Geophysics and Geophysical Exploration (지구물리와물리탐사)

Korean Society of Earth and Exploration Geophysicists (한국지구물리·물리탐사학회)
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Correction of the Sea Effect in the Magnetotelluric (MT) Data Using an Iterative Tensor Stripping During Inversion

MT 자료 역산과정에서 반복적인 Tensor Stripping을 통한 해양효과 보정

  • Yang, Jun-Mo (Deep. Sea & Marine Resource Research Division, Korea Ocean Research & Development Institute) ;
  • Lee, Chun-Ki (Groundwater & Geothermal Resource Division, Korea Institute Geoscience & Mineral Resources) ;
  • Yoo, Hai-Soo (Deep. Sea & Marine Resource Research Division, Korea Ocean Research & Development Institute)
  • 양준모 (한국해양연구원 심해해저자원연구부) ;
  • 이춘기 (한국지질자원연구원 지하수지열연구부) ;
  • 유해수 (한국해양연구원 심해해저자원연구부)
  • Published : 2008.11.30
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Abstract

When magnetotelluric (MT) data are obtained in vicinity of the coast, the sea can distort observed MT responses, especially those of deep part of subsurface. We introduce an iterative method to correct the sea effect, based on the previous topographic correction method which removes the distortions due to topographic changes in seafloor MT data. The method first corrects the sea effect in observed MT impedance, and then inverts corrected responses in a model space without the sea. Due to mutual coupling between sea and subsurface structure, the correction and inversion steps are iterated until changes in each result become negligible. The method is validated for 1-D and 2-D structure using synthetic MT data produced by 3-D forward modeling including surrounding seas. In all cases, the method closely recovers the given structure after a few iterations. To test the applicability of the proposed method to field data, we generate synthetic MT data for the Jeju Island whose 1-D conductivity structure is well known, using 3-D forward modeling. The distortions due to the surrounding sea start to appear below the frequency about 1 Hz, and are relatively severe in the electrical field perpendicular to the coastline because of the location of the observation sites. The proposed method successfully eliminates the sea effect after three iterations, and both 1-D and 2-D inversion of corrected responses closely recover the given subsurface structure of the Jeju Island model.

해양과 인접한 지역에서 MT탐사 자료를 해석하는 경우, 해양은 심부 구조의 반응을 왜곡시키는 역할을 한다. 본 연구에서는 해저면 MT탐사에서 해저 지형의 영향을 제거하는 지형보정 기법을 바탕으로, 반복적으로 해양효과를 보정하는 기법을 개발하였다. 제안된 기법은 우선 관측 MT 반응에서 주변 해양의 영향을 보정하고, 그 후 해양이 없는 모델공간에서 역산을 수행한다. 주변 해양과 지하구조와의 상호결합 때문에 이 과정은 반복적으로 수행되며, 보정 결과의 변화가 미미할 때 반복과정이 종료된다. 제안된 기법의 검증을 위해 해양을 포함하는 3차원 순산 모델링을 통하여 합성 자료를 생성하였고, 1차원 및 2차원 구조에서 보정기법을 적용하였다. 대체적으로 제안된 해양효과 보정 기법은 $2{\sim}3$회의 반복단계를 거친 후 해양이 없는 경우의 지하구조를 성공적으로 복원하였다. 실제 MT 자료와 유사한 자료를 획득하기 위해 1차원 구조로 잘 알려져 있는 제주도에 대한 3차원 모델링을 수행하여 유사 현장 자료를 생성하였다. 제주도 모델의 경우, 해양효과는 약 1 Hz 이하에서 나타나기 시작하였으며 측선의 위치 때문에 해안선과 수직한 전기장 성분에서 상대 적으로 왜곡이 심하게 나타났다. 이러한 왜곡은 3회의 해양효과 보정과정을 통해 성공적으로 제거되었으며, 1차원 및 2 차원 역산은 모델링 시 가정한 제주도의 지하구조를 성공적으로 복원하였다.

