seismic data processing

SEISMIC DATA SEISMIC DATA PROCESSINGPROCESSING

Demultiplexing Trace header generation <…observers’ dataSpherical divergence correctionDeconvolution before stackBand pass filterTrace normalization

SEISMIC DATA SEISMIC DATA PROCESSING (contd)PROCESSING (contd)

Velocity AnalysisNormal Move Out CorrectionCMP StackResidual statics estimation & applicationDip Move Out CorrectionVelocity analysis

SEISMIC DATA SEISMIC DATA PROCESSING (contd)PROCESSING (contd)

DMO stackRandom noise attenuationDecon after stackTime Variant FilterMigrationScaling

surface

Layer-1

Layer-2

Layer-3

Arrival

time

0

Depth & Time

Shotreceivers

DemultiplexingDemultiplexing

Required if the Seismic Data is recorded in multiplexed format

Conversion of scan sequential mode to trace

sequential mode.Essentially a Matrix transposition (rows to

columns and vice versa)

Ch-1Ch-2

Ch-4

Ch-3

Ch-5

Ch-12

Ch-20

Ch-48

SCAN-1

SCAN-1,ch-1

SCAN-48SCAN-47SCAN-2

SCAN-1,ch-24 SCAN-1,ch-48 SCAN-2,ch-1

SCAN SEQUENTIAL DATA

Multiplexing

SCAN-1,ch-1 SCAN-1,ch-24 SCAN-1,ch-36 SCAN-1,ch-48

SC-1200,ch-1 SC-1200,ch-24 SC-1200,ch-36 SC-1200,ch-48

SCAN-2,ch-1 SCAN-2,ch-24 SCAN-2,ch-36 SCAN-2,ch-48

SCAN-3,ch-1 SCAN-3,ch-24 SCAN-3,ch-36 SCAN-3,ch-48

SCAN-4,ch-1 SCAN-4,ch-24 SCAN-4,ch-36 SCAN-4,ch-48

SC-2500,ch-48SC-2500,ch-36SC-2500,ch-24SC-2500,ch-48

ROWS Scan sequential Multiplexing

COLUMNS Trace sequential Demultiplexing

Demultiplexing

Amp

GR

OU

ND

RO

LL

GR

OU

ND

RO

LL

GR

OU

ND

RO

LL

RE

FL

EC

TIO

NS

RE

FL

EC

TIO

NS

RE

FL

EC

TIO

NS

TYPES OF NOISETYPES OF NOISECoherent Noise (Ground roll, backscatter, multiples) Methods of reduction : - Multi channel filtration in t-x, f-k, Radon domains

- Model based (WEMA,SRME)Random Noise Methods of reduction : CMP stacking,

Predictive decon in f-x domain

Groundroll

Reflections

First Arrivals

NOISE SECTION

First Arrivals

Groundroll

Reflections

Trace Headers GenerationTrace Headers Generation

Generation of addresses to the tracesGeographical positioning Facilitates for the unique identificationSorting with respect to a common group

(common shot, common receiver, common midpoint & common offset)

GATHERS OF DIFFERENT TYPES

Shot

receivers

Common shot gather

GATHERS OF DIFFERENT TYPES

Shotsreceiver

Common Receiver gather

GATHERS OF DIFFERENT TYPES

Shotsreceiver

Common Mid Point gather

Mid Point

Spherical Divergence Spherical Divergence CorrectionCorrection

Seismic Amplitude decays as a function of time due to spherical spherical spreading and inelastic attenuation.

Compensation is done using a gain function that is inverse of the decay curve.

Objective is to see that nearly same amount of energy is reaching at every layer of the subsurface.

Time

Amplitude

Amplitude decay

Decay curve

Recovery function

Time

Amplitude

Deconvolution Before StackDeconvolution Before Stack

Earth acts as a high cut filter. Loss of high frequencies result in loss of resolution.

The High frequencies that are poorly represented in the input can be brought on par with those of better represented.

Achieved through an inverse filter application.

Frequency

AmplitudeDeconvolution

Frequency

Frequency

Amplitude

Input

Inverse opr

output

Band Pass FilterBand Pass Filter

Generally deconvolution before stack enhances frequencies.

To limit the frequencies to the seismic range a band pass filter is conventionally applied

(8-70 Hz)

Trace NormalizationTrace Normalization

The amplitude values are scaled by a scalar estimated in the user defined time window to bring them down to observable range.

Relative amplitude variation is preserved.

Static CorrectionsStatic Corrections

The elevation differences among the traces of a cmp gather cause delays.

The Low Velocity Layer(LVL) near the surface also introduces delays in the observed travel times.

The data has to be corrected to a reference surface (Datum) removing these differences.

These corrections are static; they don’t change with time; hence the name ‘Static correction’.

NMO correction is ‘Dynamic’; it is a function of time (To), source to receiver offset, and Velocity.

DATUM

Reflector

Surface

Static corrections

LVL

Velocity AnalysisVelocity Analysis

Estimation of Velocity that yields best alignment of nonzero offset travel time to its zero offset time.

Based on Hyperbolic assumption.Results in the best stacking of traces of a

common mid point gather.

Stack Power as a Function of Velocity

And Time

Normal Move Out CorrectionNormal Move Out Correction Non zero offset data is characterized by a travel time

increase with increase in offset distance from the source to the reflector.

