introduction to seismic migration
TRANSCRIPT
Introduction to Seismic Migration
One-way traveltime
V=1 m/s
Homogeneous dipping planar reflector
One-way traveltime
V=1 m/s
Homogeneous dipping planar reflector
One-way traveltime
V=1 m/s
Homogeneous dipping planar reflector
Homogeneous dipping planar reflector
One-way traveltime
V=1 m/s
Stacked position= reflection position
Migrated position=true their subsurface location
Dipping reflections
More complex structure
DefinitionProcess 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
Migrated
sectionMigrationOperationVelocity
Objectives
• Moves dipping reflections to their true dip (up dip) and subsurface location
• Collapes diffraction• Un-tie bow-tie
Seismic Velocity
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
2,12,1
2
2,12,1
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
ii
n
ii
nrms
t
tVV
1
1
2int
2,
n
ii
n
ii
nave
t
tVV
1
1int
,
RMS velocity Average velocity
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
122
int
)1()()(
nn
nrmsnrms
tt
tnVtnVnV Vint
Vrms(n-1)
Vrms(n)
TWT
tn-1
tn
CDP
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]
Exercise-2
Semicircle superposition
Impulse response migration
Diffraction summation
Kirchhoff Migration
Huygens’s secondary source
Huygens traveltime curve
Kirchhoff Summation
xin
RMS
out PtrV
xP *)(
cos
2
•Obliquity• Spherical spreading•Wavelet shaping factor
)/,0,( vrtzxP in
)0,2/,( 0 tvzxPout
220 zxxr
Kirchhoff time and depth
Kirchhoff migration parameters
• Velocity• Aperture• Maximum dip
Migration velocitiesOvermigrated Undermigrated
ZO
Desired migration
2500 m/s
5 %
10 %
20 %
Test for velocity
Test for velocity
Migration velocities
Tests for maximum dip to migrate
a. ZO sectionb. Desired migrationc. 4 ms/traced. 24 ms/trace
c
d
Tests for maximum dip
Undermigration
Migration strategy (Yilmaz)
2D versus 3D migrationPost- versus post- migrationTime versus depth migration
Case Migration Case Migrationdipping 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
prestack migration
3D structure 3D migration
ZO versus stack /CMP stack section
1. Complex structure nonhyperbolic moveout
2. Conflicting dips
Pre-stack migration
Migration algorithm
• Integral solution to the scalar wave equation• Finite-difference solution• Frequency-wavenumber implementation: Stolt,
phase-shift/Gazdag1. Handle steep dips with sufficient accuracy2. Handle lateral and vertical velocity variations3. Be implemented, efficiently
Kirchhoff depth migration