Abstract
Design of underground power transmission lines requires simple and effective techniques for estimating soil thermal resistance which is one of the important factors to determine the capacity of power lines. At first, we have performed the heat conduction numerical simulation in the underground containing of the power transmission line. As a result, it is enough to set the range of geophysical exploration to a radius of 3m around a power transmission line. Then we have tried the several geophysical explorations, the electrical, electromagnetic (the slingram), S-wave refraction, and surface wave methods, at the reclaimed land site where underground power transmission lines were buried and have examined the applicability of each exploration. Particularly, the electromagnetic method was performed by using both horizontal and vertical loops. As the results, it was confirmed that the electromagnetic method can explore the resistivity structure with the precision that is approximately equal to the electrical method. In addition, it is identified that the surface wave method is approximately equal precision as the S wave refraction method. Because the ground surface over the underground power transmission lines is almost a paved road, it is practical to use the electromagnetic and the surface wave methods. Furthermore, we have calculated the thermal resistance structure by combining the electrical resistivity with the S-wave velocity structures based on the empirical equations estimated from the laboratory tests with the artificial soil samples. Consequently, the calculated thermal resistance values quite matched to the measured values from the collected soils. However, this technique is based on the empirical equations that the reliability is insufficient, it is important to plan practical use of the new technique using the Johansson’s equation, for example. In addition, it is necessary to correct the influence caused by the resistivity of the pore water in soil samples collected from the site.
