Demultiple Routes

GLOBE Claritas™

Demultiple Approaches

www.globeclaritas.com

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


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


PRT_DEMULT Example : Input data

700ms FOM used


PRT_DEMULT Example : Harlan Demultiple


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 appliedApply 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


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)


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


#1 Stack : No Demultiple


#2 Stack :Tau-P Decon.


#3 Stack : Tau-P Decon, Radon


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


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


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


#7 Stack : SRME, Tau-P Decon


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


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


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


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


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

Demultiple Routes

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