Two-Dimensional Magnetotelluric Modeling of the Sabalan Geothermal Field, North-West Iran

Abstract

During 2007, a magnetotelluric (MT) survey in the frequency range of 0.002-320 Hz was carried out on southwestern of Sabalan geothermal region (Moeil valley, Ardabil); the aim of which was modeling of the shallow and deep electrical resistivity structures related to the local geothermal reservoirs and heat system recharge at depth. Twenty eight soundings were conducted in the study area, and the collected MT data were found to be two-dimensional (2D), based on dimensionality (skew parameter) analysis. The NNW-SSE (30°W) direction was identified as the dominant electrical strike in the area. Data along a profile crossing the hot springs with seven MT stations, have been implemented for modeling and inversion. Dimensionality analysis shows that a 2D interpretation of the data is justified, although the presumed geoelectric strike direction is not consistent over the whole profile and frequencies. MT data were analyzed and modeled using MT2DInvMatlab inversion source codes and the finite elements (FEM) method for forward modeling. Inversion parameters as an input file and appropriate mesh blocks design are prepared before start of the modeling and inversion. MT2DInvMatlab software includes a topography file into a forward model for terrain effects compensation in the inversion process. After setting up the model parameter, 2D inversion of the Sabalan magnetotelluric data was performed. Smoothness–constrained least square methods with a spatially regularization parameter estimation and the ACB (Active Constraint Balancing) algorithm were employed in MT2DInvMatlab to stabilize the model. Both apparent resistivity and phase data were used to have models with minimum misfit for TM, TE and joint TE+TM mode data. The TM mode apparent resistivity and phase are better fitted than the TE mode, as a consequence of the inductive nature of the 2D TE response in a 3-D geothermal field structures. However, the apparent resistivity and phase data are also well fitted in the joint inversion of TM and TE mode data. Although the TM mode data is often used for 2-D modeling of MT data in geothermal field studies, we have shown the other two dimensional electrical resistivity models, using apparent resistivity and phase data of TM, TE and joint TE+TM mode data. These models resolved a good correlation between the features of the geothermal field and resistivity distribution at depth. The resulting models reveal the presence of a resistive cover layer (Cap-rock) underlain by an anomalous conductive layer and other geological structures such as fluid-filled faults (about 500-1000 m below the ground surface). A very low resistivity (3-5 ohm-m) feature was found at the depths below 2000 m, bounded by two more resistive (100-500 ohm-m) features that can be interpreted as the main reservoir of the geothermal system in the area. At shallow depths, the resistivity model obtained from the MT data is consistent with the general conceptual resistivity model proposed for high-temperature geothermal systems. The deeper electrical structure was found to be more resistive (100 ohm-m) due to the presence of metamorphic rock formations. According to this results, heat source of the geothermal structure and heat transition zone from deep sources to shallow reservoir, is predicted at 2~7Km at depth.