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14.1
Map Migration
Map
Migration
Map Migration
What is map migration?
What forms does it take?
When should we use it?
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14.2
Map Migration
Depth conversion takes three forms:
Vertical scaling, for flat lying strata (or when the seismic
data have been depth migrated and scaled back to time) -
this is what we have considered so far.
Image ray migration for 3D time migrated data.
Normal incidence ray migration for stacked
(unmigrated) data or 2D grids of time migrated data - after
demigration of the picks.
Ray migration can take place in 2D as event migration on aline-by-line basis, or in 3D as map migration.
Map Migration
Depthscaling
Macro-
velocity
model
Normal
incidence
migrationDepth maps
Image
ray
migration
Seismic
horizontimes
WellVelocity
Z.O. or
image ray
modelling
Compare
SeismicVelocities
Analyticfunctions
Velocity
Maps
Calibrate
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14.3
Map Migration
Depth scaling works well in the presence of small structural
dips (less than 10) or when the seismic events are in their
correct lateral position from depth migration.
In the former case we need to know when depth scaling is
inadequate and event (2D) or map (3D) migration is required.
Depth Scaling
Map Migration
For 3D time migrated data the need for map migration is not
obvious since there are no misties in the data when we make
our interpretation. For such data image ray migration is
adequate for intermediate dips ( say 10 to 25).
For image ray maps, i.e. maps made from time migrated
sections, the need for map migration will become obvious in
the case of 2D data as misties manifest themselves in areasof steeper dip.
For 2D data the usual approach is to demigrate the
interpreted events, using migration velocities, in order to
create a normal incidence map which is then migrated.
Map Migration
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14.4
Map Migration
In the case of 2D data misties are caused by sideswipe.
Image ray migration will not overcome the problem, but
demigration of the 2D profiles to form a normal incidence
map, followed by map migration, will solve the problem.
Image ray lies within the
plane of the seismic
section.
Image ray does not
lie within the plane of
the seismic section.
Image Ray Bending
Seismic line
Reflector
Map Migration
Ray migration is a recursive process, i.e. we have to migrate
one layer at a time. The first layer, however, needs no
migration, the ray path is vertical. it is sufficient to depth scale
the image ray structure-in-time map.
We have to:
determine the direction of the image ray in the second
layer, according to Snells law, ray trace to find where it intersects the second horizon,
depth convert along the ray path and then remap the
second horizon.
The process is then repeated for horizon three, and so on.
Image Ray Migration
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14.5
Map Migration
We can estimate the need for map migration:-
Lateral positioning error is estimatedby:
xm - x t2Sin1(V1 - V2)(1 + 3Tan2)
Where
Tan1 V1dt1/dx1Tan VRMSdt2/dx2
For multiple layers:
Find 1 Tan-1( VA1dt1/dx1 )
1i = angle of incidence
Find angle of refraction 1r (Sin-1(V2Sin 1i /V1)
Displacement at 2nd layer V2t2tan(1r- 1i )
Find 2 Tan-1
( VA2dt2/dx2 )Angle of incidence 2i 2 + (1r- 1i )
Find angle of refraction 2r (Sin-1(V3Sin 2i /V2)
Etc.
x
t
2
1
V1
V2
xmx
t2
dt2
/dx2
dt1/dx1
Image Ray Migration
Map Migration
The need for image ray map migration :-
Two seismic
lines with their tie
positions
marked.
Data courtesy of Amoco
Image Ray Migration
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14.6
Map Migration
The need for image ray map migration :-
The two ties made
vertically showing
the misties.
One section is
pivoted about the tie
position until the
dipping horizons of
interest tie.
Data courtesy of Amoco
Image Ray Migration
Map Migration
Data courtesy of Amoco
Approximating map
migration:
One section is pivoted about
the tie position until the
dipping horizons of interest
tie.
The lateral displacement isannotated on the map.
The procedure is repeated
on the other line
Image Ray Migration
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14.7
Map Migration
The two measured displacements are the approximatecomponents of the Map Migration vector: -
Surface line
positionsDisplacement
Displacement
Approximate
Map Migration
Vector
Image Ray Migration
Map Migration
Do exercise 14.1 on the next page.
Image Ray Migration
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Map Migration
The seismic lines on the next two sheets tie at the location marked by the heavy line.
Use the technique described earlier to investigate the approximate map migration
vectors at different seismic reflection times.
