Interpretation of magnetic anomalies of geological and archaeological origins in a volcanic area (Tusculum site, Lazio, Italy): Methodological proposals

A detailed magnetic survey combined with the study of magnetic properties and spectral analysis in the Tusculum archaeological site (Alban Hills, Italy) indicates the existence of magnetic anomalies (total field and gradient, showing amplitudes up to thousands of nT) resulting from combined geological and archaeological features. In this paper we propose a two-fold analysis based on both manual and automatic procedures for a precise characterization of those elements from total magnetic field and magnetic gradient profiles. The first, automated step consists in spectral analysis of a long (1.2 km) profile to define the dominant wavelengths related to geological and/or archaeological features. The second, manual step consists of the definition of a hierarchical pattern of magnetic anomalies, according to their amplitude and pervasiveness, in the selected (90 x 90 m) area. The defined anomaly pattern includes 1st-order, total field, magnetic anomaly (1000 nT, decreasing towards the North) related to the geological (volcanic) background, 2nd order anomalies (tens to hundreds of nT), showing a marked periodicity, revealed by spectral analysis, in the magnetic gradient in the 1.2 km long profile, and, finally, superimposed 3rd order anomalies (on the order of several nT). Most second-order and some of the third order anomalies are normal dipoles arranged in an orthogonal pattern in plan view, and can be interpreted as linked to the main roman walls of public buildings or roads. The magnetic susceptibility of most materials used in Roman constructions (volcanic tuff and basalt, whose lithology was characterized by means of thin sections) ranges from 600 to 110,000 x 10(-6) S.I., contrasting with the overall lower susceptibility of soil fillings between walls, what explains most of the anomalies found. Magnetite and other iron oxides and sulphides are the main ferromagnetic phases in construction materials. Koenigsberger ratios of roman construction materials vary between 0.25 and 107 what explains for magnetic remanence to have played a certain (but limited) role in some particular magnetic anomalies. (C) 2020 Elsevier B.V. All rights reserved.