Why seismic processing ?

Processing Steps

aghi What’s a seismic trace?

Sandstone

Carbonate

* S(t) * R(t) Seismic trace + Noise (t) =Rf(t)

Filtering

Stacking

Deconvolution

Wave propagation

aghi Land dataMarine data

Split shot gather

0 500 1000 1500 2000 2500

Direct

Ground roll

Head wave (refraction)First multiple

Primary

Seismic eventsNon-primary events

Primary

Earth’s surface

Subsurface reflector

Ground roll Direct P-wave

Head wave (refraction)

First multiple

Seismic eventsNon-primary events

CDP Fold =Number of receivers x receiver interval

2 x shot interval

CDP gather NMO Stack

Migration

“The goal of migration is to make the stacked section appear similar to the

geologic cross-section”

Oz Yilmaz

A step in seismic processing in which reflections in seismic data are moved to their correct locations in the x-y-time space of seismic data, including two-way traveltime and position relative to shotpoints

Typical ProMax flow for velocity analysis.

Examining the normal moveout equation, it is possible to analyze NMO velocities by plotting reflections in T2 X2 space

Concept of Constant Velocity Stack as an aid to stackingvelocity estimation.

One method to determine stacking velocity is to use a Constant Velocity Stack (CVS) for several CDP gathers

Same CVS panel of traces as before switching to variabledensity color for the traces to utilize dynamic range

Same as previous color panels with velocity range nowhalved to better pick correct velocities

Another term for Normal Moveout Equation.

Options in the ProMax Velocity Analysis Routine.

Demonstration of the velocity spectra

CDP gather with NMO applied (center) surrounded by panelshaving progressively lower velocity (left) or higher velocity.

From left to right are panels for Semblance, Gather, DynamicStack, Flip Stacks, and Velocity Function Stack.

The ProMax routine ‘Velocity Analysis’ has it all – from left toright: velocity spectra, interactive cursor with CDP gather,dynamic stack, and a variation on CVS

The Semblance Panel shows the semblance plot, the pickedvelocity function, guide functions, and the interval velocitycomputed from the picked function.

Dix equation converts stacking velocities to interval velocities.

aghi However, you get RMS velocities, one can continue to

calculate interval velocities, interval thicknesses, and average velocities.

Remaining three panels in Velocity Analysis routine.

Use of ProMax routine Velocity Viewer and Editor

A common problem with stacking is residual NMO on theCDP gathers resulting from imperfect velocity specification.

Example of the data/velocity Interleave Display usingLandmark’s SeisCube program.

Progressive Mute Analysis

Prestack CDP gather with a horizon plotted along an eventthat is not perfectly flattened by NMO; other causes might bestatics, noise, and/or lithology that is affecting the phase.

ProMax routine CDP/Ensemble Stack vertically stacksinput ensembles of traces.

Definition of multiplies as it applies to processingseismic reflection data using ProMax.

Example of a surface multiple on left in red and intrabedmultiple on the right in blue.

Comparison of short-path and long-path multiples.

Conceptual abstraction of the Tau – P domain

Organizing seismic reflection data into ray-parameter domainhas certain advantages that are elaborated here.

Working definition of the Radon Filter commonly used for multiple suppression – working in the intercept-time (T) / ray parameter (p) or slowness domain.

Use of the radon transform for the removal of multiples bydiscriminating on the basis of moveout – here no rejection.

Use of the radon transform for the removal of multiples bydiscriminating on the basis of moveout – rejection shown.

More on the use of the Radon Filter.

Migration

aghi Migration

“Migration is an inversion operation involving rearrangement of seismic information elements so that

reflections and diffractions are plotted at their true locations.”

R.E Sheriff

“The goal of migration is to make the stacked section appear similar to the geologic cross-section”

Oz Yilmaz

Unmigrated

Migrated

Migration

• Collapses diffractions

• Corrects for dip– Moves dipping events in the updip direction

• Removes effects of surface curvature– “unties the bowties”

aghi Reconstructing the wavefield

Constant velocity migration

aghi Schematic that shows the imaging problem for a

simple anticline.

aghi Schematic that shows the imaging problem for a

simple syncline.

Schematic that shows the imaging problem for a vertical fault.

aghi Schematic that shows the imaging problem for a 30-

degree fault.

aghiSchematic that shows the imaging problem for a reef model.

• Kirchoff migration (diffraction stacking)

• Finite difference method

• F-K migration

– integral solution of wave equation

– derivative solution of wave equation

– Fourier domain solution of wave equation

Migration Methods

Kirchoff Migration(Diffraction Summation)

For every point (x,z), collapse all energy from hyperbola with vrms

2 4rmsv

Kirchoff Migration(Diffraction Summation)

Factors to consider before summing energy in diffraction:

• Obliquity factor– A cos

• Spherical divergence factor– A 1/r

• Wavelet shaping factor– phase correction

Migration collapses diffractions to reveal structure

Finite Difference Migration

• Solving the wave equation by stepping down discrete intervals from z=0

• Downward continue wavefield to “exploding reflector”

• Define an angle for width of cone for to be included in migration for each point– wider cone more accurate– narrow cone faster, better approximations

sintan a

Migration steepens and moves dipping reflectors

Apparent dip in time section is related to true dip:

(migrator’s equation)

aghi Collapsing diffraction and relocating

dipping surface

Diffraction D Apex P

Reflector B A

F-K Migration• Events can be separated by their

dips in F-K space

• Transform according to migrator’s equation tan a=sin

• Advantage: very computationally efficient!

