Summary
During June, 1997, a helicopter geophysical survey was carried out over Krokskogen, an area immediately northwest from Oslo. The purpose of the survey was to provide geophysical information to be used with geological datao to help determine the best route for tunneling from Sandvika to Hønefoss. Approximately 900 line-kilometers of radiometric, magnetometric and electromagnetic data were acquired, coverinbg an area of approximately 180 sq km. The average flying height was 80 m. Nominal line spacing was 200 meters. The data were processed by The Geological Survey of Norway using software developed by Geosoft, 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. Electromagnetic data, measured as parts per million of the primary field 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. Very low frequency electromagnetic (VLF) data was processes by subtracting a low passed form og each flight line from the measu- red data to obtain a residual. It was necessary to apply decorrugation filter to VLF, radiometric, and electromagnetic data sets to remove small line-to-line errors remaining in the processed data. All data was gridded using square cells with 40-m sides. All geophysical maps were produced at a scale of 1:25 000. This report covers aspects of data processing. Interpretation of the geophysi- cal data will be included in a later report, in which will also be included a bedrock geological map of the area.
