عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Increasingly high quality satellite magnetic measurements have been providing reliable magnetic data with crustal field models of improved resolution. This paper presents a new approach to delineate geothermal potential sites based on estimates of Curie depth from new crustal field models.
After a period of 20 years without high quality, low-orbit satellite magnetic coverage, since the implementation of Magsat (1979-1980), a new generation of satellites has been launched and is measuring the magnetic field with unprecedented accuracy and resolution. Two satellites were launched into 800 km and 700 km altitude orbits, namely Ørsted (launched in 1999 and still providing scalar data) and SAC-C (launched in 2001, scalar magnetometer active until 2004), respectively, while the CHAMP satellite was launched in July 2000 into a lower orbit, initially at 450 km altitude.
Temperature dependence of magnetic properties of rocks allows magnetic data to be used for estimating the thermal state of the crust in areas with few heat flow measurements. Rocks below their curie temperature may sustain induced or remanent magnetization, but above their curie temperature become practically non-magnetic. It is possible to calculate curie depth from magnetic data. The results of aeromagnetic studies presented by numerous authors show that curie depths obtained from magnetic measurements correlate well with the heat flux; high heat flux is found in areas of shallow curie depths and vice versa.
Geothermal heat flux is one of the main parameters in determining geothermal potential sites. Direct heat flux measurements are scarce because they are difficult and expensive to obtain. This paper presents an indirect method to delineate geothermal potential sites. Considering the proven relation between curie depth and heat flux, the method presented in this paper is based on satellite magnetic data. In this method, curie depth is calculated from satellite magnetic field models. Therefore, one of the thermal boundary conditions is obtained for deep crust. Then, using a one dimensional thermal model, estimation of surface heat flux is discussed.
A comprehensive investigation program is required for geothermal exploration projects. This program may include field investigations, geological, geophysical and geochemical studies. However, the potential application of satellite magnetic field models discussed in this paper can be widely used as a preliminary reconnaissance tool in the early stages of geothermal potential site selection projects.
The study area in this paper covers a wide portion of eastern Iran, located within 28-36◦N and 56-64◦E. The calculations are based on satellite magnetic field model MF5 (developed from CHAMP mission). The results show that the curie depth in the study area is located within 24- 36 km indicating that the curie isotherm is shallower than the moho, and lies within the earth’s crust. The results show two areas with high heat flow. These areas are promising for detailed investigations.
Increasingly high quality satellite magnetic data makes this new method very significant in future studies of geothermal site selection projects. Using a field model instead of raw satellite magnetic field observations is highly productive as the magnetic field measured by a satellite contains contributions from several different sources: the core, the crust, the ionosphere, and the magnetosphere. Only the field from the crust is related to the heat flux. Field modeling allows separation of various sources, thus allowing the crustal field to be isolated. Crustal magnetic field models will experience significant gains in resolution and accuracy over the coming years. The CHAMP mission is expected to continue, providing excellent quality data at solar minimum conditions and at steadily decreasing altitudes. Scheduled for 2011, the European Space Agency’s Swarm mission, a constellation comprising 3 satellites, will further improve the data basis for mapping crustal magnetic anomalies by providing direct measurements of the magnetic field gradient for the analysis.
All the programming of this research was done in MATLAB R2008a.