1D & 2D Interpretation of The Magnetotelluric data for Detecting Geological Subsurface Structures along an E-W profile in Arak



Reflection and refraction of EM signals at both horizontal and vertical interfaces separate media of different electrical parameters, geoelectromagnetic 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 imagine the subsurface structures electrically.
In 2006 wide frequency range of magnetotelluric measurements were carried out at the eastern part of the city of Arak in Iran to understand the crustal electrical conductivity of the region by putting emphasis on locating the fault zones. The electric and magnetic field components were acquired along a profile across the geological trend at 15 stations. A robust single site processing followed by the inversion and one dimensional as well as two dimensional modeling was performed. The inversion results revealed electrical conductivity structures in correlation with geological features. As a significant result, true locations of two major faults, Talkhab and Tabarteh Faults and a conductive block in between were recognized in the Arak area.
Introduction: The area of study is a part of the Arak watershed located in two Central-Iran and Sanandaj-Sirjan Zones. Two parallel faults named Talkhab and Tabarteh pass through the region and divide it into three blocks. The seismicity of the area is controlled by these two parallel faults, especially the Talkhab Fault which is presumed to be the source for seismicity activity in the region (Mirzaei and Ghadimi, 2006). Anomalous crustal conductors are occasionally associated with seismic activity. Fluid is an important factor in the fault zone and many of the active faults are characterized by a great volume of groundwater (Gundmundsson, 2000).
The magnetotelluric 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. Measurements of the horizontal components of the natural electromagnetic field are used to construct the full complex impedance tensor, Z, as a function of frequency. Using the effective impedance, determinant apparent resistivities and phases are computed and used for the inversion. Also the apparent resistivities for both TE and TM mode are computed and used for 2D inversion.
Data Processing, Inversion and Conclusions: 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 urban 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. Since the quality of the determinant data was acceptable, we performed 2D inversion of the determinant data using a code from Siripunvaraporn and Egbert (2000). Besides an extra 2D inversion of MT data for TE and TM modes was performed using a code from Rodi and Mackie (2001).
The 2D models significantly illustrate two conductive zones, two resistive blocks and a large conductive zone hidden under the Quaternary alluviums along the profile. As significant results, in collaboration with geological information about the presence of the Talkhab and Tabarteh faults the conductivity features can be attributed to the faults. Besides, a probable hidden fault is also recognizable.