Characterization of near-surface sediments based on Electrical Resistivity Tomography measurements in the vicinity of the Wawel Hill (Krakow, Poland)

Abstract

The Electrical Resistivity Tomography (ERT) method in the south western part of the foot of the Wawel Hill was applied. The survey was carried out along five survey lines (P1 – P5), 50 m long each. The Wenner alpha array with spacing a = 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.5, 7.5, 10.0 m was used. The basic electrode spacing was 0.5 m. In apparent resistivity contours and sections we can distinguish two zones, which have noticeably different resistivity values. The low resistivity zone dominates in the deeper part of the section on all survey lines, especially on the profiles located in the close neighbourhood of the Vistula river. The relative high resistivity zone is probably the effect of complex local geology, as well as the influence of the calcareous Hill. Based on ERT inversion results, three resistivity zones were distinguished. Then the probable lithological or/and anthropogenical character was assigned to them. The shallowest zone has the thickness of about 0.5 m. From the geological point of view these are probably zones of accumulation of weathered limestone fragments from the Wawel Hill. It may be that high resistivity zones have anthropogenic character. Deeper, the zone of water Bearing sands was indicated. It might be also the effect of redevelopment and reclamation of this area. Their thickness grows forwards the Vistula river. On survey lines P4 and P5 another zone, which can be identified with limestone, was identified. It could also be the progeny of accumulation of the calcareous debris for effect to flatten this area. In order to generalize and simplify the characterization of the quaternary sediments, 1D interpretation was made. The method allows us to estimate the depth of groundwater, which in that place is about 5 m.