## 2 Materials and Methods
### Underground Exploitation of Hard Coal Seams  
The underground operation of the 404/3, 404/5, and 405/1 seams of hard coal took place in the years 2013-2019, using a system involving the cave-in of roof rocks into the void formed after extraction of the deposit from the longwalls. The mining workings are to a medium depth of 505 m and a medium height of 3.2 m (Figure 3a).  
Figure 1: Location of the research area on the map: (**a**) Europe; (**b**) Poland (source: own study).  
Figure 2: A view of the research area: (**a**) bird’s-eye view (source: Google Maps); (**b**) situational–altitude map (source: own study).  
Two longwalls (1/II and 2/II) were exploited in the 404/3 hard coal seam. The heights of the longwalls were 4.0 m and 3.4 m, respectively. The depth of the operation was from 410 m (east) to 575 m (west). The seam declined at an angle of 17\({}^{\circ}\). The exploitation in the 1/II longwall took place during the time period of May-December 2013 and in the 2/II longwall in August 2016-February 2017.  
The extraction from the 404/5 hard coal seam started in November 2015 in the 1/II longwall. It had a depth of 470 m and a height of 3.3 m. Exploitation of this longwall ended in June 2016. Exploitation of the 2/II longwall was conducted for less than a year (October 2017-April 2018). It was 3.5 m high and had a depth of 550 m. The declination of the deposit was 18.5\({}^{\circ}\).  
Operations on the 405/1 seam of hard coal started in February 2019 in the 1/II longwall. The average depth of the exploitation was 550 m and its height was 1.7 m. The declination of the hard coal seam was 18\({}^{\circ}\). The exploitation of this longwall was finished in August 2019.  
The characteristics of the operations carried out in the 404/3, 404/5, and 405/1 seams of the hard coal are presented in Table 1.  
The overburden is approximately 220 m thick and is formed by layers of sands, gravels, Quaternary clays, Miocene clays, and sands. Thus, the rock mass here is not the most compact, and therefore, to a very large extent, it can be treated as a loose medium. Such a structure also partially explains the appearance of discontinuous linear deformations on the ground surface, which is described in the next section.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Feature** & \multicolumn{2}{c}{**The 404/3 Seam**} & \multicolumn{2}{c}{**The 404/5 Seam**} & \multicolumn{2}{c}{**The 405/1 Seam**} \\ \hline Longwall & 1/II & 2/II & 1/II & 2/II & 1/II \\ Depth (m) & 410–500 & 500–575 & 430–510 & 500–600 & 495–605 \\ Height (m) & 4.0 & 3.4 & 3.3 & 3.5 & 1.7 \\ Declination (\({}^{\circ}\)) & 18.8 & 15.8 & 16.9 & 21.1 & 18.0 \\ Run (m) & 460 & 245 & 500 & 300 & 443 \\ Length (m) & 220 & 243 & 230 & 233 & 240 \\ Time & May 2013– & August 2016– & November 2015– & October 2017– & February 2019– \\ December 2013 & February 2017 & June 2016 & April 2018 & August 2019 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Underground operation of the 404/3, 404/5, 405/1 seams of a hard coal in analyzed area.  
Figure 3: The exploited region against the background of a situational–altitude map: (**a**) edges of longwalls in the 404/3 (green lines), 404/5 (red lines), and 405/1 (blue lines) hard coal seams; (**b**) linear discontinuous deformations (ground steps and faults) caused by the extraction of hard coal (source: own study).