On the magnetic anomaly and its long-term stability caused by dug the ground - A case study of magnetic anomaly in the Kanoya's absolute observation house -
Yamazaki, A., Shigeno, N., Yamamoto, T., Kumagai, Y. & Itoh, N.
Abstract
A newly designed absolute observation house of the Kanoya Magnetic Observatory was built in 1995. Two months after completion of the house, in June 1995, we conducted a magnetic survey inside the house by a proton magnetometer. As the survey result, we found relatively large magnetic anomaly, of which the amplitude is about 20 nT at height of 1.5m from the floor level, spread out entirely over the house. Since there was not such magnetic anomaly before the construction, it was caused by the construction of the absolute observation house.
We supposed the magnetic anomaly was produced by dug the ground for setting up three pillars. To confirm this assumption, we made a computation of magnetic anomaly by using a shape model of the dug ground. The computational results almost coincided to the observed anomaly. Furthermore, we estimated the distribution of the three components magnetic anomaly inside the house.
Almost 10 years after the first survey, in August 2004 and March 2005, we conducted the second and the third surveys to check the status of magnetic anomaly in the house. As a result, the distribution of anomaly was almost the same as the first survey. This fact suggests that the magnetization of the dug soil is stable almost 10 years, and we suppose the magnetic anomaly caused by dug the ground is not so serious for secular magnetic observation.
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