GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 639 the electrical. The data acquired are expressed in form of apparent resistivity ( ρ ) and then they are interpreted in terms of real resistivity and depth by means of inversion software. The aim of the inversion procedure is to compute the ‘best’ set of resistivity values, which satisfies both the measured dataset and some a priori constraints, in order to stabilize the inversion and constrain the final image (deGroot-Hedlin and Constable, 1990). GPR is an active geophysical technique based on the introduction of electromagnetic (EM) waves that, in presence of variation of EM impedance of the investigated medium, is able to identify scattering phenomena occurring in the subsoil. In detail variations of physical properties of the investigated medium as dielectric permittivity (ε, F/m), electrical conductivity (σ, S/m) and magnetic permeability (μ, H/m) are recorded as reflections characterized by a particular amplitude or phase. Variations of physical properties influence strongly the velocity of propagation of e-m waves and energy attenuation. Analyses of variations of velocity are used to create permittivity map according to the fundamental relationship for non-dispersive medium v=c/√ε Where v in the propagating velocity of the material, c in velocity of em wave into the void (2.998 × 108 m/s) and ε represents the relative permittivity of the material (Annan, 2005). Study Cases. Two precast concrete buildings affected by settlement phenomena are studied with electric and electromagnetic techniques. In particular, in the first case 3D-ERT and GPR 2D radargram are used to identify the possible causes that have induced the deterioration of façade panels. In the second case, the use of only GPR and ERT 2D data has allowed to identify the possible cause of a settlement phenomena of a precast concrete bus garage. All the obtained data are constrained with direct measurements and have supported the interpretation of the geological models as showed for one study case in figure 1. Fig. 1 - GPR and ERT corendered representation with overlap of geological model. Geophysical investigations were realized with a multichannel georesistivimeter ABEM SAS-1000 that has provided to make acquisitions using maximum 64 electrodes distributed along 4 cables. An external source (a battery of 12 V) was adopted for the measurements. Further, to monitor real study cases, a series of drillings in the asphalt layer was performed before to insert the electrodes. For reducing the contact resistance between the electrodes and