Keywords

References

  1. 남명진, 2006, MT 탐사의 3차원 지형효과 분석 연구, 서울대학교 박사학위 논문, 서울대학교
  2. 송윤호, 조인기, 김정호, 정승환, 전정수, 1992, 제주도에서의 MT 탐사, 전기, 전자탐사 연구, 과학기술처, KR-92-1G-5, 한 국자원연구소
  3. 송윤호, 이태종, T. Uchida, 2006, 가청주파수 대역 MT 탐사에서의 원거리 기준점의 효과, 한국지구시스템공학회지, 43, 44-54
  4. 양준모, 2006, 자동제거 기법을 통한 MT 및 GDS 자료처리와 한반도 및 주변의 지전기 이상 해석, 서울대학교 박사학위 논문, 서울대학교
  5. 이상규 외 36인, 1995, 제주도 지열자원 탐사 및 최적 활용방안 연구(III), 통산산업부, 한국자원연구소
  6. 이춘기, 이희순, 권병두, 조인기, 오석훈, 송윤호, 이태종, 2007a, 자기지전류 탐사 자료에 나타나는 옥천대의 전기적 이방성 구 조, 한국지구과학학회지, 28, 227-239 https://doi.org/10.5467/JKESS.2007.28.2.227
  7. 이춘기, 권병두, 이희순, 조인기, 오석훈, 송윤호, 이태종, 2007b, 한반도 횡당 자기지전류 탐사에 의한 상부 지각의 지전기적 구조, 한국지구과학회지, 28, 187-201 https://doi.org/10.5467/JKESS.2007.28.2.187
  8. 이태종, 송윤호, Uchida, T., 2005, 심부 지열자원 개발을 위한 원거리 기준점 MT 탐사 자료의 2차원 역산 해석, 물리탐사, 8, 145-155
  9. 정승환 외 26인, 1993, MT 및 GDS 탐사에 의한 경상분지 내 지열 및 탄화수소 탐사(I), 과학기술처, KR-93(T)-6, 한국자원연구소
  10. 최지향, 김희준, 남명진, 이태종, 한누리, 이성곤, 송윤호, 서정희, 2007, 2차원 MT 자료의 3차원 역산을 통한 제주도 지전기구조 연구, 물리탐사, 10, 268-274
  11. Baba K. and Chave, A. D., 2005, Correction of seafloor magnetotelluric data for topographic effects during inversion, J. Geophys. Res., 110, B12105 https://doi.org/10.1029/2004JB003463
  12. Baba, K. A., Chave, A. D., Evans, G. Hirth, and Mackie, R. L., 2006, Mantle dynamics beneath the East Pacific Rise at $17^\circS$: Insights from the Mantle Electromagnetic and Tomography (MELT) experiment, J. Geophys. Res., 111, B02101
  13. Bailey, R. C., 1977, Electromagnetic induction over the edge of a perfectly conduction ocean: the H-polarization case, Geophys. J. Astr. Sco., 48, 385-392 https://doi.org/10.1111/j.1365-246X.1977.tb03678.x
  14. Bapat, V. J., Segawa, J., Honkura, Y., and Tarits, P., 1993, Numerical estimations of the sea effect on the distribution of induction arrows in the Japanese island arc, Phys. Earth Planet. Inter., 81, 215-229 https://doi.org/10.1016/0031-9201(93)90132-S
  15. Berdichevsky, M. N., Vanyan, L. L., and Dmitriev, V. I., 1989, Methods used in the U.S.S.R to reduce near-surface inhomogeneity effects on deep magnetotelluric sounding, Phys. Earth Planet Inter., 53, 194-206 https://doi.org/10.1016/0031-9201(89)90003-4
  16. Berdichevsky, M. N., Dmitriev, V. I., and Pozdnjakova, E. E., 1998, On two dimensional interpretation of magnetotelluric soundings, Geophys. J. Int., 133, 585-606 https://doi.org/10.1046/j.1365-246X.1998.01333.x
  17. Berdichevsky, N. M., 1999, Marginal notes on magnetotellurics, Surv. Geophys., 20, 341-375 https://doi.org/10.1023/A:1006645715819
  18. Boerner, D. E., Kurts, R. D., Craven, J. A., Ross, G. M., Jones, F, W., and Davis, W. J., 1999, Electrical conductivity in the Precambrian lithosphere of western Canada, Science, 283, 668-670 https://doi.org/10.1126/science.283.5402.668
  19. Chave, A. D. and Thomson, D. J., 1989, Some comments on magnetotelluric response function estimation, J. Geophys. Res., 94, 14,215-14,225
  20. Chave, A. D. and Smith, J. T., 1994, On electric and magnetic galvanic distortion tensor decompositions, J. Geophys. Res., 99, 4,669-4,682 https://doi.org/10.1029/93JB02386
