Model Stacking (MOST) technique applied in cross-hole ERT field data for the detection of Thessaloniki ancient walls' depth

•Detection of depth of the ancient walls of Thessaloniki with cross-hole ERT.•Resistivity models with various cross-hole electrode arrays•Combined interpretation of data with various cross-hole electrode arrays•Model Stacking (MOST) interpretation technique.•Comparison of models with combined interpretation and Model Stacking technique

A cross-hole Electrical Resistivity Tomography (ERT) study was undertaken near the center of Thessaloniki in order to detect the depth of the existing city walls in the planned route of the new city underground train. This cross-hole setup was used for a study of measurements with various electrode arrays in real urban field conditions to evaluate the resolution of the models which is produced by each array and the reliability of the models which is produced by the newly published “MOST” technique. The pole–tripole array (C2–C1P1P2) produces high resolution models, even when only borehole electrodes are used. The bipole–bipole C1C2–P1P2 array, when used for cross-hole measurements only, produces higher resolution models compared to the C1P1–C2P2 array, even with a lower signal-to-noise ratio, which can result in extremely high RMS error, when noise, systematic or not, must be faced. The models of both arrays are greatly improved by the use of surface electrodes. The pole–bipole array (C1–P1P2) is proved to be less accurate in imaging and quite unstable to the noisy urban environment and to systematic errors. Furthermore, the Model Stacking (MOST) interpretation technique leads to better results with models of greater resolution and fewer artifacts compared even with the combined data inversion. Finally, the ERT cross-hole analysis has been reliable in detecting the city walls.