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)
17
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.
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
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