1 Local Reverse Time Migration: Salt Flank Imaging by PS Waves Xiang Xiao and Scott Leaney 1 1 Schlumberger UTAM, Univ. of Utah Feb. 8, 2008

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




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)


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


6

Outline




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


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



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


Comparison with Migration Methods

14

Outline

Introduction Numerical Tests Conclusion



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


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

17

Z-Component VSP DataD

epth

(m

)

Traveltime (s)

2652

3887

1.2 3.0

Salt

Direct P

Reflected P

Reverberations


18

X-Component VSP DataD

epth

(m

)

Traveltime (s)

2652

3887

1.2 3.0

Salt

Direct P

Reflected P

Reverberations Direct S


19

Processing WorkflowOriginal Data

Rotate components

Pick desired events

Median filtering

Migration (KM, RM, IM, RTM)


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)


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)


Offset (m) Offset (m)

23

Conclusions

• Local RTM improves salt flank imaging.


• 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.

24

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;

25

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


IM

0 200Offset (m)

27

Future works


28

Dep

th

(m)

Offset (m)4878

0 1829

0

Local RTM Image


Documents

1 Local Reverse Time Migration: Salt Flank Imaging by PS Waves Xiang Xiao and Scott Leaney 1 1 Schlumberger UTAM, Univ. of Utah Feb. 8, 2008