is effective in attenuating some free surface and peg-leg multiples. Images show effective attenuation of strong multiple energy, but, POIC does not work well for ...
The application of primary-only imaging condition to SMAART data Min Zhou*, Hongchuan Sun and Gerard T. Schuster, University of Utah Summary The primary only imaging condition (POIC) is tested on 2-D SMAART JV data associated with a complicated velocity model. This data set is one of the most difficult for any demultiple algorithm. Results indicate that POIC is effective in attenuating some free surface and peg-leg multiples. Images show effective attenuation of strong multiple energy, but, POIC does not work well for multiples with weak energy such as interbed multiples and the multiples associated with the dipping bottom boundary of the salt bodies. The next step is to adjust POIC to handle these weaker events.
Introduction In earlier reports (Wang et al., 1999; Sun, 2001; Liu and Sun, 2002), the primary-only imaging condition (POIC) was developed and used to discriminate primary reflections from multiple reflections. POIC was applied to both 2-D SEG/EAGE salt and Mobil marine data. Unlike those moveout-based methods which transform the data to either F-K or Tau-P domains (Foster and Mosher, 1992), POIC discriminates primary from multiple reflections by using a traveltime constraint, an incidence angle constraint and a take-off angle constraint. The angle constraints are in fact similar to the beam focusing criteria used by Heinze et al., 2001. In addition, POIC is different from some of the prediction and subtraction methods (Verschuur and Berkhout, 1997) because it does not subtract multiple events from the primary events, instead it migrates only the primary reflections by using prestack wavepath migration (Sun and Schuster, 1999). In this paper, we apply POIC to the Pluto 1.5 data, a 2D synthetic elastic dataset provided by the SMAART JV (www.smaartjv.com). The dataset has strong surfacerelated and interbed multiples associated with the water bottom and salt interfaces. The multiples are frequently non-hyperbolic and the deep water sediments are slow so that the moveout-based methods can not work well. However, some of the prediction and subtraction methods can do a good job here (Verwest et al., 2001; Hill et al., 2002). Our prestack depth imaging results show that POIC is effective in attenuating both free surface multiples, and some of the peg-leg multiples associated with the complex Pluto 1.5 model. POIC, in its current incarnation, does not work well for the weak interbed multiples and the multiples associated with the dipping bottom boundary of the salt bodies due to their weak signals. Possible
improvements to POIC include picking events with conflicting dips, developing more accurate ray-tracing algorithm which can handle large velocity variations in salt boundaries, using it in an iterative mode, and also incorporating geologic information to predict the multiples.
Depth Imaging The SMAART JV Pluto 1.5 data were computed on a 25 by 25 foot grid, there are 1387 shot gathers, each of which contains 540 traces with a recording length of 9000 samples and a sample inverval of 8 ms. The synthetic model is shown in Figure 1. Both the source and the receiver intervals are 75 feet. In the paper, we use the true velocity model in all migrations, but with a 75 by 75 foot grid. Figure 2 (upper) shows the Kirchhoff prestack depth migration image without applying POIC. Figure 2 (lower) shows the depth image from POIC, where up to 100 events in each trace are picked, evaluated, and then migrated if they are associated with primary reflection energy. POIC does not work well for the near-offset data (Sun, 2001) because the differences of incidence angles between primary and multiples decrease with the offset. For POIC, only the traces with offsets larger than 4000 feet are used. The total cost of POIC is almost the same as KM migration or much less due to the efficiency of WM migration (Sun and Schuster, 1999). The migration artifacts generated by the free-surface multiples associated with the water bottom (see the zoomed views in Figure 3), and the top boundary of the salt bodies (see zoomed views in Figure 4) are successfully removed. The peg-leg multiples associated with the top of the salt are also attenuated as shown in Figure 5. These encouraging results indicate that the POIC algorithm works well to separate the primaries from the multiples. However, POIC does not work well with some of the multiples, such as those free-surface multiples associated with the dipping bottom of the salt body and some of the interbed multiples (see zoomed views in Figure 6). These interbed multiples are very close to the strong primaries associated with the bottom of the salt bodies.
A Robust POIC Strategy Some of the results indicate that POIC eliminates both primaries and multiples. We need an additional test to put these primaries back into POIC migrated section. This additional test is as follows:
The application of primary-only imaging condition to SMAART data
1. Denote as section A the POIC migration image (these events passed the POIC test) and the migrated events that does not pass the POIC test as B (this can be constructed by subtracting the wavepath migration image from the panel A). 2. Resort the migrated images that composed panel B into common image gathers. Primaries should be flat, multiples should be curved. Filter these CIG’s with a 5-trace median filter and the result should be primaries only. Add these events back into section A. Many variations of this simple strategy can be developed. This strategy and similar approaches are perhaps the key to making POIC robust and simple.
