This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/authorsrights

Author's personal copy

3-D inversion of MT data from the Sabalan geothermal field, Ardabil, Iran

Reza Ghaedrahmati a,⁎, Ali Moradzadeh b, Nader Fathianpour c, Seong Kon Lee d, Soheil Porkhial e

a Shahrood University of Technology, Shahrood, Iranb Faculty of Mining, Petroleum and Geophysics, Shahrood University of Technology, Shahrood, Iranc Mining Engineering Department, Isfahan University of Technology, Isfahan, Irand Korea Institute of Geoscience and Mineral Resources (KIGAM), Republic of Koreae Renewable Energy Organization of Iran (SUNA), Iran

a b s t r a c ta r t i c l e i n f o

Article history:Received 5 September 2012Accepted 17 March 2013Available online 28 March 2013

Keywords:Magnetotelluric3-D inversionGeothermalSabalan

Since the true Earth is 3-D in nature, a three-dimensional (3-D) inversion has clear advantages over lower dimen-sional inversions. We utilized a 3-Dmagnetotelluric (MT) inversion code,WSINV3DMT, to obtain a realistic resis-tivity model using a long period MT data set collected in the Northwest Sabalan geothermal field in Ardabil, Iran.The apparent resistivity and phase curves, the magnetic induction vectors, the impedance polar diagrams and therotational invariant of impedance tensor, indicate a complex 3-D conductivity structure. After setting up themodelparameters and designing the appropriate block discretization, we performed the 3-D inversions for two sets ofobserved data; one set includes the full MT impedance tensor and another set contains only off-diagonal elementsof the MT impedance. The final model was selected according to the relative magnitude of the data misfit and themodel normwith respect to various Lagrangianmultipliers. The results of this study illustrate the 3-D inversion ofthe off-diagonal elements ofMT impedance tensor is precisely enough to explain the structures related to the geo-thermal source. The obtained results were compared with the results of available 2-D models and they are theninterpreted using all of the geological and drilling data of the area. The main outcome of this study is the precisedelineation of the geometry of geothermal source that is located at the center of the study area with a surfacecoverage of about 7 km2.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Magnetotelluric (MT) as a deep geophysical technique (Cagniard,1953; Vozoff, 1991) is widely being used for the assessment of geother-mal fields in many regions (e.g. Arnason et al., 2010; Harinarayana et al.,2006; Heise et al., 2008; Oskooi et al., 2005; Wannamaker, 1997).

In manyworks, various two-dimensional (2-D) inversion techniquesare employed to interpret the MT data. It is clear, however, that the trueEarth is 3-D in nature and assumption of a 2-D model for the Earth isachieved by simplification of the true structures. In order to increasethe accuracy of MT data interpretation, the 3-D inversion techniquesare now being more commonly used in data interpretation. In the pastfew decades, there have been various studies from practical applicationof 3-D MT inversion (e.g., Arnason et al., 2010; Cumming and Mackie,2010; Farquharson and Craven, 2009; Heise et al., 2008; Newman et al.,2008; Sasaki, 2004; Tuncer et al., 2006; Turkoglu et al., 2009; Uchida,2005; Xiao et al., 2010; Zhdanov et al., 2011). Although a 3-D inversionhas clear advantages over one- or two-dimensional inversions, it is stillchallenging since it demands high performance computing power.Hence, most 3-D inversion codes can only be implemented either onhigh performance workstations or parallel machines.

Siripunvaraporn et al. (2005) developed a 3-D MT inversion algo-rithm based on the classical Occam approach (Constable et al., 1987),which is referred to asWSINV3DMT. In this algorithm, the computationalcosts are significantly reduced by transforming the solution of the in-verse problem from model space to data space (Siripunvaraporn andEgbert, 2000) so that we can invert modest 3-D MT data sets even on apersonal computer (PC) in a relatively reasonable time. Due to this capa-bility, this 3-D inversion code has become one of the standard tools for3-D inversion of MT data (e.g., Heise et al., 2008; Tuncer et al., 2006;Turkoglu et al., 2009; Xiao et al., 2010 among others).

In this study, it is attempted to use this 3-D inversion code to modeland interpret a set of MT data collected in Sabalan geothermal field inArdabil, Iran. Previous geophysical works in this area (Section 2) indi-cates that almost all of the interpretations are based on the modelsobtained from1-D and2-D inversions of theMTdata. In addition analysisof the MT data indicates a complex geoelectrical structures for the area(Section 3). Therefore we think the 3-D inversion of the MT data shouldbe better for further interpretation of the geothermal resource and relat-ed structures.

2. Sabalan geothermal field

The Sabalan geothermal field which is now under investigation forgeothermal electric power generation lies at the Northwest of Mt.

Journal of Applied Geophysics 93 (2013) 12–24

⁎ Corresponding author. Tel.: +98 9166622752.E-mail address: r.ghaedrahmati@yahoo.com (R. Ghaedrahmati).

0926-9851/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jappgeo.2013.03.006

Contents lists available at SciVerse ScienceDirect

Journal of Applied Geophysics

j ourna l homepage: www.e lsev ie r .com/ locate / jappgeo

Author's personal copy

Sabalan in Ardebil province at the NW of Iran (Fig. 1). This geothermalproject is an effort to explore and develop geothermal resources associ-ated with high surface thermal activity located on the Western slopesofMt. Sabalan. The area has beenunder geo-scientific exploration studiessince 1978 (Fotouhi, 1995).

Fig. 1 shows a geological map of this area. Mt. Sabalan is a largestratovolcano, consisting of an extensive central edifice built on a proba-ble tectonic horst of underlying intrusive and effusive volcanic rocks.Enormous amounts of discharged magma caused the formation of acollapsed caldera about 12 km in diameter, and a depression of about400 m. The lava flows in the Sabalan are mostly trachy-andesite and

dacite with alternating explosive phases (Kingston Morrison, 1999). Thefour major units used for the original geologic study and mapping ofNorthwest Sabalan are Quaternary alluvium, fan and terrace deposits;Pleistocene post-caldera trachy-andesitic flows; Pleistocene syn-calderatrachy-dacitic to trachy-andesitic domes and Pliocene pre-calderatrachy-andesitic lavas, tuffs and pyroclastic (SKM, 2005). The geologicalstructure of the area can be divided into arcuate ring and linear structures.Three arcuate ring structures are identified, including the inner and outercaldera structures and the strongly developed arcuate structureinterpreted to be a decollement fault that is very obvious in the field,occurring as a curved scarp, up to 100–200 m high (SKM, 2005).

Fig. 1. A geological (based on Kingston Morrison, 1999, detail faults map is based on EDC, 2010) and topographic maps of the NW Sabalan geothermal field. The study area is markedby a green square on geological map. The dotted A, B, C, D and E zones marked on geological map are identified zone related to geothermal resource that will be discussed in the 3-Dresults. The crosses symbol associated with numbers on topographic map are selected MTmeasurement sites for the 3-D inversion. Pink solid circles show the drilling well pads thatthe images of NWS7D and NWS8D wells have been shown by thick gray lines. Green arrows indicate the hydrological flow path (EDC, 2010); the hotter zone introduced by EDC(2010) has been shown by a brown dashed circle. The location of 2-D cross-sections including their names has been shown with red lines to be used in interpretation of the 3-Dmodel. (For interpretation of the references to color in this figure legend, the reader is referred to the web of this article.)

13R. Ghaedrahmati et al. / Journal of Applied Geophysics 93 (2013) 12–24