The freeze-thaw cycle of near-surface soils significantly affects energy and water exchanges between the atmosphere and land surface. Passive microwave remote sensing is commonly used to observe the freeze-thaw state. However, existing algorithms face challenges in accurately monitoring near-surface soil freeze/thaw in alpine zones. This article proposes a framework for enhancing freeze/thaw detection capability in alpine zones, focusing on band combination selection and parameterization. The proposed framework was tested in the three river source region (TRSR) of the Qinghai-Tibetan Plateau. Results indicate that the framework effectively monitors the freeze/thaw state, identifying horizontal polarization brightness temperature at 18.7 GHz (TB18.7H) and 23.8 GHz (TB23.8H) as the optimal band combinations for freeze/thaw discrimination in the TRSR. The framework enhances the accuracy of the freeze/thaw discrimination for both 0 and 5-cm soil depths. In particular, the monitoring accuracy for 0-cm soil shows a more significant improvement, with an overall discrimination accuracy of 90.02%, and discrimination accuracies of 93.52% for frozen soil and 84.68% for thawed soil, respectively. Furthermore, the framework outperformed traditional methods in monitoring the freeze-thaw cycle, reducing root mean square errors for the number of freezing days, initial freezing date, and thawing date by 16.75, 6.35, and 12.56 days, respectively. The estimated frozen days correlate well with both the permafrost distribution map and the annual mean ground temperature distribution map. This study offers a practical solution for monitoring the freeze/thaw cycle in alpine zones, providing crucial technical support for studies on regional climate change and land surface processes.
Electronics and other anthropogenic sources of noise in urban environments interfere with the early time signals of traditional transient electromagnetic (TEM) surveys due to the mutual inductance effect of transmitter and receiver coils. This poses problems for the detection of shallow geohazards such as karst dissolution features that lead to the subsidence and subsequent damage to infrastructure. The opposing-coils transient electromagnetic method (OCTEM) provides an alternative to traditional TEM surveys that is less sensitive to anthropogenic noise, and which is applied in this study to characterize shallow geohazards in a residential area responsible for subsidence and ground collapse. An investigation in Xiacun Town, China, was supplemented by ground-penetrating radar (GPR), drilling, and groundwater level monitoring to verify the OCTEM results and develop a conceptual model relating site hydrogeological factors to the ground collapse. OCTEM accurately identified shallow Quaternary gravel aquifers across the study area. However, OCTEM failed to identify additional subsidence structures near the collapsed pit demonstrated by the GPR results or the presence of a large, soil-filled cave below the pit determined from drilling. The site was concluded to be at further risk of subsidence and ground collapse associated with groundwater erosion driven by extreme precipitation events and excessive groundwater abstraction.