Download - Introduction to Seismic Migration
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Introduction to Seismic Migration
One-way traveltime
V=1 m/s
Homogeneous dipping planar reflector
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One-way traveltime
V=1 m/s
Homogeneous dipping planar reflector
One-way traveltime
V=1 m/s
Homogeneous dipping planar reflector
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Homogeneous dipping planar reflector
One-way traveltime
V=1 m/s
Stacked position= reflection position
Migrated position=true their subsurface location
Dipping reflections
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More complex structure
Definition
Process which moves dipping reflections to their true subsurface position and collapes
diffractions
Process which reconstructs seismic image from stack section so that reflections and difractions are plotted at their true location
Stacked section
Migratedsection
MigrationOperationVelocity
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Objectives
• Moves dipping reflections to their true dip (up dip) and subsurface location
• Collapes diffraction
• Un-tie bow-tie
Seismic Velocity
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Seismic Velocity
• Instantaneous• Represents actual velocity
• Similar to the well log velocity
• Interval• Instantaneous velocity over a defined interval
• Root mean square (RMS)• Used during NMO and diffraction modeling
• Average• Total distance with a total traveltime
dt
dzVins
2
1
2
1
2
2,1
2,1
2
2,1
2,1
1
1
T
T
insins
T
T
insins
dttVT
TV
dttVT
TV
Tt
t
insrms dttVT
tV0
22 1
Tt
t
insave dttVT
TV0
)(1
)(
RMS and Average Velocity
n
i
i
n
i
i
nrms
t
tV
V
1
1
2
int2
,
n
i
i
n
i
i
nave
t
tV
V
1
1
int
,
RMS velocity Average velocity
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How to derive velocity
Pre-stack seismic gather stacking velocityVelocity analysis
RMS velocity
)cos(dipVV stackrms
Interval velocity
Dix equation
Dix Equation(Dix,1955)
Assumption• Horizontal planar reflectors
• Small offset
2/1
1
1
22
int
)1()()(
nn
nrmsnrms
tt
tnVtnVnV Vint
Vrms(n-1)
Vrms(n)
TWT
tn-1
tn
CDP
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Exercise-1
Compute RMS and average velocities at reflector B,C and D!
Z=1000 m
Z=2000 m
B
Vab=2000 m/s
Vcd=6000 m/s
Vbc=4000 m/s
C
D
A
Z=3000 m
Solution-1
Depth Vint DTi V_ave V_rms
1000 2000 0.5 2000.0 2000.0
2000 4000 0.25 2666.7 2828.4
3000 6000 0.167 3272.7 3618.1
V_aveV_rms
V_int
Velocity [m/s]
TWT
[s]
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Exercise-2
Semicircle superposition
Impulse response migration
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Diffraction summation
Kirchhoff Migration
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Huygens’s secondary source
Huygens traveltime curve
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Kirchhoff Summation
x
in
RMS
out PtrV
xP *)(
cos
2
• Obliquity• Spherical spreading• Wavelet shaping factor
)/,0,( vrtzxPin
)0,2/,( 0 tvzxPout
22
0 zxxr
Kirchhoff time and depth
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Kirchhoff migration parameters
• Velocity
• Aperture
• Maximum dip
Migration velocitiesOvermigrated Undermigrated
ZO
Desired migration
2500 m/s
5 %
10 %
20 %
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Test for velocity
Test for velocity
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Migration velocities
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Tests for maximum dip to migrate
a. ZO sectionb. Desired migrationc. 4 ms/traced. 24 ms/trace
c
d
Tests for maximum dip
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Undermigration
Migration strategy (Yilmaz)
2D versus 3D migration
Post- versus post- migration
Time versus depth migration
Case Migration Case Migration
dipping event time migration strong lateral velocity variations associated with complex overburden structure
depth migrationconflicting dips with different stacking velocities
prestack migration
3D behavior of fault planes and salt flanks
3D migration
complex nonhyperbolic moveout
prestackmigration
3D structure 3D migration
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ZO versus stack /CMP stack section
1. Complex structure nonhyperbolicmoveout
2. Conflicting dips
Pre-stack migration
Migration algorithm
• Integral solution to the scalar wave equation
• Finite-difference solution
• Frequency-wavenumber implementation: Stolt, phase-shift/Gazdag
1. Handle steep dips with sufficient accuracy2. Handle lateral and vertical velocity variations3. Be implemented, efficiently
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Kirchhoff depth migration
