A simple form of MT impedance tensor analysis to simplify its decomposition to remove the effects of near surface small-scale 3-D conductivity structures

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسنده

School of Mining, College of Engineering, University of Tehran, Tehran, Iran

چکیده

Magnetotelluric (MT) is a natural electromagnetic (EM) technique which is used for geothermal, petroleum, geotechnical, groundwater and mineral exploration. MT is also routinely used for mapping of deep subsurface structures. In this method, the measured regional complex impedance tensor (Z) is substantially distorted by any topographical feature or small-scale near-surface, three-dimensional (3-D) electrical inhomogeneity. The effects of this local galvanic distortion should be separated and removed from the regional response to improve the accuracy and reliability of the data interpretation. In this paper, it is attempted to introduce an effective form of tensor analysis to facilitate the process of GB (Groom-Bailey) tensor decomposition on MT data. This approach was used to recover the regional response of conductivity structures beneath 12 MT sounding sites of a sedimentary basin in South Australia. The results of this study clearly indicate that the regional structures beneath these sites are two-dimensional (2-D) and their strike are mainly in NS (±100) direction which are geologically supported. The obtained results also show that the distortion parameters of the surficial bodies are fairly constant for the whole frequency band or its two or, at most, three subsets. In addition, the low misfit values between the measured impedances and those produced by a hypothetical 3D/2D model confirm that the regional structures beneath all these 12 MT sites are 2-D and some local surficial 3-D features are superimposed on them.

کلیدواژه‌ها


عنوان مقاله [English]

A simple form of MT impedance tensor analysis to simplify its decomposition to remove the effects of near surface small-scale 3-D conductivity structures

نویسنده [English]

  • Ali Moradzadeh
چکیده [English]

Magnetotelluric (MT) is a natural electromagnetic (EM) technique which is used for geothermal, petroleum, geotechnical, groundwater and mineral exploration. MT is also routinely used for mapping of deep subsurface structures. In this method, the measured regional complex impedance tensor (Z) is substantially distorted by any topographical feature or small-scale near-surface, three-dimensional (3-D) electrical inhomogeneity. The effects of this local galvanic distortion should be separated and removed from the regional response to improve the accuracy and reliability of the data interpretation. In this paper, it is attempted to introduce an effective form of tensor analysis to facilitate the process of GB (Groom-Bailey) tensor decomposition on MT data. This approach was used to recover the regional response of conductivity structures beneath 12 MT sounding sites of a sedimentary basin in South Australia. The results of this study clearly indicate that the regional structures beneath these sites are two-dimensional (2-D) and their strike are mainly in NS (±100) direction which are geologically supported. The obtained results also show that the distortion parameters of the surficial bodies are fairly constant for the whole frequency band or its two or, at most, three subsets. In addition, the low misfit values between the measured impedances and those produced by a hypothetical 3D/2D model confirm that the regional structures beneath all these 12 MT sites are 2-D and some local surficial 3-D features are superimposed on them.

کلیدواژه‌ها [English]

  • Impedance tensor
  • near-surface inhomogeneity
  • regional structure
  • tensor analysis
  • tensor decomposition
  • galvanic distortion
Bahr, K., 1988, Interpretation of the magnetotelluric impedance tensor: regional induction and local telluric distortion: J. Geophys., 62, 119-127.

Bahr, K., 1991, Geological noise in magnetotelluric data: a classification of distortion types: Phys. Earth Planet.  Inter., 66, 24-38.

Berdichevsky, M. N., and Dmitriev, V. I., 1976, Basic principles of interpretation magnetotelluric sounding curves: Geoelectric and Geothermal Studies, 165-221, Budapest.

Berdichevsky, M. N. and Zhdanov, M. S., 1984, Advanced Theory of Deep Geomagnetic Sounding: Elsevier, 408.

Bibby, H. M. Caldwell, T. G. and Brown, C., 2005, Determinable and non-determinable parameters of galvanic distortion in magnetotellurics: Geophys. J. Int., 163, 915–930.

Booker, J. R., 2014, The magnetotelluric phase tensor: A critical review: Survey in Geophysics, 35, 7–40.

Cai, J. T., Chen, X. B. and Zhao, G. Z., 2010, Refined techniques for data processing and two-dimensional inversion in magnetotelluric I: Tensor decomposition and dimensionality analysis: Chinese J. Geophys., 53(6), 1060-1071.

Caldwell, T. G., Bibby, H. M. and Caldwell, C., 2004, The magnetotelluric phase tensor: Geophys. J. Int.,158, 457–469.

Cerv, V., Pek, G., and Menvielle, M., 2010, Bayesian approach to magnetotelluric tensor decomposition: Annals of Geophysics, 53(2), 21-32.

Chakridi, R., Chouteau, M., and Mareschal, M., 1992, A simple technique for analysing and partly removing galvanic distortion from the magnetotelluric impedance tensor: application to Abitibi and Kapuskasing data (Canada): Geophys. J. Int., 108, 917-929.

Chave, A. D., and Smith, J. T., 1994, On electric and magnetic galvanic distortion tensor decompositions: J. Geophys. Res., 99, 4669-4682.

Chave, A. D., and Jones, A. G., 1997, Electrical and magnetic field Galvanic distortion decomposition of BC87 data: J. Geomag. Geoelectr., 49, 767-789.

Chouteau, M., and Bouchard, K., 1988, Two-dimensional terrain correction in magnetotelluric surveys: Geophysics, 53, 854-862.

Cowley, W., 2010, Summary geology of South Australia: Government of South Australia, Department of State Development, South Australia Earth Resources information sheet, M51, 1-11.

Eggers, D. E., 1982, An eigenstate formulation of the magnetotelluric impedance tensor: Geophysics, 47, 1204 - 1214.

