A study on the geothermal reservoirs in Mahallat area, Markazi Province by 1D and 2D inversion of the magnetotelluric data

Magnetotelluric (MT) method is an important passive surface geophysical method whichuses the Earth’s natural electromagnetic fields to investigate the electrical resistivitystructure of the subsurface. In thermal areas, the electrical resistivity is substantially of adifferent form and generally lower than in areas with colder subsurface temperatures. Theselected MT profile in the region crosses over the hydrothermally altered zones anddifferent geological structures. Reflection and refraction of EM signals at both horizontaland vertical interfaces separates the media of different electrical parameters.Electromagnetic methods have been developed and employed to recognize the geologicalfeatures and particularly fault zones in many regions. To achieve a 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. For subsurface mapping purposes, the long period natural-field MT method proved to be veryuseful. The MT method, due to a high penetration depth, is one of the most effectiveelectromagnetic methods to recognize deep geothermal systems.In this study, the geothermal reservoirs were conducted using Magnetotelluric (MT) data. Mahallat in Markazi Province was chosen as the case study area and the MT surveywas carried out at 17 sites with a 500-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) directions.Measurements of the horizontal components of the natural electromagnetic field wereused to construct the full complex impedance tensor, Z, as a function of frequency. Usingthe effective impedance, determinant apparent resistivities and phases were computed and used for the inversion. The 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 the study was populated and close to electric noise sources and travertine mines, the recorded data did not have a good quality to justify the low coherency between the electric and magnetic channels. Since it was assumed that the earth structure was largely 2D for the purpose of a 2D inversion, the 3D structure would appear in the data as noise.We performed a 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 better the model resolution. The 2D inversion of the TE-,TM-, TE+TM 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 TMmodes.The determinant provides a useful average of the impedance for all current directions. Since the quality of the determinant data was acceptable, 2D modeling of the determinant data would be expected to provide a more reasonable approximation of thetrue subsurface structure. Therefore, we used the model obtained from the DET-modedata as an interpretation model. The resistivity model obtained from the DET-mode isconsistent with the geological model of the Mahallat region down to two kilometers.From surface down to about 400 m depth, there is a conductive layer (<30 ohm-m)showing a variable thickness along the profile, which is hydrogeologically interpreted asthe penetrated zone for water. The surface was covered by clay and sand making it a good condition for keeping water. The conductive zone located in the middle part of the profile was interpreted as a geothermal reservoir that its estimated depth ranged from 800 m down to 2000 m. The conductive zone was hidden under the Quaternary alluviums and travertine stones along the profile.