Transcript Snorre Time lapse Processing and analysis

Suppression of multiples from
complex sea-floor by a waveequation approach
Dmitri Lokshtanov
Norsk Hydro Research Centre, Bergen.
Outline
 Main features of WE approach
 Suppression of water-layer multiples and peg-legs
 Prediction of water-layer multiples and peg-legs
 Conclusions
WE approach - the main features
 Suppresses water-layer multiples and peg-legs. Requires approximate
knowledge of the water-bottom geometry.
 The predicted multiples are split into three terms. Each term contains
multiple events which require the same amplitude correction. All
multiples of all orders are suppressed simultaneously in one
consistent step (in one or a few time windows).
 The prediction of multiples is performed in the same domain as used
for multiple suppression. Both multiple reflections and diffractions
are predicted.
Subtraction of water-layer multiples - 1
1. Remove pure multiples and receiver - side peg - legs : F1  (1  Pg ) D , where
D are input data along the profile; Pg is the receiver - side extrapolat ion operator.
2. Remove source - side peg - legs : F2  (1  Ps ) F1 , where Ps is the source - side
extrapolation operator.
3. Correct for the first - order water - layer multiple : F (1  Ps )(1  Pg ) D  Ps Dw ,
where Dw is the primary reflection from the water - bottom.
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).
Subtraction of water-layer multiples - 2
F (1  Ps )(1  Pg ) D  Ps Dw ,
(1)
We apply the ‘scaled version’ of (1) trace by trace to the tau-p transformed CMP
or common shot gathers. For each p-trace the operator has the form:
f (t )  d (t )  rg (t )  d g (t )  rs (t )  d s (t )  rsg (t )  d sg (t ),
( 2)
where d (t ), d g (t ), d s (t ), d sg (t ) are p - traces for the input data and the results
of extrapolation through the water-layer from the receiver-side, source-side (of
muted input data) and source-side after receiver-side respectively.
Two approaches for multiple prediction
 Simple ‘locally’ 1D sea-floor; arbitrary 2D structure
below it. The procedure starts from the Radon
transformed CMP gathers
 Complex sea-floor; arbitrary 2D structure below it. The
procedure starts from the Radon transformed CS gathers.
 T. Shetland
 T. Draupne
 T. Brent
Stack before multiple suppression
Stack after WE multiple suppression
Constant P sections (angle at the surface is about 10º)
Input
After WE multiple suppression
Stack before multiple suppression (left) and after WE 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.
Constant P sections (angle at the surface is about 8º)
Input
After WE multiple suppression
Difference
Prediction of multiples from the receiver side for irregular sea-floor
1. Extrapolate Radon transformed CS gather D ( p r , x s , ) down to the sea-floor:
W  x, z ( x), xs  

D ( p , x ) expi  p ( x  x )  q z ( x)dp ,

2
r
s
r
s
r
r
2. Calculate the amplitude Dg ( psc , xs ) of the reflected/scattered plane wave with
slowness psc (Wenzel et al., 1990)

dz p sc 
Dg ( p sc , x s )   W  x, z ( x ), x s   1 

 expi  p sc ( x  x s )  q sc z ( x )dx.
dx q sc 

Prediction of multiples from the source side for irregular sea-floor
1.
Use FFT to decompose the input data D ( p r , x s ,  ) into contributions with
different propagation angles (wavenumbers) from the source side:
R( pr , k , ) 
 D( p
r
, x s ,  ) exp  ikxs dx s ,
where the source-side wavenumber k s is defined as: k s   p r  k .
2.
Extrapolate the results of decomposition down to the sea-floor:
W  x, z ( x ), p r  
3.
1
R( p

2
r
, k ) exp ik s x  k sz z ( x ) dk .
Calculate reflected / scattered responses for each k and then use inverse FFT to define
Ds ( p r , x s , )
. All steps are performed in a double loop over pr and over .
Velocity model for FD modelling (with ProMax)
Input P-section (zero angle)
Receiver-side prediction
Input P-section (zero angle)
Receiver-side prediction
Input P-section (zero angle)
After prediction + subtraction
Input P-section (angle 20 degrees)
Receiver-side prediction
Input P-section (20 degrees)
Receiver-side prediction
Source-side prediction
Input P-section (20 degrees)
After prediction + subtraction
Input CS gather
After prediction + subtraction
Input CS gather
After prediction + subtraction
Recent papers on WE approach
 Jiao J., Leger P. and Stevens J, 2002, Enhancements to
wave-equation multiple attenuation, 72nd SEG Meeting,
Expanded Abstracts.
 Hill R., Langan R., Nemeth T., Zhao M. and Bube K.,
2002, Beam methods for predictive suppression of
seismic multiples in deep water, 72nd SEG Meeting,
Expanded Abstracts.
 Hugonnet P., 2002, Partial surface related multiple
elimination, 72nd SEG Meeting, Expanded Abstracts.
Conclusions
 WE approach – performs well if the main free-surface multiples are
water-layer multiples and peg-legs and if the structural variations in the
crossline direction are not severe
 Both multiple reflections and multiple diffractions are accounted for.
All predicted multiples of all orders are suppressed simultaneously in
one consistent step
 The prediction of multiples is performed in the same domain as used for
multiple suppression
Acknowledgements
 Many thanks to Norsk Hydro and CREWES for one
year of freedom in a fantastic country – Canada !
 Thanks to Norsk Hydro for permission to present the
paper.