Determination of the boundaries of Sabalan geothermal reservoirs using 1D and 2D inversion of the magnetotelluric data

In magnetotelluric (MT) methods, perpendicular and horizontal components of the fluctuations of the magnetic and electrical fields are measured on the earth surface. The earlier MT surveys (for example Singh and Drolia, 1983), provided qualitative information with limited narrow banded data and limited quantitative results due to noisy electric field data. Through improving the interpretation schemes and lowering the cost of data acquisition, the MT data has been traditionally obtained in profiles targeting the geological structures and then interpreted with one and two-dimensional inversions. In such an interpretation, one fits the off-diagonal impedances (Zxy and Zyx), generally after rotating the coordinate system so that the main diagonal components (Zxx and Zyy) are minimum or at least as small as possible. It rarely happens that we can find a single strike angle optimal for the full frequency range and all sites. Therefore, the potential impacts of off-profile structures must always be considered. By MT measurements, the electrical conductivity distribution can be determined. Based on the hydrodynamic specification of the geothermal sources, the flow can play the role of an initial parameter in the resistivity contrast of the geothermal source and its surroundings. Using this feature, MT is capable of determining the boundary between a geothermal system and its neighboring medium.The Mt. Sabalan region lies on the south Caspian Plate, which underthrusts the Eurasian Plate towards the north. It is in turn underthrusted by the Iranian Plate, which produces compression in a northwestern direction. Geological structures in this area are complicated further by a dextral rotational movement caused by the northward underthrusting of the nearby Arabian Plate beneath the Iranian Plate.The Sabalan geothermal area lies at the northwest slope of Mt. Sabalan, an immense stratovolcano located in the province of Ardebil in northwestern Iran. For the first time in Iran, MT surveys for geothermal investigations have been carried out at around the Sabalan volcano. This study aimed to delineate the shallow as well as deep conductive anomalies related to the geothermal systems in Sabalan area. To investigate the Sabalan geothermal reservoirs more closelyand to determine the injection and exploration wells, the magnetotelluric data was scheduled in two phases. MT measurements, in the Sabalan area, could clearly highlight the geothermal reservoir. In this work, the boundaries of the reservoirs were investigated by using integration of MT data. The resistivity sections derived from a 2D inversion in conjunction with exploration wells and geology surveys showed that the Sabalan geothermal system is in agreement with Johnston’s (1992) model in which the thicker conductive layers are found in the output areas. Two main reservoirs have been detected by using an interpretation of resistivity maps resulted from 1D and 2D inversions of the MT data. The main reservoir is located on the western side of Mt. Sabalan, which extends from south and southwest of Sabalan summit towards the west and Mooeil valley. The dimensions of this reservoir are two times bigger than the estimated volume resulted from earlier researches. The other reservoir with relatively smaller dimensions exists in northern part of Mt. Sabalan.