  21. Chave, A. D. and Thomson, D. J., 2004, Bounded Influence estimation of magnetotelluric response functions, Geophys. J. Int., 157, 988-1,006 https://doi.org/10.1111/j.1365-246X.2004.02203.x
  22. Constable S. C., Parker, R. L., and Constable, C. G., 1987, Occam's inversion: A practical algorithm for generating smooth models from electromagnetic sounding data, Geophysics, 52, 289-300 https://doi.org/10.1190/1.1442303
  23. Dosso, H. W., Chen, J., Chamalaun, F. H., and McKnight, J. D., 1996, Difference electromagnetic induction arrow responses in New Zealand, Phys. Earth Planet. Inter., 97, 219-229 https://doi.org/10.1016/0031-9201(95)03133-2
  24. Egbert, G. D. and Booker, J. R., 1986, Robust estimation of geomagnetic transfer functions, Geophys. J. R. Astr. Soc., 87, 173-194 https://doi.org/10.1111/j.1365-246X.1986.tb04552.x
  25. Egbert, G. D., 1997, Robust multiple station MT data processing, Geophys. J. Int., 130, 475-496 https://doi.org/10.1111/j.1365-246X.1997.tb05663.x
  26. Evans, R. L., et al., 1999, Asysmetric electrical structure in the mantle beneath the East Pacific Rise at $17^{\circ}S$, Science, 286, 752-756 https://doi.org/10.1126/science.286.5440.752
  27. Filloux, J. H., 1977 Ocean-floor magnetotelluric sounding over North Central Pacific, Nature, 262, 297-301 https://doi.org/10.1038/262297a0
  28. Gamble, T. D., Goubau, W. M., and Clarke, J., 1979, Magnetotellurics with a remote reference, Geophysics, 44, 55-68
  29. Groom, R. W. and Bailey, R. C., 1989, Decomposition of magnetotelluric impedance tensors in the presence of local three-dimensional galvanic distortion, J. Geophys. Res., 94, 1,913-1,925
  30. Groom, R. W. and Bahr, K., 1992, Corrections for near surface effects: Decomposition of the magnetotelluric impedance tensor and scaling corrections for regional resistivities: A tutorial, Surv. Geophys., 13, 341-379 https://doi.org/10.1007/BF01903483
  31. Heison, G. and Constable, S., 1992, The electrical conductivity of the oceanic upper mantle, Geophys. J. Int., 110, 159-179 https://doi.org/10.1111/j.1365-246X.1992.tb00719.x
  32. Heison, G. and Lilley, F. E. M., 1993, An approach of thin-sheet electromagnetic modeling to the Tasman Sea, Phys. Earth Planet Inter., 81, 231-251 https://doi.org/10.1016/0031-9201(93)90133-T
  33. Koyama T., 2002, A study of the electrical conductivity of mantle by voltage measurements for submarine cables, Ph. D. thesis, Univ. Tokyo., Tokyo
  34. Lee, T. J. Song, Y., Uchida, T., Mitsuhata, Y., Oh., S. and Graham, G. B., 2004, Sea effect in three-dimensional magnetotelluric survey: An application to geothermal exploration in Pohang, Korea, Proceeding of the 7th SEGJ International Symposium, November 2004, Sen-dai, 279-282
  35. Lee, T. J., Song, Y. and Uchida, T., 2007, Three-dimensional mangetotelluric surveys for geotherman development in Pohang, Korea, Exploration Geophysics, 38, 44-49 https://doi.org/10.1071/EG07010
  36. Likelybrooks, D. W., 1986, Modeling earth resistivity structure for MT data: A comparison of rotationally invariant and conventional earth response functions (abs.) EOS Trans. Amer. Geophys. Union, 67, 918
