Aeromagnetic survey in an area of weak magnetic anomaly and its interpretation

Hasegawa, Ken; Matsuoka, Toshiyuki

We approached the defining of geological structures with an aeromagnetic survey. The target is the low magnetic anomaly area known as the Ryoke belt. The survey specifications were as follows: Aircraft altitude from the surface is 400m, Survey line interval is 400m, Length and number of survey lines are 38km and 95 respectively. The fundamental data processing such as data compilation, diurnal correction and datum correction were done by means of the "equivalent anomaly" 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 (2 10 (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 way 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.