

)

Traveltime (s)

2652

3887

1.2 3.0

Salt

Direct P

Reflected P

Reverberations Direct S

Introduction Theory Numerical Tests Conclusions

19

Processing WorkflowOriginal Data

Rotate components

Pick desired events

Median filtering

Migration (KM, RM, IM, RTM)

Introduction Theory Numerical Tests Conclusions

20

Raypath Coverage

2000

4200

0 200

Dep

th

(m)

Migration of PPS

Salt

Offset (m)Introduction Theory Numerical Tests Conclusions

39receivers

21

Migration of PPS

Salt

RM IM

0 200 0 200

KM

2000

4200

0 200

Dep

th

(m)

Offset (m)

Introduction Numerical Tests ConclusionTheory

Offset (m) Offset (m)

22

Migration of PPS

Salt

IM, sediment flood Local RTM

0 200 0 200

RM

2000

4200

0 200

Dep

th

(m)

Offset (m)

Introduction Numerical Tests ConclusionTheory

Offset (m) Offset (m)

23

Conclusions

• Local RTM improves salt flank imaging.

Introduction Theory Numerical Tests Conclusions

• Imaging improvement is attained with a 1D velocity model for GOM data.

• Local RTM doesn’t suffer from the source statics and incorrect overburden and salt velocity model.

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Separation While Imaging

• Step 1: Elastic backward propagation of the whole wavefield;

• Step 2: P- and S- wavefield separation; Devaney and Oristaglio (1986)Dellinger and Etgen (1990)

• Step 3: Crosscorrelate the P- and S- waves;

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Thank you!

• Thank the sponsors of the 2005 UTAM consortium for their support.

26

Dep

th

(m)

Offset (m)4878

0 200

0

Local RTM Image

Introduction Numerical Tests ConclusionTheory

IM

0 200Offset (m)

27

Future works

Motivation Theory Numerical Tests Conclusions

28

Dep

th

(m)

Offset (m)4878

0 1829

0

Local RTM Image

Introduction Numerical Tests ConclusionTheory