Example of applying aeromagnetic survey technique to low magnetic anomaly areas

Hasegawa, Ken

The Japan Atomic Energy Agency has been performing the geoscientific research project, the "Regional Hydrogeological Study" (RHS) Project began in 1992. The study site for this project is located in an area that includes the Tono Geoscience Center (TGC), Toki City, Gifu Prefecture. This project is intended for the development of an understanding of the geological environment and development of analytical techniques and methods for systematic geological studies. As one of the RHS Project, TGC approached the defining of geological structures with an aeromagnetic survey. The target is the low magnetic anomaly area known as the Ryoke belt. The fundamental data processing such as data compilation and diurnal correction were done, and the datum correction were also done by means of the "equivalent source" method. Final data shows the magnetic anomaly which may be caused by the Toki granite. To make clear the cause of this anomaly, magnetic susceptibility measurement of rock samples of the Toki granite from some boreholes were done. The results of this measurement indicated that the Toki granite is divided into two zones from the view point of magnetic susceptibility, one zone has relatively high magnetic susceptibility (210(SI)), another has quite low magnetic susceptibility (510(SI)). Forward modeling using this result gave the 3-D model of the Toki granite which has relatively high magnetic susceptibility, and this model can almost explain the result of the aeromagnetic survey. Final data shows also additional magnetic anomalies, one may be caused by the normal remanent magnetization of the Hachiya and Nakamura formation above the Kani basin, another one may be caused by unknown intrusive rock above the Mino sedimentary complex rock area. Our approach shows that an aeromagnetic survey can be used to define geological structures for low magnetic anomaly areas such as the Ryoke belt.