## 3 Results and Discussion
### Surface Resistivity Measurements--Recognition of Subsoil and Hydrotechnical Structures  
Electrical resistivity methods are often applied to investigations of hydrotechnical structures because resistivity values obtained by these methods are very valuable for estimating the soil conditions of levees, e.g., [8; 10; 11]. Below are presented example results of surface surveys using electrical resistivity tomography (ERT) to detect weakened and damaged areas of the embankment body and its base, quality control of a vertical sealing system on a modernized levee, and recognition of the geological structure of a building foundation. Figure 12: Results of RCPT tests; electrical resistivity of sands with calculated porosity of sandy clay. In the first case, the tests aimed to determine the condition of a selected section of the embankment, i.e., areas susceptible to dangerous filtration effects. The tests were performed on the Vistula river right-bank embankment. The area has been subject to numerous leakages through the embankment body and base. The embankment was 6 m high, the crest was 3 m wide, and the slope gradient was 1:1.7 to 1:2. The embankment fill is composed of fine sands with local inclusions of silt and silty clay, and the base is made of alluvial sands locally covered with a layer of flood deposits (Figure 13). Figure 13 shows the example results of electrical resistivity imaging. Along the entire length of the analyzed embankment section, the electrical resistivity of its fill was between 600 and 800 \(\Omega\)m. The analysis included the embankment sections, in which electrical resistivity was significantly different from the background values (~600 \(\Omega\)m). An area of significantly higher electrical resistivity up to 2000 \(\Omega\)m was observed on the left side of the cross-section. High electrical resistivity may indicate loosened soil in this area and significant leakage through the embankment body. Control tests using a CPTU probe to validate the electrical resistivity results in this area indicate the presence of loosened soil. The CPTU graph shows the decrease in the cone resistance (q\({}_{c}\)) at the depth of 3.4 to 5.2 m. The cone resistance in this area was reduced from 6 to 2 MPa, while the friction ratio (R\({}_{l}\)) did not change and varied between 1.2 and 1.6. The weakened zone in the embankment body is slightly different from the range defined on the basis of geophysical research. Despite these differences, geophysical surveys have indicated places for detailed geotechnical investigations with the CPTU probe. The results confirmed the need for modernization of this embankment section. In 2015, restoration works included construction of a vertical sealing system--DSM (Deep Soil Mixing) columns in the embankment body. Geophysical surveys were conducted again; this time they were aimed at indicating the sealing continuity. The electrical resistivity method was used to distinguish the less permeable vertical layer in the embankment body. The purpose of the tests was also to investigate the depth of the DSM columns and confirm the sealing continuity. For a more accurate diagnosis, direct soundings could then be used [48]. The interpretation and testing results are presented in Figures 14 and 15. In the figure below (Figure 14), the central part of the cross-section through the embankment body includes a zone with lower electrical resistivity, which is less than 50 \(\Omega\)m. The left and right sections had electrical resistivity values of more than 160 \(\Omega\)m (250 \(\Omega\)m to the right), which indicates the occurrence of sandy deposits. A cross-section across the embankment (Figure 12) shows the presence of a hydraulic window. Electrical resistivity in the sealing zone is about 50 \(\Omega\)m, while to the left (between 8 and 18 m of the cross-section), the values of electrical resistivity correspond to values in the embankment body.