Data Acquisition and Processing - Helicopter Geophysical Surveys, Østersund, Sweden
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- Rapporter 
\u00CFn December, 2000 and March, 2001, helicopter geophysical surveys were flown over three areas in the vicinity of Østersund, central Sweden. The purpose of the surveys was to provide geophysical information for mineral exploration. The data were collected and processed by The Geological Survey of Norway (NGU). A combined total of about 2830 line-km of electromagnetic (EM), very low fre- quency EM (VLF), radiometric, and magnetic data were acquired using a nominal 200-m line spacing. The nominal flying height was 60 m above ground level (AGL) and lines were flown in alternating directions at headings of 45o and 225o. Noise levels were within survey specifications. All initial processing was carried out on a flight-by-flight basis. Magnetic data, consisting of total field measurements collected by a cesium vapor magnetometer, were corrected by removing diurnal variations as recorded at a magnetic base station at a heli- port in Østersund. Radiometric data were reduced using three-channel processing according to procedures recommended by the International Atomic Energy Asso- ciation. VLF data were reduced by removing a first-order trend along each line. EM data were leveled using data from frequent high altitude excursions above 300-m AGL. All final processed data, apart from VLF, were gridded using 50-m square cells. VLF grids have 100-m cells. Geophysical maps were produced at a scale of 1:50 000 for each of the three areas. This report covers aspects of data acquisition and processing.