2013 Volume 133 Issue 9 Pages 694-699
The propagation of lightning current underground was investigated by the remanent magnetization of materials. The study was conducted around a tower struck by lightning.
(1) The remanent magnetization of the mortar layer of a transmission tower in the city of Hakusan, Ishikawa Prefecture, was studied. The distribution of magnetization showed a clockwise circular direction, indicating that a magnetic field was induced by the positive lightning current. The magnetic intensity of the mortar layer varied between the areas surrounding each of the four iron tower feet, which indicates that the shunting of the lightning current to the tower is anisotropic.
(2) The behaviors of lightning current in the counterpoise and electrode were investigated through a rocket-triggered lightning experiment. The samples near the counterpoise placed at 1.3m from the tower foot had two magnetic components of different polarities, whereas the samples at 8.8m had only one component. This implies that the lightning currents of both polarities propagated in the counterpoise up to a distance of 1.8m; however, only the positive lightning current reached the electrode at a distance of 8.8m from the tower foot.
The method using remanent magnetization is found to be effective for investigating the propagation of current after a lightning strike.