Determination of the dips, and segregation, of geological units from airborne magnetic data

Various geological units in an area often exhibit different magnetic properties. Due to this distinction in magnetic properties of the geological units, a correlation between the type of a geological unit and its magnetic response can be established. To determine the lateral and vertical extents of geological units from the magnetic data in a survey area, a number of filters for processing magnetic data can be used.
Magnetic responses of deep anomalies are generally obtained by applying upward continuation filter on the magnetic data. If the upward continuation filter in contact between two layers having different magnetic properties is applied, the magnetic profiles with increasing height are displaced laterally. This displacement is carried out in the direction of the dip between two layers. By combining the upward continuation processing results in the form of a three-component combination image (RGB), we can obtain the dips of geological layers from the magnetic data or map. In this regard, airborne magnetic data acquired from Kalateh-Reshm area, Damghan, have been used and a map indicating the dips of geological layers for the area has been derived. To determine the interface between two adjacent geological units, ZS-Edgezone filter, developed by Zhiqun Shi, have been applied. By analysis of the results of applying the ZS-Edgezone filter on the upward continuation processed magnetic data, we can quite well determine the directions of the dips of geological units, and also distinguish the geological units having low dips from those having high dips. This is due to the fact that a geological unit having a low dip indicate a relatively high horizontal displacement as we increase the height in the upward continuation process. However, a relatively low horizontal displacement can be seen for a geological unit having a relatively higher dip as the height in the upward continuation process is increased to the same amount as in the previous case of the low dip geological unit.
The above-mentioned method has been used in this paper to determine the dips of geological units from airborne magnetic data in Kalateh-Reshm area, located at 120 kilometers south of Damghan city. The aeromagneic data have been acquired by Geological Survey of Iran (GSI) in 2004. In general, the survey area geologically consists of metamorphosed carbonated units and volcanic rocks. To determine the dips of geological units from the airborne magnetic data in the survey area, first we have applied reduction to the pole (RTP) processing filter on the magnetic data. Then, upward continuation filter on the RTP processed magnetic data, and finally, ZS-Edgezone filter on the obtained RTP and upward continuation processed magnetic data have been applied. All these processed magnetic data and maps have been obtained using “Profile Analyst” software, developed by Encom geophysical company. The magnetic field variations in the area are mainly attributed to the changes in the primary and/or secondary magnetite contents of the rocks. As a result of some kinds of alterations in the rocks, the secondary magnetite is occurred while the primary magnetite is origininally occurred in the rocks when they are just formed. The obtained map of the dips of units has been shown by three main colours, i.e. red, green and blue (RGB). Where these three colours in the map are well separated, the dip is considered to be low. However, for the case of high dip, the separation of these three colours in the map is is less made. In other words, in the case of high dip, these three colours in the map have more overlapping than in the case of low dip. Based on this procedure and the results of applying the above processing methods on the magnetic data from the area, presented in the form of RGB colour map, we conclude that the dips of the geological units in central parts of the survey area are comparatively higher. Also, in northeastern parts of the area, the dips are intended towards northeast while in southwestern parts of the area, the dips are seen towards south. Also, two anticlines and a syncline are observed in the central and northern parts of the area. Also, an anticline structure can be seen in the centrer towards south of the area.
The obtained results are in good agreement with the geological map information of the area, and indicate the significance of the method in determination of the dips, and segregation, of the geological units of the extensive study area covered by airborne magnetic surveys.