Drape-related problems in aeromagnetic surveys: the need for tight-drape surveys

Abstract

The trend towards higher flight height and loose-drape fixed-wing surveys has important implications for high-resolution aeromagnetic surveys for mineral exploration, especially for targets such as kimberlites. Recent trends regarding safety considerations, implemented by leading airborne geophysical contractors and some Governments, make it virtually impossible to acquire adequate data in areas of even moderate relief using conventional aircraft. Variations in survey elevation give rise to changes in magnetic relief and magnetic texture that are not related to magnetic sources but are simply artefacts produced by varying depth-to-source. Important high frequency and low amplitude magnetic signals are absent in areas of high survey elevation and cannot be recovered by drape corrections, which involve intrinsically unstable downward continuation. Geophysicists are well aware of the loss of horizontal spatial resolution as survey elevation increases and the loss of subtle information on depth extent may be equally serious in exploration for pipe-like bodies such as kimberlites. Aeromagnetic data are relatively insensitive to depth extent because of the rapid decay of the kernel function with depth. At shallow depths there is a clear difference in anomaly shape between a pipe source and a thin sheet but as depth-to-source increases these differences become much less obvious. For cost-effective high-resolution aeromagnetic surveys for kimberlites we need to fly tight-drape surveys at low level, using aircraft designed to fly at low level such as the Pacific Aerospace Cresco 750 and Fletcher FU-24. The terrain-following ability of these aircraft is impressive. Comparison of tight-drape and simulated loose-drape data shows a significant loss of high frequency content for loose-drape surveys over shallow magnetic sources. The loose-drape examples show significant variations in magnetic resolution due entirely to variations in survey elevation. Profile-based drape corrections removed most of these artefacts but fail to restore high frequency signals. Analysis of magnetic signatures of kimberlite models shows a significant deterioration in detection limits as survey elevation increases from 80 m to 150 m. Model studies comparing pipe models with thin-sheet models shows that our ability to distinguish pipe and sheet sources deteriorates as source depth increases. At low level there are clear differences in profile shapes, with the sheet models showing rapid decay away from the source, and pipe models showing slower decay rates. However, as source depth increases these differences are much less obvious.