Abstract
Magnetic susceptibility anomaly was found in faults. Magnetic susceptibility in fracture zones was often higher than that in non-fracture zones. In faults, in addition, anomalous low gamma-ray dose was often observed. To confirm whether the factor that increases magnetic susceptibility can be used as an index of the gamma-ray screening effect, the gamma-ray attenuation experiment was conducted. The gamma-ray absorption increased corresponding to grain refining and high FeO content that increases magnetic susceptibility. Therefore, the decrease in the gamma-ray dose in the fault is attributed to the screening effect associated with high magnetic susceptibility in the fracture zone in some cases. Change of magnetic susceptibility is caused by change of magnetic mineral such as grain refining, density increase, weathering, and alteration. The mechanism of forming gamma-ray anomaly in the faults appears to differ among different faults depending on the magnetic characteristic of the fault material.
