GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.2 159 intensity of the injected current. A reason could be the fact that its specific behavior is observed only in the lab where employed current densities are orders of magnitude greater than those normally available in field measurements, mainly for safety and portability of current sources. In addition, the non-linearity dependence on charging time was observed using longer charging times than those normally used in field resistivity surveys. In the above-cited papers, Zadorozhnaya published a physical-mathematical model which accounts for the observed non-linear behavior in the tested rock samples. She showed that the non-linearity can be explained by a specific solution of the equations governing the phenomenon of membrane polarization invoking the concept of constrictivity of pores. In her solution, the observed non-linearity depends on the pore-size spectrum, which, if extended successfully to field-scale surveys, shall gain relevance especially for hydrogeological studies (i.e. to be able to indirectly measure porosity from the analysis of the non-linear behavior of specific geoelectric surveys). In this paper, we show that the non-linearity can be observed in the field too, provided a suitable setup of field measurements. Some hints to the (partly) published model by Zadorozhnaya shall be given in the following. Direct current experiment. Laboratory studies of Zadorozhnaya were focused on clay-free samples so as to avoid any IP contribution due to the well-known mechanism of membrane polarization. Therefore, our field experiment was carefully planned, firstly by selecting a site where a clay-free, saturated aquifer of very shallow depth (i.e. nearly outcropping), so that measurements certainly involve it and the reduced investigation depth allows that current densities, available from a common, portable geo-resistivity meter, could approach the lab conditions. The test site was found in the northern Apennines, where fresh potable water is exploited from a sandstone aquifer. This site is located in the municipal territory of Serramazzoni (Modena Province, N. Italy, Fig. 1a). The acquiclude layer is represented by fine-grained sediments (clay) underlying the sandstone rocks. The hydrogeological unit belongs to the flysch Fig. 1 – Simplified geological maps of the two test sites tackled in the text.