Detection of Ancient Subsurface Structures using Magnetic Data in Teppe-Hissar Area, Damghan

Summary: Geophysical methods are effective tools in archaeological investigations. Sensitive magnetometers have been used for many years to locate and characterize archaeological sites. Magnetometry is one of the most widely geophysical methods in this application as it contains no harmful environmental effects. Magnetization contrast in ancient sites is created by natural causes and human activities. Studies have shown that topsoil has generally a higher magnetic susceptibility than most bedrocks and sub-soils. Vegetation fires and fermentation effects increase background magnetic susceptibility, and oxidation-reduction cycles associated with alternate wetting and drying of the soil; all of these tend to convert the iron compounds to strongly magnetic oxide maghaemite. In this paper, subsurface structures have been investigated in the Tepe-Hissar ancient area of Damghan. In this regard, synthetic and real magnetic data and fuzzy filters have been used to investigate these subsurface structures. Moreover, normalized derivatives respect to vertical derivative (NAVD or NA) filter has been introduced that enhance edges in potential field methods.
Magnetization contrast has been examined between metal items, building materials, ditches, pits, and their surrounding environment. This magnetization can be either remanent magnetization, induced magnetization, or both, that creates a contrast in magnetization between the interested features and the surrounding environment. Remanent magnetization can be locked into archaeological materials such as kilns, ovens and floors that are baked (Clark et al., 1988). Induced magnetization also plays a large part in the ability to detect archaeological features. Fire has long been known to increase the magnetic susceptibility of soil. Weakly magnetic iron oxides in the clay and silt particles are transformed into highly magnetic oxides through burning. When the organic matter in a soil burns at 200 0C, it produces a reducing atmosphere, which can change hematite to magnetite, and probably maghaemite on reoxidation as the burn ceases (Aspinall et al. 2008). For better interpretation, noise must be removed. After necessary corrections, the total magnetic intensity map is seen to be dominated by long wavelength magnetic anomalies. The anomaly appears to be about 100nT that represents the amount of magnetization of the area.
Methodology and Approaches: Local phase filters provide an approach to detect the edges of the anomalies but conventional phase functions need to be unwrapped to remove phase ambiguity (Fitzgerald et al., 1997). Therefore, detection of the boundary of chambers or walls and the location of sources can be obtained from derivative based filters such as the horizontal gradient magnitude, tilt-angle, theta-map, Laplacian and tangent hyperbolic filters, however, these filters typically fail for archaeological investigations due to the high noise content of the acquired data. In this paper, similarly to prospect an area, a synthetic model is prepared which combines structures and the lane or rooms that have been filled with alluvium and soil. One of the conventional phase filter that is used for edge detection is the tilt angle (Miller and Singh, 1994). The tilt angle is effective in balancing the amplitudes of different anomalies, but it is not primarily an edge-detection filter. The theta map uses the analytic signal amplitude to normalize the total horizontal derivative (Wijns et al. 2005). The amplitude of the response of this filter from the deeper and shallow source bodies is similar, although the response from the deeper bodies is rather different. The hyperbolic tilt angle (HTA) filter uses of the real part of the hyperbolic tangent function in the tilt angle. Using this filter, better delineation of the edges of the anomalous body than the other filters is achieved. The maximum value of the HTA gives location of the body edges (Cooper and Cowan, 2006). Edge enhancement in potential field methods helps to make proper geological and archaeological interpretation. There are many methods for enhancing edges, most of which are high-pass filters based on the horizontal or vertical derivatives of the field. Normalized angle respect to vertical derivative (NAVD) filter, which is a new edge detection filter, is based on ratios of the horizontal derivatives of the field. The NAVD filter has been demonstrated on synthetic and real magnetic data from the archaeological site of Tepe-Hissar. Compared with other filters, the NAVD filter detects edges very well.
Results and Results of this research work indicate that the obtained information from magnetometry is compatible with the subsurface information before excavations in the area. The results of magnetic surveys in the archaeological site of Teppe-Hissar show that the site has an irregular construction with meandering lanes. The main lane in this archaeological site appears to be along northeast-southwest direction. This fact is clearly understood from the magnetic images of the site.