An Efficient 3D Inversion of MT Data Using Approximate Sensitivities

효율적인 3차원 MT 역산을 위한 다양한 감도의 이용

  • Han, Nu-Ree (Dept. of Civil, Urban and Geosystem Eng., Seoul National University) ;
  • Nam, Myung-Jin (Dept. of Petroleum and Geosystems Eng., The University of Texas at Austin) ;
  • Kim, Hee-Joon (Dept. of Environmental Exploration Eng., Pukyong National University) ;
  • Lee, Tae-Jong (Groundwater and Geothermal Resources Div., KIGAM) ;
  • Song, Yoon-Ho (Groundwater and Geothermal Resources Div., KIGAM) ;
  • Suh, Jung-Hee (Dept. of Civil, Urban and Geosystem Eng., Seoul National University)
  • 한누리 (서울대학교 지구환경시스템공학부) ;
  • 남명진 ;
  • 김희준 (부경대학교 환경탐사공학과) ;
  • 이태종 (한국지질자원연구원 지하수지열연구부) ;
  • 송윤호 (한국지질자원연구원 지하수지열연구부) ;
  • 서정희 (서울대학교 지구환경시스템공학부)
  • Published : 2007.11.30

Abstract

An efficient algorithm for inverting static-shifted magnetotelluric (MT) data has been proposed to produce a three-dimensional (3D) resistivity model. In the Gauss-Newton approach, computational costs associated with construction of a full sensitivity matrix usually make 3D MT inversion impractical. This computational difficulty may be overcome by using approximate sensitivities. We use four kinds of sensitivities in particular orders in the inversion process. These sensitivities are computed 1) analytically for an initial, homogeneous earth, 2) exactly for a current model, 3) approximately by the Broyden method, and 4) approximately using the previous adjoint fields. Inversion experiments with static-shifted synthetic and field MT data indicate that inversion results are highly dependent on characteristics of data and thus applying various combinations of sensitivities is helpful in obtaining a good image of the subsurface structure with reasonable computation time.

정적효과가 포함된 MT 자료를 효율적으로 역산하기 위해 새로운 근사 감도(sensitivity)를 제시하였다. 근사 감도를 이용하여 감도 행렬을 구하는데 필요한 막대한 계산 시간을 합리적인 수준으로 줄이면서 정확한 감도를 이용한 경우와 비슷한 수준의 역산 결과를 얻을 수 있었다. 이 연구에서 제시한 근사 감도 외에 초기 균질 모형에 대한 감도, Broyden 법을 이용하여 업데이트한 감도, 정확한 해에 의존한 감도 등을 여러 가지 조합으로 3차원 역산을 수행할 수 있는 효율적인 방법에 대해 연구하였다. 인공합성자료 및 포항 지역에서 획득한 현장 자료에 대하여 3차원 역산한 결과, 네가지 감도의 조합으로 성공적인 역산이 가능함을 확인하였다. 이로부터 다양한 감도를 적용하여 역산을 수행하는 것이 보다 효율적이고 합리적인 역산 결과를 얻는데 도움이 됨을 알 수 있었다.

