Airborne electromagnetic mapping of surficial deposits in Finland

Abstract

Aerogeophysical measurements hove been carried one by the Geological Survey of Finland (GTK) for the last 50 years. During this time the measurement techniques have been modified and improved several times (see Puranen 1963; Peltoniemi 1992; Poikonen et al. 1998). Airborne magnetic, electromagnetic (AEM) and radiometric surveys of Finland with a flight altitude of 150 m and a line spacing of 400 m were completed in 1972, after which surveys were restarted at a lower altitude (nominally 30 m) with a closer spacing (200 m). The low-altitude aerogeophysical mapping now covers more ~than 80% of Finnish territoty. Originally the aerogeophysical data were mainly used in exploration for base metal ores, but more recently the results of low-altitude aeromagnetic surveys have proved to be extremely useful in geological mapping of bedrock. During the past decade, tow-altitude AEM data have also been increasingly applied in environmental and engineering studies of surficial deposits. This is natural because most AEM anomalies in Finland are associated with clay dtposits, peatlands and lakes which cover about half of the country. Numerous applications have been presented for different AEM systems. In Austria AEM surveys have been used in a country-wide search for prospective clay deposits (Hubl et al. 1996). The AEM method has also proved to be useful in clay thickness mapping (Gamey et al. 1996), groundwater exploration (Paterson & Reford 1986) and saltwater intrusion mapping (Fitterman & Deszcz-Pan 1998) in the USA, where the method has further been tested in locating ordnance disposal (Irons 1989) and waste disposal sites (Nyquist & Beard 1996). In Alaska the AEM system has been used in measurement of sea-ice thickness (Kovacs & Holladay 1990)and in Canada the method was tested in sea-bottom profiling of coastal areas (Becker et al 1986). In Japan AEM measurements have been applied in landslide surveys (Konishi 1998) and in Australia the feasibility of AEM surveys for dryland salinity mapping is being tested (Coppa et al 1998). The AEM system of GTK has been used in investigations of various surficial targets in Finland. Soininen et al.(1998) examined the feasibility of the system for measurement of ice thickness in the Baltic Sea. The system has also been used in mapping and monitoring landfill sites to detect possible leakages (Jokinen & Lanne 1996). The areal extent of polluted soils around the waste pond formed by pulp mill effluents could be rapidly defined from AEM measurements (Puranen et al. 1996). AEM data have also been used to assess the occurrence of fine-grained sulphidic sediments (Astrom 1996), which can be oxidized to harmful acid sulphate soils (see Palko 1994; Dent & Dnwson 1999). The first tests of using AEM measurements for mapping of conductive overburden were made by Peltoniemi (1982). Thickness mapping of peat and clay deposits would be beneficial for peatland inventories and various construction works. However, for reliable AEM modelling of these deposits we need data of their in situ conductivity structures, which have been recently gathered by electric conductivity probing (see Puranen et al 1997). In this article we present conductivity data and relate them to modelling and interpretation of AEM data over surficial deposits in Finland.