جستجوی مقالات مرتبط با کلیدواژه "برگردان به قطب" در نشریات گروه "زمین شناسی"

Studied area is located in Kerman province, Iran. This area belongs to the Urumia-Dokhtar magmatic arc (UDMA) zone. Several porphyry copper deposits were known in this magmatic arc. UDMA is marked by voluminous Tertiary volcanic sequences of up to 3000 m thickness. It seemed that these Cu occurrences are related to structures, especially major and main faults.In this study, magnetic structures were extracted by using the aeromagnetic data. This data was extracted by Atomic Energy Organization of Iran (AEOI) during 1977 and 1978. The flight lines distance and the sensor altitude were about 500 and 120 m, respectively. Airborne magnetic method is among the most efficient geophysical techniques for the detection of buried anomalies.In the first step, total magnetic intensity map was prepared and reduction to pole transformation was done on it. Reduction to the pole (RTP) is a standard part of magnetic data processing method, especially for large-scale mapping. RTP operation can transform a magnetic anomaly caused by an arbitrary source into the anomaly that the same source would produce if it is located at the pole and magnetized by induction only. Interpretation of magnetic data can further be helped by RTP in order to remove the influence of magnetic latitude on the anomalies, which is significant for anomalies caused by crust. Most of the studied areas are covered by mafic volcanic rocks. These rocks produce moderate to high magnetic anomalies. In some parts, the effect of demagnetization can be observed in these rocks because of the spread of alteration in this area. A low magnetic anomaly is observed in the northern part of the studied area related to sedimentary rocks in this area. In the next step, body magnetic anomalies were extracted from reduction to pole map. A tilt angle filter used to extract magnetic lineaments was applied to the reduction to pole data. Upward continuation filter was implemented at 200 m, 500 m and 1000 m on the tilt angle map and magnetic lineaments were extracted from tilt angle maps. Several magnetic lineaments and magnetic bodies were extracted from magnetic maps (tilt angle maps and reduction to pole maps). There is good correspondence between magnetic structures and copper occurrences. The P-A curve confirms this result.

Sangan iron deposit is a magnetite iron skarn. Magnetometry is the most common method for the exploration of such kinds of deposits based on the magnetic properties of these minerals. This method specifies the depth, dip, shape, and strike of the sources causing the anomaly. Many explorations of this ilk have been done of which giant Gol e Gohar in Iran is an example. Based on geology and following magnetometry, a massive iron deposit was predicted during Sangan iron exploration, where 558 boreholes in 50×50 m net were designed and drilled in the west ore body, revealing very useful subsurface information. Intrusive bodies are mainly granitic and oxidant types which, because of the presence of fine grain and disseminated magnetite, have higher magnetic susceptibility (K) in comparison to other granites. These bodies are considered as source rocks for the creation of gigantic Sangan mines. Multiphase ground magnetic survey points (19376) with different specifications were equalized and interpreted in this study. The causative sources of most magnetic anomalies in the rotation to the pole (RTP) map were magnetite mineralization, confirmed by the drilling data. In this research, filters of first vertical gradient and upward continuation were used to guide drilling owing to the presence of the remnant magnetism in the magnetite skarn and the unavailability of such measurements that are necessary for specifying the accurate depth of the magnetic anomaly sources. The information pertaining to vertical gradient and upward continued maps correlated well with the depth obtained from drilling and 3D block modelling. The amount of ore reserve and its depth increased from the western anomaly (A') to the central anomaly (C) and had a dip direction to the east. Such change in the depth of mineralization correlated with upward continued responses which represent the central magnetic anomaly (C) as the deepest anomaly in Sangan iron ore deposit. Magnetic responses of the Upward continuation to 1000 m of this magnetic anomaly can be related to either the high depth of the mineralization (620 m drilled so far) or the deeper intrusive body which needed deeper drilling (more than 1000 m) for confirmation.

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 short-wavelength 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.

With detection of the magnetic minerals along with the chromite, it is possible to prospect the chromite deposits. Chromite has no magnetic properties but from the genetic aspects, there is a close relation between chromite and magnetite. With this respect and the magnetic differences of chromite minerals with the country rocks, the effort was to prospect the chromite mineralization of 1: 50000 Abdasht sheet by the use of air born geophysical data. In the study of the geophysical air born of the area, the observed magnetic piles in the area has been investigated by using of the digital filters of reduce to pole, first and second derivations, analytical signals and upwards continuation on the map of the magnetic field of the total intensity. With the study of these maps and compare them with the area, especially ophiolite and ultramafic bodies and chromite-included areas have been studied and the final results are compared with the magnetic pattern and with respect to the maps of the vertical derivation and upwards continuation, 25 areas for the detailed exploration and field sampling area suggested in the entire sheet of the Abdasht.

In March 9th، 2008 an earthquake of ML=5. 1 occurred in Aryan-Shahr area، 45 km north of Birjand، East of Iran. More than 100 aftershocks were recorded within 8 days after the main shock. Primarily، the Sedeh fault with N76 trend was nominated as source of this earthquake. But the distribution of aftershock dispersals has been occurred in an ellipsoid district which is not superimposed on Sedeh Fault. The long axis of distribution ellipsoid of aftershocks (N120) is nearly perpendicular to the Sedeh Fault trend. Based on investigation of morphotectonic-neotectonic evidences، aftershock epicenter dispersal and the interpretation of aeromagnetic data reveal the presence a blind fault parallel to long axis of distribution ellipsoid of aftershocks، which has been named Gheysar blind fault.