Draw a series of sketches below to illustrate the vectors at the different depths.
Explain your observations.
Exercise 14.1
14.8
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Map MigrationExercise 14.1
14.9
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Map Migration
14.10
Exercise 14.1
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Map Migration
14.11
Exercise 14.1
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Map Migration
14.12
Exercise 14.1
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14.13
Map Migration
The need for image ray
migration is greatest where
we have a structure beneath
a strongly dipping
overburden which contains
high velocity contrasts.
1 km
2 km
1 km
2900
2600
4350
4700
5400
5200
5000
51004550Example after Larner et al, Depth migration of imaged
time sections, Geophysics, May 1981
Image Ray Migration
Map Migration
Migrated depth map
with displacement
vectors and the
before and after
fault positions.
From a Paradigm Geophysical brochure.
Image Ray Migration
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14.14
Map Migration
For normal incidence maps we can estimate the effects ofmigration as follows:-
x
z
dx
dz
t
Depth error
dz = dT / VRMSwhere dT is the vertical time
displacement given by
dT = T{1 - [1 - (VRMS2.tan2)/4]1/2}.
The horizontal displacement is
dx = (VRMS2T.tan)/4,
tan = T / x (time dip) from theunmigrated section,
T the two way time to the reflector.
Normal Incidence Migration
See Chun and Jacewitz 1981 for full details
Map Migration
As with image ray migration, normal incidence ray migration
is a recursive process, but this time, we need to migrate the
first horizon.
Lasthorizon?No
Time
picks
Velocities
Emergence
angle for
horizon
Time dip
for horizon
Migrate to
depth by
ray tracing
Next layer
Final model Yes
Normal Incidence Migration
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14.15
Map Migration
The rays are traced into a
layer for a distance given by
L = To x velocity / 2
starting at an emergence
angle given by
sin = time dip x Ve/ 2.
The local time dip of the
event is given by
tan = t / x
Ve is the velocity with whichthe ray emerges from the
layer. T
XV1
Vi = V0 + Kz
VO
t
xTo
To
Normal Incidence Migration
Map Migration
Migrated depth map with
displacement vectors and
the before and after fault
positions.
From Sattlegger GmbH brochure.
Normal Incidence Migration
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14.16
Map Migration
Ray Migration
Image ray migration is most sensitive
to errors in velocity, and hence errors
in angles.
Normal incidence migration is most
sensitive to the emergence angle
which depends on velocity gradient
and surface dip.
Map Migration
Summary
Vertical scaling is adequate when dips are less than 10.
Image ray map migration is adequate for dips between about
10 and 25 with 3D time migrated data.
For 2D time migrated data with dips over 10 and 3D time
migrated data with dips over ~25 use normal incidence map
migration.
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14.17
Map Migration
Summary
StabilityDip
Handling
HighLow ( 20)
NIRay
Migration
Map Migration
Map Migration
Integration
Analyticfunctions
/ Mapping
Depthscaling
Average,Interval,
InstantaneousMacro-
velocity
model
Image Ray
migration Depth maps
NI Raymigration
Seismic
horizon
times
Velocity log
Sonic logCheckshot
or VSP
Z.O. or
image ray
modelling
Compare
Seismic
Velocities
Dip,
Interpolation
Invert
(Dix/Bias)
Edit,
Smooth
Calibrate
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14.18
Map Migration
Summary
Stack
Conventional
Processing
Time
Migration
Stacking /
Time migration
Velocity
Calibrated
Velocities
Horizon
Picks
Well
Data
Macrovelocity
Model
Depth
Scale / MapMigrate
Forward
Model
Map Migration
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Map Migration
Exercise: compute the image ray bending.
In the following model the reflection times and time dips are observed on a time
migrated dip section at the flank of a salt dome and the interval velocities come
from a nearby well. You do not have map migration software available.
Should you consider map migration to depth convert the 4th event (the target
horizon)?
Hint: Find the average velocity down to each event, convert the time dips into
structural dips and then compute the lateral displacement of the image ray at
each interface.
0
720
1145
1315
1870
TWT msec
VI = 6760 ft/s
VI = 7246 ft/s
VI = 8130 ft/s
VI = 8889 ft/s
time dip = 400 msec over 280 traces
time dip = 400 msec over 230 traces
time dip = 400 msec over 220 traces
time dip = 500 msec over 30 traces
trace spacing = 75 ft.
Exercise 14.2
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