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Multiple Removal with Local Plane Waves. Dmitri Lokshtanov. Content. Motivation WE multiple suppression operator Fast 2D/3D WE approach for simple sea-floor 2D/3D WE approach for irregular sea-floor Conclusions. Motivation. Seismic processing and imaging - main challenges:

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content
Content
  • Motivation
  • WE multiple suppression operator
  • Fast 2D/3D WE approach for simple sea-floor
  • 2D/3D WE approach for irregular sea-floor
  • Conclusions
motivation
Motivation
  • Seismic processing and imaging - main challenges:
    • Velocity model building for sub-salt and sub-basalt imaging
    • Removal of multiples from strong irregular boundaries
slide6

Depth migration with water velocity

multiple suppression
Multiple suppression
  • For multiples from complex boundaries the methods based on periodicity or kinematic discrimination usually don’t work or are not sufficient.
  • In such cases the main demultiple tools are based on the Surface Related Multiple Elimination (SRME) or Wave-Equation (WE) techniques.
srme berkhout 1982 verschuur 1991 advantages and limitations
SRME(Berkhout, 1982; Verschuur, 1991) – advantages and limitations
  • Does not require any structural information. Predicts all free-surface multiples
  • As a rule becomes less efficient with increased level of interference of multiples of different orders
  • Requires the same dense sampling between sources as between receivers
  • Noise in data and poor sampling significantly degrade the prediction quality
  • Missing traces required by 3D SRME are reconstructed with least-square Fourier or Radon interpolation; residual NMO correction; DMO/inverse DMO; migration/demigration
we approach versus srme
WE approach versus SRME
  • SRME is the method of preference for data from areas with deep sea-floor, especially when a thick package of strong reflectors is present below the sea-floor
  • WE approach is especially efficient when the main free-surface multiples are just ‘pure’ water-layer multiples and peg-legs. Gives usually better results than SRME when several orders of multiples are involved
  • 3D WE approach has less sampling problems than 3D SRME and it gives a flexiblility in methods for wavefield extrapolation depending on complexity of structure
slide11

The operator Pg transforms the primary reflection event recorded at receiver 1 into the multiple event recorded at receiver 2 (Wiggins, 1988; Berryhill & Kim, 1986).

The operator Pg transforms the primary reflection event recorded at receiver 1 into the multiple event recorded at receiver 2 (Wiggins, 1988; Berryhill & Kim, 1986).

wave equation approach main features
Wave-equation approach – main features
  • All predicted multiples are split into 3 terms, where each term requires the same amplitude correction
  • All source-side and receiver-side multiples of all orders are suppressed simultaneously in one consistent step
  • The prediction and the adaptive subtraction of multiples are performed in the same domain
  • Fast version (WEREM) for a simple sea-floor. Slower version for irregular sea-floor
why in the tau p domain
Why in the tau-p domain
  • Easier to apply antialiasing protection
  • No problems with muting of direct arrival
  • Easier to define ‘multiple’ zone of tau-p domain and mute it away
  • Estimated reflection coefficients are explicitly angle dependent
slide23

T. Shetland

 T. Draupne

 T. Brent

Stack before multiple suppression Stack after Werem

slide25

Stack before multiple suppression (left) and after Werem multiple suppression (right).

The pink line shows the expected position of the first-order water-layer peg-leg from the

Top Cretaceous (black line). The multiple period is about 140 msec.

slide26

Constant P sections (angle at the surface is about 8º

Input After Werem multiple suppression Difference

improving the results local prediction subtraction of multiples
Improving the results - local prediction / subtraction of multiples
  • Within the same prediction term, for the same CMP and the same p we have events reflected at different positions along the water bottom
  • Inconsistency between prediction and subtraction in case of rapid variation of sea-floor reflectivity
  • The problem is partly solved by applying adaptive subtraction in different time windows
  • Or by making prediction dependent on both p and offset (window)
3d werem basic features
3D WEREM – basic features
  • 3D data can be represented as a sum of plane waves with different vertical angles and azimuths from the source-side and receiver-side.
  • Current quasi 3D marine acquisition does not allow full 4D decomposition
  • Decomposition uniquely defines the direction of propagation from the receiver-side and is an integral over crossline slownesses from the source-side
  • The result of decomposition are used for exact prediction of multiples from the receiver-side and approximate prediction from the source-side
  • The approximation is that the crossline slowness from the source-side is the same as from the receiver-side (the same azimuth for 1D structures). The approximation allows us to mix data for flip flop shooting
werem conclusions
Werem - conclusions
  • Very efficient when the main assumptions are met: strongest multiples are water-layer multiples and peg-legs and the sea-floor is simple
  • Very fast - each predicted p trace is simply obtained as a sum of time-delayed input traces with the same p from the neighbour CMPs
we for irregular sea floor
WE for irregular sea-floor
  • Kinematic prediction of multiples (extrapolation through the water layer) takes into account coupling between incident and reflected / scattered plane waves with different slownesses
  • Both multiple reflections and diffractions are predicted
  • The procedure starts from the Radon transformed CS gathers (no interleaving is required)
  • In 3D exact prediction from the receiver side; approximate prediction from the source side
slide59

Input Constant P section R-side prediction

slide60

Input Constant P section R-side prediction

slide61

Input Constant P section After adaptive subtraction

slide65

primary

peg-legs

Modelled CS gathers

slide67

Constant P sections (small angle) for line with crossline offset 250m.

Input 3D prediction 2D prediction

slide68

Constant P sections (larger angle) for line with crossline offset 250m.

Input 3D prediction 2D prediction

slide69

Constant P sections (small angle) for line with crossline offset 250m.

Input quasi 3D prediction 2D prediction

slide70

Constant P sections (larger angle) for line with crossline offset 250m.

Input quasi 3D prediction 2D prediction

we for irregular sea floor71
WE for irregular sea-floor
  • Both multiple reflections and diffractions are predicted
  • Exact 3D prediction of pure water-layer multiples and peg-legs from the receiver-side
  • Quasi 3D prediction of peg-legs from the the source side
  • 3-5 times slower than WEREM
conclusions
Conclusions
  • SRME is the method of preference for data from areas with deep sea-floor, especially when a thick package of strong reflectors is present below the sea-floor
  • As a rule the method becomes less efficient when several orders of multiples are involved
  • For such data we use the wave-equation schemes, especially when the main free-surface multiples are just water-layer multiples and peg-legs
  • The 3D WE approach has fewer sampling problems than 3D SRME and it allows us to use different WE extrapolation schemes for different complexities of sea-floor and structure below it