عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In general, the airborne magnetic method compared to other airborne geophysical methods has more applications in prospecting and exploration for mineral deposits, and mainly leads to better results. Nowadays, geophysical prospecting for most mineral deposits, especially for metallic deposits such as base metals, iron and chromite, in large areas is often carried out using the airborne magnetic method. Three main stages in this method, like other geophysical methods, are: 1. data acquisition, 2. data processing, and 3. data interpretation. The data acquisition stage includes various methods, which can generally be classified in two categories: data acquisition using helicopters and data acquisition using aircraft. In the data processing stage, necessary corrections and also filters or transformations are applied on the data. The data interpretation stage includes different methods of interpretation, modeling and determination of earth parameters including depth of observed magnetic anomalies.
In order to prospect chromite deposits in the Sabzevar ophiolithic complex area, airborne magnetic data from the area acquired by an aircraft along a flight pattern with a flight interval of 1000 meters, have been processed and interpreted. The Sabzevar ophiolithic complex in the 1:100000 geological map or sheet of the area (provided by the Geological Survey of Iran) is seen in the north of the map as an extensive band elongated in an east-west direction with a length of approximately 300 kilometers. In this study, the geological sheet of the area with the scale of 1:100000 has been divided into 4 larger scale sheets (with the scale of 1:50000), and prospecting of chromite deposits has been made in each of these 4 sheets by tracing magnetic minerals associated with chromite, where chromite itself does not have a considerable magnetic property. Although geological and geochemical investigations of the area have already been made, there has been no attempt to carry out detailed processing and interpretation of airborne magnetic data from the area. This research work aims to prospect chromite deposits in the area by processing and interpretation of the acquired magnetic data. To fulfill this aim, in the first step, we have digitized all the magnetic data as well as the geological maps, geochemical data and other exploration information from the area.
In the primary processing stage, necessary corrections, such as parallax, leveling, microleveling and diurnal corrections have been made on the airborne magnetic data acquired from the Sabzevar area. Then, the total magnetic map of the area from the corrected magnetic data has been presented in which the Sabzevar ophiolithic complex has generally higher magnetic intensity in comparison with other parts of the area that are mainly covered by sediments and sedimentary rocks. Furthermore, various filters or transformations such as reduction to the pole, vertical first and second derivatives, analytical signal, upward continuation and downward continuation have been applied to the magnetic data, and as a result, magnetic anomalies have been detected. Applying downward and upward continuation filters on the magnetic data, we increase the detection of shallow and deep magnetic anomalies, respectively, and hence, these anomalies will be discriminated. The filter of reduction to the pole causes magnetic induction to be made vertically, and thus, the magnetic data are changed so that they have been acquired in the pole (i.e. in the magnetic latitude of 90 degrees). The detection and resolution of shallow magnetic anomalies are increased by applying vertical derivatives on the magnetic data. Finally, applying the analytical signal filter on the magnetic data, we determine the location and shape or geometry of the detected magnetic anomalies. To remove or considerably reduce regional effects from, and thus increase residual anomalies, on the magnetic data, we can use vertical derivative or downward continuation maps in which this condition has been provided.
Qualitative interpretations of the total magnetic and filtered magnetic maps indicate the existence and location of sedimentary basins in the area as they can be recognized by low magnetic relief zones. These sedimentary basins have been surrounded by high magnetic relief zones. Also, magnetic lineaments are determined as the boundaries between the zones having different magnetic relief. Geological faults are an example of such magnetic lineaments. Note that the existence of a magnetic lineament in a location does not necessarily indicate the existence of a geological fault in the location.
By accurately marking the locations of active chromite mines of the area on the total magnetic map (or other magnetic maps) of the area, we can easily observe that these active chromite mines have been located on the places where magnetic intensity is relatively moderate (or even relatively low). This indicates that these active chromite mines are not mainly situated on the places having high magnetite contents. Moreover, as a result of accurately studying the locations of the geochemical samples of chromite taken from the area, and investigating the results of chemical analysis of the chromite samples, and also considering the locations of the active chromite mines on the interpreted magnetic maps of the area, we have been able to obtain a suitable magnetic pattern or model from the area. The magnetic pattern, and integration of this magnetic pattern with geological and geochemical information, has led to the determining of 20 prospective chromite areas, which will be considered for the next exploration stages.