Data Acquisition and processing - Helicopter Geophysical Surveys, Hurdal, 2000
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.