## 1 Introduction  
Post-mining areas, resulting from both underground and open-cast operations, experience negative consequences even decades after the end of mining operations [1, 2, 3, 4, 5]. These effects include, among other things: flooding of the subsided areas [6, 7]; sinkholes and other forms of discontinuous deformations, as a result of sudden destruction of underground workings, especially shallow ones [8, 9]; secondary deformations (uplift), as a result of the restoration of groundwater levels [10, 11, 12, 13]; erosion of mining waste heaps; fires related to coal mining waste heaps; pollution caused by waste heaps [14, 15, 16, 17]; difficulties with the introduction of vegetation and degradation of vegetation conditions due to unfavorable soil characteristics; and subsidence and flooding [18, 19, 20, 21]. These processes pose a threat for new development and use of post-mining areas. As precise information on the location and extent of former excavations is often incomplete or lost, it is difficult to properly plan measures to counteract and protect against the potential, and often sudden, secondary changes of post-mining areas. In addition, as these areas are no longer monitored, e.g., by mine surveying services, the extent and intensity of these processes remain largely unknown. Secondary subsidence is one of the most threatening processes in post-mining areas. It is deformation of the land surface caused by the readjustment of rocks above voids left by underground mining activity, and is usually sudden. The collapse of a mine working could be the result of factors such as: changes in hydrogeological conditions related to water infiltration into the rock mass; a loss of support for old, shallow excavation workings; weakening of the rock mass properties, as a result of weathering and rheological processes; development of voids, as a result of displacement of backfill material in improperly liquidated shafts; dynamic loading of the rock mass with communication vibrations; or mining tremors [22; 23; 24; 25]. Studies of post-mining areas with a focus on establishing of the extent of former underground mining activity, mapping present-day deformations, modelling and predicting the potential for secondary deformations, and risk assessment of post-mining areas have been conducted, with different measurement, processing, and analytical methods. Land deformation measurements in post-mining areas are carried out with terrestrial (e.g., levelling, geophysical), as well as satellite and/or airborne techniques [8; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. Geophysical surveys are an important aspect of studies of post-mining areas, as they enable the continuous and non-invasive measurement of specific parameters of the rock mass. These parameters, depending on the chosen technique, are usually determined from the processed data. Ground-based surveys may take one of many forms [35; 36], e.g., seismic surveys based on measurement of the propagation of seismic waves produced by human-generated impact or vibration in the ground; gravity surveys using a gravimeter to measure the gravity field to determine variations in rock density; electromagnetic surveys, which involve examination of changes in ground conductivity induced by electromagnetic fields; Ground Penetrating Radar (GPR) measurements; or resistivity methods, based on measurements of the variation of electrical properties of the ground. These methods have been applied for many years to define lithological boundaries and tectonic features, as well as to locate post-mining voids and underground infrastructure in the rock mass, such as drifts or shafts, and the results aid in the assessment of risk of ground subsidence. A review of geophysical methods for the detection and mapping of underground mine workings has been presented by [37].