GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 However, geotechnical investigations are often used to levee analysis whenever possible, these kinds of data show the maximum information accuracy only with a punctual validity. On the other hand, geophysical methods are non-intrusive and provide physical information over larger areas (Francese & Santos, 2014). In this case, geophysical surveys are useful to identify and mitigate embankment failures since geotechnical tests can often be considered time-consuming, destructive, and expensive compared to geophysical methods (Cosenza et al., 2006). Geophysical information is often interpreted in a complementary way with geotechnical data according to the physical properties. Electromagnetic Method (EM) is a fast and accurate geophysical method that can be used in this approach. EM systems return qualitative values for electrical conductivity because of instrument calibration difficulties (Lavoué et al., 2010; Minsley et al., 2012). In this case, electrical conductivity values obtained from ERT and EM cannot be compared directly. Although both methods measure electrical conductivity values, ERT inversion returns lateral and vertical electrical conductivity values. In contrast, EM returns values that reflect cumulative electrical conductivity distribution over a specific depth range (Lavoué et al., 2010). Therefore, for a quantitative approach, calibration is requested. A good solution for calibration is the use of ERT (Lavoué et al., 2010; McLachlan et al., 2020). The test area identified for the experiment is very well known from a geophysical and a geotechnical point of view and was preferred because of serious stability concerns represented by sand boils. Thus, having a clear response to one of such structures, based on ERT results, we have calibrated EM data in order to delineate sand intrusions inside the levee. 2. STUDY AREA AND GEOPHYSICAL ACQUISITION Our target comprises part of a levee of the Brenta River, located in Campolongo Maggiore (VE) in Italy. The stratigraphy of the subsoil mainly consists of alternations of clayey silt, sandy silt with silty clay levels of 0.5-1.5 m thick, and some lenses of fine silty sand of a thickness of 0.15-0.35 m. The levee top surface is around 7.5 m above sea level (a.s.l), and close to 2 m a.s.l presents a level of fine sand. Usually, in geophysical EM surveys, frequencies from Hz to kHz are considered low frequencies or low induction numbers (LIN). Some assumptions derived from McNeil (1980) for LIN approximations include the need for instruments to operate at low frequencies below 15 kHz and zero elevation. Typical source-receiver orientations to measure electrical conductivity by EM instruments are vertical magnetic dipole (VMD) or horizontal coplanar loops (HCP) where both transmitter and receiver coil are oriented horizontally; horizontal magnetic dipoles (HMD) or vertical coplanar loops (VCP) where both transmitter and receiver coils are oriented vertically; perpendicular loops (PRP) where transmitter coil is oriented vertically and the receiver coil is oriented horizontally.