MT data inversion and dimensionality analysis in Varton geothermal central Iran

Abstract

The Varton geothermal is located circa 70 kilometers northeast of Isfahan, central Iran. The study area consists of highly fractured rocks with argillic alteration zones and some hot springs. Here, we conduct a magnetic and magnetotelluric data survey to study the effect of the fractures on the hydrothermal convection system. The magnetic data processing exposed two parallel main faults with NW-SE strike in the northern and southern borders of the area. Here, we used nine magnetotelluric (MT) stations along a profile perpendicular to the main faults. Dimensionality analysis of MT impedance and phase tensors in the stations indicated a media with two-dimensional structures. However, 1D MT data inversion revealed a three-layered earth beneath most stations, though the layers' thickness and resistivities varied along the profile. Also, 2D MT data inversion results showed that the resistivity between the two main faults was significantly smaller than zones beyond the faults. However, a large zone with low resistivity was close to the southern fault. Besides,  a shallow layer overlain a thin conductive layer with a resistivity of one ohmm or less. In depths more than 500 m, the resistivity noticeably increased gradually to 1000 ohmm. According to the mentioned results, we deduce that the southern fault is the main feeding fault for the geothermal system. Furthermore, the layer with the lowest resistivity correlates with clay alteration and could be the clay cap of the reservoir. Therefore, the deeper resistive rock is the geothermal reservoir.