Magnetic anomaly caused by soil excavation and its long-term stability : Case study of a magnetic anomaly in the absolute observation house at Kanoya Magnetic Observatory
Akira YAMAZAKI, Nobuaki SHIGENO, Teruaki YAMAMOTO, Yoshiko KUMAGAI and Nobukazu ITOH
A newly designed absolute observation house was built in 1995 at the Kanoya Magnetic Observatory. In June 1995, two months after completion, we conducted a magnetic survey inside the house with a proton precession magnetometer. We found a relatively high-amplitude magnetic anomaly of about 20 nT at 1.5 m above floor level, spread over the entire house. Because there was no such magnetic anomaly before construction, the magnetic anomaly was evidently caused by the construction of the absolute observation house.
We assumed that the magnetic anomaly resulted from the excavation and backfilling of soil necessary for installing pillars to support measurement instruments. To confirm this assumption, we used a model based on the shape of the excavated section of ground to calculate the magnetic anomaly that would arise upper the area of excavated ground. The results of the calculation based on this model agreed well with the observed anomaly. We also used the model to estimate the distributions of three components of the magnetic anomaly inside the absolute observation house.
Almost 10 years after the first survey, in August 2004 and March 2005, we conducted second and third surveys to check the magnetic anomaly inside the absolute observation house. The profiles of the magnetic anomaly found during those surveys was almost the same as that found in the first survey. This fact suggests that the magnetization of the excavated and backfilled soil has remained stable for almost 10 years, and we expect that the magnetic anomaly caused by the excavated soil will pose no serious problems for secular magnetic observation.