One of the major problems in the forward modelling of magnetic anomalies is the assessment of a minimum level of acceptable accuracy in the fit between observed and theoretical anomalies. We present a new approach to the analysis and interpretation of archaeological magnetic anomalies, based on classical algorithms of forward modelling and a new technique of error assessment. This approach allows us to determine geometry, physical properties, and location of buried archaeological features, as well as the occurrence of fires or other historical events that may have affected the observed magnetic signal. Our method starts from the acquisition of total field data, usually in a regular grid arrangement, and proceeds through their reduction to archaeological magnetic anomalies. This reduction is performed subtracting from the observed total field data a polynomial representation of the regional field, on the basis of a rigorous criterion that tries to separate archaeological anomalies from geological (crustal) contributions. At the next step, a map of the maximum allowed misfit is built, which depends from the estimated uncertainty at each point of the magnetic anomaly field. This map specifies the maximum allowed deviation of theoretical anomalies from the observed values. The last step is the analysis of these anomalies through a new forward modelling tool, with the objective to reconstruct the three-dimensional arrangement of buried features and possibly obtain some information about the history.

We perform an analysis of the errors that affect magnetic anomaly data in archaeological geophysics, arising from both survey time pro− cedures and common potential fields methods of magnetic data processing. Specifically, there are errors due to: 1. positioning of total field readings, 2. the estimated diurnal drift of the Earth's magnetic field, 3. the selected gridding algorithm, 4. the process of reduction of total field data to magnetic anomalies, 5. the application of decorrugation filters, and 6. knitting of two or more survey rectangles within the same archaeological area. Our analysis shows that in normal conditions these errors can have a magnitude up to few tens of nT and a lower limit exists to the amplitude of the anomalies that can be interpreted archaeologically. A correct error assessment is especially required when the magnetic anomalies must be interpreted quantitatively through a forward modelling procedure. We illustrate an application of these concepts to a magnetic data set acquired at the Roman settlement of Urbs Salvia (Central Italy). We show that forward modelling provides a powerful tool for the reconstruction of ancient buried settlements. FIGURE 4. Errors associated with the choice of the gridding algorithm. The three maps show residual grids obtained subtracting from a bi−directional grid grids obtained by IDW (A), Kriging (B), and Minimum curvature (C).

Hadrian’s Villa is an ancient Roman archaeological site built over an ignimbritic tuff and characterized by abundant iron oxides, strong remnant magnetization, and elevated magnetic susceptibility. These properties account for the high-amplitude magnetic anomalies observed in this site and were used as a primary tool to detect deep archaeological features consisting of air-filled and soil-filled cavities of the tuff. An integrated magnetic, paleomagnetic, radar, and electric resistivity survey was performed in the Plutonium-Inferi sector of Hadrian’s Villa to outline a segment of the underground system of tunnels that link different zones of the villa. A preliminary paleomagnetic analysis of the bedrock unit and a high-resolution topographic survey by aerial photogrammetry allowed us to perform a computer-assisted modelling of the observed magnetic anomalies, with respect to the archaeological sources. The intrinsic ambiguity of this procedure was reduced through the analysis of ground penetrating radar and electric resistivity profiles, while a comprehensive picture of the buried archaeological features was built by integration of the magnetization model with radar amplitude maps. The final subsurface model of the Plutonium-Inferi complex shows that the observed anomalies are mostly due to the presence of tunnels, skylights, and a system of ditches excavated in the tuff.

We report on two magnetic surveys performed in July and September 2015 at the ancient Hellenistic city of Antigonea,\ud located in southern Albania. The main objective of the two surveys was to find the city theatre and determine possible\ud sites of future excavations. We suggest a new technique for dense collecting of magnetic data along difficult terrains,\ud with minimization of the topographic effect. Evidence of a possible location of the theatre was found along the\ud southern slope of the Jermë hill, just outside the city walls. Other interesting structures indicate the presence of many\ud other buildings in this part of the Antigonea settlement

The UNESCO World Heritage Hadrian’s Villa lies over the Colli Albani volcanic district near Rome. Magnetic, paleomagnetic, radar, and electric resistivity surveys were performed in the Plutonium–Inferi sector to detect buried buildings and outline a segment of the underground system of tunnels that link different zones of the villa. In particular, a paleomagnetic analysis of the bedrock unit allowed to accomplish an accurate geomagnetic field modelling and characterize the archaeological sources of the magnetic field anomalies. We used a computer-assisted forward modelling procedure to generate a structural model of the sources of the observed anomalies. The intrinsic ambiguity of the magnetic field modelling was reduced with the support of ground penetrating radar amplitude slices and an analysis of radar and electric resistivity profiles. The bedrock lithology in this area is an ignimbrite tuff characterized by abundant iron oxides. The high-amplitude magnetic anomalies observed in the Plutonium–Inferi area are due to strong bedrock remnant magnetization and susceptibility contrasts between topsoil infill of cavities and the surrounding tuff. The resulting magnetization model of the Plutonium–Inferi complex shows that the observed anomalies are mostly due to the presence of tunnels, skylights and a system of ditches excavated in the tuff.