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 (Kurimo, et al, 1986; Peltoniemi, 1982; Poikonen, 1991; Vironmäki, et al, 1982). 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 low-altitude measurements the following systems are in use: magnetic, gamma radiation and vertical coplanar EM and VLF eleciromagnetic. Total magnetic field is measured with a wingtip gradiometer. The horizontal spacing between the sensors is 21 meters. Earth's gamma radiation is measured with a spectrometer which utilises a 25 1 NAI crystal detector. In the electromagnetic unit a vertical coplanar coil configuration is used. The frequency is 3112Hz (from 1995 also 14368 Hz) and coil separation is 21.4 m. GSF processes data with in-house software and profile, contour, shaded colour and grey scale maps are drawn (Kurimo, et al. 1986). The survey results are used in mineral exploration and geological mapping, recently also increasingly in environmental applications.

The POLAR Profile in the northern Baltic Shield was analyzed by two- and three-dimensional quantitative gravity modelling, interpretation of Fourier amplitude spectra, qualitative interpretation using Bouguer anomalies and second vertical derivative and horizontal gradient maps, and reference to petrophysical data. Densities of outcropping crustal units show several distinct modes ranging from 2600 to 2990 kg/m 3 . In relation to the mean background density of 2690 kg/m 3 used in the gravity modelling, the granitic crustal units (2600 kg/m 3 ) produce well-defined gravity lows of up to 20 mGals, and the granulites (2750 kg/m 3 ) and greenstones (2940 kg/m 3 ) cause the most pronounced gravity highs of 30 and 45 mGal. The estimated thickness of the Kittilä Greenstone Belt is about 6 km. In the cross section along the POLAR Profile the base of the Lapland Granulite Belt dips to the northeast and reaches a maximum thickness of about 16 km. The thickness of the Vainospää granite body is approximately 6 km. Evidence of buried anomaly sources also exists. Our interpretation predicts units of increased density in the basement under the Varanger Peninsula sedimentary rocks. The general pattern of a gravity high near the Barents Sea coast coupled with a gravity low seawards implies a thick layer of sedimentary rocks under the sea which may be compensated by crustal thinning. According to the seismic refraction interpretation there is a depression in the Moho under the southwestern edge of the Lapland Granulite Belt explaining, together with other features of the seismic model, the regional background on which the gravity anomaly of the granulite belt is superposed. According to the Fourier analysis of gravity anomalies there are two dominant depth clusters of density discontinuities in the southwestern (Karelian Province) and middle parts (Lapland Granulite Belt) of the POLAR profile. The shallower discontinuity varies between 6 and 10 km, and the deeper one rises from about 20 km in the southwest to about 10 km in the middle area. In the northwestern part (Inari and Sørvaranger Terrains and Varanger Peninsula) there are many separate depth values and clusters exist only at depths between 13 and 15 km.

ABSTRACT Statistical multivariate methods for the integrated processing of airborne geophysical data were tested. The data consisted of magnetic, electromagnetic and gamma radiation measurements, to which cluster analysis, principal components analysis and discriminant analysis were applied. Also, auxiliary variables were derived from the original ones and their value was tested. Although the frequency distributions of the data do not favour statistical analysis, the practical results are acceptable. Principal component analyses show geological and technical aspects that are difficult to obtain from the original observations. In cluster analyses, the sources of measured fields control the grouping of variables. Discriminant analysis was applied to the automatic identification of rocks by geophysical data. The rocks investigated are metasediments and metavolcanics, some magnetic and others conductive. When all available geophysical data were included, correct identifications were made in more than 60% of cases. In particular, gamma ray observations were found to improve the discrimination of non‐magnetic and non‐conductive rocks. The geophysical similarity of rocks studied by cluster analysis depends on electrical and magnetic properties as well as on their origin; the content of radioactive elements in turn is related to the origin.

An automatic interpretation program package, using least-squares fit, is described. Starting with models provided either automatically by the program or by the user, the parameters of up to 20 plates or prisms can be found in the non-linear optimization option. Alternatively, the susceptibilities or densities of up to 1000 plates or prisms can be computed. Automatic fit is superior to the methods using characteristic points, since all measured values are utilized and the basic models, plates and prisms can be combined to form highly complicated structures. The program package has been used mainly for structural interpretation on the regional scale. The examples show two-dimensional interpretations of several parallel profiles and of the same profile measured by different geophysical methods, as well as three-dimensional models. Automatic interpretation tends to yield bodies which are broad, are located near the earth's surface and have a low value of the material constant. This has led to the use of the program package as a tool for prognostic ore interpretation. The compact, more highly magnetized units yielded by the automatic process are replaced by thinner bodies having the susceptibility of iron ores. A re-interpretation tells us whether this new, prognostic model is able to explain the measured anomalies or not.