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بررسی معیار معرفی‌شده در کد WALDIM به‌منظور شناسایی ناهمسانگردی الکتریکی در ساختارهای زمین‌شناسی پیچیده؛ مطالعة موردی: حاشیة قاره‌ای | ||
| فیزیک زمین و فضا | ||
| مقاله 5، دوره 41، شماره 2، مرداد 1394، صفحه 219-228 اصل مقاله (1.24 M) | ||
| شناسه دیجیتال (DOI): 10.22059/jesphys.2015.52816 | ||
| نویسندگان | ||
| منصوره منتهایی* 1؛ بنفشه حبیبیان دهکردی2 | ||
| 1استادیار، گروه فیزیک زمین، مؤسسة ژئوفیزیک دانشگاه تهران، ایران | ||
| 2استادیار بخش ژئومغناطیس، مؤسسة ژئوفیزیک دانشگاه تهران | ||
| چکیده | ||
| از میان روش‌های متعددی که برای تحلیل بعدیت داده‌های مگنتوتلوریک (MT) به کار می‌روند، تنها معیاری که در برنامة WALDIM برمبنای مقادیر ناورداهای چرخشی WAL معرفی شده، قادر بوده است اثر حضور ساختارهای ناهمسانگرد الکتریکی را تشخیص دهد. شناخت ساختارهای ژئوالکتریک ناهمسانگرد، پیامدهای فراگیری در ارزیابی ذخایر اقتصادی، تفسیر جریانات هیدرولوژی و درک سیر تکاملی ساختارهای زمین‌شناسی دارد. در این مقاله اعتبار و توانایی ناورداهای چرخشی WAL در استخراج اثرات ناهمسانگردی موجود در داده‌ها، بررسی می‌شود. به این منظور مدل‌های مصنوعی حاوی ساختارهای ژئوالکتریک ناهمسانگرد از یک حاشیة قاره‌ای در نظر گرفته می‌شوند که به‌علت تقابل بسیار زیاد هدایت‌ویژه در دو طرف خط ساحل، برهم‌کنش قوی بین اثرات ناهمسانگردی و ناهمگنی الکتریکی وجود دارد. در ادامه قابلیت این ناورداها در شناسایی ناهمسانگردی ساختاری پوستة پایینی مربوط به حاشیة قاره‌ای جنوب مرکز شیلی ارزیابی می‌شود که پیش‌تر از شواهد دیگر (مهم‌ترین آن‌ها: توابع تبدیل ژئومغناطیس) استنباط شده است. | ||
| کلیدواژه‌ها | ||
| تحلیل بعدیت؛ حاشیة قاره‌ای؛ مگنتوتلوریک؛ ناوردای چرخشی؛ ناهمسانگردی الکتریکی | ||
| عنوان مقاله [English] | ||
| An investigation of the proposed WALDIM criteria to identify electrical anisotropy in complex geological region; case study: continental margin | ||
| نویسندگان [English] | ||
| Mansoore Montahaei1؛ Banafshe Habibian Dehkordi2 | ||
| 1Assistant Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran | ||
| 2Assistant Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran | ||
| چکیده [English] | ||
| In an electrically anisotropic media, current density is not aligned with the electric field and varies with the E-field direction. This can be considered as the spatial aliasing effects aroused by the subsurface structures whose individual dimensions are smaller than the inductive length scale of diffusively propagating EM fields. The role of these structures is particularly significant in tectonically active regions, where tectonic processes induced penetrating fabrics. Accordingly, identification, characterization and interpretation of electrical anisotropy have large implications to understand evolutionary aspects of geological structures, evaluate the economic resources and interpret hydrological flows (Wannamaker, 2005). Marti (2014) provides a comprehensive review of the works conducted to recognize electrical anisotropy imprints during MT data analysis and also different strategies used to model this property in case studies. Dimensionality analysis is a preliminary stage in MT data interpretation procedure to recover the strike direction of the regional geo-electric structure, characterize the distortion effects of superficial conductive bodies and also to adopt an appropriate modeling approach (1D, 2D or 3D), coincident with the intrinsic dimension of the measured data. The application of the preliminary dimensionality tools, Swift’s and Bahr’s skews for synthetic and real MT data affected by anisotropy shows that they are disable to identify the anisotropy footprints and distinguish between structural and anisotropy strike directions (Heise and Pous, 2001). Weaver et al., (2000) suggested a family of rotationally invariant parameters characterizing the dimensionality properties of the underlying geo-electric structures. Marti et al. (2009, 2010) published the WALDIM code based on these invariants and extended them to provide proper conditions from which isotropic and anisotropic structures could be differentiated. The main criteria proposed by the WALDIM code to differentiate anisotropic media from isotropic ones are as follows: The WAL rotational invariant values indicate a 2D regional structure, while the strike directions estimated from the first and second columns of impedance tensor are inconsistent. This situation is mentioned as “3D/2D anisotropy” in subsequent table and figures. We report here on the application of this scheme to analyze the dimensionality of MT responses from some principal anisotropic models representing complex geological settings at continental margins and also for MT