Contribution of the resistivity method to characterize mud walls in a very dry region and comparison with GPR

摘要

Mud walls are frequent in many archaeological sites, including sites located in semi-desert regions. In cases with buried walls, the geophysical methods can be useful tools for their investigation, since they provide maps of the subsoil in a non-invasive way and in relatively short times. This article deals with the characterization of mud walls in very dry soils through the resistivity method, discussing a methodology to satisfactorily apply the method in this kind of adverse environment and how to interpret the results. The case of the Palo Blanco archaeological site, a pre-Incaic village located in the NW region of Argentina, is analyzed. In very dry soils, it is frequent that the electrical current cannot be injected due to the high values of electrical resistivity at the shallowest portions of soil, which makes the resistivity method useless. Here we show how this problem can be systematically overcome by wetting a shallow thread between each pair of injection electrodes, and the manner in which the resulting profiles can be analyzed. We perform numerical simulations of the resistivity profiles considering 3D models of mud walls and wetted threads that reproduce experimental situations, and compare them to 2D field data. The resistivity profiles are analyzed for different orientations and distances between the wall and profile, and fluctuations of the soil parameters. The experimental profiles are interpreted from these simulations and compared to ground penetrating radar (GPR) profiles. The resistivity methodology shows effective for characterizing the size and depth of the mud walls, even in cases with collapsed or much eroded walls, for which GPR often fails.