Data Driven Interbed Multiple Attenuation

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Data-driven Interbed Multiple Removal: Strategies and Examples Roald van Borselen*, PGS Geophysical

Summary

Removal of multiples from seismic reflection data is anessential pre-processing step before seismic imaging in

many marine environments. Surface-Related MultipleElimination (SRME) has proven to be a valuable tool toremove multiples that have been generated by the free-surface. This paper discusses the application of SRMEwhere, instead of free-surface multiples, multiples

generated by internal surfaces are removed. Particularattention is paid to implementation issues. The proposedstrategy requires limited user-interaction, and iscomparable in performance and speed to ‘standard’ SRME.

Application to data sets from the Gulf of Suez, and from

the North Sea leads to encouraging results.

Introduction

Although free-surface multiples usually exhibit stronger

amplitudes than interbed multiples, strong impedancecontrast at internal reflectors (such as top / base of salt orchalk, unconformities, disconformities etc.) may still createa complicated set of interbed multiples that can easily

obscure primary reflections from relatively weaksedimentary reflectors.

Similar to the removal of free-surface multiples, there are

two generic strategies to tackle interbed multiples: model-driven methods, that make use of statistical assumptions

and/or a priori information about the subsurface (such as alocal 1D assumption, detailed velocity- and/or reflector

information), and data-driven methods, that use themeasured data itself to predict and, subsequently subtractinterbed multiples.

Although model-driven approaches have been appliedsuccessfully, the reliability of the inherent assumptions andthe user-provided priori information, as well as the level ofuser-interaction required makes these methods less suitable

for efficient, 3D production processing.

Surface Related Multiple Elimination (SRME) removes allmultiples that are introduced by a particular surface in theEarth. To remove these multiples, some form of a prioriinformation from this surface (such as the geometry, andthe reflection coefficients) is required. Since this

information is readily available for the water surface, it is possible to remove all multiples that have been generated by the water surface, without using any additionalinformation about the subsurface. This application is fully

data driven, meaning that only the data itself is used to

predict the multiples. Because user interaction isminimised, SRME has been successfully applied to removefree-surface related multiples in production processingenvironments, in most geological regions (see, for example,

Van Borselen et al. (1997), King et al. (2000), Long et al.(2000)).

This abstract discusses the application of SRME to removemultiples generated by an internal surface. The approach

taken is based on work proposed by Verschuur andBerkhout (1997) and Jacubowitz (1998), and requires onlythe identification (i.e. picking) of the multiple generator, orthe identification of a pseudo boundary along which

internal multiples have crossed during wavefield

propagation.

Methodology

The application of SRME to remove interbed multiples has

been discussed by various authors. In Berkhout (1982), it isdescribed that, by means of downward continuation of boththe source- and receiver wavefield to the multiplegenerator, interbed multiples can be predicted using the

well-known feedback loop. Later, in Verschuur andBerkhout (1997), this approach was reformulated makinguse of common focus point gathers. Also, an extension ofthe method was presented that allowed for the prediction of

all multiples that crossed a particular boundary. In Matsonet al. (1999), a method based on inverse scattering is

presented that is not interface specific and attenuates allinterbed multiples of a given order at once.

In Jakubowicz (1998), it was shown that the process ofdownward continuation could be circumvented bydecomposing interbed multiples into three wavefield

constituents that can be readily obtained from the measureddata via a simple muting procedure. It is shown howinterbed multiples can be composed by cross-correlating awavefield from reflectors below the multiple generator with

the primary of the multiple generator, followed by aconvolution of this result with the wavefield from reflectors

below the multiple generator. In Jakubowicz (1998), it isalso shown how this method can be extended to sequencesof multiple reflections, rather than just individual events.Using this approach, all internal multiples generated by thechosen boundary are predicted.

In Van Borselen (2002), an extension of this procedure isillustrated to remove interbed multiples that have crossed a

pseudo-boundary that is chosen to lie between two strong

internal reflectors. At the same time, it is shown that this

SEG Int'l Exposition and 72nd Annual Meeting * Salt Lake City, Utah * October 6-11, 2002



Data-Driven Interbed Multiple Removal: Strategy and Examples

method must be applied in a top-down fashion to prevent‘leakage’. Leakage occurs when internal multiples

generated by a reflector above the chosen multiplegenerator are mistaken for primary events, and generate

during the prediction process non-physical events.

Implementation

Cost and turnaround time continue to play an importantrole in production processing of large 3D seismic data sets.In the proposed processing flow, the application of internalSurface-Related Multiple Elimination is preceded by the

removal of all non-physical noise, regularisation of themeasured data to obtain a constant grid of sources andreceivers, the interpolation of missing near- andintermediate offsets and missing shots, removal of the

direct wave and its surface reflection, and the removal ofall free-surface related multiples.

As discussed in the previous section, the identification of

either the internal multiple generating boundary, or a pseudo boundary is crucial. Only by minimizing the userinteraction in the process of boundary identification andmuting, “production processing” of larger 3D data volumes

becomes feasible. New visualization techniques, andsophisticated automatic horizon pickers play a crucial role

in the reflector identification process and quality-control.Once the relevant reflector information is retrieved, mutedshot- and receiver gathers are generated automatically, afterwhich the interbed multiples are generated through

temporal and spatial convolutions of these gathers. Finally,the predicted interbed multiples are subtracted from theinput data, using the minimum energy criterion, which

states that, after the subtraction of the multiples, the totalenergy in the seismogram should be minimized.

