عنوان مقاله [English]
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 1000-meter distance between stations using GMS05 (Metronix, Germany) systems. Three magnetometers and two pairs of non-polarizable electrodes were connected to this ﬁve-channel 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 DET-mode 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.