PreStack Depth Migration Shafini's Group
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PreStack Depth Migration (PSDM)PreStack Depth Migration (PSDM)
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Where PSDM Stand?
PreStack Depth Migration
1. Demultiplex
2. Edit
3. F-K filtering
4. Sort
5. Elevation Statics
6. Deconvolution
7. NMO correction
8. Mute
9. Static corrections
10. Velocity analysis and stack
11. Additional F-K filtering
12. Filtering
13. Migration
14. Zero-phasing
15. Final filter for display
Typical Processing
PSDM
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Where PSDM Stand?
PreStack Depth Migration
Typical ProcessingPSDM
1. Demultiplex
2. Edit
3. F-K filtering
4. Sort
5. Elevation Statics
6. Deconvolution
7. NMO correction
8. Mute
9. Static corrections
10. Velocity analysis and stack
11. Additional F-K filtering
12. Filtering
13. Migration
14. Zero-phasing
15. Final filter for display
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PreStack Depth Migration
‣ A processing technique that moves seismic reflections to their correct locations in space.
‣ Critical in areas where there are significant and rapid lateral or vertical changes in velocity that distort images acquired in the time domain.
‣ Eg: The Gulf of Mexico and thrusted parts of the Rocky Mountains.
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Results of PSDM
PreStack Depth Migration
PSDM PSTM
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PreStack DepthMigration
PreStack Depth Migration
‣ PSDM is the current and cutting-edge method that is eagerly practiced
‣ The conventional method is PSTM
‣ PSDM is preferred as it can produce better quality image in complex structure (salt and fault complexity)
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PreStack Depth Migration Algorithms
Wavefield Extrapolation MigrationWavefield Extrapolation Migration
Kirchhoff PreStack Depth MigrationKirchhoff PreStack Depth Migration
Reverse Time Migration (RTM)Reverse Time Migration (RTM)
Common Azimuth Wavefield Extrapolation MigrationCommon Azimuth Wavefield Extrapolation Migration
Beam MigrationBeam Migration
Enhanced Migration Amplitude NormalisationEnhanced Migration Amplitude Normalisation
PreStack Depth Migration
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PreStack Depth MigrationPreStack Depth Migration
PSDM vs PSTMPSDM vs PSTM
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Prestack DEPTH migration Prestack TIME migration
• Done after PSTM• Has better imaging• More accurate• Can interpret complexity
structures • More interpretative
• Image is not clear enough• Better lateral resolution• Better coherency
PSDM is done after PSTM
Both needs high quality data
Comparison
PreStack Depth Migration
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PreStack Depth Migration
Midpoint (km) Midpoint (km)
PSTM PSDM
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PSTM using commercial processing system
PreStack Depth Migration
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Image of faults using PSDM
PreStack Depth Migration
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Over thrust model of PSTM
PreStack Depth Migration
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Over thrust model of PSDMPreStack Depth Migration
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PSTM PSDM
PreStack Depth Migration
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PreStack Depth Migration
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Advantages of PSDM
✓ Can be used at complex subsurface.
✓ Precise and accurate seismic solution.
✓ Clear expressions/images.
✓ Reduce structures positioning errors.
PreStack Depth Migration
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Disadvantages of PSDM
๏ More expensive compared to others.
๏ Time consuming.
๏ Sometimes cannot handle any reliable amplitude data.
๏ Requires a good geologically velocity model.
๏ Results are not really representative in highly complex tectonic.
PreStack Depth Migration
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PreStack Depth MigrationPreStack Depth Migration
Reverse TimeMigration
Reverse TimeMigration
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Pre-Stack Reverse Time Depth Migration
✓ Aims to construct an image of subsurface from reflection image recording
✓ Base on full wave seismic modelling on numerical grid.
