The influence of the Electrodes-Spacing-to-Diameter-Ratio (ES2DR) on ERT measurements: an operational approach

Electrical Resistivity Tomography (ERT) is widely used in archaeo-geophysics due to its capability to non-invasively detect buried structures and stratigraphic heterogeneities. However, when ERT is applied to shallow, small-scale archaeological targets requiring high spatial resolution, the finite geometry of electrodes and the reduced inter electrodes distance can introduce systematic errors that may be misinterpreted as cultural features. This study investigates the impact of the electrode spacing to diameter ratio (a/φ) on apparent resistivity measurements, with particular attention to conditions typical of archaeological prospection, where electrode spacing is often limited by site constraints and preservation requirements. Six electrode types with diameters ranging from 4 to 16 mm were tested using dipole–dipole and pole–dipole arrays at four electrode spacings (10, 30, 50, and 100 cm), generating 48 ERT datasets acquired over a mainly homogeneous test area, characterised by the presence of a higher resistive target. The analysis focused on apparent resistivity values to avoid uncertainties introduced by inversion procedures. Results demonstrate that electrode material does not significantly influence resistivity measurements; instead, geometric factors dominate. In particular, when a/φ il lower than 25 (φ/a ≥ 4%), corresponding to dense electrode setups frequently used in archaeological ERT, apparent resistivity is strongly affected, particularly at shallow depths and in the presence of resistive anomalies (e.g., conditions typical of buildings, voids, pavements, foundation remains, or anthropogenic stratigraphy). When a/φ is higher than 31.5 (φ/a ≤ 3.2%), electrode diameter has negligible impact, confirming that standard ERT configurations at meter-scale spacing are generally robust. Increased variability and systematic deviations were observed up to approximately three times the electrode spacing, potentially generating artificial resistive highs that could be erroneously interpreted as archaeological features. Intermediate spacing values (30–50 cm) show transitional behaviour, with distortions decreasing progressively with depth. The study provides an operational framework for archaeo-geophysical practice: (1) electrode diameter becomes critical when small electrode spacing is required; (2) reliable shallow imaging in archaeological contexts demands maintaining sufficiently high a/φ ratios or explicitly modelling electrode geometry; and (3) resistive archaeological targets are most susceptible to artifact generation under inadequate a/φ conditions. These findings support improved survey design, more reliable interpretation of near-surface ERT data, and reduced risk of false positives in cultural heritage investigations.