تفسیر دوبعدی داده‌های مگنتوتلوریک دشت میقان اراک

روش مگنتوتلوریک (mt) به سبب تفکیک‌پذیری جانبی و عمق نفوذ زیاد از موثرترین روش‌های الکترومغناطیسی برای تصویرسازی الکتریکی از ساختارهای زیرسطحی است. به‏دلیل تغییرات مقاومت ویژه در لایه‌های مختلف زمین و ساختارهای زمین‏شناسی مانند گسل و سنگ کف، می‏توان با استفاده از روش مگنتوتلوریک به مطالعة این ساختارها پرداخت.در بهار 1390 اندازه‌گیری‌های مگنتوتلوریک در بازة فرکانسی وسیعی در جنوب استان مرکزی، به منظور مشخص‌کردن هدایت الکتریکی پوسته با تاکید بر مشخص‌کردن ساختارهای زمین‌شناسی و شناسایی سنگ بستر و گسل احتمالی انجام گرفت.مولفه‌های میدان‌های الکتریکی و مغناطیسی در طول یک پروفیل و در شش ایستگاه با فاصله‌های 1000 متر اندازه‏گیری شدند. پردازش داده‌ها و به دنبال آن وارون‌سازی یک‏بعدی برای هرکدام از سایت‌ها صورت گرفت و در ادامه وارون‌سازی دوبعدی این داده‌ها اجرا شد. نتایج به‏دست‌آمده از وارون‌سازی و مدل به‏دست‌آمده از مد te، هدایت الکتریکی ساختارها را در توافق خوبی با داده‌های زمین‌شناسی مشخص کرده است. از مهم‌ترین این نتایج می‏توان به شناسایی یک گسل احتمالی پنهان و همچنین سنگ‏کف در عمق با مقاومت ویژه زیاد اشاره کرد. مدل دوبعدی ضمن تفکیک لایه‏های رسوبی براساس تباین مقاومت‏ویژه آن‌ها، به‌خوبی نشان‏دهنده ساختار گسلی است.

2D interpretation of the magnetotelluric data on Mighan plain of ARAK

Magnetotelluric (MT) method is a passive electromagnetic technique that uses the natural, time varying electric and magnetic field components measured at right angles at the surface of the earth to make inferences about the earth’s electrical structure which, in turn, can be related to the geology tectonics and subsurface conditions. Reflection and refraction of electromagnetic (EM) signals at both horizontal and vertical interfaces separate media of different electrical parameters. Electromagnetic methods have been developed and employed to recognize the geological features and particularly fault zones in many regions. To achieve higher lateral resolution and also greater depth penetration, the MT method is one of the most effective electromagnetic techniques to create image of the subsurface structures electrically.    In 2011 wide frequency range of magnetotelluric measurements were carried out at Mighan plain in the southern part of the Markazi province in Iran to understand the crustal electrical conductivity of the region by putting emphasis on locating the geological structures and recognizing the bedrock and probable fault. The electric and magnetic field components were acquired along a profile at 6 stations with a 1000meter distance between stations using GMS05 (Metronix, Germany) systems. Three magnetometers and two pairs of nonpolarizable electrodes were connected to this fivechannel data logger. The experimental setup included four electrodes distributed at a distance of 100 m in north–south (Ex) and east–west (Ey) direction.    Measurements of the horizontal components of the natural electromagnetic field were used to construct the full complex impedance tensor, Z, as a function of frequency. Using the effective impedance, determinant apparent resistivities and phases were computed and used for the inversion. MT data were processed using a code from Smirnov (2003) aiming at a robust single site estimate of electromagnetic transfer functions. As the area of study is populated and close to noise sources, the recorded data has not good quality which justifies the low coherency between the electric and magnetic channels. We performed 1D inversion of the determinant data using a code from Pedersen (2004) for all sites. The 2D modeling was applied to the data to explain the data if their responses fitted the measured data within their errors. Generally, the better the fit between measured and predicted data, the more reliable model. The 2D inversion of the TE and DETmode data using a code from Siripunvaraporn and Egbert (2000) were performed. The data were calculated as apparent resistivities and phases. Apparent resistivity and phase data exhibited fairly different characteristics in the TE and DET modes.  we used the model obtained from the TE mode data as an interpretation model. The resistivity model obtained from the TE mode is consistent with the geological model of the Mighan region down to five kilometers.The 2D models significantly illustrate two conductive blocks and a fault structure and resolved layers with sharp resistivity contrasts. As significant results, in collaboration with geological information about the presence of the Tabarteh fault, the conductivity features can be attributed to the fault. Besides, a probable hidden fault is also recognizable. The bedrock was also detected with high apparent resistivity by the two dimensional model.