At an archaeological site (Pilisszentkereszt Cistercian Monastery, Hungary) we carried out 3D tensorial geoelectric mapping measurements. We applied the well-known tensorial form of Ohm's differential law, where a 2 × 2 resistivity tensor relates to the horizontal current density vector and the corresponding electric field vector. In the DC apparent resistivity tensor there are three independent rotational invariants, and we defined two alternative sets. In the field, the current electrode distance was fixed to 15 m; two perpendicular AB directions were used, and 16×15=240 potential electrodes (with an equidistant space of δx=Δy=50 cm) were put in the central (nearly square, 7.5 m × 7 m) area between the current electrodes. Due to a four-channel measuring system, it was possible to determine both components of a horizontal electric vector at the same time. The time needed to measure all potential differences between the neighbouring potential electrodes (thus to obtain 15×14=210 resistivity tensors), was about 40 minutes. So far, about 239 such maps have been completed, thus we have more than fifty thousand resistivity tensors around the monastery. The tensorial results are shown together with the results of traditional measurements. In field conditions, any resistivity estimation provides reliable information about the subsurface (both the tensor invariants and the traditional mean values). At the same time, the multidimensional (2D and 3D) indicators proved to be informative only in case of significant subsurface inhomogeneities.
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