Local MVA for VSP Data Progress Report
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Transcript of Local MVA for VSP Data Progress Report
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Local MVA for VSP Data
Progress ReportSanzong Zhang, Xin
Wang and Xian XiaoJan. 7, 2010
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Outline Local VSP
Migration Local VSP MVA Challenges Numerical
Examples Future Work Acknowledgements
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Local VSP Migration
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Conventional VSP Migration
s
x
g
BackwardForward
s
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Defocusing in VSP Migration
s
x
g
Errors in the overburden
and salt body velocity model
Defocusing
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Limitations in VSP Migration
Overburden or salt velocity model is required, but hard to build.
Errors due to imperfect velocity models.
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Local VSP Migration
s
x
R(g’|s)
g’
T(g|s) g
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(1) Crosscorrelation imaging condition
Imaging Condition
(2) Deconvolution imaging condition
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(a) VSP data: P(g|s)=T(g|s)+R(g|s)
T(g|s)
s
gR(g|s)
x
s
(b) Backward reflection
R(g|s)g
x
R(x|s)= G(x|g)*R(g|s)g
(c) Backward transmission
T(g|s)
s
g
x
T(x|s)= G(x|g)*T(g|s)
g
(d) Crosscorrelation
m(x)= R(x|s)*T(x|s)s
R(g|s)g
x
Steps of Local VSP Migration
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Benefits Local VSP migration is oriented to our
target . Only a local velocity model near the well is needed.
Complex overburden and salt body are avoided.
Source statics are automatically accounted for.
Immune to salt-related interbed cross-talk.
Fast and easy to perform.
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Local MVA for VSP Data
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Local VSP MVA (LVM) LVM combines VSP migration and velocity
model updating
LVM is based on the local VSP migration obtained by using reflected and transmitted waves.
Depth residuals from common image gathers (CIGs) are transferred to traveltime residuals.
Traveltime tomography is used to update the local velocity model near the well.
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Challenges in Local VSP MVA
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Comparison of Three Migration Methods in Local
VSP MVA.
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CIGs using the background velocity model 2000m/s.
Depth Residuals
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Numerical Examples
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Sigsbee P-wave Velocity Model m/s
0
Dep
th (
km
)
9.2
4500
1500-12.5 12.5Offset (km)
279 shots, interval of 45.7m
150 receivers,Interval of
30m
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Pressure component of a common receiver gather for the receiver at the depth of 4.6 km
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Local VSP Migration Results4.6
9.2
Dep
th (
km)
-3 3Offset (km)
True modelMigration image
f = fault
f
d
d
d = diffractor
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Offset (m)
Marine 2D Offset VSP dataD
ep
th
(m)
48780 1829
0Source @150 m offset
2800 m
3200 m
Salt
82 receivers with 15.3-m interval
@600 m offset @1500 m offset
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Velocity Profile
4500
P Wave
Dep
th
(m)
0
0 5000
2800 m
3200 m
Salt
Incorrect velocity model
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
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150 m offset
3.3
3.9
0 100
Dep
th (
km)
Offset (m) 0 100Offset (m)
Without deconvolution
With deconvolution
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600 m offset
3.3
4.4
0 600
Dep
th (
km)
Offset (m) 0 600Offset (m)
Without deconvolution
With deconvolution
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1500 m offset
3.3
4.4
0 600
Dep
th (
km)
Offset (m) 0 600Offset (m)
Without deconvolution
With deconvolution
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Future work Method to recognize minor
depthresiduals
Interactive velocity updating based
on time residual tomography
Conversion between depth residual
and time residual
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Acknowledgements
Thanks to the 2009 sponsors of UTAM Consortium for their support. Thanks to Jerry for providing me excellent working conditions at KAUST. Thanks to Xian Xiao for providing me his data and code.