## 3 Results
### Surface Morphology Analysis
#### 3.1.2 Cross-Sectional Analysis of Different Samples  
The quantitative characterization of the microstructure of coal reservoirs is a core scientific issue for optimizing unconventional natural gas development and greenhouse gas sequestration technologies. Based on the technical requirements for characterizing the nanoscale pore structure of coal rocks, atomic force microscopy (AFM) cross-sectional analysis provides key methodological support for revealing the heterogeneity of coal  
Figure 1: **(A)** Demonstrates the three-dimensional morphology of AH-1; **(B)** Demonstrates the three-dimensional morphology of HB-1; **(C)** Demonstrates the three-dimensional morphology of SX-1; **(D)** Demonstrates the three-dimensional morphology of SX-2; **(E)** Demonstrates the three-dimensional morphology of SX-3; **(F)** Demonstrates the three-dimensional morphology of SX-4. The brighter parts indicate depressions, while the darker parts indicate protrusions.  
matrices. Compared with traditional two-dimensional surface morphology observation, cross-sectional analysis, through line scanning along a set vector path, can quantitatively analyze the three-dimensional topological characteristics of subsurface pores [29]. Studies have shown that the vertical distribution of nanoscale pores in coal exhibits significant heterogeneity, and this structural characteristic directly affects the gas desorption kinetics process [30]. [PERSON] et al. used AFM cross-sectional analysis to find a nonlinear relationship between the depth distribution gradient of vitrinite pores and methane adsorption capacity, revealing the control mechanism of pore geometric parameters on gas occurrence states [31]. [PERSON] et al. confirmed through multi-cross-sectional superimposition techniques that the tortuosity of effective percolation channels in coal is closely related to the local variation in pore size distribution, which cannot be captured by traditional two-dimensional images [32]. In this study, two samples with significant differences in V\({}_{\text{daf}}\) were selected for cross-sectional analysis to investigate their pore structural characteristics.  
By conducting profile studies on the HB-1 and SX-4 samples (Figure 2), we can observe that when V\({}_{\text{daf}}\) is relatively high, the pores in coal are predominantly represented by a small number of large pores, as shown in Figure 2B. Among them, the maximum pore width reaches 169.50 nm and the depth reaches 19.08 nm, classifying it as a macropore. The smallest pore width in the profile is 39.25 nm, with a pore depth of 9.40 nm, classifying it as a mesopore. When V\({}_{\text{daf}}\) is relatively low, as seen in Figure 2D, the pores are characterized by a large number of smaller pores. However, macropores with diameters exceeding 100 nm still exist, such as pore 1 with a width of 109.19 nm and a depth of 5.95 nm. More commonly, there are mesopores with smaller pore diameters, with the smallest pore diameter in the cross-section being 13.10 nm and a depth of 4.45 nm. Comparing the profile analyses of the two samples, we find that when V\({}_{\text{daf}}\) is high, the pore diameters are also larger, accompanied by deeper pore depths. However, when V\({}_{\text{daf}}\) is low, although macropores with larger diameters still exist, their proportion significantly decreases, and the pore depths are also greatly reduced. There is a notable increase in the number of pores, which are primarily macropores and mesopores with smaller diameters.  
Figure 2: (**A**) represents the plan view of the HB-1 sample, with the highlighted line indicating the section location. (**B**) represents the section view of the HB-1 sample, with the highlighted line indicating the section location. (**C**) represents the plan view of the SX-4 sample. (**D**) represents the section view of the SX-4 sample.