Measurements of the dielectric effective permittivity applied to archaeology

Abstract

Archaeological studies require high resolution radar techniques. A good knowledge of the analyzed media and conditions are necessary (material, water content and porosity) because of the influence of these conditions on the propagation and reflection of electromagnetic waves. Several in situ measurements have been performed in different archaeological places by using Ground-Penetrating Radar (GPR), allowing us to obtain the effective permittivity for several materials. Measurements were also performed in areas where different conditions of water content were observed. Wave speed variations are determined when the water content is lightly increased, mainly in porous materials. This variations seems to be caused due to a change in dielectric permittivity and conductivity of media. Several authors have carried out laboratory experiments to detect changes depending on water porosity and clay content (e.g. Robert, 1996; Saarenketo, 1996; Knoll and Knight, 1994). In this job we present the preliminary results of several measurements performed on known sites, where the exact materials of every layer and the contact depths between materials, lateral changes and water contents are well known. The analysis of the arrival times of the reflected waves on the surface, where the medium properties change, allow us to calculate the wave velocity in such a medium. Comparing the two way travel time of the reflected waves in radar records, obtained for the same material with different water contents, it is possible to obtain the different dielectric permittivities. Some of these measurements were performed on marble floor, located over a layer of concrete (which is placed over ground materials), allowing to obtain several experimental values for the dielectric permittivity of different media (several materials, water and air content). Comparing the results among them and using different models to relate the dependence of the dielectric permittivity on the porosity and saturation, it has been possible to obtain differences on the product of the water content by the porosity of the two emplacements using the simplified relation: S PΔ =     Δ 2 8 1 ε , where ε is the effective value of the dielectric permittivity of the medium measured during the in situ experiences, P is the porosity of the materials and ΔS is the difference in water content in the medium.