Analytic signal vs. reduction to pole: solutions for low magnetic latitudes

Abstract

As mineral exploration activity shifts to regions at low magnetic latitudes, interpretation skills acquired at high latitudes become harder to apply, thus leading to under-utilisation of expensive magnetic survey data. Changes in anomaly shape, reduction in overall amplitude, and changes in map textures make the ready interpretation of geology from magnetic data difficult. These problems are worst for magnetic inclinations within 20° of the equator. Reduction to pole (RTP) is the best theoretical solution because it normalizes the effect of induced magnetization and strike on the shape of the magnetic anomaly while preserving dip and textural information (the "normal" magnetization and ambient field direction being the vertical). However, in practice, the standard RTP transform is difficult to apply at very low latitudes and produces variable-quality maps, sometimes dominated by declination-parallel artefacts. Additionally, the transform cannot completely reconstruct north-south trending anomalies. The 3D Analytic Signal is a function of magnetic gradients and is easy to compute at all latitudes. The magnitude or amplitude of the 3D Analytic Signal can be computed easily and accurately for any ambient and source magnetization and can be readily imaged. The magnitude is almost, but not entirely, independent of magnetization direction. However, it lacks the resolution that horizontal and vertical gradient maps provide, and lacks the dip (and therefore structural) and textural information that total magnetic intensity and RTP maps contain. Although neither is the perfect solution, both transformations are of use, particularly at low latitudes. In this paper, I illustrate their differences, advantages, and disadvantages.