Application of Magnetotelluric method in exploration of geothermal reservoirs with an example from Iceland

Magnetotellurics (MT) is a geophysical passive technique for exploring geothermal reservoirs. It utilizes a broad spectrum of natural geomagnetic fields for electromagnetic induction in the Earth. This method is also preferred over DC-resistivity methods, particularly where the exploration of subsurface deep aquifers is considered. The role of MT method in the exploration of geothermal reservoirs is highlighted in this paper.
As a practical example, it focuses on the results of a recent MT study performed on a geothermal region in Iceland. Because it is crossed by the Mid-Atlantic Ridge and its associated rift and fault zones, Iceland is very active both tectonically and magmatically. In order to determine the deep structure between two neighboring geothermal fields: Hengill and Brennisteinsfjoll MT data were collected. one- and two- dimensional inversions of these data are done and the results are presented. In a good agreement with geological information, the two-dimensional inversion model declares a highly conductive Smectite-Zeolite zone followed by a less conductive Epidote-Chlorite zone. Also, a highly conductive deep zone is seen in the middle of the profile which is interpreted as a cooling partial melt representing the main heat source of the geothermal system.
Introduction: Geothermal resources are renewable source of heat and of economic interest. Geophysical exploration of geothermal fields using electromagnetic (EM) methods has received increased attention over the past few years. The electrically conductive water reservoir surrounded by a relatively resistive host is efficiently imaged using EM methods. In particular, because of its capability in the large-scale imaging of lateral conductivity variations and greater depth of investigation, Magnetotellurics (MT) is preferred over
other electromagnetic methods. The main focus of the paper presented here is one- and two- dimensional interpretation of the MT data over a geothermal field in South-West of Iceland.
Geothermal systems: Geothermal gradient and thermal conductivity of rocks are chief elements which cause the heat flow within the Earth crust. In addition, both conduction and convection processes occur within a geothermal field. Because of density differences caused by varying temperature, water moves within the reservoir by convection. Also, the conduction process gives a linkage between the magma body and permeable reservoir rocks (Barbier, 2002).Through four types of geothermal systems introduced in this paper, we focused on the hydrothermal system and discussed its two Water-dominated and Vapor dominated types.
Geological settings: The Icelandic crust is mostly of volcanic origin, with both intrusive and extrusive rocks (mainly oceanic-type flood basalts, tuffs, hyaloclastites and some acidic rocks) that were erupted under rift conditions (S?mundsson, 1979). The main geological features and distribution of geothermal systems are shown in Fig 1. As it is shown in Fig 1. geothermal fields occur in regions of young volcanism and along active plate boundaries. Because the abundant geothermal systems in Iceland are the results of volcanic activities, two basic models of alteration associated with volcanic geothermal systems: Acid sulfate and Adularia sericite are also presented (Fig 2).
MT Data acquisition: In September 2000, an MT survey was carried out at 21 sites along a 12 km line in southwest Iceland (Fig 1). The MT profile is almost perpendicular to the axis of the active tectonics and volcanism, and in correspondence of the high-temperature systems of the Hengill volcanic complex and the Brennisteinsfjoll geothermal area.
Inversion and Interpretation: 1D inversion of determinant impedance data and 2D inversion of joint TE- and TM- mode data are performed. As it is shown by the 1D inversion results in Fig 5-a, a top resistive layer with a resistivity value greater than 100 ohm-m changes to a conductive structure of about 10 ohm-m. Also, a transition into a more resistive zone is seen at about 1.2 km depth. This resistive unit with a thickness of 2 km changes into a conductive structure at a depth of 4 km. As for the 2D inversion results, a resistive layer (>400 ohm-m) is recognized at the top (Fig. 6-b). The second layer is very conductive (<10 ohm-m), and shows a variable thickness along the 2D section, passing from a few hundred meters at Site 20 to about 1800 m at Site 03. Below this conductor there is an increase in resistivity with depth along the whole profile. The southern part of the profile is characterized by a high resistivity (?1000 ohm-m) basement, whereas in the middle of the profile the top conductive layer (<10 ohm-m) is followed by a resistive layer (30–100 ohm-m), which in turn is overlying a very conductive structure (<5 ohm-m). The very resistive layer at the top can be interpreted as the porous basalt layer near the surface. At about 400 m depth, the conductive layer, showing variable thickness along the profile, is most naturally interpreted as the smectite–zeolite zone. The less conductive zone below this conductor is interpreted as the chlorite-epidote mineralization zone. Considering the characteristics of the neovolcanic zone in Iceland, this conductive bulk (<5 ohm-m) located at the middle of the profile, can be interpreted as either partial melt or a porous region with hot ionized fluids located on top of a magmatic heat source. Since this conductive structure is located where the Hengill fissure swarm intercepts the profile, it is most naturally interpreted as magmatic intrusions acting as a heat source for the geothermal system.