تصویرسازی کانسار آهن اجت‌آباد با استفاده از داده‌های مغناطیس‌سنجی

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

1 دانشکده مهندسی معدن، نفت و ژئوفیزیک، دانشگاه صنعتی شاهرود

2 دانشکده‌های فنی، دانشگاه تهران

چکیده

کانسار آهن اجت‌آباد در شمال شرق شهر سمنان واقع شده است. در این محدوده بیشترکانی‌سازی آهن از نوع هماتیتی و بعد مگنتیتی است. به منظور شناسایی وضعیت عمقی و گسترش جانبی کانسار از روش مغناطیسسنجی استفاده شد. پس از برداشت داده‌ها و انجام تصحیحات لازم بر روی این داده‌ها و اعمال صافی برگردان به قطب چندین بی‌هنجاری در محدوده شناسایی شد. نتایج نشان می‌دهد که هشت بی‌هنجاری مثبت مغناطیسی در این منطقه وجود دارد. با انجام صافی گسترش به سمت بالا و مدل‌سازی وارون سه‌بعدی داده‌های مغناطیسی، گسترش جانبی و عمقی توده‌های بی‌هنجاری تصویرسازی گردید. بررسی‌های انجام گرفته نشان می‌دهد که از بین این هشت توده، تعداد هفت عدد آن‌ها با کانی‌سازی همراه بوده و تنها یک توده بی‌هنجاری موجود به احتمال زیاد توده نفوذی است. همچنین مدل‌سازی نشان داد که عمق کانی‌سازی بین 10تا 100متر است.
 

کلیدواژه‌ها


عنوان مقاله [English]

Imaging OjatAbad iron ore using magnetic data

نویسندگان [English]

  • Mohamad Rezaie 1
  • Ali Moradzadeh 2
  • Hamid Aghajani 1
  • Ali Nejati 1
چکیده [English]

OjatAbad iron ore located in north east of Semnan city. Old indications of mining are evident in the area. Belich and Bragin (1993) introduced Semnan iron ores as hydrothermal deposits. Recent studies show that the iron ores are related to Oligo-Miocene magmatism. Most iron ores have magnetite which has high magnetic susceptibility; therefore, magnetic method is a conventional method for geophysical exploration of iron. In order to identify and detect this deposit, a magnetic survey was carried out in OjatAbad area. The magnetic data corrected for diurnal change of magnetic field and then total magnetic field of the Earth has been reduced. To do this, a reduce to pole (RTP) filter was implemented on the grid for locating the anomalies and their sources. This method entails removing the dependence of magnetic data to the magnetic inclination, i.e., converting the data which were recorded in the inclined Earth’s magnetic field to what they would have been if the magnetic field had been vertical. This method simplified the interpretation because for sub-vertical prisms or sub-vertical contacts (including faults), it transforms their asymmetric responses to simpler symmetric and anti-symmetric forms. The symmetric “highs” are directly centered on the body, while the maximum gradient of the anti-symmetric dipolar anomalies coincides exactly with the body edges. For depth estimation of anomalies, the upward continuation filter was implemented. This is a mathematical technique that projects the data taken at an elevation to a higher elevation. The effect is that the shortwavelength features are smoothed out because one is moving away from the anomaly. The upward continuation is a way of enhancing large scale (usually deep) features in the survey area. It attenuates the anomalies depending on their wavelengths; the shorter the wavelength, the greater the attenuation. Also upward continuation tends to accentuate the anomalies caused by deep sources at the expense of the anomalies caused by shallow sources. For 3D imaging of magnetic data, we chose the inversion method of Li and Oldenburg (1998) that minimizes a function composed by (1) the data-misfit function defined in the data space as the L2 norm of the difference between the observed and predicted data, and (2) the stabilizing function defined in the parameter model space as the L2 norm of the first-order derivative of the weighted density distribution in both vertical and horizontal directions. They introduced a depth-weighting function to counteract the spatial decay of the kernel function with depth by giving more weight to rectangular prisms as depth increases. On the RTP magnetic map of study area we can see 8 magnetic anomalies (A, B, C, D, E, F, G and H) which are located at the northeast to the southwest trend. The H one has the lowest amplitude. The results of upward continuation filter show that the anomalies of F and G are shallower than the other anomalies. Also, H and B are only deeper than 100m. The inversion results recovered all of the 8 anomalous bodies and confirm the above results. They showed that the anomalous body H has lower magnetic susceptibility and is deeper than the others and it seems likely that it is an intrusive body. So, iron mineralization is probably happened in the other bodies. The anomalous body B is the deepest mineralized body which elongates to 100 meter.
 