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, 1913-1925.

Groom, R. W., and Bailey, R. C., 1991, Analytic investigations of the effects of near-surface three-dimensional Galvanic scatterers on MT tensor decompositions: Geophysics, 56, 496-518.

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. Survey in Geophysics, 13, 341-379.

Groom, R. W., Kurtz, R. D., Jones, A. G., and Boerner, D. E., 1993, A quantitative methodology to extract regional magnetotelluric impedances and to determine the dimension of the conductivity structure: Geophys. J. Int., 115, 1095-1118.

Hohmann, G. W., 1975, Three-dimensional induced polarisation and electromagnetic modeling: Geophysics, 40, 309-324.

Li, Y., Yu, P., Zhang, L., Wang, J., and Wu, J., 2010, An Improved Approach on Distortion Decomposition of Magnetotelluric Impedance Tensor: 2010 SEG Annual Meeting, 17-22 October, Denver, Colorado, SEG-2010-0824.

Lilley, F. E. M., 1995, Strike direction: obtained from basic models for 3D magnetotelluric data: Three-dimensional Electromagnetics (Eds. M Oristaglio and B. Spies), 359-370. Ridgefield, Conn. USA.

Lilley, F. E. M., 1998, Magnetotelluric tensor decomposition: Part I. Theory for a basic procedure: Geophysics, 63(6), 1885–1897.

Lilley, F. E. M., 2012, Magnetotelluric tensor decomposition: insights from linear algebra and Mohr diagrams: In: New achievements in geoscience (Ed: Hwee-Sam Lim) doi: 10.5772/2066.href=’’http://www.intechopen.com/books/new-achievements-in- geoscience”.

Lilley, F. E. M., and Weaver, J. T., 2010, Phases greater than 90° in MT data: analysis using dimensionality tools: J. Appl. Geophys., 70, 9-16.

Jiracek, G. R., 1990, Near-surface and topographic distortions in electromagnetic induction: Survey in Geophysics, 11, 163-203.

Jiracek, G. R., Reddig, R. P., and Kojima, R. K., 1989, Application of the Rayleigh-FFT technique to magnetotelluric modelling and correction: Phys. Earth Planet. Inter., 53, 365-375.

Jones, A. G., and Dumas, I., 1993, Electromagnetic images of a volcanic zone: Phys. Earth Planet. Inter., 81, 289-314.

Jones, A. G., and Groom, R. W., 1993, Strike-angle determination from the magnetotelluric impedance tensor in the presence of noise and local distortion: rotate at your peril: Geophys. J. Int., 113, 524-534.

Kannaujiya, S., and Israil, M., 2012, Determination of Geoelectric Strike and 2D inversion of Magnetotelluric Data from Himalayan Region: 9th Biennial International Conference and Exposition on Petroleum Geophysics, P-008, Hyderabad, India.

Kaufman, A. A., 1988, Reduction of the geological noise in magnetotelluric soundings: Geoexploration, 25, 145-161.

Larsen, J. C., 1977, Removal of local surface conductivity effects from low frequency mantle response curves: Acta Geodaet. Geophys. et Montanist. Acad. Sci. Hung., 12, 183-186.

LaTorraca, G. A., Madden, R. T. and Korringa, J., 1986, An analysis of the magnetotelluric impedance for three-dimensional conductivity structures: Geophysics, 51, 1819-1829.

LeMouel, J. L., and Menvielle, 1982, Geomagnetic variation anomalies and deflection of telluric currents: Geophys. J. Roy. astr. Soc., 68, 575-587.

McNeice, G. W., and Jones, A. G., 2001, Multisite, multifrequency tensor decomposition of magnetotelluric data: Geophysics, 66,158–173.

Moradzadeh, A., 1998, Electrical Imaging of the Adelaide Geosyncline using Magnetotellurics (MT): Ph.D. Thesis, Flinders Univ. of South Australia.

Moradzadeh, A., and White, A., 2005, An assessment of the geoelectric dimensionality of subsurface structures using magnetotelluric data: J. Sci. & Tech., Shahrood University of Technology, 6, 59-65.    

Moradzadeh, A., 2003, Static shift appraisal and its correction in magnetotelluric (MT) surveys: The 21st Symposium on Geosciences, 197-201, Tehran, Iran.

Pellerin, L., 1988, Use of Transient Electromagnetic Soundings to Correct Static Shifts in Magnetotelluric Data: MSc. Thesis, Univ. Utah.

Smith, J. T., 1995, Understanding telluric distortion matrices: Geophys. J. Int., 122, 219–226.

Swift, C. M., Jr., 1967, A Magnetotelluric Investigation of an Electrical Conductivity Anomaly in the Southwestern of United States: Ph.D. Thesis, Mass. Inst. Tech.

Vozoff, K., 1991, The magnetotelluric method. in: Nabighian, M. N. (Ed), Electromagnetic Methods in Applied Geophysics. Society of Exploration Geophysicists, 641-712.

Wannamaker, P. E., Hohmann, G., W., and Ward, S. H., 1984, Magnetotelluric responses of three-dimensional bodies in layered earths: Geophysics, 49, 1517-1533.

Ward, S. H., 1967, Electromagnetic Theory for Geophysical Application, Mining Geophysics: Society of Exploration Geophysicists, 10-196.

Yee, E., and Paulson, K. V., 1987, The Canonical decomposition and its relationship to other forms of magnetotelluric impedance tensor analysis: Geophysics, 61, 173-189.

Zhdanov, M. S., 1987, Application of space analysis of electromagnetic fields to investigation of the geoelectrical structure of the earth: Phys. App. Geophys., 125, 483-497.

Zhang, P., Roberts, R.G., and Pedersen, L. B., 1987, Magnetotelluric strike rules: Geophysics, 52, 267-278.