## 3 Construction of Spatial Database
Permeability was classified as classes 1-6, representing very highly (>1 cm/s), highly (1-10\({}^{-2}\) cm/s), moderately (10\({}^{-2}\)-10\({}^{-3}\) cm/s), slightly (10\({}^{-3}\)-10\({}^{-5}\) cm/s), and very slightly (10\({}^{-5}\)-10\({}^{-7}\) cm/s) permeable and practically impermeable (<10\({}^{-7}\) cm/s), respectively. In this study, the permeability grade ranged from 4-4.5 (slightly permeable). The groundwater data were collected from a report published in May 1996 by the Coal Industry Promotion Board. Borehole point data should be converted into raster data for spatial analysis, and the accuracy of a raster map depends on the number of data points. However, the available borehole data were limited in this study. Therefore, raster maps from the limited borehole data were constructed using an inverse distance weighting (IDW) interpolation method, which is useful for predicting values at unmeasured locations where data are insufficient [11]. Eight control factors influencing land subsidence were constructed with 2 m \(\times\) 2 m grid data, resulting in 775 columns and 860 rows, for a total of 666,500 cells within the study area. In total, 24 land subsidence areas as 24 vector-type polygons were converted to 2 m \(\times\) 2 m grid data for a total of 3863 cells with a value of 1. The 3863 cells of land subsidence were randomly classified into training and validation sets, with a 50% (1931 cells) and 50% (1932 cells) distribution, respectively, to evaluate model performance. Figure 2: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Geological** & \multirow{2}{*}{**Formation**} & **Thickness** & **Description** \\
**Age** & & **(m)** & **Description** \\ \hline Quaternary & Alluvium & \multirow{2}{*}{\(\sim\)20} & - Gravel, sand, and clay \\  & (Qa) & & - Mainly milky white–light green coarse–very coarse sandstone with greenish-gray–gray shale interpbeds. Intercalations of pinkish sandstone, purple shale, and grayish-green sandy shale in the upper part. The sandstone is less compact than that of the Hambaegsan Formation. \\ Permian & Hambaegsan (Ph) & 70–250 \\  & Jangseong & & - Four–five cyclothems consisting of dark-gray sandstone, black shale, and coal seam. Abundant plant fossils occur in the shale above the coal seam, the most valuable anthracic bed, of the 3 rd–4 th cyclothem from the bottom. \\  & Geumcheon (Cg) & 50–100 \\ Carboniferous & Manhang (Cm) & & - Mainly dark-gray–black shale and dark-gray fine sandstone intercalated with dark-gray limestone lenses and two to three thin coal seams \\  & Manhang (Cm) & & - Mainly purple, greenish-gray, or light-green shale and light-green–green or light-gray medium–very coarse sandstone intercalated with three–four limestone lenses. \\  & & 250–300 \\  & & & - Conglomerates with a thickness of a few meters occur at the base in some places. \\ Ordovician & Makgol (Om) & & - In the upper part, gray–dark gray limestone intercalated with dolomite \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of geological stratigraphy in Taebaek [30]. Figure 2: Eight factors influencing coal mine subsidence were used as input data in this study: (**a**) Geology, (**b**) land use, (**c**) slope, (**d**) distance from drift, (**e**) distance from lineament, (**f**) groundwater depth, (**g**) rock mass rating (RMR), and (**h**) permeability.