GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 719 levee geometry and inner structure on the measured data. The 3D model of the embankment was built using geometrical and available information about the construction material of the levee segment and the pseudosection was calculated along the line corresponding to the long- term ERT profile (Fig. 2a). The same model was introduced to 2D forward modelling (Fig. 2b). The correction factor can be calculated “pixel by pixel”, resulting in a correction section obtained dividing the calculated apparent resistivity section from 3D modelling by the one from 2D modelling (Fig. 2c). To correct the data, the measured apparent resistivity sections are divided by the correction section. This procedure accounts for the 3D geometry of the levee, the heterogeneity in inner materials and the boundary conditions (presence of air or water). 3D effects on ERT measurements on levees are severe, most of all if external conditions (such as water level) change. In the Colorno site, since the levee has a wide series of banks, normally the river does not reach the monitored zone that is in contact with water only during Fig. 1 - a) Chosen site in Colorno; b) piping phenomenon that provoked the instability during the flood of December 2017; c) long-term geoelectrical and weather monitoring system installed in La Penza site, Colorno. Fig. 2 - a) Pseudosection from Res3dmod; b) pseudosection from Res2dmod; c) 3D effects “pixel by pixel”.