Compilation of various airborne geophysical data in the Oslofjord area
NGU has been acquiring airborne data in the Oslofjord area from early eighties until 2011. Some of the airborne surveys were performed by NGU itself and some of them were contracted to surveying companies e.g. Fugro, SGU. These airborne data were presented in various reports and stored as separate grids and databases for individual surveys. Later, it seemed necessary to compile this data together and present them as a single grid data. Therefore, various grids for magnetic and radiometric data were stitched together from airborne surveys performed during 1981-2011. Stitching of magnetic data was straightforward. It was done by suture method of grid knitting available in Geosoft software. However, stitching of radiometric data could not be stitched automatically because some of earlier surveys were not calibrated. Some of the surveys had only counts of gamma radiations for K, eU and eTh windows as gridded data. So scatter plot of the overlapping areas of various grids was made and regression analysis was performed to get slope and intercept of the fitting line. The regression parameters were used to bring two grids at similar level of ground concentration and then all the grids were stitched together one by one to get the final stitched grid.