North Sea Example

Large areas of the North Sea suffer severe multiple

problems, which are particularly frustrating whencombined with the low reflectivity of the prospectiveCretaceous section. Figure 1 shows an example from theJæren High Ula North area. The water bottom in the survey

area is relatively shallow (125 m), resulting in a strong trainof short-period, free-surface multiples. In addition, the

presence of strong internal reflectors, such as the top- and

base of Chalk, generate interbed multiples that are maskingdipping primary reflectors. Removal of multiple energy

remains the foremost obstacle to successful seismicimaging in this part of the world.

Figure 1a shows the stacked section of the raw data andFig. 1b shows the result after the removal of free-surface

multiple using ‘standard’ SRME. Note the significantreduction of multiples in areas indicated by the white

arrows. Next, the top of chalk was chosen as reflector fromwhich to remove the corresponding interbed multiples.

Figure 1c shows the result after the application of ISRME. Note the additional reduction of multiple energy, especially

beneath the crest and along the flanks of the structure.Figure 1d shows the true-amplitude difference betweenFigs. 1b and 1c. It is noted that hardly any primary energycan be detected, indicating that primaries remained

unaffected.

Gulf of Suez Example

Seismic exploration in the Gulf of Suez is often hindered by the presence of both free-surface, and interbedmultiples. Figure 2 shows an example from the Octoberarea. The water bottom depth varies from 50-75 meters,

resulting in strong, short-period water layer reverberations.In addition, internal reflectors, such as the top- and base ofsalt and the Zeit formation, generate interbed multiples thatobscure any primary energy at target levels.

Figure 2a shows the stacked section of the pre-processeddata before demultiple. Figure 2b shows the result after theremoval of free-surface multiple using ‘standard’ SRME.

Note the significant reduction of multiple energy. Next, pseudo boundary A, hung off the water bottom, was chosen

to remove all interbed multiples that crossed this boundary(see Figure 2c). Figure 2d shows the predicted interbedmultiples that have crossed pseudo boundary A. Figure 2eshows the stacked result after adaptive subtraction of the

multiples, and Fig. 2f shows the true-amplitude difference between Figs. 2b and 2e. Note the significant reduction ofmultiple in the target area. Next, pseudo-boundary B above

the base of salt was chosen to remove any remaininginterbed multiples that have been reflected back into theearth by the Zeit formation. (see Figure 2c). Figure 2gshows the corresponding interbed multiples after stacking.Figure 2h shows the stacked result after adaptivesubtraction, and Fig. 2i shows the difference between Figs.

2e and 2h. Note that hardly any primary energy can bedetected in Figs. 2f and 2i, indicating that primariesremained unaffected.

Conclusions

Surface-related multiple elimination has successfully been

applied to remove interbed multiples from a North Sea-,and a Gulf of Suez data set. An optimized processing

strategy leads to efficient removal of either interbedmultiples that are generated by a (chosen) internal reflector,or interbed multiples that have crossed a (chosen) pseudo

boundary during wavefield propagation. Because user-interaction is minimized, the method is suitable to remove

interbed multiples from large 3D data volumes in a production processing environment.





Acknowledgments

The author thanks PGS Geophysical for permission to publish this work, and Ian Threadgold, Norman Allager

(GUPCO), Ken Matson, Narul Kabir (BP UTG), and EricVerschuur (DELPHI) for many fruitful discussions onmethodology- and application-related issues.

References

Berkhout, A.J., 1982, Seismic Migration, Imaging ofAcoustic Wave Energy by Wave Field Extrapolation, A:

Theoretical Aspects, second edition, Elsevier SciencePublications;

Borselen, van R.G., Hanna, M., Nekut, T., Berg van der,

P.M., Fokkema J.T., 1997, Surface-related MultipleRemoval – Applications to the Gulf of Mexico, SEG 1997Expanded Abstracts;

Borselen, van R.G., 2002, Fast track, Data Driven InterbedMultiple Removal – Application to the North Sea, EAGE2002 Expanded Abstracts;

Jakubowicz, H., 1998, Wave Equation Prediction andRemoval of Interbed Multiples, EAGE 1998 Expanded

Abstracts;

King, D., Høy, T., Borselen, van R.G., Brittan J., 2001,Multiple Removal in the Norwegian Sea – A Comparisonof Methods, EAGE 2001 Expanded Abstracts;

Long, A.S., Borselen, van R.G. and Fountain L., 2001,Surface-Related Multiple Elimination - Applications toOffshore Australia Data Sets; ASEG 2001 ExpandedAbstracts;

Matson, K. Corrigan D., Weglein A. and Young C., 1999,Inverse Scattering Internal Multiple Attenuation: Resultsfrom Complex Synthetic and Field Data Examples, SEG

1999 Expanded Abstracts;

Verschuur, D.J., and Berkhout, A.J., 1997, Estimation ofMultiple Scattering by Iterative Inversion, Part II: Practical

Aspects and Examples, Geophysics, 62, No. 5, 1596-1611.

Top

Chalk

(a) (b)

(d)(c)

Figure 1: a) Stacked section before demultiple, b) Stacked section after the removal of free-surface multiples,

c) Stacked section after the removal of interbed multiples that were generated by the top of Chalk, d)

Difference between before and after interbed multiple removal.





← A

← Zeit← B← Bos

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 2: a) Stacked section before demultiple, b) After the removal of free-surface multiples, c) Pseudo boundaries

and B, d) Predicted interbed multiples crossing pseudo boundary A, e) After adaptive subtraction, f) Difference

between b and e, g) Predicted interbed multiples crossing pseudo boundary B, h) After adaptive subtraction, I)

Difference between e and h.


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Data Driven Interbed Multiple Attenuation