Unfrozen water content of permafrost during thawing by the capacitance technique

Abstract

The capacitance method is portable, safe in terms of no radiation hazard, fast in terms of response time, and cheap, and has been widely used to measure the water content of unfrozen soils. However, it is sensitive to installation, salinity, temperature, bulk density, and clay content. Calibration for specific soil types of interest is essential for improved accuracy. This paper explores using the capacitance technique to measure the unfrozen water content of permafrost. It presents a detailed capacitance sensor calibration process, data, and regression equations for describing the relationship between volumetric water content and the sensor output. The subfreezing temperature effects on the sensor electronics response and on the relative permittivity or dielectric constant of water, ice, and oven-dry soil minerals are investigated. The obtained unfrozen water content results are used for estimating the heat capacity of permafrost, which is compared with that measured by using the Transient Line Heat Source method at thawed status. It is found that the temperature effects on the relative permittivity of ice and soil minerals are small and can be ignored. However, the temperature effect on the sensor electronics response is almost linear, that on the relative permittivity of water is significant and should be accounted for. The unfrozen water content for permafrost of silty sand, silt and clay nature was assessed for temperatures ranging from −20 °C to 10 °C, and found to be consistent with previous findings; empirical relationships between the unfrozen water content and temperature were obtained. In addition, the heat capacity of permafrost samples was estimated based on the weighted average method and the unfrozen water content, and a good agreement was found between the estimated and measured heat capacity values for all samples at thawed status.