A Recent Magnetotelluric Investigation of the Sabalan Geothermal Field in North-Western Iran

Bollettino di Geofisica Teorica ed Applicata Vol. 57, n. 3, pp. 261-274; September 2016 DOI 10.4430/bgta0174 A recent magnetotelluric investigation of the Sabalan geothermal field in north-western Iran B. OSKOOI1, M. TAKALU1, M. MONTAHAEI1 and M.R. RAHMANI2 1 Institute of Geophysics, University of Tehran, Iran 2 Geothermal Exploration Division, Renewable Energy Organization, Tehran, Iran (Received: April 29, 2015; accepted: March 23, 2016) ABSTRACT Magnetotelluric measurements of the Sabalan geothermal field in north-western Iran could clearly highlight the geothermal reservoirs. This study is aimed at delineating the shallow as well as deeper conductive anomalies related to the geothermal systems in the Sabalan area. Dimensionality analysis showed a two-dimensional (2D) behavior for the data, and therefore 2D inversion was performed. The resistivity sections showed that the Sabalan geothermal system is in agreement with the results of Johnston et al. (1992), in which thick conductive layers are found close to the surface. Two reservoirs have been located. The main reservoir is located in the western side of the Sabalan volcano, extending from south and SW of the Sabalan summit towards the west and Moeil valley. The dimensions of this reservoir are approximately two times greater than the estimated volume that resulted from earlier studies. The other reservoir, with smaller dimensions, is located to the north of the peak. Key words: geothermal reservoir, magnetotelluric, Sabalan, resistivity, Iran. 1. Introduction Geophysical methods, especially magnetotelluric (MT), play a key role in geothermal exploration, since naturally occurring high temperatures, fluid-filled faults, and other elements of geothermal systems produce strong variations in underground physical properties. Natural MT signals come from a variety of natural phenomena, including thunderstorms and solar wind. The total frequency range of MT data can be from 40 kHz to less than 0.0001 Hz. The data are acquired in a passive mode using a combination of electric sensors and induction magnetic coils and can detect changes in resistivity at great depths. Experiences have shown that the correlation between low resistivity and fluid concentration is not always correct, since alteration minerals very often produce either comparable or a greater reduction in resistivity. Moreover, although water-dominated geothermal systems have an associated low resistivity signature, the opposite is not true and the analysis requires the inclusion of geological and possibly other geophysical data, in order to limit uncertainties (Spichak and Manzella, 2009). Subsurface conductive hydrothermal fluid, as a characteristic feature of geothermal systems, causes a surface electric field potential. Such an electric field is due to the streaming of the potential field, caused by the movement of hydrothermal fluids around the subsurface heat source (Fitterman and Corwin, 1982). In geothermal areas, © 2016 – OGS 261 Boll. Geof. Teor. Appl., 57, 261-274 Oskooi et al. Fig. 1 - The geographic location and geological map of Sabalan in Ardebil province, with distribution of the MT stations and MT profiles. 262 Magnetotelluric investigation in NW Iran Boll. Geof. Teor. Appl., 57, 261-274 the electrical resistivities are substantially different from and generally lower than in areas with colder subsurface temperature (Oskooi et al., 2005). Because of this, MT is capable of determining the boundary between the geothermal system and the neighboring medium. In order to investigate the Sabalan geothermal reservoir and to locate the injection and exploration wells, MT surveys were scheduled in two phases. The first phase was carried out at 28 MT stations in 2007 and the second phase involved 50 MT stations in 2009. The area lies at the NW slope of Mt. Sabalan, an immense stratovolcano located in the province of Ardebil in north-western Iran. The geological map of the Sabalan area with main surface manifestation and geological structures is shown in Fig. 1. The area has been the subject of geo-scientific exploration studies since 1978 (Fotouhi, 1995). In 1998, a semi-detailed regional MT survey was conducted at 212 stations around Sabalan (KML, 1998). Three deep exploratory wells and two shallow reinjection wells were drilled between 2002 and 2004. The Moeil low-resistivity anomaly was identified by the 1998 MT survey (Porkhial et al., 2010; Takalu et al., 2014). 