DOI QR코드

DOI QR Code

Improvement of Reverse-time Migration using Homogenization of Acoustic Impedance

음향 임피던스 균질화를 이용한 거꿀시간 참반사보정 성능개선

  • Lee, Gang Hoon (Department of Energy Resources Engineering, Inha University) ;
  • Pyun, Sukjoon (Department of Energy Resources Engineering, Inha University) ;
  • Park, Yunhui (Department of Energy Resources Engineering, Inha University) ;
  • Cheong, Snons (Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources)
  • 이강훈 (인하대학교 에너지자원공학과) ;
  • 편석준 (인하대학교 에너지자원공학과) ;
  • 박윤희 (인하대학교 에너지자원공학과) ;
  • 정순홍 (한국지질자원연구원 석유해저연구본부)
  • Received : 2016.03.17
  • Accepted : 2016.05.31
  • Published : 2016.05.31

Abstract

Migration image can be distorted due to reflected waves in the source and receiver wavefields when discontinuities of input velocity model exist in seismic imaging. To remove reflected waves coming from layer interfaces, it is a common practice to smooth the velocity model for migration. If the velocity model is smoothed, however, the subsurface image can be distorted because the velocity changes around interfaces. In this paper, we attempt to minimize the distortion by reducing reflection energy in the source and receiver wavefields through acoustic impedance homogenization. To make acoustic impedance constant, we define fake density model and use it for migration. When the acoustic impedance is constant over all layers, the reflection coefficient at normal incidence becomes zero and the minimized reflection energy results in the improvement of migration result. To verify our algorithm, we implement the reverse-time migration using cell-based finite-difference method. Through numerical examples, we can note that the migration image is improved at the layer interfaces with high velocity contrast, and it shows the marked improvement particularly in the shallow part.

탄성파 자료의 영상화 과정에서 입력자료인 속도 모델에 불연속면이 있는 경우 반사파에 의해 참반사보정(migration) 결과가 왜곡될 수 있다. 따라서 참반사보정을 위한 속도 모델은 지층 경계면에서 샘 파동장과 수신기 파동장을 구할 때 발생하는 반사파를 제거하기 위해 평활화(smoothing)하여 사용하는 것이 일반적이다. 그러나 속도 모델을 평활화할 경우 지층 경계면에서 속도 정보가 달라져 지하구조 영상이 왜곡될 가능성이 있다. 본 연구에서는 이러한 단점을 최소화하기 위해 속도가 불연속인 층간의 음향 임피던스를 일정하게 만들어 샘 파동장과 수신기 파동장을 구할 때 발생하는 반사파를 줄이고자 하였다. 음향 임피던스를 일정하게 만들기 위해 속도 차이를 보상하는 가상의 밀도(fake density)를 정의하고 참반사보정에 사용하였다. 음향 임피던스가 모든 층에서 일정할 때, 반사면에서 수직 입사파의 반사계수가 영이 되고 반사파가 최소화되어 참반사보정 결과를 향상시킬 수 있다. 이를 검증하기 위해 셀기반 유한차분법을 이용하여 거꿀시간 참반사보정(reverse-time migration) 알고리듬을 구현하였다. 수치예제를 통해 속도 대비가 큰 지층 경계면에서 참반사보정 영상의 품질이 향상되는 것을 확인할 수 있고, 특히 천부 지층에서 성능 개선효과가 큰 것을 관찰할 수 있다.

