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PreStack Depth Migration (PSDM)PreStack Depth Migration (PSDM)

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

Where PSDM Stand?


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


‣ 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.

Results of PSDM


PSDM PSTM

PreStack DepthMigration


‣ 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)

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 MigrationPreStack Depth Migration

PSDM vs PSTMPSDM vs PSTM

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



Midpoint (km) Midpoint (km)

PSTM PSDM

PSTM using commercial processing system


Image of faults using PSDM


Over thrust model of PSTM


Over thrust model of PSDMPreStack Depth Migration

PSTM PSDM


Advantages of PSDM

✓ Can be used at complex subsurface.

✓ Precise and accurate seismic solution.

✓ Clear expressions/images.

✓ Reduce structures positioning errors.


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.



Reverse TimeMigration

Reverse TimeMigration

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)


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Reverse Time Depth Migration Model

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‣ 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

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


Forward Process

Earth ReflectionCoefficients

Wavelet WaveletSuperposition

Recorded Trace

Seismic Section

Impedance


Inverse Process

ReflectionCoefficients Wavelet

Seismic Section

Impedance Deconvolved Section

Integrated Section


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


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



Kirchhoff AlgorithmKirchhoff Algorithm

What IsKirchhoff Algorithm

‣ Apply integral method

‣ Widely used in the industries

‣ Accept high angles : 90˚

‣ Accept turning waves (more than 90˚)

‣ Preserves amplitude


The Equation ofKirchhoff Algorithm


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

The Use OfKirchhoff Algorithm

‣ to better repositioning of data

‣ to better imaging accuracy / repositioning

‣ improve seismic image at steeply dipping subsurface structures


Source Receiver

CommonMid-Point

Simplifying Flat

ReflectionPoint

Bedrock


CommonMid-Point

Sometimes, Not Flat!

Bedrock


The Migration


The Upside


✓Enhance image quality

✓Distinctive subsurface structures

✓Fidelity of the algorithm

✓Good on handling lateral velocity changes

The Downside

Migration Noise


๏ Migration artifacts

๏ Migrate non-primary

๏ Damage primary reflections

๏ Sensitive to errors in migration velocity

๏ Time consuming (3D)

๏ High cost


Wavefield ExtrapolationMigration

Wavefield ExtrapolationMigration

★ 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


What Is W.E.M


Superior Image of W.E.M

Concept of W.E.M



Finite-difference Explicit solutions

Implicit solutionsSplit-step Fourier

Concept of W.E.M


Single-arrival Kirchhoff impulse response

Shot profile migration of WEM


Which one is ‘which’?

Propagation Effects

Diffraction, multiple arrivals, amplitude decay in shadow zone


Advantages

• Can tolerate irregular acquisition geometry

• Accurate dips handling up to 90° showing superior imaging

• High quality at low frequency

• Cost effective


Can tolerate irregular acquisition geometry

Detailed interpretationof an intensely faulted base salt event


Advantages




• Cost effective



Kirchhoff Migration W.E.M

Advantages




• Cost effective


Limitation of WEM

๏ Relied most on Velocity model

๏ Expensive yet preferable



Case Study (1)Case Study (1)

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


North Sea


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.


Seismic Before and After PSDM


Interpretation from PSDM data



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

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.



Case Study (2)Case Study (2)


PSDM method applied:

✓ Kirchhoff PreSDM

✓ W.E. Migration

Figure 1: Field layout of the 3D seismic survey conducted over the salt structure.

N


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.


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.


Figure 2: PSTM

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Figure 3: Kirchhoff preSDM image of the same crossline

Figure 2: PSTM


PSTM Kirchhoff PSDM

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Figure 4: The salt flanks exhibit a small improvement

compared to the Kirchhoff image.


Figure 3: Kirchhoff PSDM

Kirchhoff W.E.M

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.


Kirchhoff W.E.M

The Kirchhoff preSDM image of a crossline shows a very good image of the base salt


Kirchhoff PSDM

The W.E. preSDM image shows not only a good base-of-salt image, but also better defined subsalt reflections.


W.E.M PSDM

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.



ConclusionConclusion

Conclusion

✓ Production depth imaging combines established Kirchhoff technology with new-generation wavefield extrapolation migrations.

✓ The two technologies are complementary.



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