PSI - Issue 84

Ilaria Catapano et al. / Procedia Structural Integrity 84 (2026) 119–126

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low-frequency antennas, and UAV-based deployments still represent a frontier rather than a consolidated practice, mainly due to signal attenuation in conductive soils, complex geometries, and the lack of standardized acquisition protocols. ERT shows the opposite pattern: it is widely recognized as a standard technique for landslide characterization and monitoring, enabling the reconstruction of slip surfaces, hydrogeological architectures, and time-lapse water-content variations in both soil and rock slopes. In contrast, its use in bridge engineering is mainly confined to delineating foundation soils, scour-prone zones, permafrost degradation, and moisture-affected masonry. Resolution and array choice constraints hinder the reliable imaging of slender structural elements and complex foundation systems. 3.2. Forward modeling of electrical resistivity data Regarding the assessment of ERT capability of supporting the identification of foundation structures, the pursued objective is twofold: to define improved protocols for ERT acquisitions and to optimize them for different geological conditions and construction technologies. To achieve these goals, 2D and 3D synthetic apparent resistivity pseudo-sections were computed using various combinations of array configurations. Two simplified structures were simulated: a masonry plinth foundation (CS1) and a pile foundation (CS2). Specifically, CS1 consists of a rectangular masonry plinth (1.5 x 2.0 x 1.0 m), whereas CS2 represents a square reinforced concrete (RC) plinth (3.0 x 3.0 x 1.0 m) supported by two circular piles with a diameter of 0.5 m, center to-center spacing of 2.5 m, and length of 20.0 m. Table 1 shows the 2D arrays adopted to evaluate the capability of the ERT performed on the surface and in boreholes. For surface configurations (S), 48 electrodes were employed, spaced 0.5 m apart for CS1 and 2.0 m for CS2. In borehole acquisitions (B 2 ), electrodes were spaced 0.2 m and 1.0 m for the two analyzed cases, respectively. These distances were chosen to optimize resolution for targets located at the surface (plinths) and at depth (piles), depending on the case studied. For SB 2 , a Dipole-Dipole array was used, with the transmitting electrodes (Tx) placed on the surface and the receiving electrodes (Rx) positioned in both common-hole and cross-hole configurations within the borehole. In B 2 and SB 2 , the positions of potential and current electrodes are alternated to have cross measurements according to an “X- scheme” (in other words , Tx and Rx quadrupoles are never placed in the same borehole).

Table 1. Configuration of electrodes for 2D acquisition adopting surface and borehole electrodes. Code Type

Electrode Array

Mixed: Wenner, Wenner-Schlumberger, Dipole-Dipole, Pole-Pole, Pole-Dipole, Multi-Gradient

S

Surface-ERT

B 2

Cross-hole ERT with two Boreholes

Dipole-Dipole

Surface – Cross-hole ERT with two Boreholes and two surface acquisitions

Dipole-Dipole

SB 2

Two different geological scenarios were simulated. In scenario (a), the foundations are embedded in a low resistivity medium (50 Ω∙m), representative of loamy or fine -grained subsoil. In scenario (b), a moderately resistive medium (1000 Ω∙m) is considered, typically consisting of sand and gravel. In (a), the low electrical resistivity contrast makes it more difficult to detect RC structures. Conversely, in the more resistive background (b), detecting the masonry foundation is expected to be more challenging. Preliminary tests assigned electrical resistivity values of 100 Ω∙m and 500 Ω∙m for the RC and masonry structures, respectively. The left panel of Fig. 2 and Fig. 3 shows the ERT measurement configurations considered for surveying CS1 and CS2, respectively. Specifically, the first row shows the surface configuration; the second row depicts the cross-hole