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

The Electrical Resistivity Tomography (ERT) method was applied at the SW 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 m, 1.0 m, 1.5 m, 2.0 m, 2.5 m, 4.0 m, 5.5 m, 7.5 m, and 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 limestone of the Wawel Hill. Based on ERT inversion results, three resistivity zones were distinguished. Then the probable lithological or/and anthropogenic character was assigned to them. The shallowest zone has the thickness of about 0.5 m and results probably from accumulation of weathered limestone fragments derived from the Wawel Hill. It may be that high resistivity zones have anthropogenic character. Below, a zone of water-bearing sands was recorded, which may result from redevelopment and reclamation of this area. Thickness of this zone increases towards the Vistula River. Another zone, which can be identified with limestone, was identified on survey lines P4 and P5. This could also be result of accumulation of the calcareous debris for the purpose of planation of the area. In order to generalize and simplify the characterization of the quaternary sediments, ID interpretation was made. The method allows us to estimate the depth of groundwater, which in that place is about 5 m.