γ線スペクトロメトリーによる断層調査

長野県山口村の阿寺断層を例として

抄録

The manborn and carborn γ-ray survey using a multichannel analyzer with NaI (Tl) scintillation detectors was carried out across the Atera active fault in Yamaguti Village, Nagano Prefecture.
The manborn was investigated 880 m long along the Ohsawa River across the Atera Falts. The data of the fluctuation of²¹⁴Bi, ²⁰⁸Tl, ⁴⁰K and ratios of those three radioactivity nuclides were compared with the degrees of fracture of outcrop rocks on the river-bed in order to evaluate the parameters for locating of buried faults. The carborn was carried out 50.1 km long through the Atera active fault to reveal the regional distribution of radioactive anomalies.
The sharp anomalie of the fluctuation ratio which was a measured value to moving average of²¹⁴Bi/²⁰⁸Tl, ²¹⁴Bi/⁴⁰K on the ground surface was appeared above fault gouge. From viewpoint of fault mapping the most important parameteres which can indicate location of fault are²¹⁴Bi/²⁰⁸Tl and ²¹⁴Bi/⁴⁰K. Corrections for elimination of Compton scatterring is found more effective to emphasize anomalies.
In the area of granitic rocks, three geological settings of uranium anomaly are known: the fault zone, the boundary of host rocks of contact metamorphism and the roof pendant. The anomalies of²¹⁴Bi/²⁰⁸T1 fluctuation ratio in investigated area from carborn distinguish the fault zone from the boundary of host rocks of contact metamorphism.
From analysis of the γ-ray spectrometry around these anomalies, it was clarified that the anomalies of²¹⁴Bi/²⁰⁸Tl and²¹⁴Bi/⁴⁰K around faults are due to a little increase of y-ray activities of²¹⁴Bi and decrease of²⁰⁸Tl and⁴⁰K. On the other hand, the anomalies around the boundary of host rocks of contact metamorphism are due to only increase of²¹⁴Bi.