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,

Fig. 1 - The geographic location and geological map of Sabalan in Ardebil province, with distribution of the MT stations and MT profiles.

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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).

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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

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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.

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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

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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. 7a). Along the profile P02, two conductive zones (less than 30 Ω·m) are detected, one within well NWS-7D, in the western part beneath MT stations 249 and 24, and another one beneath station 216 in the eastern part. The conductive anomaly on the west is part of the conductive layer observed in P01. A more resistive block (more than 100 Ω·m)

Fig. 7 - Joint 2D inversion of TE- and TM-mode data along profiles a) P01, b) P02, c) P03, d) P08, e) P06, and f) P09.

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is modelled, separating the conductive zones whose boundaries are marked by steep resistivity gradients. The shallowest level of this resistive body is found beneath stations 109, 219, and 218, at elevations of about 1500 m a.s.l. (Fig. 7b). Along the profile P03, highly resistive (more than 200 Ω·m) cap layers were detected to the north (Toas) and to the south. They are underlain by two conductive anomaly zones (less than 30 Ω·m) at elevations of 900-1200 m beneath stations 15 and 14 and beneath the Kasra dome. The anomaly at Toas is open towards the north. The same resistive body (more than 100 Ω·m) that was detected in P02 separates these anomalies and comes close to the surface beneath station 219, at an elevation of about 1500 m a.s.l. (Fig. 7c). Fig. 7d shows the resistivity structure resulting from 2D inversion of line P08. Notable features in this profile are the three low-resistivity anomalies (more than 30 Ω·m) found beneath stations 18, 209, and 204,�� at elevations of between 2800 and 1200 m. This is underlain by increasing resistivity values of more than 70 Ω·m which were detected to be shallowest beneath station 203 at elevations of about 2700 m a.s.l. (Fig. 7d). A distinct feature along profile P06 is an interconnected minimally resistive layer with variable thickness, whose base moves up beneath station 12 and is underlain by a more resistive structure. Along profile P09 an almost continuous conductive (less than 30 Ω·m) anomaly is found between elevations of 3000 to 1000 m a.s.l., while on the eastern side, a resistive block is found from the surface down to lower elevations. There are several faults crossing profile P09, visible along the fault line (Fig. 7e). Profile P09 has been chosen to demonstrate the influence of the faults on the resistivity sections, and the results are shown in Fig. 8a for further consideration. These major faults provide routes for surface water to penetrate into and out of the geothermal reservoir, i.e., they are conveying water to and from the geothermal reservoir. A geological section corresponding to profile P09, shown in Fig. 8b, illustrates the location of the faults, geological formations, and heat source of the Sabalan geothermal reservoir, whose location on the western part of the Kasra dome has been determined.

4. Discussion

The conductive zone (less than 20 Ω·m) detected within Mt. Sabalan between elevations of 3000 and 2800 m a.s.l. is an indication of the hydrothermal activities in the region (Fig. 7) and is consistent with the study performed by Johnston et al. (1992) (Fig. 9), wherein the thickest conductive layers are usually found in the outflow regions. Johnston et al. (1992) evaluate different electromagnetic resistivity methods for geothermal exploration. The authors concluded that detection of the geothermal reservoir, even by the MT method, was marginal at best, given the likely resistivity contrasts, resolution, and level of measurement error. However, their analysis assumed a horizontal interface between the conductive layer and the high-temperature reservoir. If constraints imposed by the hydrology of the geothermal system are used in the resistivity interpretation process, a coherent and consistent resistivity model can be developed that reliably delineates the geothermal target (Anderson et al., 1999). The interpreted major outflow directions in the Sabalan area are towards the west (Moeil Valley) and the north (Shabil), as indicated by the presence of the conductive zones that persist at deeper elevations. These conductive zones are about 600 to 1000 m thick beneath pad D

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Fig. 8 - 2D resisitivity model (a) and the geological section (b) along profile P09 (redrawn from Khosrawi, 2008).

