Data Acquisition and processing - Helicopter Geophysical Surveys, Hurdal, 2000
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In August, 1998 and July, 2000, helicopter geophysical surveys were carried out over an area of about 560 km2 immediately west of Hurdalssjøen and the Garder- moen airport. The purpose of the surveys was to provide geophysical informa- tion for improved geological mapping. The data were collected by The Geological Survey of Norway (NGU) personnel and processed at NGU. A combined total of 2780 line-km of VLF, radiometric, and magnetic data were acquired using a nominal 200-m line spacing. Poor performance of the EM system in 1998 yielded unusable EM data collected in 2000 are reported. These data amount to 1925 line-km. The nominal flying height was 60 m above ground level (AGL), and lines were flown in alternating directions, east-to-west or west-to-east. Noise levels were within survey specifications. All initial processing was carried out on a flight-by-flight basis. Final processing was carried out on all flight combined. Magnetic data, consisting of total field measurements collected by a cesium vapor magnetometer, were leveled by removing diurnal variations as recorded at a magnetic base station at Eggemoen airfield near Hønefoss. Radio- metric data were reduced using three-channel processing according to procedures recommended by the International Atomic Energy Agency. VLF data were reduced by removing a first-order base along each line. EM data were levelled using data from frequent high altitude excursions above 300-m AGL. Final processes data were gridded using square cells with 40-m sides. Geophysical maps were produced at a scale of 1:50 000. This report covers aspects of data acquisition and processing.