Reducing cross-profile aliasing in aeromagnetic data

Abstract

Image processing of high-resolution aeromagnetic data is an essential tool in mineral exploration and is finding increasing application in petroleum exploration. However interpolation of the flight line data onto a regular grid can lead to loss of high frequency information and introduction of gridding artifacts which can obscure subtle anomalies of interest. There is a high degree of anisotropy in sampling with closely spaced measurements along flight lines and much wider spacing between flight lines. For surveys conducted over shallow magnetic basement, there is significant high frequency information in the located total magnetic field data profiles which is lost or distorted in gridding. Gridding of line data is always a compromise between honouring closely spaced data along line and producing smooth and continuous interline interpolation between widely spaced flight lines. Cross-line aliasing is a minimum when flight lines are perpendicular to the geological strike but where multiple trends are present or where we have local trends, aliasing is directional and cross profile aliasing may be severe. A problem, common to all gridding methods, is that linear trends at an acute angle to lines tend to produce `bull's-eyes' at line intersections. Linear and curvilinear anomalies appear as stepladder or string of beads effects. This paper examines methods of improving line to line correlation and concludes that gradient enhanced gridding using measured transverse gradient provides the best solution to interpolating between flight lines as this provides direct information on magnetic anomaly variation between flight lines. However, when measured gradients are not available it is still possible to use calculated gradients or other search techniques to improve line to line correlation and reduce stepladder and string of beads effects to an acceptable level.