عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The aeromagnetic data of Iran was surveyed by Aeroservice Company (Houston, Texas) under auspicious of Geological Survey of Iran during 1974-1977. The survey was done with a two-engine airplane using a cesium vapor magnetometer with a sensitivity of 0.02 gama. The data was collected along flight lines with average line spacing of 7.5 km over 62 flight blocks mostly with a constant barometric flight height. A barometric elevation range of 1000-3600 m was used (Yousefi and Feridberg, 1977) which translated to about a 500-1000 m height from the Earth surface.
Saleh (2006) used the raw data of the 62 blocks and produced an aeromagnetic composite map of Iran. The raw data had already been corrected for daily variations of the geomagnetic field. To produce a composite grid of the aeromagnetic map of Iran from the original rather raw data, Saleh (2006) first implemented the detailed leveling and micro-leveling procedures for each block. The fully leveled blocks were stitched initially to eight geologically coherent larger blocks and finally the larger blocks were stitched. The final 1 km by 1 km grid of the aeromagnetic map of Iran was produced from the combined data set using a bidirectional interpolation scheme.
Magnetic anomalies in Iran latitudes do not correlate directly with their corresponding causative magnetic bodies because the direction of the geomagnetic field and magnetization are not normal to the Earth surface. The asymmetry between the magnetic anomalies and their causative magnetic bodies increases from north to south of Iran. The deviation could reach to tens of kilometers for aeromagnetic anomalies located in the south of Iran. For geological interpretation purposes, it is very desirable to derive aeromagnetic anomalies that are positioned over their causative magnetic bodies, quite similar to that expected from gravity anomalies, or an induced magnetic body located in the North Pole. Baranov and Naudy (1964) introduced a procedure called reduction-to-the-pole (the standard RTP method) which converts magnetic anomalies in mid-latitudes to that produced by the magnetic bodies having vertical magnetization, and lying at the north geomagnetic pole.
The standard RTP is only valid for regions in which the direction of the geomagnetic field is almost constant. Therefore, the standard RTP method is not applicable to produce an RTP map of the aeromagnetic field of Iran. The RTP methods which allow for variations of geomagnetic field are called differential reduction-to-pole methods (DRTP).
In this study, the revised aeromagnetic map of Iran (Saleh, 2008) was reduced to the pole considering the variations of inclination and declination of the geomagnetic field over Iran. The new aeromagnetic map was produced using the differential reduction to the pole (DRTP) method developed by Arkani-Hamed (1988). The DRTP operator shifts aeromagnetic anomalies in different geographical latitudes to the top of their causative sources, thus facilitating an easier geological interpretation of the magnetic anomalies. We first applied the DRTP method to the synthetic magnetic anomalies of three identical spheres lying in north, centre and south of Iran assuming induced magnetization for the spheres. We found that the DRTP method did not show any instability; thus it is appropriate to use in Iran. The DRTP aeromagnetic map of Iran showed significantly a better correlation between the magnetic anomalies and the boundaries of the main tectono-stratigraphic units of Iran (e.g. Alborz, Jazmurain Depression), volcanoes (e.g. SabalanMountain) and major active faults (e.g. Tabriz and Doruneh faults).