GNGTS 2018 - 37° Convegno Nazionale

642 GNGTS 2018 S essione 3.2 realize in complex contexts, such as strong topographic variations, dense vegetation, rough terrain around landslide areas. To overcome such problems and to reduce costs, HR 3D ERT data acquisitions with non-conventional geometries of electrodes have been proposed. In this work, the results of a 3D ERT survey carried out by a new layout of electrodes that makes use of cross-cable quadrupoles (i.e., all possible combinations of transmitting and receiving dipoles that are not on the same line) is shown. The proposed technique is particularly suitable in presence of strongly three-dimensional features that necessarily require 3D surveys, limited availability of space for electrode disposition, limited budget for surveys, logistical difficulties. In this work, a non-conventional 3D electrode configuration is proposed for a high-resolution characterization of the conditions of the coverage overlying a tunnel being excavated (Fig.1). The adopted scheme has been able to provide a very detailed image of the investigated survey area allowing to discriminate of the phenomena of instability in place on the site in question. Fig. 1 - Acquisition geometry. Method. Conventional design for a 3D ERT survey requires data acquisitions along parallel profiles (2.5D modeling) or along a grid of electrodes (3D modeling). The latter has usually small size being limited by the number of electrodes that can be managed by the modern georesistivimeters, generally 48, 72 or 96, which, therefore, allow high-resolution investigations only for the shallowest portions of the subsoil. To obtain both HR resistivity distribution and greater investigation depth, in this work an unconventional 3D electrode geometry is proposed for the electrical characterization of the subsoil, whose complex topography prevented the use of conventional electrode arrays. The adopted 3D acquisition scheme is based on a sequence of measures designed ad hoc, which allows to use the electrodes of each cable both as transmission and as measuring devices. This means that the position of the transmitting (TX) and receiving (RX) dipoles is continuously modified to obtain a uniform coverage along the measurement profiles and along all three spatial directions. Specifically, the ERT-Lab Sequencer software (distributed by Geostudi Astier S.r.l.) has been used that allows to set complex acquisition geometries by choosing cross- cable sequence generation according to the specific requirements for the survey area. Such a procedure is thus able to provide a real volumetric distribution of apparent resistivity values and not an interpolated one, like that obtained from a pseudo-3D (or 2.5D) ERT technique. Finally, the successive processing of the acquired data by suitable inversion software provides the exact nature of the subsoil 3D model.