✓ Made up 2 processes :+ seismic forward modelling+ reverse time modelling of shot record (time sample ➔ model scheme at their respective position as boundary condition)
PreStack Depth Migration
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QuickTime™ and aH.264 decompressor
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PreStack Depth Migration
Reverse Time Depth Migration Model
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PreStack Depth Migration
‣ High amplitude attribute in the respective subsurface location
‣ Down-going wave field from the source side and Up-going wave field towards the receivers at the same time
Forward-Reverse Modelling
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Earth
The Picture at Vertical Incidence
Reflection Coefficient
Acoustic Impedance
Z = Velocity X Density
R =Z - Z 12
Z + Z 121
Z = Z1 + R i
i + 1 1 - R ii
PreStack Depth Migration
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Forward Process
Earth ReflectionCoefficients
Wavelet WaveletSuperposition
Recorded Trace
Seismic Section
Impedance
PreStack Depth Migration
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Inverse Process
ReflectionCoefficients Wavelet
Seismic Section
Impedance Deconvolved Section
Integrated Section
PreStack Depth Migration
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Below shows reprocessing and RTM providing better imaging of the pre-salt data, and better imaging and truncations of the sediments on the salt flanks
Conventional Migration
RTM Migration
PreStack Depth Migration
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Conventional๏ Propagating data downward
through a velocity model into the earth
๏ Limit by structure
๏ Velocity field generate more complex arrival like prism wave that cause noise in image data
Reverse Time Migration๏ Propagate both events (downward and
upward) through earth model
๏ Explicitly handling turning wave and complex propagation wave
๏ Allow imaging of poor direct illumination of subsurface
๏ More accurate focusing, positioning and amplitudes in complex areas
๏ Improved imaging of complex plays- Steep dips- Complex overburdens, regardless of dip or rugosity
PreStack MigrationComparison
PreStack Depth Migration
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PreStack Depth Migration
Kirchhoff AlgorithmKirchhoff Algorithm
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What IsKirchhoff Algorithm
‣ Apply integral method
‣ Widely used in the industries
‣ Accept high angles : 90˚
‣ Accept turning waves (more than 90˚)
‣ Preserves amplitude
PreStack Depth Migration
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The Equation ofKirchhoff Algorithm
PreStack Depth Migration
V (M )= w(ξ,M )U•(
Ω∫∫ ξ,τD (,M ))dξ1dξ2
V (M )=Migrated Image at point M
w(ξ,M ) =Migration Weighting Function
U•
(ξ,τD (,M )) =Time−differentiated seismic data
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The Use OfKirchhoff Algorithm
‣ to better repositioning of data
‣ to better imaging accuracy / repositioning
‣ improve seismic image at steeply dipping subsurface structures
PreStack Depth Migration
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Source Receiver
CommonMid-Point
Simplifying Flat
ReflectionPoint
Bedrock
PreStack Depth Migration
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CommonMid-Point
Sometimes, Not Flat!
Bedrock
PreStack Depth Migration
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The Migration
PreStack Depth Migration
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The Upside
PreStack Depth Migration
✓Enhance image quality
✓Distinctive subsurface structures
✓Fidelity of the algorithm
✓Good on handling lateral velocity changes
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The Downside
Migration Noise
PreStack Depth Migration
๏ Migration artifacts
๏ Migrate non-primary
๏ Damage primary reflections
๏ Sensitive to errors in migration velocity
๏ Time consuming (3D)
๏ High cost
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PreStack Depth MigrationPreStack Depth Migration
Wavefield ExtrapolationMigration
Wavefield ExtrapolationMigration
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★ A new generation of more advanced algorithms
★ Base on Wave Equation Migration (WEM)
★ Improvement over Kirchhoff method in image quality
★ Implement differential solutions base on downward extrapolation
PreStack Depth Migration
What Is W.E.M
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PreStack Depth Migration
Superior Image of W.E.M
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Concept of W.E.M
PreStack Depth Migration
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PreStack Depth Migration
Finite-difference Explicit solutions
Implicit solutionsSplit-step Fourier
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Concept of W.E.M
PreStack Depth Migration
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Single-arrival Kirchhoff impulse response
Shot profile migration of WEM
PreStack Depth Migration
Which one is ‘which’?