  37. Mackie, R. L., Madden, T. R., and Wannamaker, P. E., C, Three-dimensional magnetotelluric modeling using difference equations - Theory and comparisons to integral equation solutions, Geophysics, 58, 215-226 https://doi.org/10.1190/1.1443407
  38. McNeice, G. W. and Jones, A. G., 2001, Multisite, multifrequency tensor decomposition of magnetotelluric data, Geophysics, 66, 158-173 https://doi.org/10.1190/1.1444891
  39. Nolasco, R. P., Smith, J. T., Filloux, J. H. and Chave, A. D., 1998, Magnetotelluric imaging of the Society Island hotspot, J. Geophys. Res., 103, 30,287-39,309 https://doi.org/10.1029/98JB02129
  40. Pedersen, L. B. and Engels, M., 2005, Routine 2D inversion of magnetotelluric data using the determinant of impedance tensor, Geophysics, 70, 33-41
  41. Pringle D., Ingham, M., McKnight, D. and Chamalaun, F., 2000, Magnetovariational soundings across the South Island of New Zealand: difference arrows and the Southern Alps conductor, Phys. Earth Planet. Inter., 199, 285-298
  42. Rikitake, T. and Honkura, Y., 1985. Solid earth geomagnetism, Terrapub, Tokyo
  43. Rodi, W. L. and Mackie, R. L., 2001, Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion, Geophysics, 66, 174-187 https://doi.org/10.1190/1.1444893
  44. Santos, F. A. M., Nolasco, M., Almeida, E. P., Pous, J., and Mendes-Victor, L. A., 2001, Coast effects on magnetic and magnetotelluric transfer functions and their correction: application to MT soundings carried out in SW Iberia, Earth Plant. Sci. Lett., 186, 283-295 https://doi.org/10.1016/S0012-821X(01)00237-0
  45. Shimoizumi M., Mogi, T., Nakada, M., Yukutake, T., Handa, S., Tanaka, Y. and Uchida, H., 1997, Electrical conductivity anomalies beneath the western sea of Kyushu, Japan, Geophys. Res. Lett., 24, 1551-1554 https://doi.org/10.1029/97GL01542
  46. Siripunvaraporn W., Egbert, G., and Uyeshima, M., 2005, Interpretation of two-dimensional magnetotelluric profile data with three-dimensional inversion; synthetic examples, Geophys. J. Int., 160, 804-814 https://doi.org/10.1111/j.1365-246X.2005.02527.x
  47. Unsworth, M., Soyer, W., Tuncer, V., Wagner, A., and Barnes, D., 2007, Hydrogeologic assessment of the Amchitka Island nuclear test site (Alaska) with magnetotellurics, Geophysics, 72, 47-57
  48. Wannamaker P. E., Hommann, G. W., and Ward, S. H., 1984, Magnetotelluric responses of three-dimensional bodies in layered earths, Geophysics, 49, 1,517-1,533
  49. Wannamaker, P. E., Stodt, J. A., and Rijo, L., 1986, Twodimensional topographic response in magnetotelluric modeled using finite elements, Geophysics, 51, 2,132-2,144
  50. Wannamaker, P. E., Booker, J. R., Jones, A. G., Chave, A. D., Filloux, J. H., Waff, H. S., and Law, L. K., 1989, Resistivity cross section through the Juan de Fuca subduction system and its tectonic implications, J. Geophys. Res., 94, 14,127-14,144
  51. Weaver, J. T., and Agarwal, A. K., 1991, Is addition of induction vectors meaningful?, Phys. Earth Planet. Inter., 65, 163-181
  52. Weaver, J. T., and Dawson, T. W., 1992, Adjustment distance in TM mode electromagnetic induction, Geophys. J. Int., 108, 293-300