2. General description of the study area Mt. Sabalan, known as a large trachyandesitic composite volcano, lies on the south of the Caspian Plate. It is located near the triple junction of the Eurasian, Iranian, and Arabian plates. Relative movements of the plates, like the dextral rotational movement caused by the northward underthrusting of the Arabian plate beneath the Iranian plate, determine a structurally complex compressional tectonic zone in this region (Emami, 1994). Geological studies (KML, 1998) identified two major types of structures: a set of linear faults and several arcuate ring faults which strike predominantly toward the NW and NE (Fig. 2). Faults have always been considered of great importance in geothermal systems (Hanano, 2000) because their associated fractures provide highly permeable zones which are likely preferential flow paths for the ascent of geothermal fluids toward shallower geological formations. The four major stratigraphic units of the studied area, arranged in a descending order of age, are as follows (SKM 2003): a) Pliocene pre-caldera trachyandesitic lavas, tuffs, and pyroclastics (Valhazir Formation); b) Pleistocene syn-caldera trachydacitic-to- trachyandesitic domes, flows, and lahars (Toas Formation); c) Pleistocene post-caldera trachyandesitic flows, domes, and lahars (Kasra Formation); d) Quaternary alluvium, fan, and terrace Fig. 2 - Rose diagram for fault strikes in the study area deposits (Dizu Formation). (KML, 1998). 263 Boll. Geof. Teor. Appl., 57, 261-274 Oskooi et al. Fig. 3 - MT data of six MT sites, 247, 239, 231, 226, 225, and 223, displayed as apparent resistivity and phases in both polarizations of TE- mode (red) and TM mode (blue). 3. Processing, inversion, and interpretation of the MT data 3.1. MT data processing and dimensionality analysis Three Phoenix MTU-5A data acquisition systems with a standard frequency range from 0.0005 to 380 Hz were used in this study. The raw time series data were processed using the Phoenix Geophysics, Ltd. SSMT2000 software, and the resulting data were edited, analyzed, and modelled using the Geosystem’s WinGlink software. The MT data were generally of good 264 Magnetotelluric investigation in NW Iran Boll. Geof. Teor. Appl., 57, 261-274 Fig. 4 - Apparent resistivity and phase data of two MT sites of 218 and 109 a) before editing and b) after editing. Fig. 5 - Impedance polar diagrams for MT sites 24 and 215 for selected frequencies. 265 Boll. Geof. Teor. Appl., 57, 261-274 Oskooi et al. quality down to 0.01 Hz. Fig. 3 shows the MT data in unrotated coordinates for TE and TM mode polarizations at six stations: 247, 239, 231, 226, 225, and 223. Poor-quality data (with large error bars), outliers, and scattered data were removed from the data set prior to inversion modelling (Fig. 4). An impedance polar diagram is divided into a matrix of small plots. Each of these plots corresponds to one frequency. It is drawn in a polar form as functions of rotation angles. In each plot, a vector indicating the rotation angle for a certain frequency is displayed. The off-diagonal components, represented by the red lines, show a strong 2D response except at the highest frequencies, with a stable direction of maximum electric field aligned along the axis of the trough (Fig. 5). Dimensionality analysis of the measured data, based on the phase-sensitive skew (Bahr, 1988), reveals that skew values are less than 0.3 for most stations and periods and the whole data set may be regarded as 2D (Fig. 6a). The phase tensor scheme (Caldwell et al., 2004) employed to provide estimates of regional geo-electric strike direction for all profiles evinces that a major strike direction of 0˚ is quite reasonable for the study area (Fig. 6b). Accordingly, 2D inversions were applied on the unrotated impedance data. Fig. 6 - a) Dimensionality analysis based on phase-sensitive skew method and b) estimation of the strike direction based on phase tensor approach for the data along profile P02. 3.3. 2D inversion The 2D smooth inversion algorithm of Rodi and Mackie (2001) was used for the 2D inverse modelling. The inversions were computed by employing a uniform half space with 100 Ω·m resistivity as a starting model. An error floor of 2% for the apparent resistivities and 5% for the phases was applied. For 2D inversion, TE- and TM-mode data were both considered. The normalized root mean square (rms) misfit was at a level of around 4 for the joint TE- and TM- mode data inversion. The maximum penetration depth for the MT signals is calculated roughly at about 13 km. In order to achieve the best coverage of the 2D study, six profiles of P01, P02, P03, P06, P08, and P09, which cover the Shabil area in the north, the Moeil valley in the west, and the Alvarez area in the south and SW of Sabalan volcano, are considered (Fig. 1). Profile 266 Magnetotelluric investigation in NW Iran Boll. Geof. Teor. Appl., 57, 261-274 P01 crosses the boreholes NWS-8D and NWS-7D. Fig. 7 displays the models of the joint 2D inversion of the TE- and TM-mode data along profiles P01, P02, P03, P06, P08, and P09. The results along line P01 are shown in Fig. 7a. Resistivity of the top layer varies from 50 to more than 250 Ω·m. An anomalous conductive layer extending from the Moeil valley to wells NWS-7D and NWS-8D was observed to about 1000 m a.s.l. This conductive layer has a thickness of about 500-1000 m and is underlain by a moderately resistive layer of less than 50 up to 250 Ω·m (Fig.

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