2D inversion of the magnetotelluric data from Mahallat geothermal field in Iran using finite element approach
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ORIGINAL PAPER
2D inversion of the magnetotelluric data from Mahallat geothermal field in Iran using finite element approach Behrooz Oskooi & Mehrdad Darijani
Received: 27 October 2012 / Accepted: 17 February 2013 # Saudi Society for Geosciences 2013
Abstract The natural-field magnetotelluric (MT) method has proven very useful for mapping the geothermal fields as resistivity sections. The depth of investigation of the MT method is sufficiently large to penetrate deep into the upper crust. MT soundings along two transects across Mahallat geothermal field in Iran were carried out to determine the crustal structure in the region. The selected MT profiles in the region cross over the hydrothermally altered zones and different geological structures. Data were acquired along two profiles crossing the Mahallat hot springs with a total of 28 MT stations in a frequency range of 8,000 to 0.008 Hz. Spacing between stations was kept 500 m for a good resolution. We have used the code MT2DInvMATLAB for inversion using the method of finite elements for forward modeling. Apparent resistivity and phase data of transverse electric (TE), transverse magnetic (TM), and TE + TM modes along each profile were modeled. The geothermal fluid reservoir is resolved at 1,000 to 3,000 m depth and the geothermal resource is estimated to be located at 7,000 m or deeper. Keywords Electrical resistivity . Geothermal . Interpretation . Magnetotelluric . Mahallat . 2D inversion
Introduction Magnetotellurics is a passive electromagnetic method, which uses the natural occurring electromagnetic field. The long and B. Oskooi (*) : M. Darijani Institute of Geophysics, University of Tehran, Kargar shomali, Tehran, Iran e-mail: [email protected] M. Darijani e-mail: [email protected]
short periodic signals originate from fluctuations in the intensity of the solar wind and global lightning activity, respectively. The electromagnetic energy released in discharges propagates with slight attenuation over large distances in a waveguide between the ionosphere and Earth’s surface. At large distances from the source, this is a plane wave with frequencies from about 10−5 to 105 Hz. The magnetotelluric fields can penetrate the Earth’s surface and induce telluric currents in the subsurface. Magnetotelluric (MT) studies are important; they constrain the fluid content and thermal structure, which are the key parameters for defining the rheology of the crust and upper mantle (Unsworth 2010). This method has been proven to be useful for widespread applications. For example, MT is extensively being used in imaging the fluids in subduction zones and volcanic belts (Jones and Dumas 1993), orogenic regions (Unsworth 2010), delineation of ancient and modern subduction zones (Jones 1993), lithospheric studies (Patro and Sarma 2009), and geothermal studies (Johnston 1992). The aim of the present study was to analyze the potential of twodimensional (2D) magnetotellurics for the detection of geothermal resources in the Mahallat geothermal in general. Geothermal resources are i
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