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PreStack Depth Migration (PSDM)

Where PSDM Stand?Typical Processing1. 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

PSDM

PreStack Depth Migration

Where PSDM Stand?Typical Processing1. 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

PSDM

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.

PreStack Depth Migration

Results of PSDM

PSDMPreStack Depth Migration

PSTM

PreStack Depth MigrationPSDM 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

PreStack Depth Migration AlgorithmsWavefield Extrapolation Migration Kirchhoff PreStack Depth Migration Reverse Time Migration (RTM) Common Azimuth Wavefield Extrapolation Migration Beam Migration Enhanced Migration Amplitude Normalisation

PreStack Depth Migration

PSDM vs PSTMPreStack Depth Migration

ComparisonPrestack DEPTH migration Done after PSTM Has better imaging More accurate Can interpret complexity structures More interpretative Prestack TIME migration Image is not clear enough Better lateral resolution Better coherency

PSDM is done after PSTM Both needs high quality data

PreStack Depth Migration

Midpoint (km)

Midpoint (km)

PSTM

PSDM

PreStack Depth Migration

PSTM using commercial processing systemPreStack Depth Migration

Image of faults using PSDMPreStack Depth Migration

Over thrust model of PSTMPreStack Depth Migration

Over thrust model of PSDMPreStack Depth Migration

PSTMPreStack Depth Migration

PSDM

PreStack Depth Migration

Advantages of PSDMCan be used at complex subsurface. Precise and accurate seismic solution. Clear expressions/images. Reduce structures positioning errors.

PreStack Depth Migration

Disadvantages of PSDMMore 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

Reverse Time MigrationPreStack Depth Migration

Pre-Stack Reverse Time Depth MigrationAims 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

Reverse Time Depth Migration Model

r

QuickTi H.264 c r ss r t s t is ictur .

PreStack Depth Migration

Forward-Reverse ModellingHigh amplitude attribute in the respective subsurface locationQuickTime and a H.264 decompressor are needed to see this picture.

Down-going wave field from the source side and Up-going wave field towards the receivers at the same time

PreStack Depth Migration

The Picture at Vertical IncidenceEarth

Reflection Coefficient

R 1=

Z 2- Z Z 2+ Z

1 1

Acoustic Impedance

Z = Velocity X Density Z i + 1= Z 1 +Ri i i

1 - R

PreStack Depth Migration

Earth

Impedance

Reflection Coefficients

Wavelet

Wavelet Superposition

Recorded Trace

Seismic Section

Forward ProcessPreStack Depth Migration

Seismic Section

Impedance

Reflection Coefficients

Wavelet

Deconvolved Section

Integrated Section

Inverse ProcessPreStack Depth Migration

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 MigrationPreStack Depth Migration

PreStack Migration ComparisonConventionalPropagating 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 MigrationPropagate 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 Depth Migration

Kirchhoff AlgorithmPreStack Depth Migration

What Is Kirchhoff AlgorithmApply integral method Widely used in the industries Accept high angles : 90 Accept turning waves (more than 90) Preserves amplitude

PreStack Depth Migration

The Equation of Kirchhoff AlgorithmV (M ) !

;

w(\, M )U (\,X D (, M ))d\1d\ 2

V (M ) ! Migrated Image at point M w(\, M ) ! Migration

eighting Function

U (\,X D (, M )) ! Time differentiated seismic data

PreStack Depth Migration

The Use Of Kirchhoff Algorithmto better repositioning of data to better imaging accuracy / repositioning improve seismic image at steeply dipping subsurface structures

PreStack Depth Migration

Simplifying FlatSource Common Mid-Point Receiver

Bedrock

Reflection Point

PreStack Depth Migration

Sometimes, Not Flat!Common Mid-Point

PreStack Depth Migration

The Migration

PreStack Depth Migration

The UpsideEnhance image quality Distinctive subsurface structures Fidelity of the algorithm Good on handling lateral velocity changes

PreStack Depth Migration

The DownsideMigration artifacts Migrate non-primary Damage primary reflections Sensitive to errors in migration velocity Time consuming (3D) High cost

Migration Noise

PreStack Depth Migration

Wavefield Extrapolation MigrationPreStack Depth Migration

What Is W.E.MA 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

Superior Image of W.E.M

PreStack Depth Migration

Concept of W.E.M

PreStack Depth Migration

Finite-difference

Explicit solutions

Split-step Fourier

Implicit solutions

PreStack Depth Migration

Concept of W.E.M

PreStack Depth Migration

Single-arrival Kirchhoff impulse response

Which one is which?Shot profile migration of WEM

PreStack Depth Migration

Propagation Effects

Diffraction, multiple arrivals, amplitude decay in shadow zone

PreStack Depth Migration

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

Can tolerate irregular acquisition geometry

Detailed interpretation of an intensely faulted base salt event

PreStack Depth Migration

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

PreStack Depth Migration

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

Kirchhoff Migration

W.E.M

PreStack Depth Migration

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

Limitation of WEM

Relied most on Velocity model Expensive yet preferable

PreStack Depth Migration

Case Study (1)PreStack Depth Migration

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 coverPreStack Depth Migration

North Sea

PreStack Depth Migration

SolutionsReprocessing 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

Seismic Before and After PSDM

PreStack Depth Migration

Interpretation from PSDM data

PreStack Depth Migration

Interpretation from PSDM data

The interpreted section,

The Boundaries,NBR: near basem

Dark grey: Mesozoic

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