High-resolution microgeophysics : a fascinating challenge. I. Detection of thin patinas

In this paper, the framework of a high-resolution integrated project is presented. Its aims are to attempt to minimize the limits of equivalence and thus decrease those of suppression in the detection by GPR techniques of external growth patinas, intrasample fractures, oxidation surface layers, surface irregularities, etc. Two different sites were investigated (site A and site B) using techniques of crystallography and microgeophysics. Site A shows, with remarkable clarity, over calcarenitic blocks, the transition of the process of patina growth (gypsum and calcium oxalate dihydrate) from areas that are patina-free to those which, instead, are patina-rich. The amplitudes of the waves reflected by the surfaces covered with gypsum and oxalate are sistematically smaller than those from the areas that are patina-free. Furthermore, the waves reflected by the inner planes are characterized by smaller values of amplitudue when the patinas are present and such a decrease peaks in the 1.4 - 2 MHz frequency range. Site B is characterized by large calcarenitic blocks which are considerably more decayed than those of site A. Gypsum is the primary patina constituent. In practice, the site consists of patina-rich (often in the course of detachment) and patina-detached areas, since much gypsum has fallen apart, thus exposing, once again, the calcarenitic substrate. Here, as was the case for site A, the values of the amplitudes are smaller for the areas covered with patina. Such a behavior, however, can vary considerably since it is controlled by the degree of compactness and attachment of the patinas to their substrates. Furthermore, in the areas covered by patina the spectra that are produced by the more internal reflections are distorted and amplified as a result of resonance effects from shallow patina exfoliation phenomena.