Keywords

References

  1. 김희준, 남명진, 한누리, 최지향, 이태종, 송윤호, 서정희, 2004, MT 자료의 3차원 역산 개관, 물리탐사, 7, 207-212
  2. 이태종, 송윤호, Uchida, T., 2005, 포항 지열개발 지역 MT 탐사 자료의 2차원 및 3차원 해석, 한국지구시스템공학회지, 297-307
  3. Constable, S., and Weiss, C., 2006, Mapping thin resistors and hydrocarbons with marine EM methods: Insights from 1D modeling, Geophysics, 71, G43-G51
  4. deGroot-Hedlin, C., 1991, Removal of static shift in two dimensions by regularized inversion, Geophysics, 56, 2102-2106 https://doi.org/10.1190/1.1443022
  5. Farquharson, C. G., and Oldenburg, D. W., 1999, Approximate sensitivities for multidimensional electromagnetic inversion, in Three-dimensional Electromagnetics, Oristaglio M. L., and Spies, B. R. (eds), SEG, 256-264
  6. Guptasarma, D., and Singh, B., 1997, New digital linear filters for Hankel $J_0\;and\;J_1$ transforms, Geophys. Prosp., 45, 745- 762 https://doi.org/10.1046/j.1365-2478.1997.500292.x
  7. Jones, A. G., 1988, Static shift of magnetotelluric data and its removal in a sedimentary basin environment, Geophysics, 53, 967-978 https://doi.org/10.1190/1.1442533
  8. Lee, T. J., Song, Y., and Uchida, T., 2007, Three-dimensional magnetotelluric surveys for geothermal development in Pohang, Korea, Exploration Geophysics, 38
  9. Lee, T. J., Song, Y., and Uchida, T., 2007, Three-dimensional magnetotelluric surveys for geothermal development in Pohang, Korea, Butsuri-Tansa, 60
  10. Lee, T. J., Song, Y., and Uchida, T., 2007, Three-dimensional magnetotelluric surveys for geothermal development in Pohang, Korea, Mulli-Tamsa, 10, 44-49
  11. Lock, M. H., and Barker, R. D., 1996, Practical technique for 3D resistivity surveys and data inversion, Geophys. Prosp., 44, 499-523 https://doi.org/10.1111/j.1365-2478.1996.tb00162.x
  12. Mackie, R. L., and Madden, T. R., 1993, Three-dimensional magnetotelluric inversion using conjugate gradients, Geophys. J. Int., 115, 215-229 https://doi.org/10.1111/j.1365-246X.1993.tb05600.x
  13. McGillivray, P. R., Oldenburg, D. W., Ellis, R. G., and Habashy, T. M., 1994, Calculation of sensitivities for the frequencydomain electromagnetic problem, Geophys. J. Int., 116, 1-4 https://doi.org/10.1111/j.1365-246X.1994.tb02121.x
  14. Newman, G. A., and Alumbaugh, D. L., 2000, Three-dimensional magnetotelluric inversion using non-linear conjugate gradients, Goephys., J. Int., 140, 410-424 https://doi.org/10.1046/j.1365-246x.2000.00007.x
  15. Ogawa, Y., and Uchida, T., 1996, A two-dimensional magnetotelluric inversion assuming Gaussian static shift, Geophys. J. Int., 126, 69-76 https://doi.org/10.1111/j.1365-246X.1996.tb05267.x
  16. Sasaki, Y., 2001, Full 3-D inversion of electromagnetic data on PC, Journal of Applied Geophysics, 46, 45-54 https://doi.org/10.1016/S0926-9851(00)00038-0
  17. Sasaki, Y., 2004, Three-dimensional inversion of static-shifted magnetotelluric data, Earth Planets Space, 56, 239-248 https://doi.org/10.1186/BF03353406
  18. Sasaki, Y., and Meju, M. A., 2006, Three-dimensional joint inversion for magnetotelluric resistivity and static shift distributions in complex media, J. Geophys. Res., 111, B05101
  19. Siripunvaraporn, W., Egbert, G., and Lenbury, Y., 2002, Numerical accuracy of magnetotelluric modeling: A comparison of finite difference approximations, Earth Planets Space, 54, 721-725 https://doi.org/10.1186/BF03351724
  20. Siripunvaraporn, W., Egbert, G., Lenbury, Y., and Uyeshina, M., 2005, Three-dimensional magnetotelluric inversion: dataspace method, Physics of the Earth and Planetary Interiors, 150, 3-14 https://doi.org/10.1016/j.pepi.2004.08.023
  21. Smith, J. T., 1996, Conservative modeling of 3-D electromagnetic fields, Part I: properties and error analysis, Geophysics, 61, 1308-1318 https://doi.org/10.1190/1.1444054
  22. Torres-Verdin, C., and Bostick, F. X. J., 1992, Principles of spatial surface electric field filtering in magnetotellurics: Electromagnetic array profiling (EMAP), Geophysics, 57, 603-622 https://doi.org/10.1190/1.1443273
  23. Unsworth, M. J., Travis, B. J., and Chave, A. D., 1993, Electromagnetic induction by a finite electric dipole source over a 2- D earth, Geophysics, 58, 198-214 https://doi.org/10.1190/1.1443406
  24. Yee, K. S., 1966, Numerical solution of initial boundary value problems involving Maxwell's equation in isotropic media, IEEE Trans. Anten. Prop., AP-14, 302-307
  25. Zhdanov, M. S., Fang, S., and Hursan, G., 2000, Electromagnetic inversion using quasi-linear approximation, Geophysics, 65, 1501-1513 https://doi.org/10.1190/1.1444839