# A Case Study Comparing Methods for Coal Thickness Identification in Complex Geological Conditions
## 2 Geological Test  
### Seismic Geological Setting  
The research area of the 24 Mining District in Shanxi, China is characterized by low-to mid-mountain erosion and structural terrain. The mining field is located on the eastern foothills of the Luyashan Range, in the northern section of the Luliang Mountains, with the Fen River to its east. The overall topography of the mining field is higher in the northwest and lower in the southeast. The highest point in the area has an elevation of 1715.079 m, while the lowest point is at 1402.472 m, resulting in a maximum relative height difference of 312.607 m. The surface layer mainly consists of exposed bedrock and Quaternary loess coverage, with localized colluvial deposits. Typical landforms of the exploration area are shown in Figure 1.  
Structurally, the area is characterized by the Ningjing syncline, which trends northeast. Most of the field lies on the northwest limb of this syncline, where the strata are relatively gentle, with small dips. The coal seam dip angles within the exploration area range from 2\({}^{\circ}\) to 4\({}^{\circ}\), dipping towards the southeast. Fault structures are not well-developed in the mining field. Some parts of the area are covered by Quaternary loess, while bedrock outcrops can be found in the western region. The coal-bearing strata belong to the Middle Jurassic Datong formation, primarily comprising the No. 2 and No. 3 coal seams. The main stratigraphy in the area, from oldest to youngest, includes the Triassic, Jurassic, and Quaternary formations. The Jurassic No. 2 coal seam is the primary mineable coal seam in this area, with an average thickness of 3.28 m. The coal seam thickness ranges from 0.12 to 6.84 m, with an average of 3.02 m, classifying it as a medium-thick coal seam. The overall mineable index for the entire field is 84%, with a coal thickness variation coefficient of 52%. Within the mineable range, the coal thickness varies from 0.73 to 6.84 m, with an average of 3.79 m and a variation coefficient of 23%.  
In this region, the roof and floor of the coal seam are mainly composed of sandstone and siltstone, creating differences in wave impedance, and producing a set of continuous reflection waves (T\({}_{2}\) waves), which are the primary target waves for seismic exploration. The No. 3 coal seam, located near the No. 2 coal seam, also exhibits significant wave impedance differences with the surrounding rock, resulting in another set of continuously traceable reflection waves (T\({}_{3}\) waves). The T\({}_{2}\) waves have some influence on the T\({}_{3}\) waves, making the T\({}_{3}\) waves a set of composite waves, as shown in Figure 2.  
Figure 1: Typical landforms of the exploration area.  
Overall, the mid-deep seismic geological conditions in the research area are favorable. Figure 2 shows a typical single-shot record and seismic time section of this area. While the near-surface and shallow seismic geological conditions are complex, the deep seismic geological conditions are relatively favorable.  
For areas with complex mountainous erosion structures characterized by rugged terrain, numerous ravines, and significant lateral variation in surface layers, various excitation and reception combination techniques were employed. In shallow loess-covered areas (with a thickness of less than or equal to 4 m), shallow boreholes reaching 1-2 m into the bedrock were used. In medium-thick loess areas (with a thickness greater than 4 m but generally not exceeding 10 m), single boreholes were drilled into the bedrock surface. In bedrock-exposed areas, single boreholes were drilled to a depth of 3 m; in weathered bedrock areas, the depth was set to 4 m. The excitation was carried out with three geophones connected in series in a clustered pattern; an observation system was arranged to ensure uniform distribution of stacking directions and source-receiver distances.