## 3 Results and Discussion
### Evolution of Frozen Soil Distribution in the Study Area
#### 3.3.2 Field ERT Test Inversion  
High-density resistivity inversion profiles for two survey lines across different months are illustrated in Figure 10. The profiles from July, September, and December display distinct vertical stratification. The first layer (0-10 m) is characterized by low resistivity (\(\rho<\) 100 \(\Omega\cdot\)m), corresponding to gravelly coarse sand and sandy gravel strata. The second layer (10-15 m) serves as a transitional zone with resistivity increasing from low to high (100-150 \(\Omega\cdot\)m), marking the boundary between seasonal and perennial permafrost. The third layer (below 15 m) exhibits high resistivity (\(\rho>\) 150 \(\Omega\cdot\)m), indicative of perennial permafrost.  
Comparative analysis of the profiles reveals minimal changes between July and September, with consistent resistivity distributions and interface depths, suggesting stable thermal and moisture conditions from late summer to early autumn. By December, the second-layer transition zone notably thickens, and the resistivity gradient becomes broader and more gradual. Simultaneously, the high-resistivity interface of the third layer slightly ascends, reflecting the downward expansion of seasonal frost and increased resistivity at the upper boundary of the perennial permafrost due to declining ground temperatures in winter [58; 59]. Further examination of the second-layer transition zone indicates significant horizontal extension in addition to vertical expansion. In July, this zone is primarily concentrated between 100 and 180 m along the profile; by September, it shows minor shifts but maintains its overall form; by December, it extends horizontally from 50 to 230 m.  
Comparing Figure 10a,b, both survey lines exhibit consistent patterns, with the shallow low-resistivity areas diminishing as the seasons transition from summer to winter, indicating progressive freezing of the shallow seasonal permafrost. Additionally, both lines show that the deep high-resistivity regions do not exhibit gradual color variation indicating warming with seasonal changes. This phenomenon is attributed to the reduction in initial volumetric water content in the deep soil layers from summer to winter, leading to decreased resistivity upon freezing. This observation aligns with the latter part of the \"U\"-shaped curve in Figure 8, where frozen soil with a higher initial water content shows decreasing resistivity as the initial volumetric water content decreases [60; 61].  
In summary, high-density resistivity methods effectively capture the distribution and evolution of permafrost layers across the three surveyed months. The minimal differences between the July and September profiles suggest stable interfaces between seasonal and perennial permafrost during late summer to early autumn. The December profile's significant thickening of the second-layer transition zone and the upward shift in the high-resistivity layer interface reflect deepening freezing processes in winter. The seasonal variability of the second-layer transition zone implies that its thickening and increased resistivity reduce the hydraulic conductivity of the soil, limiting the downward infiltration of moisture from the upper layers. This process provides a stable moisture environment for surface vegetation roots, particularly in December, where the thickened transition zone further impedes the infiltration of meltwater and shallow moisture into deeper layers, positively influencing surface water conservation and vegetation survival [62; 63].  
Figure 10: Inverted high-density electrical resistivity profiles along different survey lines: (**a**) Line A–A\({}^{\prime}\), (**b**) Line B–B\({}^{\prime}\).