Summary
During June, 1997, a helicopter geophysical survey was carried out over part of Nordmarka, an area immediately northwest from Oslo. The purpose of the survey was to provide geophysical information to improve geological mapping in the area. Approximately 3260 line-kilometers of VLF, radiometric, magnetometric, and electromagnetic data were acquired, covering an area of approximately 650 sq km, mostly over the area covered by NGUs Oppkuven mapsheet. An additional 2020 line-kilometers of radiometric, magnetometric, and VLF data were collected in an adjoining area immediately to the north of the Oppkuven are (Gran mapsheet), covering approximately 400 sq.km. In both areas, the average flying height was 80 m above ground level and the nominal line spacing was 200 meters. The data were collected by Geological Survey of Norway (NGU) personnel and processed at NGU using software developed by Geopsoft. Inc. 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 the Eggemoen airfield. Radiometric data were reduced using procedures recommended by the International Atomic Energy Association. Electro- omagneic data, measured as parts per million of the primary field with in-phase and quadrature components, were reduced by subtracting an estimated zero level from the beginning of each flight, then correcting for drift under the assumption of linear drift. Is was necessary to apply decorrugation filters to VLF and electromagnetic data sets to remove small line-to-line errors remaining in the processed data. All data were gridded using square cells with 40-m sides. All geophysical maps were produced at a scale of 1:50 000. This report covers aspects of data acquisition and processing.
