Compilation of the Namibian airborne magnetic surveys: procedures, problems and results

Abstract

Forty one airborne magnetic surveys flown between 1962 and 1992 over different areas of Namibia, with a total of about 700,000 km, have been compiled as part of a German-Namibian co-operation project to produce a refined compatible countrywide magnetic data set. It was required to compile the data as if the whole country was flown by a single survey at 100 m terrain clearance at the same magnetic epoch. Emphasis was therefore given to the selection of the magnetic reference field model. The creation of a digital terrain model (DTM) was compulsory. Levelling of the survey blocks was impeded since adjacent blocks were only rarely overlapping. The magnetic data set is available as both coloured contour maps (scales 1:250 000 and 1:1 000 000) and digital grids with varying cell sizes (2000 m, 500 m and 200 m). Shield areas, fold belts and anorogenic intrusions dominate the magnetic anomaly pattern of Namibia. The fold belts are characterized by parallel running, narrow (<100 km) magnetic and virtually non-magnetic zones. Shield areas, covered by a thick sequence of mostly non-magnetic sediments, are distinguished by broad, mid-amplitude anomalies with low gradients. The analysis of magnetic linear features reveals the existence of two directional sets of magnetic lineations running across both fold belts and shield areas.