LA RADIOMÉTRIE AÉROPORTÉE, OUTIL D'EXPLORATION GÉOLOGIQUE STRUCTURALE*

This paper gives the results of an airborne radiometric survey flown in France over an area of 1,700 km² at 150 m. above ground, with parallel north‐south profiles 1 kilometre apart. Besides uranium indications discovered or recovered, this survey shows a close correlation between isorad and geological maps. The southern granitic zone is more radioactive than the northern gneissic one, with a constant step on the boundary. In the northern zone granitic intrusions have high radioactivity, and isorad contours are right on the boundaries of the main granitic units. In addition, the radiometric map shows the connections between these batholith roots and probable new satellite outcrops not shown on the geological map. Such a radiometric map is a help towards completing the geological map, and gives indications regarding probable structural connections between different granites.

The detailed survey of an anomaly discovered during the previous reconnaissance survey is given as another example. It was flown twice, with an aircraft at 75 m elevation with profiles 250 m apart, and with a helicopter at 35 m elevation with profiles 50 m apart. In this area of the main Marche fault, granulites in the south and gneisses in the north have low radioactivity, and the central shearing zone also. But, on both sides of this shearing zone, two crushed areas have high radioactivity, and uranium indications. The geological survey of the northern crushed gneisses area shows small granitic intrusions linked with the radioactive anomalies. These intrusions were unknown until they were discovered with the help of this map. Some mineral indications are connected with them.