نوع مقاله : مقاله تحقیقی (پژوهشی)
1 دانشکده مهندسی معدن، نفت و ژئوفیزیک، دانشگاه صنعتی شاهرود
2 دانشکدههای فنی، دانشگاه تهران
عنوان مقاله [English]
OjatAbad iron ore located in north east of Semnan city. Old indications of mining are evident in the area. Belich and Bragin (1993) introduced Semnan iron ores as hydrothermal deposits. Recent studies show that the iron ores are related to Oligo-Miocene magmatism. Most iron ores have magnetite which has high magnetic susceptibility; therefore, magnetic method is a conventional method for geophysical exploration of iron. In order to identify and detect this deposit, a magnetic survey was carried out in OjatAbad area. The magnetic data corrected for diurnal change of magnetic field and then total magnetic field of the Earth has been reduced. To do this, a reduce to pole (RTP) filter was implemented on the grid for locating the anomalies and their sources. This method entails removing the dependence of magnetic data to the magnetic inclination, i.e., converting the data which were recorded in the inclined Earth’s magnetic field to what they would have been if the magnetic field had been vertical. This method simplified the interpretation because for sub-vertical prisms or sub-vertical contacts (including faults), it transforms their asymmetric responses to simpler symmetric and anti-symmetric forms. The symmetric “highs” are directly centered on the body, while the maximum gradient of the anti-symmetric dipolar anomalies coincides exactly with the body edges. For depth estimation of anomalies, the upward continuation filter was implemented. This is a mathematical technique that projects the data taken at an elevation to a higher elevation. The effect is that the shortwavelength features are smoothed out because one is moving away from the anomaly. The upward continuation is a way of enhancing large scale (usually deep) features in the survey area. It attenuates the anomalies depending on their wavelengths; the shorter the wavelength, the greater the attenuation. Also upward continuation tends to accentuate the anomalies caused by deep sources at the expense of the anomalies caused by shallow sources. For 3D imaging of magnetic data, we chose the inversion method of Li and Oldenburg (1998) that minimizes a function composed by (1) the data-misfit function defined in the data space as the L2 norm of the difference between the observed and predicted data, and (2) the stabilizing function defined in the parameter model space as the L2 norm of the first-order derivative of the weighted density distribution in both vertical and horizontal directions. They introduced a depth-weighting function to counteract the spatial decay of the kernel function with depth by giving more weight to rectangular prisms as depth increases. On the RTP magnetic map of study area we can see 8 magnetic anomalies (A, B, C, D, E, F, G and H) which are located at the northeast to the southwest trend. The H one has the lowest amplitude. The results of upward continuation filter show that the anomalies of F and G are shallower than the other anomalies. Also, H and B are only deeper than 100m. The inversion results recovered all of the 8 anomalous bodies and confirm the above results. They showed that the anomalous body H has lower magnetic susceptibility and is deeper than the others and it seems likely that it is an intrusive body. So, iron mineralization is probably happened in the other bodies. The anomalous body B is the deepest mineralized body which elongates to 100 meter.