Airborne magnetic susceptibility mapping

Abstract

This paper describes an automated, three-dimensional, multibody aeromagnetic interpretation technique, which operates on the total intensity map by means of straight-forward operations. The magnetic field is reduced to the pole, and the magnetization of an equivalent layer of poles is computed at discrete points by a linear inversion procedure. Assuming induced magnetization only, the computer magnetization map can be transformed into an apparent susceptibility map by dividing it by the total intensity of the geomagnetic field. The apparent susceptibility map will approximate the true variations in susceptibility if the magnetic sources have vertical sides, flat horizontal tops lying at the same depth, and bottom at infinity. In this case the horizontal outline of the sources is obtained. In order to apply this technique to large areas, a stabilization procedure is introduced. Using theoretical data, we analyse the effects of violating some of the above assumptions: uneven top, wrong assumed depth to the top, and finite thickness. We have found that, in general, the technique produces reasonable estimates of the magnetic susceptibility and a rough outline of the sources. We applied the technique to field data from three different geological settings in the USA. However, the method may be particularly applicable in Australia, where the topography is usually gentle. Our results show that the apparent sus- ceptibility map often provides a good qualitative picture of the actual susceptibility distribution, which can be used to select favourable sites for detailed anomaly modeling. However, rough topography presents a serious obstacle, and the ambiguity involving depth and susceptibility cannot be resolved without independent geophysical or geological information.