## 2 Occurrence Conditions of Coal Rock Seams  
Occurrence conditions of coal rock seams at the coal mining face are affected by geological structure conditions. In the case of folded structures, coal rock seams are compressed due to plastic deformation of strata and exhibit the wavy bending, which results in the coal seam thickness change locally. As the geological condition is a fault structure, the fault of a coal rock seam induces the discontinuity of mining operation. However, the geological condition of magma intrusion reduces the thickness of the coal seam and increases its hardness, which hinders the continuity of shearer operation [8]. In general, occurrence conditions of coal rock seams are composed of whole coal and whole rock (through the fault) as well as coal and rock mixtures (coal and roof, coal and floor, coal and gangue, coal and inclusions) [9]. The roof and floor strata are located above and below the coal seam, respectively; complex coal seams show at least 1-2 layers of stable gangue strata, and several layers at most (Figure 2) [10]. In China, there are lots of coal bases and mining areas which exhibit distinct geological occurrence conditions of coal seams as shown in Table 1. According to coal seam thickness, the coal seam can be divided into thin coal seam (<1.3 m), medium thick coal seam (1.3-3.5 m) and thick coal seam (>3.5 m). According to the standard GB/T 18023-2000 in China, the coal is macroscopically divided into bright coal (vitreous and bright coal > 80% and presence of metallic or metal-like luster), semi-bright coal (vitreous and bright coal 50-80% and  
Figure 1: The fully mechanized coal mining face. presence of glass luster), semi-dark coal (vitreous and bright coal 20-50% and presence of dull luster) and dull coal (vitreous and bright coal! 20% and presence of silk luster). However, there are lots of classification methods of coal lithotype according to different criteria (Table 2). [PERSON] et al. [11] revealed that hard dirt bands are mostly distributed about 0.34 m away from the roof with a thickness of 0-0.4 m, of which 71% exhibits the thickness of 0-0.2 m and 21% presents the thickness of 0.2-0.4 m. They are distributed unevenly in layers. Generally, areas with hard dirt bands smaller than 0.2 m can be directly mined by the improved powerful drum-reinforced pick shearer. Areas with hard dirt bands thicker than 0.2 m need to be blasted. [PERSON] and [PERSON] [12] employed the instrument for recording acoustic signals on tape and their subsequent analysis, to obtain a \"map\" of seam inclusions. [PERSON] et al. [13] developed the model of a coal seam with distributed geological and geomechanical characteristics using inverse distance weighting, and found the hyperbolic dependences of the drum shearer advance speed and capacity on the coal seam thickness. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Mining Area** & **Coal Base** & **Geological Occurrence Conditions of Coal Seams** \\ \hline Jiaozuo & Henan & Large coal seam thickness and stable distribution indicating simple structure coal seam [14] \\ Hancheng & Huanglong & Strong in the east and south, weak in the west and north, complex in the edge shallow region, and simple in the middle deep region [15] \\ Huaibei & Lianghuai & Large coal seam thickness, unstable distribution and complex geological occurrence conditions [16] \\ Shendong & Shendong & Large coal seam thickness, good geological occurrence conditions, simple hydrogeological, engineering, and environmental geological conditions of coal seams [17] \\ Yushen & Shanbei & Stable coal seam layer, simple structure, great variation of individual coal seam thickness [18] \\ Shengli & Mengdong & Lots of coal seams, large thickness, high coal bearing ratio, shallow buried depth and good continuity [19] \\ Shizuishan & Ningdong & Lots of coal seams, large thickness, stable coal seam distribution, medium complex geological structure, gentle formation occurrence [19] \\ \hline \hline \end{tabular}
\end{table}