Fig. 9 - Scheme of a generalized geothermal system (after Johnston et al., 1992).

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based on profiles P01 and P02. Furthermore, correlation with the well data of NWS-7D (Table 1) shows that the conductive layer coincides with smectite (Sm) and illite-smectite (Il-Sm) zones, while the epidote zone coincides with the increasing resistivity values (more than 30 Ω·m; Fig. 10). Again, this is consistent with the conceptual model by Johnston et al. (1992) wherein by correlating the different alteration mineral assemblages with resistivity, it was found out that the higher temperature minerals like pure illite and epidote have resistivity values more than 20 Ω·m. The same can be observed from NWS-8D well data (Table 2); Sm, chlorine (Cl), Il- Sm zones lie within less than 30 Ω·m, while the appearance of illite and sericite coincides with more than 30 Ω·m contour. This is also consistent with the results of the previous surveys wherein the conductive layer coincides with the Sm zone (Bromley et al., 2000). Additional correlation shows that the Paleozoic metamorphics lie within layers with more than 70 Ω·m resistivity (Fig. 10).

Table 1 - Alteration mineral geothermometers and predicted temperatures, well NWS-7D (EDC, 2008).

DEPTH INDEX MINERALS PREDICTED (m MD/m VD) TEMPERATURE (°C) ~ 200 Smectite, Tridymite, Cristobalite <100 ~ 420 Chlorite, Smectite ~ 120 ~ 550 Illite-Smectite, Quartz ~ 150-180 ~ 820 Incipient Epidote ~ 180-200 ~ 960 Illite, Incipient to Anhedral Epidote ~ 200-220 1316/1280 Euhedral to subhedral Epidote ~ 240-250 2180/1982 Euhedral to subhedral Epidote veins ≥250 2480/2188 Illite-Smectite, anhedral Epidote, Laumontite ~ 220-240 2700/2260 Laumontite ~ 220-240

Table 2 - Alteration mineral geothermometers and predicted temperatures, well NWS-8D (EDC, 2010).

DEPTH ALTERATION MINERAL PREDICTED (m MD/m VD) GEO-THERMOMETERS TEMPERATURE (°C) 170 Smectite, Tridymite <100 280 Chlorite, Sphene ~ 120 300-486 Smectite, Vermiculite, Chlorite, Illite-Smectite ~ 120-150 495-696 Quartz, Illite-Smectite, incipient to anhedral Epidote ~ 150-180 706-958 Illite-Smectite, incipient to anhedral Epidote ~ 180-200 1251 Illite-Smectite, anhedral Epidote 200 1624 Illite-Smectite, Anhydrite, Sericite ~ 250 1743 Epidote, Illite, Chlorite ~ 250 1785 vein Epidote, Illite, Chlorite ~ 250-260 1815 Actinolite, Biotite, vein Epidote ~ 260 2174 Muscovite, Illite, Biotite, Epidote ≥260 2341 Muscovite, Andalusite 260-280

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Fig. 10 - Correlation of resistivity with alteration minerals along profile P01.

A broad conductive anomaly of less than 20 Ω·m mapped in the Moeil area, at elevations starting from 2400 m down to 2000 m, was also detected by the previous MT surveys and was interpreted to correspond to the relict hydrothermal alteration rocks formed by a decollement (Talebi et al., 2005). Other conductive structures (less than 30 Ω·m) are detected in Kasra and Toas, east and NE of pad D, respectively (Figs. 7b and 7c). They reveal an interconnected low-resistive layer reflecting Smectite-Zeolite alteration whose base is elevated around the central part of the profile. This kind of electrical resistivity distribution, a highly conductive surficial layer underlaid by an increase in resistivity, is a common conceptual model of high-enthalpy geothermal systems, where magma intrudes into shallow crustal levels. Volcanic centres such as the Kasra dome may be associated with the intrusive body in this region. Here the conductive overburden can be interpreted as either partial melt or enhanced circulation of hydrothermal conductive fluids into the fault-generated pathways, the elevation of the base of the conductive layer typically conforms to the top of the reservoir, and the very resistive structure at greater depth can be explained by increasing portions of pure illite and epidote as high-temperature alteration products (Cumming and Mackie, 2010; Münoz, 2014). The main resource area in the west is thus delineated based mainly on the extent of the conductive zones (Figs. 7a, 7b, and 7c). An extension of this resource area is outlined to include a part of the Alvares area, south of Mt. Sabalan, where conductive zones were also detected in this region (Fig. 7f). In order to conceptualize more accurately the Sabalan geothermal reservoir according to Johnston et al. (1992), isoresistivity maps derived from the 2D inversion results of all profiles shown in Fig. 1 are presented in Fig 11a, from ground level to 0 m a.s.l elevation. Projecting the

