Air-borne geophysical prospecting using Helicopter

The air-borne geophysical prospecting can investigate geological properties widely and quickly. In this investigation, we applied 3-types of geophysical prospecting, those were Electro-magnetic, Magnetic, and Radiometric method. We can measure apparent resistivity of ground on Electro-magnetic method, radioactivity on Radiometric method, and magnetization on Magnetic method. We can measure also resistivity distribution in the depth direction by measuring electro magnetic response to various frequencies. Conducting measurement, analysis and interpretation of above-mentioned 3-geophysical properties, we solved the geological structure from the distribution of geophysical properties in this area. We also tried to study and discuss comparison and correspondence with the present geological map. The air-borne Electro-magnetic method showed that the regions distributed with Sarabetsu Formation and Isachi Formation, which mainly consists of sandstone and conglomerate, have high apparent resistivity about 30-500 Ω·m. The regions distributed with Kowama Formation, Wakkanai Formation and Mashihoro Formation (Upper Mudstone Layer), which mainly consists of mudrock, have low resistivity 1-10 Ω·m in the depth over than dozens of meters, though some area dozens of meter below the surface have high resistivity about 50-200 Ω·m. Magnetic method showed that magnetization tends to decrease from southwest to northeast and this result corresponded with previous large scale magnetic intensity map (Geological Survey of Japan, 1992). High magnetic anomaly area, which spread from north-northwest to south-southeast, is found on a boundary between Sarabetsu and Isachi formation. Mashihoro formation, which is located east side of Nukanan fault, has also high magnetic anomaly area. Radiometric method showed that high radioactivity areas are distributed in alluvial depressions along Tenshio river and small river in the hill, while low radioactivity area are distributed in Tertiary formations. High radioactivity areas are not found on Oomagari and Nukanan faults. Consequently, the geophysical properties corresponded with the present geological map in general, so that the present geological map is supported in large scale from the point of view of measured geophysical properties. (author)