Time-Lapse electrical resistivity tomograohy to delineate mud structures in archaeological prospections

Archaeological sites in rural areas are often characterized by structural remains that are made of mud or raw brick, and that produce a very small contrast in physical characteristics, as the surrounding materials of both anthropogenic and natural origins have similar lithological and mineralogical properties. The main measurable differences are due to changes in compaction and porosity and, as a conse¬quence, in permeability, which influence the hydrological behaviour of the subsurface materials. In this paper, we present some applications based on Electrical Resistivity Tomography (ERT) surveys that are routinely applied in archaeological prospection, carried out over two different weather conditions. Measurement of the changing resistivity, corresponding to different moisture conditions in the subsoil, allowed us to identify archaeological strata and structures surrounded by natural, undisturbed soil. A number of tests carried out on a physical model permitted a further definition of the acquisition parameters and methodologies to be used to secure the best results. The field surveys were carried out in the necropolis of “Pill’e Matta” (IV BC - V AD), located in the suburbs of the metropolitan area of Cagliari, in the southern part of Sardinia, and in a Punic and Roman site near the village of Terralba, located in the Campidano plane, in the western part of Sardinia. The results show that time-lapse prospecting can increase the resistivity contrast between archaeological structures and soil.