Electrical prospecting for seasonal frozen ground

摘要

It is important to grasp freeze-thaw process and frozen depth for disaster prevention or agriculture in cold districts. Therefore, we studied the efficient survey technique for frozen ground using electrical prospecting. First, we investigated the dependence of apparent resistivity on ground temperature and the relationship between unfrozen water and temperature in our laboratory. Next, real-time electrical prospecting equipment was developed and a long-term resistivity monitoring was carried out at the frozen ground in Obihiro University, Hokkaido. Finally, each phase of the freeze-thaw process was considered by classifying the curves of normalized apparent resistivity into six types (H, Q, K, K1, A, A1). The results of our study are as follows. (1) The soil resistivity increases as the temperature decreases. Especially it becomes high rapidly at a temperature less than 0 degree. This is because the resistivity is deeply related to the amount of unfrozen-water in soil. (2) The normalized apparent resistivity curves change when the events of the freeze-thaw process occur. Two types of apparent resistivity curves are intermingled when the curve changes. Therefore it is easy to estimate the freeze-thaw process using the normalized apparent resistivity curves. (3) There is the correlation between the frost penetration rate and the slop of normalized apparent resistivity curve in a cold area without snow cover. The slop is almost the same if the frost depth is the same, even if the ground temperature is different. So we deduced the equation that calculates the frost depth from the slop of the curve for each type of six curves.