seismic data processing
TRANSCRIPT
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