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Propagation Effects
Diffraction, multiple arrivals, amplitude decay in shadow zone
PreStack Depth Migration
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Advantages
• Can tolerate irregular acquisition geometry
• Accurate dips handling up to 90° showing superior imaging
• High quality at low frequency
• Cost effective
PreStack Depth Migration
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Can tolerate irregular acquisition geometry
Detailed interpretationof an intensely faulted base salt event
PreStack Depth Migration
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Advantages
• Can tolerate irregular acquisition geometry
• Accurate dips handling up to 90° showing superior imaging
• High quality at low frequency
• Cost effective
PreStack Depth Migration
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PreStack Depth Migration
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Advantages
• Can tolerate irregular acquisition geometry
• Accurate dips handling up to 90° showing superior imaging
• High quality at low frequency
• Cost effective
PreStack Depth Migration
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PreStack Depth Migration
Kirchhoff Migration W.E.M
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Advantages
• Can tolerate irregular acquisition geometry
• Accurate dips handling up to 90° showing superior imaging
• High quality at low frequency
• Cost effective
PreStack Depth Migration
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Limitation of WEM
๏ Relied most on Velocity model
๏ Expensive yet preferable
PreStack Depth Migration
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PreStack Depth MigrationPreStack Depth Migration
Case Study (1)Case Study (1)
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Mona Lisa
• Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis (MONALISA)
• Pre-Permian sedimentary basins (SE North Sea) have been previously interpreted from potential field data but only poorly imaged on seismic sections obtained.
• It is due to the presence of salt layers and a thick Mesozoic and Cenozoic cover
PreStack Depth Migration
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North Sea
PreStack Depth Migration
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Solutions
๏ Reprocessing using pre-stack depth migration of sections of MONA LISA deep seismic has successfully imaged pre-Permian sequences whose exploration in the North Sea has been historically unknown.
๏ The difficulties in seismic imaging, are such as the overburden geology, and the presence of Zechstein salt.
PreStack Depth Migration
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Seismic Before and After PSDM
PreStack Depth Migration
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Interpretation from PSDM data
PreStack Depth Migration
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PreStack Depth Migration
The interpreted section, ‣Dark grey: Mesozoic section White: Pre-Permian basinLight grey: basement
Interpretation from PSDM data
The Boundaries,NBR: near basement reflectionPU: Permian unconformity
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Summary
✓ The standard processing at lithospheric scale features can lead to degradation of the seismic information in the upper crust.
✓ Reprocessing with PSDM of the upper crustal sections seismic gives considerable amounts of new information such as their lithology boundaries and sections.
PreStack Depth Migration
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PreStack Depth MigrationPreStack Depth Migration
Case Study (2)Case Study (2)
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PreStack Depth Migration
PSDM method applied:
✓ Kirchhoff PreSDM
✓ W.E. Migration
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Figure 1: Field layout of the 3D seismic survey conducted over the salt structure.
N
PreStack Depth Migration
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Early procedures
Gravity survey detected onshore salt body.
2D seismic survey conducted-obtain more subsurface information
Wells are drilled, oil & gas producing sands were discovered from depths
within 3000-10000 ft.
PreStack Depth Migration
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Figure 2: 3D prestack time migration (preSTM) section of a crossline, located near the center of the survey area.
The subsalt reflections are poorly focused and not correctly positioned spatially by the preSTM.
PreStack Depth Migration
Figure 2: PSTM
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Figure 3: Kirchhoff preSDM image of the same crossline
Figure 2: PSTM
PreStack Depth Migration
PSTM Kirchhoff PSDM
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Figure 4: The salt flanks exhibit a small improvement
compared to the Kirchhoff image.
PreStack Depth Migration
Figure 3: Kirchhoff PSDM
Kirchhoff W.E.M
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The Kirchhoff preSDM of an inline section, highlighting the subsalt depth interval of between 11,900 and 25,500 feet.
A more enhanced image of the subsalt structure. It is apparent that there is structure beneath the salt body forming a rollover or structural high.
PreStack Depth Migration
Kirchhoff W.E.M
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The Kirchhoff preSDM image of a crossline shows a very good image of the base salt
PreStack Depth Migration
Kirchhoff PSDM
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The W.E. preSDM image shows not only a good base-of-salt image, but also better defined subsalt reflections.
PreStack Depth Migration
W.E.M PSDM
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Summary• The Kirchhoff preSDM method will continue to be
the preferred choice for prestack depth imaging.
• Need to understand the capabilities and limitations of each PSDM method.
• Wave equation method, is not as a replacement, but as a complementary technology to Kirchhoff.
PreStack Depth Migration
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PreStack Depth MigrationPreStack Depth Migration
ConclusionConclusion
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Conclusion
✓ Production depth imaging combines established Kirchhoff technology with new-generation wavefield extrapolation migrations.
✓ The two technologies are complementary.
PreStack Depth Migration
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PreStack Depth Migration
Q & A