A study of soil thermal conductivity measurement based on the actively heated distributed temperature sensing cable

The actively heated distributed temperature sensing technology (AH-DTS) can realize distributed continuous measurement of the soil thermal conductivity in different layers through optical cables implanted in the soil. However, the accuracy and sensitivity of soil thermal conductivity measurement by AH-DTS method need to be further studied. Through designing indoor tests with the loess, the aim of this study is to compare the thermal response process and the soil thermal conductivity measured by carbon fiber heated cable (CFHC) and copper-mesh heated cable (CMHC) under different heating strategies. The numerical simulation is used to furtherly verify the influence of the optical cable structure on the thermal conductivity measurement results. The results show that the thermal response process of CFHC and CMHC can be divided into three stages: Internal heat transfer of optical cable, fiber-soil transition and stable heat transfer of soil. The difference in optical cable structure will lead to different heat transfer rates, which makes the initial time of thermal conductivity measurement of CFHC 100 s earlier than that of CMHC. The temperature rise value of CFHC is higher under the difference of optical cable size and specific heat capacity. The thermal conductivity measurement result of CFHC is more stable and accurate than CMHC under the same DTS temperature measurement accuracy. Increasing the heating power or increasing the heating time will improve the accuracy of the soil thermal conductivity measurement by CFHC and CMHC. The research results provide an important basis for further improvement and promotion of this technology.