GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 689 not the 2.5D (pseudo 3D) one for these reasons: 3D survey avoids errors due to interpolation of data between 2D lines; 2.5D techniques could be insufficient in the description of complex geological contexts (for examples in the presence of three – dimensional geometry structures); a 3D acquisition could investigate a three – dimensional volume with a good resolution, even less than 10 cm (depending on the electrode spacing); 3D methodology allows to analyse the whole volume through horizontal and vertical sections along different orientations. In this paper, we present a joined investigation between 3D resistivity and induced polarization (IP) tomography surveys led along specific measurement lines, equally spaced. The right choice of the electrodes spacing and the joined investigation, allows us to confirm or deny the presence of metallic bodies that can be associated with landmines or World War II bombs. Methodology of investigation. The combined analysis involved resistivity tomography and chargeability. In details, the three – dimensional resistivity tomography was carried out using a pole–dipole sequence because it provides better horizontal coverage, reaches a greater depth of investigation (if compared with Wenner, Wenner–Schlumberger and dipole–dipole arrays) and the outcomes are less sensitive to the telluric noise, with respect to the dipole–dipole device. More specifically, the acquisitions were carried out creating 6 spreads with 48 electrodes, for each line, fixed into the ground for about 40 cm. We used the multi–channels georesistivimeter Syscal Pro produced by IRIS Instruments (France), that is able to automatically control the current injection. The georesistivimeter defines which are the measuring electrodes and which ones of current, until the execution of all the possible combinations. We employed six 3D acquisitions, with alignments of 2 spreads of 48 electrodes spaced 4 m; the spreads 3 and 4, located immediately next to the railway lines, are spaced about 12 m (Fig. 1). The electrodes are 1.5 m spaced with the aim of generate a rectangular mesh that reach an investigation depth of about 15 m. 3D acquisitions were overlaid on the common central spread, as shown in the example of figure 1, to guarantee the accurate analysis of the whole survey area. An ad hoc measurement sequence, that allowed to acquire, for each pair of geoelectric spreads, 12841 data, was created. Then, a planimetric survey of the electrodes was led with the aim to better reconstruct the electrode geometry essential for the correct data post-processing. Modelling and analysis of geophysical data. After the acquisition, resistivity (Ω·m) and chargeability (mV/V) data were modelled. The inversion provided parameterization of resistivity and chargeability values. The data, acquired along the 6 spreads, were merged in a single dataset and implemented in a 3D model that was filtered and processed by software ErtLab_64™ (Geostudi Astier & Multi-Phase Technology) that uses a three – dimensional tetrahedral finite Fig. 1 - Scheme of acquisition of 3D surveys.