Non zero offset to zero offset conversion is achieved through a correction called as NMO (nomal move out) correction. The NMO equation for a flat layer case is :

Ti**2 = To**2 + Xi**2/V**2, where Ti = Travel time at offset distance Xi To = Zero offset travel time V = NMO velocity or stacking velocity at time To.

Ti – To = DT nmo for offset distance Xi. NMO correction is ‘Dynamic’; it is a function of time

(To), source to receiver offset, and Velocity.

Before NMO Cor. After NMO Cor.

Offset

Time

StackingStacking

Each common mid point gather after normal move out correction is summed together to yield a stacked trace.

Stacking enhances the in-phase components and reduces the random noise.

Stacking yields Zero offset section (in the absence of dipping layers in the subsurface)

Brute STACK

STACK after Dip filtering

Crooked (slalom) Profile & Mid Crooked (slalom) Profile & Mid Point distributionPoint distribution

STACK after Crooked Profile adjusting

STACK after Velocity Analysis and NMO

Residual Static CorrectionsResidual Static Corrections Field static corrections are computed using the

velocity of LVL and are based on the ray paths. Field statics alone, can not correctly account strong

near surface velocity irregularities. Residual static corrections are estimated on the filed

statics applied and NMO corrected gathers in a surface consistent approach; that is time shifts are only dependant on the source receiver locations , but not on the ray paths from shots to receivers.

Velocity Analysis and stacking performed after accounting residual static corrections yield improved resolution.

STACK after Residual Statics application

Dip Move Out CorrectionDip Move Out Correction NMO ensures non zero offset to zero offset conversion in the

absence of dipping layers. In the presence of contrasting dips, the estimated velocity will be :

V* = V/Cosine(Alfa), where

V* = dip corrupted velocity,

V = actual velocity and

Alfa = dip angle (measured wrt horizontal). Ti**2 = To**2 + Xi**2/V**2 – (Xi*Sin (Alfa))**2 /V**2 The term (Xi*Sin (Alfa))**2 /V**2 is the Dip Move Out term. This additional correction promises non zero offset to zero offset

conversion.

PmPd

Pn

Dip Move Out & Migration

(Brian Russel, 1998)

NMO STACK

DMO STACK

NMO STACK DMO STACK

Random Noise AttenuationRandom Noise Attenuation

Seismic noise can be either random or coherent.

Random noise is random. Its estimation is done in

Frequency – space (FX) domain. The predictable

nature of the Sinusoidal signals offer their removal through a deconvolution. Total field minus the

signal field gives the random noise field.

Coherent Noise AttenuationCoherent Noise Attenuation

Coherent noise attenuation is achieved through multi channel (TX, FK, Tau-P domain) filters.

The characteristics of coherent noise are velocity, frequency, etc.

Random noise is random. Its estimation is done in

Frequency – space (FX) domain. The predictable nature of the Sinusoidal signals offer their removal through a deconvolution. Total field minus the signal field gives the random noise field.

Signal EnhancementSignal Enhancement

Coherent signal can be searched in adjoining traces

in a specified narrow range of dips and can be gathered.

Multi channel input data facilitates such processes.

For structural interpretations this is permitted, but not

always a recommended practice for stratigraphic

interpretations.

Deconvolution After StackDeconvolution After Stack

NMO correction and Stacking also act as a high cut filter. Loss of high frequencies result in loss of resolution.

The deconvolution employed at the post stack stage is similar to that in the pre – stack stage, but the parameters are such that the decon action is milder.

DECON AFTER STACK

Time Variant FilterTime Variant FilterEarth consists of sets of layers (strata) that

are distinctly characterized by certain band of frequencies.

It is often advantageous to view the seismic section (which is a cross section of the earth) in tune with the characteristic band of frequencies.

Different parts of the seismic section can be subjected different sets of band pass filters in a time variant manner following tests.

TIME VARIANT FILTER

MigrationMigrationWhen the subsurface consists of dipping layers, the

Zero offset section does not represent the cross section of the earth because the reflected energies are placed at apparent spatial locations.

Moving the reflection energies from the apparent locations to the true locations is achieved through ‘Migration’. The spatial velocity distribution of the velocity is used here for the identification of these true points in the subsurface.

Migration improves the spatial disposition of the reflecting layers and hence achieves ‘Imaging’.

Apparent dip

True dip

MigrationA B

P

Q

P’

Q’

Migration Equation

Tan(app. Dip) = Sin(true dip)

Bow Tie

(before migration)

Syncline

(after migration)

Migration

During migration, trace energy is smeared along a surface of allpossible reflector positions. This means a given wavelet of certainperiod (frequency) is placed along a circle of radius equals to thetwo way time with the observed CMP location as center. Nowproblem lies in the way these smeared energies are placed on theCMP traces.

CMP

POST STACK MIGRATION

Energy placedAlong the radius

Energy placedAlong the cmp axis

CMP

MIGRATION & WAVELET DISTORTION

MIGRATION & WAVELET DISTORTION

CMP

Dip=0

Dip=90

Dip=45

Higher the Dip, larger will be wavelet stretch.High frequencies suffer more at higher dips.

CMP

MIGRATION & SPATIAL ALIASING

CMP

MIGRATION & SPATIAL ALIASING

CMP

MIGRATION & SPATIAL ALIASING

Fine input samplingunique dips

Fine output samplingunique dips

CMP

MIGRATION & SPATIAL ALIASING

Coarse input samplingNon unique dips

Coarse output samplingNon unique dips

Coarse input samplingImproper restoration

Loss of resolution

Fine input samplingProper restoration

TEMPORAL ALIASING