Environmental mapping using airborne geophysics in Finland

Geological Survey of Finland (GSF) has carried out systematic airborne geophysical surveys in Finland since 1951. The second program started in 1972 using a flight altitude of 30-40 meters and line spacing of 200( 100) meters (Lohva et al, 1996; Peltoniemi, 1986). Today 80 % of the country has been measured. The measurements have been made with fixed-wing aircraft. The differential GPS is utilised in navigation. In lowaltitude measurements the following systems are in use: magnetic, gamma radiation and EM. Total magnetic field is measured with a wingtip gradiometer. The horizontal spacing between the magnetic gradiometer sensors is 21 meters. Earth's gamma radiation is measured with a spectrometer which utilises a 41 litres NAI crystal detector. The electromagnetic unit is dual frequency vertical coplanar coil equipment (Poikonen et al, 1998). The frequencies are 3.1 kHz and 14.4 kHz and coil separation is 21.4 m. The survey results are used in mineral exploration and geological mapping, recently also increasingly in environmental applications. GSF has recently conducted aerogeophysical studies of some contaminated are as in Finland (Lerssi et al, 1997). Ämmässuo is a modern, municipallandfill of Helsinki. Landfill area has been mapped three times, first in the national airborne mapping program (1984) before the operation of the landfill started. The 1993 and 1997 monitoring surveys were carried out in the same north-south flight direction as in 1984 but using tighter (100 meters) line spacing. Magnetic anomalies indicate metallic waste and metal constructions. Comparison of 1984, 1993 and 1997 EM maps of Ämmässuo indicate small changes, which have been estimated guantitatively by layer model and 3-D model interpretation. By total radiation map, it was possible to estimate changes of moisture and soil thickness. Lievestuore pulp millled its waste effluent during 1935-1967 to the nearby Koivusensuo mire, where the Lipeälampi waste pond was formed. GSF carried out airborne measurements and ground surveys around the waste pond. The electric conductivity of the efflucent (mainly sodium lignosulfonate) is high and its spreding into the soils can thus be deciphered by electric methods. The AEM anomalies indicate that the most heavily polluted mire area extends north- and southwards from the pond, which was verified by ground conductivity logging (Puranen et al. 1997). Logging and seismie results were also used as a reference material in interpretation and modelling of effluent plume migration.