Use of Kohonen's self-organizing map to combine images of airborne geophysical data and to classify lithological units

During the last 19 years over 50 % of Finland has been surveyed by low-altitude aerogeophysical measurements. The data obtained have contributed to many kinds of geological surveys. Nowadays airborne geophysical data are also handled as interpolated images. Our study is related to making this digital data more broadly useful. One important use of images interpolated from airborne geophysical data is to assist lithological mapping of new areas at 1:100000 and to update old lithological maps. Traditionally various image data -- the interpolated magnetic field, EM in-phase and out-of-phase; and gamma radiadion channels -- have been combined as RGB colour composition images and visually interpreted.