demultiple approaches - globe claritas · shift data down by 1000ms shift data up by multiple time...

GLOBE Claritas™

Demultiple Approaches

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Primary Demultiple Approaches

• Deconvolution

– multiple considered as reverberation

– Tau-P, shot and receiver location ensemble etc.

• RADON demultiple (PRT_DEMULT)

– Velocity discrimination (parabolic approximation)

– Use primary or multiple model (reject down or flat dips)

– High resolution mode via Harlan transform

• Flattening on Offset Planes

– Digitise seafloor (and/or peg-leg generator)

– Flatten multiple via static shits and remove flat events

• SRME and subtraction

– Two good subtraction routines (MONK/WANG)

– Limitations on long offset and deep water

– Flatten multiple via static shits and remove flat events

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RADON Anti-Multiple : theory

• Data summed along parabolas

• Defined by far offset moveout (FOM) in ms

• This example : 90% NMO applied

• Multiples +ve P-values

• Primaries -ve P-Values

• Data outside of the Radon transform range is retained

• Define a modelled range (in FOM) and a noise range to remove

• “Cross” shape in Radon-space focussed to a point by Harlan mode

300ms far offset

moveout, measured via

ruler (RMB click)

Primary

Multiple

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PRT_DEMULT Example : Input data

700ms FOM used

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PRT_DEMULT Example : Harlan Demultiple

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Flattening on Offset Planes Example

Digitise multiple

Shift data down by 1000ms

Shift data up by multiple time

Multiple flat at 1000ms

Strong shallow multiple

Water-velocity FK

applied

Apply an FK filter

Reject +/- 0.5ms per trace

Applied from 950-1050ms

Remove all shifts

• Only effective where multiple crosses primaries

• Can apply again for the second bounce

• Can apply pre-stack – use FLATTEN on offset planes

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SRME Example

Original Shot Modelled multiple Subtracted result

• Model created by cross correlating shot and receiver gathers (SRME)

• Data needs to be extended to zero offset (OFFREG)

• Shot and receiver spacing needs to be the same (OFFREG, SHOTINT)

• Should not process incoming data at all – except perhaps mute and filter

• Only models reflections, not refractions

• Theoretically needs full fold shot and receiver gathers to be most effective

• In deep water, limiting the shot and receiver domain offsets can be beneficial

• Use adaptive subtraction to remove the multiple (WANGSUBT, MONKSUBT or combinations of these)

• Complex workflow, consider simple multiple modelling (MULMOD)

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Basic Processing Routes Re-sampled Shots

Stack #1

Tau-P Decon.

Hi res RadonRadon

Stack #2 Stack #3 Stack #5

SRME

Tau-P Decon.

Stack #7

Offset

Plane

Stack #4 Stack #6 Stack #8

Offset

Plane

Offset

Plane

No demultiple used RADON approaches SRME approaches

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#1 Stack : No Demultiple

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#2 Stack :Tau-P Decon.

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#3 Stack : Tau-P Decon, Radon

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#4 Stack : Tau-P Decon, Radon, Offset

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#5 Stack : Tau-P Decon, Radon/Harlan

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#6 Stack : Tau-P Decon, Radon/Harlan, Offset

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#7 Stack : SRME, Tau-P Decon

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#8 Stack : SRME, Tau-P Decon, Offset

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Examples on line TL-01

• Raw data has 50m SP, 25m groups, 120 channels

• Minimum Offset is 189.4m

• Tau-P run with 50m SP, 6.25m groups

• SRME run at 25m SP, 25m groups

• Stacks etc. created with 12.5m SP, 25m groups

• All managed via OFFREG and SHOTINT

• Spike QC important pre-interpolation

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Radon Parameters Used

• Offsets 184.9m to 3159.9m increment 25m

• Fold 120, HMAX 3159.9

• Model MS: -700 to 700

• Noise MS: 60 to 696

• 300 p-values (3Hz-80Hz range)

• Start time 1.75x water bottom, 200ms minimum

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Offset Plane Parameters Used

• Digitised Seafloor on near trace plot

• DSORTOFF to sort to common offset planes

• FLATTEN to flatten 1st multiple

• QFKPS FK filter +/-50ms around flattened multiple

• Filter set to reject flat data, 150 trace window

• Repeat for the 2nd multiple

• Sort back to CDP order via DISCGATH

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SRME Parameters Used

• Three passes of adaptive subtraction

• WANGSUBT with 80% eigen values

• MONKSUBT 300ms gated

• MONKSUBT 100ms gated

• Run using REREAD and pseudo traces