Assistant Professor, Earth Physics Department, Institute of Geophysics, University of Tehran, Iran
Ph.D. Student, Faculty of Geology, University of Sistan and Balouchestan, Iran
Electromagnetic methods are widely used for the study of subsurface resistivity structures. These methods are based on the response of the subsurface structures to electromagnetic fields. The magnetotelluric (MT) method is an electromagnetic technique that uses the natural, time varying electric and magnetic field components measured at right angles to the surface of the earth, to make inferences about the earth’s electrical structure which in turn can be related to the geology tectonics and subsurface conditions. Measurements of the horizontal components of the natural electromagnetic field are used to construct the full complex impedance tensor, Z, as a function of frequency. Using the effective impedance, determinant of apparent resistivities and phases are computed and used for the inversion. Also the apparent resistivity for DET-mode is computed and used for the 2D inversion.
The long- and 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 wave-guide 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. The MT method uses simultaneous measurements of natural time variations in the three components of the Earth's magnetic ﬁeld (Hx, Hy, and Hz), and the orthogonal horizontal components of the induced electric ﬁeld (Ex and Ey) to obtain the distribution of the electric conductivity in the Earth's interior.
Magnetotelluric studies are important. They contain information about the ﬂuid content and thermal structure, which are the key parameters for deﬁning the rheology of the crust and upper mantle. This method has been proved to be useful for widespread applications. For example, MT is extensively being used in imaging the ﬂuids in subduction zones and volcanic belts, orogenic regions, delineation of ancient and modern subduction zones and lithospheric studies.
1D and 2D inversions are conducted to resolve the conductive structures. We performed 1D inversion of the determinant data using a code from Pedersen (2004) for all sites. Since the quality of the determinant data was acceptable, we performed 2D inversion of the determinant data using a code from Siripunvaraporn and Egbert (2000).
An MT survey was carried out using MTU2000 systems belonged to Upssala University of Sweden. Data are stored on an internal hard disk and are downloaded via a connection. Power is supplied by a 12 V external battery. Three magnetometers and two pairs of non-polariable electrodes are connected to this ﬁve-channel data logger. For the registration of magnetic ﬁeld variations in the range from 10,000 to 0.001 Hz broadband induction coil magnetometers are used. The electric ﬁeld variations are registered by measuring potential differences with non-polarizable electrodes. The experimental set-up includes four electrodes, which are distributed at a distance of 100 m in north–south (Ex) and east–west (Ey) direction. They are buried at a depth of about 30 cm and coupling to the soil is improved using water. The ADU logger and magnetometers are located in the centre, whereas the three induction coils are oriented north–south (Hx), east–west (Hy) and vertical (Hz) at a distance of 10 m from the data logger and at least 1 m from electric ﬁeld wires and 5m from every conductive object. The vertical coil was buried to 4/5 of its length and covered by a plastic tube in order to prevent recordings from the inﬂuence of wind.
In this study, the subsurface structure of Havir Lake, in southeast of Damavand volcano has been studied using the Magneotelluric (MT) method. There are two ideas about its generation: activation of Quaternary glaciers due to their movement and melting, and/or a production of Mosha fault. So, we gathered the geological and geophysical evidences related to field work to find a logical reply for this matter. A north-south Magnetotelluric profile was designed in the southern part of the Lake. After acquisition, processing and 1D and 2D inversions of the MT data, with respect to the structural and geological information, a low resistivity body (60 Ohm-m) distinguished in the southern part. Its thickness is about 4000m and starts from a depth nearly of 500m to 4500m. It seems that its existence is due to shear movement of Mosha fault and the debris of glaciers movements. In the northern part of the profile (exactly near the lake) a very high resistive body (1000 Ohm-m) is recognized from the 300m of earth surface with 400m thickness which most probably is the very rigid basement of the lake which probably belongs to inseparable Jiroud and Mobarak formations.