Magneettiset lentomittaukset

1.0

The data can be applied to ore exploration, bedrock mapping and environmental studies at a regional scale.

Variation in the magnetic properties of the ground cause local magnetic anomalies in the measurement results. The position, extent, depth, attitude, and strength of the magnetisation can be interpreted from these. In interpretation, the relationships between an ore deposit and rock types, size and properties are determined.

Kar-Air

Malmilento

Suomen Ilmailuopisto

Utin Lento

Ilmatieteen laitos

Over the decades, the data have been gathered using different aircraft and various measurement systems consequently the data are not fully homogeneous (standardised) and vary in quality between years. Nevertheless, the data have been gathered and recorded systematically and combined (standardised) where possible. Positioning accuracy has also varied over the years. Positioning in 1972-1975 was based solely on fixed points that were then accurate within the 50-00 m range and there was less precision between points. In 1976, a Doppler navigation device was introduced making it possible to maintain an accuracy between fixed points approximately the same as that for individual points. The transfer to a differential GPS system was made in 1993, allowing accuracy within an order of magnitude of one metre.

To assist magnetic levelling, hourly average values for the magnetic field collected by geophysical observatories and supplied by the Department of Geomagnetism of the Finnish Meteorological Institute have been used.

Basic checks and logical corrections have been carried out on the original magnetometer readings during the year of measurement. After that the data for the whole flight area have been split according to flight line and they have been combined with coordinate information. Magnetic measurements made from an aircraft require methodical corrections to allow an elimination of interfering factors that is as precise as possible. The aircraft itself is a magnetised object and it causes an error depending on the attitude of the aircraft and the flight direction if not corrected. Automatic compensation is used to correct errors caused by the constantly changing attitude of the aircraft. Compensation was not used originally; being introduced just after the mid-1990s. An error arising from the direction of flight is minimized using predetermined heading correction values. Over the years the Earth's magnetic field changes slowly and consequently measurement results require correction so that, several decades later, the values measured can be accurately combined at the same level. Additionally the delay associated with a measurement event and with registration as well as the false anomalies caused by the aircraft's hydraulic pump have been removed from the data. Finally the data has been levelled using different methods at various times. Over time, requirements have increased and the opportunities to correct the data have been improved. There has also been development in instrumentation. Consequently, the data does not have the same quality throughout. Data originating from the 1970s and 1980s may contain differences of tens of nT between adjacent flight lines. The quality of the material will be improved in the coming years by re-levelling the data.

Regional measurement data has been interpolated into a grid with a pixel size of 50 m x 50 m. The “minimum curvature” algorithm has been used as an interpolation method. The DGRF1965.0 reference field used in different years to combine the data has been removed. Thereafter, the regional measurements have been combined in a raster image covering the whole country, in which the standard deviation of the magnetic anomaly field is in the 300 nT range. Different grid densities are available: 50 m x 50 m, 100 m x 100 m, 200 m x 200 m, 400 m x 400 m, 500 m x 500 m and 1 km x 1 km; the l km grid being available without cost.

National airborne surveys have been made using DC3 (1972-1979), Twin Otter (1980-2007) and Cessna Caravan (2000-2006) aircraft operated and owned by Kar-Air, Malmilento, Suomen Ilmailuopisto (Finnish Aviation Academy) and Utin Lento.

Magnetic and radiometric measurements have been made with commercial instruments while electrical measurements have been made with equipment built by the GTK. Initially the software has been written completely at the GTK while partial use of commercial software only began in the mid-1990s; mainly in interpolation and visualisation (Geosoft Oasis montaj, ERDAS ER Mapper and INTREPID). The mainframe computer of the GTK's Southern Finland office was used until the mid-1990s when, as the era of the microcomputer progressed, there was a transfer to using personal PCs in the office and, further, laptops in the field.

Aineisto