data from an active continental margin in South-Central Chile, where the presence of electrical anisotropy has been previously recognized (mainly from geomagnetic transfer functions). In electrical anisotropy modeling resistivity is represented as a symmetric, positive definite tensor which can be diagonalized employing Euler’s elementary rotations to obtain its principal directions and their corresponding resistivities (principal resistivities: ρxx, ρyy, ρzz). These directions are known as the strike (αs), dip (αD) and slant (αL) anisotropy angles. The non-zero values of these angles and the specified relationships between principal resistivities would determine the type and geometry of the electrical anisotropy. We restricted our study to uniaxial, azimuthal anisotropy, where αs≠0, αD= αL= 0 and also ρxx= ρzz≠ ρyy. Model responses were calculated employing the algorithm of Pek and Verner, 1997. The proposed models of geological settings are selected so that their complexity is gradually increasing. Dimensionality analysis results for the synthetic model responses and real data are depicted in figures (2, 3 and 4) and (6 and 7), respectively. The results indicate that the proposed criteria is slightly firm in the sense that they could not identify electrical anisotropy in the presence of galvanic distortions caused by superficial conductive structures and complexities of regional structures. | ||
| کلیدواژه‌ها [English] | ||
| continental margin, electrical anisotropy, magnetotellurics, dimensionality analysis, rotational invariants | ||
| مراجع | ||
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Bahr, K., 1991, Geological noise in magnetotelluric data: a classification of distortion types. Phys. Earth Planet. Inter. 66: 24–38.
Brasse, H., Kapinos, G., Li, Y., Mütschard, L., Soyer, W., Eydam, D., 2009, Structural electrical anisotropy in the crust at the South-Central Chilean continental margin as inferred from geomagnetic transfer functions, Phys. Earth Planet. In., 173: 7-16.
Chave, A., Jones, A., 2012. the Magnetotelluric method, theory and practice. Cambridge University Press.
Groom, R.W., and Bailey, R.C., 1989. Decomposition of the magnetotelluric impedance tensor in the presence of local three-dimensional galvanic distortion. J. Geophys. Res. 94, 1913–1925.
Heise, W. and Pous, J., 2001, Effects of anisotropy on the two-dimensional inversion procedure, Geophys., J. Int. 147, 610–621.
Krawczyk CM, SPOC Team , 2003, Amphibious seismic survey images plate nterface at 1960 Chile earthquake. EOS 84(32):301, 304–305.
Lilley, T., 2007. Coast Effect of Induced Currents, ENCYCLOPEDIA of GEOMAGNETISM AND PALEOMAGNETISM, Springer.
McNeice, G.W., and Jones, A. G., 2001, Multisite, multifrequency tensor decomposition of magnetotelluric data, Geophysics, 66(1), 158–173.
Martí, A., Queralt, P., and Ledo, J., 2009. Waldim: a code for the dimensionality analysis of magnetotelluric data using the rotational invariants of the magnetotelluric tensor. Comp. Geosci. 35, 2295-2303.
Martí, A., Queralt, P., Ledo, J., and Farquharson, C., 2010. Dimensionality imprint of electrical anisotropy in magnetotelluric responses. Phys. Earth Planet. Inter. 182, 139–151.
Martí, A., 2014, The role of electrical anisotropy in magnetotelluric responses: from modelling and dimensionality analysis to inversion and interpretation. Surveys in Geophysics. Doi:10.1007/s10712-013-9233-3.
Montahaei., M. and Oskooi., B., 2014, Magnetotelluric inversion for azimuthally anisotropic resistivities employing artificial neural networks, Acta Geophysica, Vol. 62, No. 1, 12-43.
Pek, J., and Verner, T., 1997. Finite-difference modeling of magnetotelluric fields in two-dimensional anisotropic media. Geophys. J. Int. 128, 505–521.
Swift, C. M., 1967, A magnetotelluric investigation of an electrical conductivity anomaly in the southwestern United States, Ph.D. thesis, MIT, Cambridge, MA.
Wannamaker, P., 2005, Anisotropy versus heterogeneity in continental solid Earth electromagnetic studies: Fundamental response characteristics and implications for physicochemical state, Surveys in Geophysics, 26, 733–765, doi:10.1007/ s10712-005-1832-1.
Weaver, J.T., Agarwal, A.K., and Lilley, F.E.M., 2000. Characterization of the magnetotelluric tensor in terms of its invarients. Geophys. J. Int. 141, 321–336.
Wiese, H., 1962. Geomagnetische Tiefentellurik Teil II: die Streichrichtung der Untergrundstrukturen des elektrischen Widerstandes, erschlossen aus geomagnetischen Variationen. Pageoph 52, 83–103. | ||
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