Investigation of the influence of soil moisture on thermal response tests using active distributed temperature sensing (A–DTS) technology

Abstract

In-situ thermal response testing (TRT) has become the most effective way to determine ground thermal parameters before developing renewable geothermal energy systems. However, these parameters may not be static when some hydrologic conditions in porous formation change with time. In this paper, we propose a method for evaluating the ground heat exchange performance using the active distributed temperature sensing (A–DTS) technology, which infers soil moisture by a thermal response caused by active electrical current. We evaluated the feasibility of this method by several lab experiments and a field thermal response test (TRT). Using this method, the relationship between thermal conductivity and soil moisture content was established for silt, clay, organic soil and sand, respectively. To quantitatively evaluate the sensitivity of soil thermal conductivity to moisture content, a new parameter called relative thermal conductivity (β), is defined to describe the effect of water bridges among soil solid particles on thermal conductivity. The lab test results demonstrate that soil moisture content has significant influence on thermal conductivity when the soil is nearly dry (β > β(cri)), but its effect becomes less evident when the soil is moist (β ≤ β(cri)). It is found that the relationship between soil moisture content and thermal conductivity can be well fitted by the Johansen model. The results of the field-scale TRT demonstrated the ability of the proposed technique to detect the effects of rainfall on soil thermal conductivity near the ground surface. The field test results also suggest that the soil thermal conductivity measured by in-situ DTS is larger than that obtained from soil samples in lab. However, for rock, the thermal conductivity acquired by DTS is less than the values collected in lab.