The seasonal freeze-thaw cycle of frozen soil regulates soil hydrothermal processes and serves as a crucial indicator of climate change in high-latitude cold regions. Monitoring the dynamic evolution of frozen soil structure and composition is essential for infrastructure development, soil conservation and carbon storage regulation. Compared to in-situ borehole measurements and remote sensing, near-surface geophysical methods offer spatially resolved insights into freeze-thaw processes at different depths. In this study, we applied electrical resistivity tomography and ambient noise seismic monitoring to investigate seasonal freeze-thaw cycles at a frozen soil test site in Northeast China. Geophysical data collected over a complete freeze-thaw cycle reveal the coupling between soil structure and hydrothermal properties, with strong consistency observed between physical parameters and hydrological information. Resistivity variations correlate with temperature, water content, and solute concentration across different freeze-thaw stages. Seismic relative velocity changes (dv/v) and surface wave phase velocity changes (dc/c) were negatively correlated with accumulated temperature and groundwater levels, reflecting soil pore freezing and the hydrothermal state of the deep subsurface environment. Meanwhile, the measured data verify that dc/c offers higher spatiotemporal resolution than dv/v. Sensitivity analysis indicate that resistivity is more responsive to shallow thermal exchange, while seismic velocity changes are more sensitive to deep hydrological variations. Integrating pore geometry and water-ice phase mechanisms, we construct a freeze-thaw evolution model for seasonally frozen soil based on combined hydrological and geophysical data. The results validate the effectiveness of geophysical methods for detecting and monitoring frozen soil, and provide technical support for quantifying phase transition mechanisms in freeze-thaw processes.

This study investigates the dynamic evolution of cracks in expansive soil under varying wet-dry cycles, employing a self-developed three-dimensional spatiotemporal crack evolution model testing system. The research includes experiments, spatial moisture migration analysis, resistivity monitoring, and crack distribution inference to elucidate the crack development mechanisms. The findings reveal distinct stages in moisture evaporation at different soil depths, characterized by initiation, stability, deceleration, and residual phases. The influence of wet-dry cycles on evaporation rates is pronounced, particularly in deep soil layers. Resistivity changes in expansive soil during moisture evaporation display specific phases, demonstrating their potential to characterize crack development. The study validates the feasibility of assessing crack development through soil resistivity changes. Crack formation initiates at weak points on the soil surface, with subsequent elongation and secondary crack development, resulting in a crack network. Further moisture evaporation and volume shrinkage widen cracks, while wetting leads to crack healing. Total crack length, average width, and area crack ratio decrease exponentially with soil depth, but increase at different depths with more wet-dry cycles. Volume crack ratio initially rises and then stabilizes, while volume shrinkage capacity diminishes until equilibrium. Wet-dry cycles promote crack development, modifying particle arrangements. This research underscores that soil cracking and crack development result from the evolving balance of moisture-induced stresses in space, stemming from non-uniform moisture distribution. In conclusion, this study sheds light on crack development mechanisms in expansive soil under wet-dry cycles, offering valuable insights for soil engineering and geotechnical applications.