GNGTS 2017 - 36° Convegno Nazionale

728 GNGTS 2017 S essione 3.3 anomaly, which increases with the top depth z 1 , while the presence of a remnant magnetization component can be easily established by the detection of one or more among the following features: 1. A magnetic anomaly amplitude exceeding a few nT; 2. A deviation of the strike of the simmetry axis of a dipolar anomaly from the present day reference field declination, D 0 ; 3. A deviation of the anomaly shape from the expected shape for the given reference field inclination, I 0 . Fig. 1 shows an example of observed anomalies that can be modelled by objects having a remnant magnetization component. In general, the observation of anomalies associated with induced magnetization requires one or more among the following conditions: 1) a strong susceptibility contrast with the surrounding soil; 2) a random arrangement of natural remnant magnetization (NRM) components (e.g., a random orientation of magnetite grain spins in a paramagnetic matrix, a random build–up of bricks, etc.); 3) a low Koenigsberger ratio Q = M R / M I , and 4) the absence of nearby objects with a significant NRM component. Examples of archaeological features whose anomalies are dominated by induced magnetization contrasts are: graves, historical iron artifacts (Bevan, 2002), ditches and limestone walls. In contrast, remnant magnetization generally produces much stronger anomalies in materials with high Koenigsberger ratio or, more often, when the archaeological structures are fired materials (e.g., bricks) or materials that have been fired at a later time during historical or natural events. A forward modelling session of any local survey anomaly should start with the selection of an object type (dipole, rectangular prism, or general vertical prism) and the creation of 1–2 magnetic profiles, as illustrated in Fig. 1. At the next step, the user should inspect the magnetic profiles, in particular the error curve, in order to start an interactive trial–and–error procedure and determine a magnetization model that can explain Fig. 2 - A rectangular prism model of observed anomalies (A) along the hill slope (Antigonea archaeological park, southern Albania, Schettino et al. , 2017). These data were acquired 0.5 m above the terrain. The average soil susceptibility was χ 0 = 500x10 -6 , while the ambient field parameters were: D 0 = 3.95°, I 0 = 56.72°, F = 46336.00 nT. Panel (B) shows the model anomalies. calculated assuming χ 0 = 3000x10 -6 , z 1 = 2 m, z 2 = 3 m, and a NRM vector wih parameters D = 90°, I = –20°, M R = 0.9 A/m. Panels (C) and (D) illustrate magnetic profiles with model and observed anomalies (green and black curves, respectively), and the error curve (in red). Finally, Panel (E) shows a N-S topographic profile through the prism.