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mapped major faults shows that most of these structures coincide with the discontinuities or the changes in the resistivities. For profile P09, these include faults FLM1, FLM2, FLM3, FLM4, FLM5, FLM6, FLM7, and FLM8 as shown in Fig. 8a. The MT survey conducted at Mt. Sabalan further confirmed the existence of a geothermal resource beneath the Sabalan volcano, which is located to the west of Mt. Sabalan (Fig. 11a). The size of the reservoir is determined to be more than twice the size estimated by earlier surveys (SKM, 2003). An extension of this body is found in Alvares, south of Mt. Sabalan. The survey also confirms the existence of a hotter region at the east of pad D, postulated to be the heat source for the NW Sabalan geothermal reservoir. This is associated with the elevation of the base of the low-resistive smectite clay zone, at about 1500-2000 m a.s.l., near the Kasra dome. Another probable geothermal resource area is delineated in the Shabil region, north of Mt. Sabalan (Fig. 7d). This area is defined by the base of the conductive layer at the elevations of about 1200-1500 m a.s.l. between stations 17 and 203. Based on the isoresistivity maps, the main heat source in Mt. Sabalan probably lies near or beneath the Mt. Sabalan volcano, where the main outflow directions are towards the Moeil valley in the west and the Shabil in the north. The hotter region detected east of pad D could be one of the localized intrusive bodies related to the main Mt. Sabalan volcanic complex and is the heat source for the present NW Sabalan geothermal reservoir (Fig. 11b).

Fig. 11 - Isoresistivity maps derived from 2D inversion MT data for different levels (a) and determined boundary of Sabalan geothermal reservoir and the hottest region (b).

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The next well to be drilled should target the delineated high-temperature reservoir area located to the east of pad D to evaluate the postulated resource area. This suggestion can be verified by conducting structural mapping in the area. Several unmapped faults have been delineated by MT coinciding with resistivity breaks or gradients.

5. Conclusions

The resistivity sections derived from 2D inversion, in combination with exploration wells and geology surveys, show that the pieces of evidence found for the Sabalan geothermal system are in agreement with those of Johnston et al. (1992), in which the thick conductive layers are found in the outer margins of the reservoirs. Two main reservoirs have been defined from 1D and 2D interpretation of the MT data on Sabalan. The major reservoir is located in the west side of the Sabalan volcano, which extends from the south and SW of Sabalan summit towards the west and Moeil valley. A hotter region detected to the east of Pad D could be one of the localized intrusive bodies related to the main Mt. Sabalan volcanic complex and is the heat source for the present NW Sabalan geothermal project. The volume of this reservoir is estimated at almost two times more than what was estimated by earlier research. Our study delineates an additional geothermal resource in the Shabil region, north of Mt. Sabalan.

Acknowledgements. We thank dr. Soheil Porkhial and the Renewable Energy Organization of Iran for putting the MT data and other information at our disposal.

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Corresponding author: Behrooz Oskooi Department of Geomagnetism, Institute of Geophysics, University of Tehran, Kargar shomali, Tehran, Iran, Phone: +98 21 61118238; fax: +98 21 88009560; e-mail: [email protected]

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