• Disadvantage: only works for constant velocity (without modifications that compromise its efficiency)

aghi Migration removes multiple-

branch reflections

• Synclines get broader

• Anticlines get narrower

“Untying the bowties”

Limitations of Migration• Insufficient spatial resolution will result in aliasing• 2-D slice of 3-D wavefield (need 3-D migration!)• Edge effects• Coherent noise• Requires knowledge of velocity structure• Time migration methods assume lateral velocity varies

slowly (otherwise need depth migration)

3-D Processing

• Binning by common midpoints in cells on a grid

• Migration can be two stage 2-D migration (in-line direction, then cross-line direction) or full 3-D wavefield solution

• Most other processing operations are unchanged

• Display is more difficult (and more fun!)

Why Deconvolution?

• Decreases ringing

• Increases resolution

• Improves appearance of stacked section and makes it easier to interpret

• Section is more like the earth and less like the seismic source

• Can remove multiples

Convolutional model of a seismograms=w*e+n

source wavelet

Earth response function seismogram

(+noise)

Spiking Filter

• Take existing wavelet and transform to a unit impulse (delta function)

• Also called “whitening” because it aims to create a white spectrum

Predictive Deconvolution

• Deconvolution with a built-in time lag

• Use to remove – Multiples– Bubble pulse

Deconvolution Example

Raw gather decon Bandpassfiltered

autocorrelograms

Raw gathers

After decon

Deconvolution

• Deterministic Inverse Filtering

• Deghosting

• Least Squares (Optimum) Filtering

• Spiking filter

• Wavelet shaping

• Predictive Deconvolution

aghi Convolutional model of a

seismogram

s=w*e+n

One equation with 3 unknowns

How can we possibly find e?

We make assumptions:– e, n are white (random)– w is minimum phase

deEarth response Wavelet Seismogram

Deterministic DeconvolutionAssume that an operator f(t) exists such that

)()(*)( ttftw

In the Fourier domain:

1)()( FW

wiw eA

1)( so

)(/1)(

The inverse operator f(t) has opposite phase and inverse amplitude spectrum from the source wavelet w(t)

Deterministic Deconvolution

• Assumptions:1 source wavelet is minimum phase2 noise is zero3 wavelet is known

• Not true, especially 2

• In practice, the Fourier domain implementation is not very good if assumptions are not met

• Other methods are more stable

Deghosting

• Eliminate source & receiver ghosts by considering them as time delayed copies of the source (and with known depths the time delays are known)

• Alternatively, hydrophones and geophones with different responses can be combined to eliminate ghosting effects

Correlation

Autocorrelation

Cross-correlation

1,,1,01

Wavelet Estimation

• In general, the source wavelet is unknown

• Source wavelet can be estimated from seismogram alone assuming:– minimum phase wavelet– white earth response spectrum

ewx rrr * autocorrelation

(with white earth response)wx rrr 0

Autocorrelation of seismogram is the autocorrelation of source wavelet (within a constant)

Optimum Weiner Filters

Want to find the optimum filter components fi that minimize the error between the desired and actual outputs in a least-squares sense:

ttt xfdL 2)(

)1(,,2,1,0,0

By setting

so 022

xxfxdf

Recognizing the terms for auto- and cross-correlation,

ii grf

titt xdxxf

ii grf

Or, in matrix form,

The autocorrelation matrix is a Toeplitz matrix, and can be inverted by Levinson recursion

are called the normal equations

The gi terms are the cross-correlation of the desired wavelet with the input wavelet (seismogram).

In the case of spiking deconvolution, the normal equations take the form

Wavelet Processing

• Attempt to shift source wavelet to some other known wavelet, to accomplish one or more of:

• Reduce variation of source (between shots, between receivers)

• Shift to another known wavelet– e.g., hydrophone response to match seismometer

• Separate wavelet and earth response more clearly

Wavelet Processing

Transform to zero phase and broaden spectrumIncrease resolution

1) Shots : 2 – 548

2) Minimum phase

3) Traces have been resampled (2ms >4ms) and decimated (384 > 192)

4) Fk filter

5) Geometry has been applied

6) Velocity file is available(By Geco)

Real data

12.5 m

7.5 m 8.5 m

Check the muteVelocity Analysis

Real data work flowSorting

Pick mute

True Amplitude Recovery

Deconvolution

Velocity Manipulation

Migration

Demultiple

aghi Real data

-A report:

-Explanation of the processing steps with proper and related snap shots(Mute, TAR, Decon, NMO, Demultipling, Stacking, Migration,etc

-Final results(a comparison study)

-Brute-stack section(s)

-Demultipled stack section(s)

-Migrated section(s)

Project results:

Why seismic processing ?

Documents

General Seismic Processing Workflow

Omega Seismic Processing

Seismic Processing and Interpretation

Seismic Data Processing With Seismic Unix

OpenCPS Seismic Processing Software

Seismic methods: Seismic reflection - IVrallen.berkeley.edu/.../Lectures/L17_SeismicReflectionIV.pdf · Applied Geophysics – Seismic reflection IV Seismic reflection processing

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Seismic data processing services from T&A Surveyta-survey.nl/pdf/TA_Survey_Seismic_data_processing_flyer.pdf · seismic data processing facility. All seismic processing is done in-house

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