Demultiple Routes

GLOBE Claritas™
Demultiple Approaches

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
www.globeclaritas.com

RADON Anti-Multiple : theory

Primary

Multiple
300ms far offset
moveout, measured via
ruler (RMB click)

• 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

PRT_DEMULT Example : Input data

700ms FOM used


PRT_DEMULT Example : Harlan Demultiple


Flattening on Offset Planes Example

Strong shallow multiple Digitise multiple Apply an FK filter Remove all shifts
Water-velocity FK Shift data down by 1000ms Reject +/- 0.5ms per trace
applied Shift data up by multiple time Applied from 950-1050ms
Multiple flat at 1000ms

• Only effective where multiple crosses primaries
• Can apply again for the second bounce
• Can apply pre-stack – use FLATTEN on offset planes


SRME Example
• Model created by cross correlating
shot and receiver gathers (SRME)
Original Shot Modelled multiple Subtracted result • 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

Tau-P Decon. SRME

Radon Hi res Radon Tau-P Decon.

Offset
Offset Offset
Plane
Plane Plane

Stack #1 Stack #2 Stack #3 Stack #4 Stack #5 Stack #6 Stack #7 Stack #8

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


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