کلیدواژه‌ها [English]

  • Iron ore
  • OjatAbad
  • magnetic data
  • Reduce to Pole
  • upward continuation
  • Inversion
مرادزاده، ع.، دولتی ارده‌جانی، ف.، و طیبی، ب.، 1385، تفسیر کیفی داده‌های مغناطیسی کانسار آهن اجت‌آباد سمنان: بیست وپنجمین همایش علوم زمین، سازمان زمین شناسی و اکتشافات معدنی کشور، 107-109.
قربانی، م.، 1381، دیباچه‌ای بر زمین‌شناسی اقتصادی ایران: سازمان زمین‌شناسی و اکتشافات معدنی کشور، پایگاه ملی داده‌های علوم زمین کشور، گزارش 2.
عابدی، آ.، فردوست، ف.، خزاعی، م.، و سعیدی، س.، 1387، بررسی فازهای کانی‌سازی آهن در معدن آهن اجت‌آباد، شمال شرق سمنان: شانزدهمین همایش انجمن بلورشناسی و کانی شناسی ایران.
آقاجانی، ح.،1388، بررسی قابلیت روش گرادیان کل نرمال‌شده داده‌های گرانی در تعیین پتانسیل هیدروکربوری تله‌های نفتی: رساله دکتری، دانشگاه صنعتی شاهرود.
Anderson, E. D., Zhou, W., Li, Y., Hitzman, M. W., Monecke, T., Lang, J. R., and Kelley, K. D., 2014, Three-dimensional distribution of igneous rocks near the pebble porphyry Cu-Au-Mo deposit in southwestern Alaska: Constraints from regional-scale aeromagnetic data: Geophysics, 79, B63–B79.
Baranov, V., and H. Naudy, 1964, Numerical calculation of the formula of reduction to the magnetic pole: Geophysics, 29, 67–79.
Belich, A. I., and Bragin, Y. D., 1993. Ore deposits of Iran: Vniizarubezh Geologiya press, 294 p (in Russian).
Carlos, D. U., Uieda, L., and Barbosa V. C. F., 2014, Imaging iron ore from the Quadrilátero Ferrífero (Brazil) using geophysical inversion and drill hole data: Ore Geology Reviews, 61, 268–285.
Cordani R., 2013, Constrain modelling in iron ore exploration: presented at 13th International Congress of the Brazilian Geophysical Society, Rio de Janeiro, Brazil.
Ganiyu, S. A., Badmus, B. S., Awoyemi, M. O., Akinyemi, O. D., Olurin, O. T., 2013, Upward continuation and reduction to pole process on aeromagnetic data of Ibadan area, South-Western Nigeria: Earth Science Research, 2, 66–73.
Hansen, P. C., 1992, Analysis of discrete illposed problems by means of the L-curve: SIAM Review, 34, 561–580.
Henderson, R. G., and Zietz, I., 1949, The upward continuation of anomalies in total magnetic intensity fields: presented at St, Louis Meeting of the Society, U. S. Geological survey.
Jacobsen, B. H., 1987, A case for upward continuation as a standard separation filter for potential-field maps: Geophysics, 52, 1138–1148.
Li, S., and Li, Y., 2012, Inversion of magnetic anomaly affected by strong remanent magnetization over rugged terrain: A case study from Daye, China: SEG Technical Program Expanded Abstracts, 1–5.
Li, Y., and Oldenburg, D. W., 1996, 3-D inversion of magnetic data: Geophysics, 61, 394–408.
Spicer, B., Morris, B., Ugalde, H., 2011, Structure of the Rambler Rhyolite, Baie Verte Peninsula, Newfoundland: Inversions using UBC-GIF Grav3D and Mag3D: J. Applied Geophysics, 75, 9–18.
Telford, W. M., Geldart, L. P. and Sheriff, R. C., 1991, Applied Geophysics: 2nd edition, Cambridge Unversity Press.
Williams, N. C., 2008, Geologically Constrained UBC-GIF Gravity and Magnetic Inversions with Examples from the Agnew-Wiluna Greenstone Belt, Western Australia: PhD Thesis, University of British Columbia, Vancouver.
Ribeiro, V. B., Louro, V. H. A., and Mantovani, M. S. M., 2013, 3D Inversion of magnetic data of grouped anomalies — Study applied to São José intrusions in Mato Grosso, Brazil: J. Applied Geophysics, 93, 67–76.