Heinze, W. D., Sherwood, J. W., and Tieman, H., 2001, An effectient 3D depth model-based, coherent noise suppression technique: 71nd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 19771980.
Conclusions
Sun, H., and Schuster, G. T., 1999, Wavepath migration versus Kirchhoff migration: 69th Ann. Internat. Mtg.,Soc. Expl. Geophys., Expanded Abstracts, 1138-1141.
The POIC algorithm was applied to 2-D SMAART JV Pluto 1.5 dataset for multiple removal. Results show that POIC is effective and efficient in attenuating some of the surface-related (free-surface and peg-leg) multiples. It is as fast as standard Kirchhoff migration when up to 100 events in each trace are picked and migrated so that it is affordable for 3-D data. Our current version of POIC does not work well with weak interbed multiples and some of the weak surface-related multiples. In order to include the weak signals in migration, more events in each trace need to be picked. The accuracy of separating primaries and multiples in POIC depends on the angle picking process and ray-tracing algorithm. A more careful picking and more sophisticated ray-tracing will improve the performance of POIC, and iterative application of POIC to the data may prove beneficial. POIC can not make use of the near offset data due to the small angle differences between the primary and multiple events in near offset data. Since the POIC criterion is applied trace by trace, for each event, success for far offset traces does not help evaluate the near offset traces. A POIC algorithm which is applied more globally – applying POIC to each event instead of each trace, can include the near offset events and improve the resolution of migration image. This flexibility of POIC makes us optimistic in improving the performance of POIC in suppressing a wider class of multiples.
Hill, R. N., Langan, R. T., Nemeth, T., and Zhao, M., 2002, Beam methods for predictive suppression of seismic multiples in deep water: 72nd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 2118-2121. Liu, Y., and Sun, H., 2002, The application of primaryonly imaging condition to Mobil data: UTAM 2001 Annual Report, 17-32.
Sun, H., 2001, Multiple removal by primary-only imaging condition: UTAM 2001 Mid-year Report, 25-37. Verschuur, D. J., and Berkhout, A. J., 1997, Estimation of multiple scattering by iterative inversion, Part I: Theoretical considerations, Geophysics, 62, 15861595. Verschuur, D. J., and Berkhout, A. J., 1997, Estimation of multiple scattering by iterative inversion, Part II: Practical aspects and examples, Geophysics, 62, 1596-1611. Verwest, B., Karazincir, M., and Cooper, N., 2001, Surface related multiple suppression and imaging of SMAART JV Pluto 1.5 dataset: EAGE 63rd Conference & Technical Exhibition, A-34. Wang, Y., Sun, H., and Schuster, G. T., 1999, Migration imaging condition for primary reflections: 69th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 1126-1129. Wiggins, J. W., 1988, Attenuation of complex waterbottom multiples by wave-equation-based prediction and subtraction: Geophysics, 53, 1527-1539.
Acknowledgements
References Foster, D. J. and Mosher C. G., 1992, Suppression of multiple reflections using the Radon transform: Geophysics, 57, 386-395.
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We are grateful for the financial support from the members of the 2002 University of Utah Tomography and Modeling/Migration (UTAM) Consortium (http://utam.gg.utah.edu). We also thank SMAART JV (www.smaartjv.com) for the dataset.
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Figure 1:
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P-wave velocity model of SMAART JV.
The application of primary-only imaging condition to SMAART data
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Figure 2: Depth images of SMAART dataset. (Upper) KM image, all data are used; (lower ) POIC image, data from CSG No. 50 to CSG No. 1337, and with the offset between 4050 and 27,000 feet are used in migration.
The application of primary-only imaging condition to SMAART data
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Figure 3: Zoom view of migration artifacts associated with the 1st order free-surface multiple of water bottom. (Upper) Velocity model; (middle) Kirchhoff image; (lower) POIC image. The multiple is successfully removed by POIC, but some weak primaries are also dimmed out.
Figure 5: Migration artifacts associated with the peg-leg multiples for the middle salt. (Upper) Velocity model; (middle) KM image; (lower) POIC image. Arrows indicate the places the peg-leg multiples are presented.
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Figure 4: Zoom view of migration artifacts associated with 1st order free-surface multiples of the salt bodies. The multiples associated with the upper boundaries of the salt bodies are greatly attenuated. The multiples associated with the lower boundary of the salt still remain.
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Figure 6: Zoom views of the multiples remaining after applying POIC. (Left) KM images; (right) POIC images. POIC fails to attenuate some of the free-surface related and interbed multiples associated with the bottom boundary of the salt body.
