Abstract
The aim of this work is to evaluate the efficiency of six electrode arrays used in electrical resistivity imaging. Pole-Pole (PP), Pole-Dipole (PD), Wenner-Alpha (WA), Wenner-Schlumberger (WS), Dipole-Dipole (DD) and multiple Gradient (MG) electrode arrays have been selected to detect underground cavities at shallow depth. Numerical simulation has been made for three synthetic models that have been generated using Res2dmod program. Each model represents three cavities with 2m diameter, spaced 6m from each other and located at a depth of 1.5m from the surface of the ground: 1) air-filled cavity, 2) half-watered cavity and 3) full-watered cavity. The background resistivity of each model was chosen equal to 10, 50 and 250 Ωm respectively. The resistivity of the air and water were set at 106 Ωm and 1 Ωm respectively. The results show that the PD, MG, PP and WS arrays gave good resolutions and clear images, and are less contaminated by noise. The DD array is very sensitive to noise and for this reason, it gave less accurate results for the first and the second synthetic models. An exception is the third synthetic model, where a good resolution model was obtained. This means, that the DD is more efficient in mapping cavities when the background environment is moderately resistive. The shapes of resistive air-filled cavities were found more clearly than those of conductive watered cavities, for the latter, however, the true resistivity values were better estimated than for the air-filled cavities. From the results of the analysis of the inverted synthetic models, the PD, MG, PP and WS arrays show the best results among the other used electrical arrays.
Time-lapse electrical resistivity imaging of the thermally affected zone of a Borehole Thermal Energy Storage system near Torino (Northern Italy)
