An assessment of the dimensionality of subsurface geoelectric structures and 1-D and 2-D inverse modelling of magnetotelluric data in NW of Sabalan geothermal region

Magnetotelluric (MT) is a geophysical exploration method that utilizes simultaneous measurements of naturally occurring magnetic and electric fields. As this method utilizes natural electromagnetic (EM) signals with a wide frequency range, its exploration depth is from several meters to several kilometers. Depending on its frequency, it is used for petroleum, groundwater, geothermal, mineral, and geotechnical explorations. To determine electrical variation of a subsurface structure using the MT survey, five components of electric and magnetic field variation are measured on the earth’s surface in each measuring site. The two components of the horizontal electric field (Ex and Ey) and also two horizontal components of the magnetic field (Hx and Hy) are normally measured in the north-south (x) and east-west (y) directions. An extra measurement of the vertical component of the magnetic field (Hz) is sometime measured in each measuring site. The relationship between the electric and magnetic fields at the earth’s surface can be written as i ij j E  Z H where, i, j  x, y and ij Z is the complex impedance tensor of order 2 2. When the resistivity of the earth is a function of depth (i.e. in a one-dimensional earth), the diagonal elements of impedance tensor (Z) are equal to zero and its off-diagonal elements are equal in amplitude but opposite in signs. In two dimensional (2- D) structures where resistivity is invariant in the strike direction, diagonal terms become zero if the EM fields are defined in a coordinate system normal to the strike of the structure. In such cases, the impedance component of the electric field which is parallel to strike (i.e. transverse electric (TE) mode) would be different from those components of the electric field perpendicular to the strike (i.e. the transverse magnetic (TM) mode). In the case of 2-D structures, if the impedance is measured in an arbitrary orientation, the angle required to rotate the measurements into TE and TM modes can be determined from the impedance tensor. In the case of the three dimensional (3-D) earth, the entire components of Z would be non-zero. Presently, most of the MT survey is performed as electrical sounding and the measured data is then modeled and interpreted to sense the details of the subsurface structure. To provide a reasonable and physically meaningful model of the subsurface structure, its dimension must be determined somehow. To determine the dimensionality of the subsurface structure using MT data, several parameters such as conventional skew, ellipticity and polar diagrams of the impedance tensor elements are used in practice. As these parameters are very sensitive to the noise of data, the phase sensitive skew and some dimensionality indices were defined. In this study, it was attempted to use various parameters such as conventional skew, ellipticity and phase sensitive skew along with dimensionality weighting indices (D1, D2, D3) to determine the structural dimension of the Sabalan geothermal field in the NW of Iran using MT data. It was also attempted to model the MT data of several sites along a profile located in Moil valley in the NW of Sabalan in order to determine any possible location of the geothermal reservoirs. The obtained results indicated that the structure up to the medium depth was 1-D and the deepest structure was 2-D. As the subsurface structure of the area was 1-D at periods lower than 1 second, the averaged data of both TE and TM modes were first inverted one dimensionally using the WinGlink software to explore and delineate the locations of any geothermal reservoirs likely to be present in the study area. The inversion results illustrated a layered structure located from the ground surface to the depth level of 1500 meters above the sea level (m.a.s.l) which in turn confirmed the shallow depth structures were 1-D. The results also showed a highly conductive layer, with resistivity lower than 40 Ω.m located beneath the MT stations of 24 to 244. The results of drilling revealed that this conductive zone could be interpreted as a clay cap over the geothermal reservoir which elongates to an approximate depth of 1000 meter. This clay zone overlays a more resistive zone, with resistivity values from 40 to 100 Ω.m, which in turn can be interpreted as a geothermal reservoir. The data from a 2-D joint inversion of the TE and TM modes confirmed the results of the 1-D inversion of the MT data for shallow to intermediate depths. It further delineated that the location of the geothermal reservoir was at a depth zone of 500-1500 m.a.s.l. under the sounding location of 7-245 in the south to south eastern part of the study area.