## 1 Introduction  
The active layer, which is the uppermost layer of soil above the permafrost, is exposed to seasonal freezing and thawing. The active layer is considered to be a variable resistor that regulates the heat flow between the atmosphere and permafrost [1]. As a representative index for heat transfer in soils, the thermal conductivity has previously been estimated; it has been shown that the addition of water to dried soils leads to higher thermal conductivity. Furthermore, the active layer during summer has a higher thermal conductivity than during winter, due to the higher water content. The water content in the active layer increases with the increase in precipitation during summer, and eventually induces an increase in the latent heat, which can delay the refreezing of the active layer during winter. In this respect, the water in the active layer plays an important role in the heat transfer process. In practice, the existence of water in the active layer can cause potential problems, such as differential ground subsidence, development of thermokarst, frost heave, and slope failure [2; 3]. Several methods have been used to measure the water content in soils. First, using the neutron moisture meter (NMM), which emits fast neutrons, the slow neutrons thermalized by hydrogen atoms that are predominantly found in water molecules can be detected. As the thermalizing effect in water is much greater than in other elements commonly found in soils, the NMM is useful for estimating the total water content [4; 5]. However, when using the NMM, it is not possible to discriminate the liquid water from the ice in the soil matrix. Second, as a reliable method for determining the water content in frozen soils, nuclear magnetic resonance (NMR) spectroscopy has been used for determining unfrozen water content, although the application of NMR has been limited to laboratory tests [6; 7; 8]. Third, the gamma ray attenuation method has also been used for the measurement of water content in soils [9]. However, these methods may cause a potential health hazard. Alternatively, time domain reflectometry (TDR) based on the propagation velocity of electromagnetic waves has also been employed over the past few decades to estimate the water content [10]. Based on the difference between the dielectric constants of water (kwater = 80) and other soil constituents (kair = 1, ksolid = 3-7), a calibration relationship between the volumetric water content and the dielectric constant proposed by [PERSON] et al. [11] has been used [12; 13; 14; 15; 16; 17; 18; 19]. Even for frozen soils, the volumetric water content can be easily estimated due to the low value of the dielectric constants of ice (kice = 3.2) when compared to that of water. Lastly, frequency domain reflectometry (FDR), also based on the dielectric constant determination of the material surrounding the probe, are popular for the evaluation of the water content in soils [20; 21]. However, the water content determined from using the FDR probe is more sensitive to soil type and salinity than that determined using the TDR probe. During the process of the repeated freezing and thawing of the active layer, the water content is a key aspect of frost heave and ground subsidence. The active layer in the cold region is formed approximately within one to two meters from the ground surface. To measure the volumetric water content in the shallow-depth soil, [PERSON] et al. [22] and [PERSON] et al. [23] developed a combined cone penetrometer which included the electrodes of a TDR probe in the shaft of the penetrometer. A coiled-type combined TDR-penetrometer has also been used to improve the accuracy of travel time measurements with an increased number of electrodes [24; 25; 26; 27]. To coincide with the position of the penetrating cone, a conductive coil for detecting the water content was located at the cone tip [28]. A series of curved plate-type combined TDR-penetrometers was designed to investigate the optical configuration for waveguides [29].