Acknowledgement

Supported by : 한국에너지기술평가원(KETEP), 한국지질자원연구원

References

  1. Alkhalifah, T., 2000, An acoustic wave equation for anisotropic media, Geophysics, 65(4), 1239-1250. https://doi.org/10.1190/1.1444815
  2. Aminzadeh, F., Burkhard, N., Long, J., Kunz, T., and Duclos, P., 1996, Three dimensional SEG/EAEG models-an update, The Leading Edge, 15(2), 131-134. https://doi.org/10.1190/1.1437283
  3. Baysal, E., Kosloff, D. D., and Sherwood, J. W. C., 1983, Reverse time migration, Geophysics, 48(11), 1514-1524. https://doi.org/10.1190/1.1441434
  4. Baysal, E., Kosloff, D. D., and Sherwood, J. W. C., 1984, A two-way nonreflecting wave equation, Geophysics, 49(2), 132-141. https://doi.org/10.1190/1.1441644
  5. Chang, W. F., and McMechan, G. A., 1986, Reverse-time migration of offset vertical seismic profiling data using the excitationtime imaging condition, Geophysics, 51(1), 67-84. https://doi.org/10.1190/1.1442041
  6. Claerbout, J. F., 1971, Toward a unified theory of reflector mapping, Geophysics, 36(3), 467-481. https://doi.org/10.1190/1.1440185
  7. Díaz, E., and Sava, P., 2016, Understanding the reverse time migration backscattering: noise or signal?, Geophysical Prospecting, 64(2), 581-594. https://doi.org/10.1111/1365-2478.12232
  8. Du, Q., Gong, X., Zhang, M., Zhu, Y., and Fang, G., 2014, 3D PS-wave imaging with elastic reverse-time migration, Geophysics, 79(5), S173-S184.
  9. Etgen, J., 1986, Pre-stack reverse time migration of shot profiles: Sep-50, 151-170.
  10. Etgen, J., and Brandsberg-Dahl, S., 2009, The pseudo-analytical method: Application of pseudo-Laplacians to acoustic and acoustic anisotropic wave propagation, 79th Annual International Meeting, SEG, Expanded Abstracts, 2552-2556.
  11. Etgen, J., Gray, S. H., and Zhang, Y., 2009, An overview of depth imaging in exploration geophysics, Geophysics, 74(6), WCA5-WCA17. https://doi.org/10.1190/1.3223188
  12. Fletcher, R., Du, X., and Fowler, P. J., 2008, A new pseudoacoustic wave equation for TI media, 78th Annual International Meeting, SEG, Expanded Abstracts, 2082-2086.
  13. Gray, S., 2000, Velocity smoothing for depth migration: How much is too much, 70th Annual International Meeting, SEG, Expanded Abstracts, 1055-1058.
  14. Jones, I. F., 2014, Tutorial: migration imaging conditions, First Break, 32(12), 45-55.
  15. Lee, H. Y., Min, D. J., Kwon, B. D., and Yoo, H. S., 2008, Time-Domain Elastic Wave Modeling in Anisotropic Media using Cell-Based Finite-Difference Method, Journal of the Korean Society for Geosystem Engineering, 45(5), 536-545.
  16. Levin, S. A., 1984, Principle of reverse-time migration, Geophysics, 49(5), 581-583. https://doi.org/10.1190/1.1441693
  17. Loewenthal, D., and Mufti, I. R., 1983, Reversed time migration in spatial frequency domain, Geophysics, 48(5), 627-635. https://doi.org/10.1190/1.1441493
  18. McMechan, G. A., 1983, Migration by extrapolation of timedependent boundary values, Geophysical Prospecting, 31(3), 413-420. https://doi.org/10.1111/j.1365-2478.1983.tb01060.x
  19. Min, D. J., Shin, C., and Yoo, H. S., 2004, Free-surface boundary condition in finite-difference elastic wave modeling, Bulletin of the Seismological Society of America, 94(1), 237-250. https://doi.org/10.1785/0120020116
  20. Paffenholz, J., McLain, B., Zaske, J., and Keliher, P. J., 2002, Subsalt multiple attenuation and imaging: Observations from the Sigsbee2B synthetic dataset, 72nd Annual International Meeting, SEG, Expanded Abstracts, 2122-2125.
  21. Park, Y., and Pyun, S., 2013, Application of Effective Regularization to Gradient-based Seismic Full Waveform Inversion using Selective Smoothing Coefficients, Jigu-Mulliwa-Mulli-Tamsa, 16(4), 211-216.
  22. Ravasi, M., Vasconcelos, I., Curtis, A., and Kritski, A., 2015, Vector-acoustic reverse time migration of Volve ocean-bottom cable data set without up/down decomposed wavefields, Geophysics, 80(4), S137-S150. https://doi.org/10.1190/geo2014-0554.1
  23. Sava, P., and Hill, S. J., 2009, Overview and classification of wavefield seismic imaging methods, The Leading Edge, 28(2), 170-183. https://doi.org/10.1190/1.3086052
  24. Sun, R., McMechan, G. A., Lee, C. S., Chow, J., and Chen, C. H., 2006, Prestack scalar reverse-time depth migration of 3D elastic seismic data, Geophysics, 71(5), S199-S207. https://doi.org/10.1190/1.2227519
  25. Tang, B., Xu, S., and Zhou, H., 2014, A fast RTM implementation in TTI media, 84th Annual International Meeting, SEG, Expanded Abstracts, 3892-3897.
  26. Versteeg, R. J., 1993, Sensitivity of prestack depth migration to the velocity model, Geophysics, 58(6), 873-882. https://doi.org/10.1190/1.1443471
  27. Whitmore, N. D., 1983, Iterative depth migration by backward time propagation, 53th Annual International Meeting, SEG, Expanded Abstracts, 382-385.
  28. Yan, J., and Sava, P., 2008, Isotropic angle-domain elastic reversetime migration, Geophysics, 73(6), S229-S239. https://doi.org/10.1190/1.2981241
  29. Yan, R., and Xie, X. B., 2012, An angle-domain imaging condition for elastic reverse time migration and its application to angle gather extraction, Geophysics, 77(5), S105-S115. https://doi.org/10.1190/geo2011-0455.1
  30. Yoon, K., and Marfurt, K. J., 2006, Reverse-time migration using the poynting vector, Exploration Geophysics, 37(1), 102-107.
  31. Zhang, H., and Zhang, Y., 2008, Reverse time migration in 3D heterogeneous TTI media, 78th Annual International Meeting, SEG, Expanded Abstracts, 2196-2200.
  32. Zhou, H., Zhang, G., and Bloor, R., 2006, An anisotropic acoustic wave equation for modeling and migration in 2D TTI media, 76th Annual International Meeting, SEG, Expanded Abstracts, 194-198.