Abstract
We tested an off-ground ground-penetrating radar (GPR) system at a fixed location over a bare agricultural field to monitor the soil freeze-thaw cycles over a snow-covered surface. The GPR system consisted of a monostatic horn antenna combined with a vector network analyzer, providing an ultra-wideband stepped-frequency continuous-wave radar. An antenna calibration experiment was performed to filter antenna and back scattered effects from the raw GPR data. Near the GPR setup, sensors were installed in the soil to monitor the dynamics of soil temperature and dielectric permittivity at different depths. The soil permittivity was retrieved via inversion of time domain GPR data focused on the surface reflection. Significant effects of soil dynamics were observed in the time-lapse GPR, temperature and dielectric permittivity measurements. In particular, five freeze and thaw events were clearly detectable, indicating that the GPR signals respond to the contrast between the dielectric permittivity of frozen and thawed soil. The GPR-derived permittivity was in good agreement with sensor observations. Overall, the off-ground nature of the GPR system permits non-invasive time-lapse observation of the soil freeze-thaw dynamics without disturbing the structure of the snow cover. The proposed method shows promise for the real-time mapping and monitoring of the shallow frozen layer at the field scale.
Highlights
The interactions between seasonal soil frost and snow cover have a substantial impact on the soil moisture balance and energy exchange processes between the soil and atmosphere, as the hydraulic and thermal properties of unfrozen and frozen soils are quite different
A monostatic off-ground ground-penetrating radar (GPR) system was tested to monitor the temporal development of soil freezing and thawing processes in a natural setting over a snow-covered surface
Over the course of nine days, several freeze-thaw cycles of the shallow subsurface could be detected by the in situ sensors in terms of soil temperature and dielectric permittivity changes
Summary
The interactions between seasonal soil frost and snow cover have a substantial impact on the soil moisture balance and energy exchange processes between the soil and atmosphere, as the hydraulic and thermal properties of unfrozen and frozen soils are quite different. The influence of snow cover on the ground thermal regime depends on the accumulation, duration, timing and thickness of the seasonal snow layer, as well as melting processes and geographical location [1]. The snow cover retained by vegetation on the ground surface can enhance the insulation of the soil, which may reduce periodic variation in soil temperature, slowing the freezing and thawing processes [3] and even altering the density of the snow layers [4]. Investigation of the freeze-thaw cycles are mainly characterized by using invasive in situ point-measurements (e.g., frost tubes, thermistors, dielectric permittivity sensors), which provide limited local scale information and disturb the natural soil system during installation. The TDR method requires the installation of several vertically- and horizontally-positioned probes at fixed soil depths, thereby restricting its applicability in larger spatial investigations
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