The determination of boundaries of layers and geothermal bodies using 2D inversion of magnetotelluric (MT) data, in Sabalan area

^*نگارنده رابط:         تلفن: 61118238-021         دورنگار: 88630479-021                               E-mail: boskooi@ut.ac.ir






Geophysical methods play a key role in geothermal exploration since many objectives of geothermal exploration can be achieved by these methods. The geophysical surveys are directed at obtaining indirectly, from the surface or from shallow depth, the physical parameters of the geothermal systems. The other geophysical techniques like gravity, magnetic, self-potential studies, and shallow seismic refraction also provided valuable information about the shallow geothermal zone. The earlier magnetotelluric (MT) that was survey carried out (Singh and Drolia, 1983), provided qualitative information with limited narrow band data and limited quantitative result due to noisy electric field data. Due to both limitations in interpretation methods and the cost of data acquisition, magnetotelluric (MT) data have been traditionally obtained in profiles targeted to the geology, and then interpreted with two-dimensional inversion. In such an interpretation, one fits the off-diagonal impedances (Zxy and Zyx), generally after rotating the coordinate system so that the main diagonal components (Zxx and Zyy) are minimum, or at least small. It is seldom possible to find a single strike angle that is optimal for the full frequency range and for all sites, and possible impacts of off-profile structure must always be considered.
MT is an appropriate tool for identification of the deep subsurface structures. In this method, recording the erpendicular to horizontal components the fluctuations of the magnetic and electrical fields are measured at the earth surface. Using these measurements, the electrical conductivity distribution can be determined.
Geothermal resources are ideal targets for EM methods since they produce strong variations in underground electrical resistivity. Geothermal waters have high concentrations of dissolved salts that result in conducting electrolytes within a rock matrix. The resistivities of both the electrolytes and the rock matrix (to a lesser extent) are temperature dependent in such a way that there is a large reduction in the bulk resistivity with increasing temperature. The resulting resistivity is also related to the presence of clay minerals, and can be reduced considerably when clay minerals and clay-sized particles are broadly distributed. On the other hand, resistivity should be always considered with care. Experience has shown that the correlation between low resistivity and fluid concentration is not always correct since alteration minerals produce comparable, and often a greater reduction in resistivity. Moreover, although water-dominated geothermal systems have an associated low resistivity signature, the opposite is not true, and the analysis requires the inclusion of geological and, possibly, other geophysical data, in order to limit the uncertainties (Spichak, and Manzella, 2009). Geothermal energy has been harnessed by using the steam or hot water stored underground at high temperatures and pressures for the generation of electric power in conventional steam turbines, and by the direct use of the heat content of the resources in heat exchangers in industrial or domestic utilizations. Temperature and the circulation of subsurface hydrothermal fluids, both of which are characteristic features of geothermal systems, are capable of generating a surface electrical potential field. Such electric fields are the result of streaming potential, caused by the movement of hydrothermal fluids around the subsurface heat source (Fitterman and Corwin, 1982). Based on hydrodynamic geothermal sources, the flow can play the role of on initial parameter in the resistivity contrast of the geothermal source and its surrounding. using this feature, MT is capable of determining the boundary between geothermal system and the neighboring medium.
In order to investigate more closely Sabalan geothermal reservoirs and determine the injection and exploration wells, the magnetotelluric data was scheduled in two phases. The first phase was carried out at 28 MT stations in 2007. The second phase 50 magnetotelluric stations were taken in 2009. MT measurements, in Sabalan area, could clearly highlight the geothermal reservoir. The results of the MT survey are presented through isoresistivity maps sliced at different elevations to show the resistivity changes with depth, and through cross sections to show the resistivity structures that were modeled. The changes in resistivity with elevation and observed resistivity layers are discussed in detail. Interpretation of these results will help in delineating the arbitrary boundaries geothermal resource at Mt. Sabalan and pinpoint the best drilling targets in the area.
After dimensional analysis using skew parameters (for skew below 0.2) study area shows a two-dimensional behavior. After removing data outside of category that caused by environmental noise, magnetotelluric inversion was performed. The aim designing of the two profiles S01 and S02 including some part of reservoir and we also wanted to S01 profile to pass the exploration wells.  Profiles S01 and S02 cover the Moil Valley and the present development block of the Mt. Sabalan Geothermal Project.
Along the profile S01, resistivity of the top layer varies from 50 to >250 Ω-m. An anomalous conductive layer extending from Moil Valley to wells NWS-7D and NWS-8D was observed to about 1000 m above sea level (a.s.l.) This conductive layer has a thickness of about 500-1000 m and is underlain by a moderate to highly resistive layer with resistivity values >50 to 250 Ω-m.
Along the profile S02 two conductive zones (<30 Ω-m) are detected, one within the well NWS-7D, in the western portion, beneath MT stations 249 and 24, and another one beneath station 216, on the eastern portion. The conductive anomaly on the west is part of the conductive layer observed in P01. A high resistivity block (>100 Ω- m) is modeled separating the conductive zones, its boundaries marked by steep resistivity gradients. The shallowest portion of this resistive body is found beneath stations 109, 219 and 218, at elevations of about 1500 m a.s.l.
The resistivity sections derived from 2D inversion in conjunction with exploration wells and geology surveys showed that Sabalan geothermal system is in agreement with Johnston’s studies (1992) in which the thicker conductive layers are found in the outer areas.