## 1 Introduction
Noteworthy examples of the application of these methods in studies of post-mining areas include:  
* Location of abandoned transport and ventilation shafts in a post-mining lead-zinc area in the USA, with seismic reflection and GPR methods [38];
* Location of underground coal mine workings in India to assess the risk of crown hole subsidence occurrence, with multi-electrode resistivity imaging (RI) and Ground Penetrating Radar (GPR) [39];
* Characterization of derelict coal mine workings in the UK, that included locating an abandoned coal mine shaft [40];
* Detection of post-mining voids using the microgravity technique in karstic environment [41];
* Long-term microgravity and geotechnical monitoring of relict salt mines in the UK [42];
* Estimation of sinkhole occurrence risk in post-mining areas of shallow hard coal exploitation in Poland, with gravimetric and seismic measurements [43];
* Assessment of mine collapse (sinkhole) risk due to voids in underground shafts and the surrounding rock mass [24];
* Detection of abandoned strontiumite mine workings in Germany, with a combination of seismic refraction tomography and resistivity tomography [44];
* Identification of voids from underground coal mining which pose a threat of sinkhole occurrence on the surface in Upper Silesia (Poland), with GPR methods [45];Mapping of subsidence and sinkholes related to post-mining voids resulting from underground coal mining in South Africa, with High-Resolution Seismic Reflection surveys [46];
* Mapping of abandoned mine underground workings and unconsolidated zones in abandoned coal mining areas in Bulgaria, with application of electrical resistivity tomography (ERT) [47];
* Analysis of the occurrence of voids or loosening zones in the rock mass near an abandoned mining shaft [48];
* Combination of geodetic, GNSS, and geophysical measurements for complex monitoring of mining areas [49]. Geophysical methods have advantages and disadvantages. The main advantage is the possibility of \"seeing without drilling\". All of the mentioned geophysical methods provide the distribution of petrophysical parameters below the surface in the form of cross-sections and maps. For studies of post-mining areas, the gravity and seismic methods are the most valuable, because density and/or velocity models can be obtained. Changes in the density of rocks are directly related to the remains of underground works. Resistivity methods are useful for hydrogeology, as electrical parameters depend on the conductivity of the subsurface. Considering the disadvantages of geophysical methods, the problem of resolution should be mentioned here. The more a priori information (for instance, from existing boreholes), the better the obtainable results from the modelled data. However, for so-called \"shallow geophysics\", very trustable information can be provided from surface geophysics, without boreholes. As proved in the mentioned publications, a combination of geophysical methods can be used for the development of models describing the underground situation in post-mining areas. The research presented in this paper aimed at creating a digital inventory of the selected post-mining workings, located in a unique postglacial environment, and mapping the extent of the rock mass disturbed by underground brown coal mining, with the objective of identifying weaker areas susceptible to present-day mine collapse and secondary land deformation. For these purposes, a 3D geospatial database of underground workings, based on available historic mining documentations, was created in GIS, and 2D/3D microgravity, followed by electrical resistivity tomography (ERT) geophysical measurements, was performed, and their results were analyzed and interpreted. The study area, methodology, and results, with their discussion, are described in Sections 2-4, respectively.