## 4 Conclusions  
This study systematically investigated the freezing characteristics and geophysical properties of clayey sand from the Qinghai-Tibet Plateau through laboratory freeze-thaw tests, and conducted long-term field monitoring using the ERT test in a typical permafrost area to explore the evolution of frozen ground distribution. The main conclusions are as follows:  
1. During the freezing process, clayey sand samples with different initial volumetric water contents exhibited S-shaped curves characterized by gentle slopes at both ends and a steep transition in the middle. This process can be divided into three stages: rapid heat release, rapid freezing, and stable freezing. A higher initial volumetric water content led to a higher freezing temperature and faster freezing rate, whereas the melting temperature remained consistent at approximately \(-4\)\({}^{\circ}\)C.
2. As soil temperature decreased, the unfrozen water content gradually declined, accompanied by a significant increase in resistivity. An inverse relationship between these two parameters was observed within the test range, confirming the dominant influence of unfrozen water content on the resistivity of frozen soil. This finding provides a basis for distinguishing seasonal and perennial frozen soils in the Qinghai-Tibet Plateau. For fully frozen soils, resistivity exhibited a U-shaped variation with respect to initial volumetric water content, reaching a minimum at an optimal initial volumetric water content. Additionally, the resistivity decreased with increasing dry density.
3. Forward modeling demonstrated that both Wenner-\(\alpha\) and Schlumberger arrays exhibited good resolution for the upper and lower boundaries of frozen soil layers. However, the Wenner-\(\alpha\) configuration showed stronger resistance to interference. Considering the harsh geological conditions in the study area, Wenner-\(\alpha\) was selected as the primary electrode arrangement for field electrical surveys of frozen soils.
4. Field high-density resistivity profiles revealed minimal differences between summer (July) and early autumn (September). In contrast, by winter (December), the intermediate transition layer exhibited a pronounced increase in vertical thickness and horizontal extent, indicating the seasonal evolution of frozen soil. Once frozen, this transition layer acts as a \"hydraulic barrier\", effectively reducing the downward infiltration of surface and shallow water, thereby supporting surface vegetation survival.  
Conceptualization, H.Y. and J.Y.; methodology, H.Y.; software, Y.X.; validation, H.W., R.C. and Q.D.; investigation, H.Y., S.Z., M.S. and Y.X.; resources, H.Y.; writing--original draft preparation, Y.X.; writing--review and editing, H.Y.; supervision, J.Y.; funding acquisition, H.Y. and J.Y. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (Projects No. 42374082 and 42230811) and the Fundamental Research Funds for the Central Universities (Project No. 2024 KYJD2009).  
Not applicable.  
The datasets used or analysed during the current study available from the corresponding author on reasonable request.  
The authors have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.