Data acquisition and processing - Helicopter Geophysical Survey, Storsjön, Sweden
In June, 2001, a helicopter geophysical survey was flown over an area in the vicinity of Storsjö, central Sweden. The purpose of the survey was to provide geophysical information for mineral exploration. The data were collected and processed by Geological Survey of Norway (NGU). A combined total of about 1690 line-km of magnetic, very low frequency EM (VLF), and radiometric data were acquired using a nominal 150 m line spacing. The nominal flying height was 60 m above ground lever (AGL), and lines were flown in alternating directions at headings of 180 grader and 360 grader. All initial processing was carried out on a flight-by-flight basis. Magnetic total field were collected using a cesium vapor magnetmometer. These data were corrected by removing diurnal variations as recorded at a magnetic base station at the base at Storsjö kapell. VLF data were reduced by removing a firs-order polynomial trend along each line. A circular differential median filter was applied to VLF data to remove line errors. Radiometric data were reduced using tree-channel processing according to procedures recommended by the International Atomic Energy Association. Noise levels were within survey specifications for magnetic and VLF data. Due to technical installations, the radiometric data were partly influenced by the helicopters fuel content and data quality was seriously reduced. All final processed data, apart from VLF, were gridded using 45 m square cells. VLF grids have 90 m cells. Geophysical maps were produced at a scale of 1:20 000 and 1:10 000 except for radiometric data (only 1:20 000 due to poor data quality). This report covers aspects of data acquisition and processing.