Geophysical investigations: Krogskogen, Oppkuven and Gran, 1998
During June, 1997, a helicopter geophysical survey was carried out over parts of Krokskogen, Nordmarka, and northward into Hadeland. The sources of many prominent anomalies from the helicopter survey were not immediately evident. Ground visual and geophysical follow-up was necessary in order to identify them. The methods employed were magnetic profiling and magnetic susceptibility measurements, DC resistivity sounding, borehole resistivity logging, VLF profiling, and radiometric profiling. The DC resistivity soundings confirmed the accuracy of resistivities computed from the helicopter EM measurements. The borehole resistivity log provided resistivities for several important lava flow units in the Krokskogen area. Magnetic profiles confirmed a major remanently magnetized dike or series of dikes in Hadeland, and also confirmed minor magnetic anomalies, also most probably dikes. Radiometric measurements confirm that radiometric uranium counts fall off abruptly from west to east near Brandbukampen.