Integrated Geophysical Search for 19th-Century Underground Tanks in Krakow (Poland): Analysis of 2D, 2.5D and 3D ERT Surveys

The paper presents the results of a noninvasive geophysical search for 19th-century underground tanks located in the former Austrian imperial barracks complex of Archduke Rudolf in Krakow (Poland). The location of the tanks was necessary to assess their condition for safety reasons. Complementary geophysical methods in terms of field conditions and data acquisition, that is, magnetometry, ground penetrating radar (GPR) and electrical resistivity tomography (ERT), were used to locate the tanks. Interdisciplinary surveys made it possible to identify the subsurface structure, locate sewage settling tanks and verify archival documentation. The paper presents the problems that arise in imaging underground structures using applied geophysical methods. The aim of the study was also to assess the effectiveness of these methods in subsurface identification, including 2D, 2.5D and 3D ERT surveys. The applied sequence of measurements (magnetometric, GPR and ERT) proved to be correct and the most effective. It allowed for narrowing down the research area. Magnetometric measurements allowed for the location of iron elements related to the construction of the tanks. GPR measurements determined their range. The ERT results showed that 2D and 2.5D surveys conducted in the parallel-line arrangement are as effective in determining the location of an anomalous object as the 3D orthogonal-line arrangement. The 3D method enabled the identification of additional anomalies not detected by the 2.5D method. From the point of view of the practicality of both techniques, the 2.5D method turned out to be better than the 3D method, due to the simplicity of the fieldwork, lower spatial and hardware requirements, a greater depth range and lower complexity of data acquisition and processing.