Magnetisation mapping in rugged terrain

Abstract

Magnetisation mapping aims to derive a map of the surface magnetisation (or equivalently, apparent susceptibility) distribution to assist in geological mapping through a more precise delineation of magnetisation boundaries and by providing a map of magnetisation levels that can be directly related to rock properties. The mapping surface can be either the magnetic basement, which generally corresponds to the crystalline basement, or, where non-magnetic overburden thickness is small, the ground topography. When the mapping surface and the survey altitude are both constant, magnetisation mapping reduces to a downward continuation of the observed magnetic field. Draped-survey flying in rugged terrain presents problems for magnetisation mapping due to variable sensor altitude and changing ground topography. Changes in the depth to the magnetic sources lead to variable attenuation of the magnetic anomalies. If the dominant wavelengths of the ground topography are comparable to changes in magnetisation, then topographically-induced anomalies will be indistinguishable from the effects of magnetisation changes. Therefore, the standard magnetisation mapping approach is modified in order to address these problems: 1) the varying sensor height is dealt with by using an approximate equivalent geometry from which the magnetic data can be more easily inverted; 2) the change in ground elevation is overcome by using an iterative inversion procedure that takes into account the varying source-to-observation distance; 3) finally, a correction is applied that accounts for errors introduced by the transformation of geometries. This approach is demonstrated on a recent aeromagnetic survey of the Tatla Lake region, British Columbia, Canada.