Topographic correction of magnetic data on rugged topography with application to Río Blanco-Los Bronces and El Teniente porphyry copper districts, Southern Andes, Chile

Abstract

Airborne magnetic surveys over rugged topography generate noticeable magnetic signatures that are likely equivalent in amplitude with the signal of geological interest. Synthetic models demonstrate that this magnetic terrain effect has the same wavelength as the topography and cannot be overcome by drape flying. The magnetic terrain effect amplifies negative and positive magnetic signals over steep valleys and ridges, respectively. These magnetic artefacts may induce incorrect geological interpretation of magnetic features. In order to remove these spurious signals, we develop a semiquantitative methodology based on 3D magnetic modelling of the topographic effect. Observed total magnetic field is then corrected by subtracting the synthetic field related to the topographic effect. The key element in this approach is the appropriate estimate of the magnetisation associated with the topography, which is especially difficult to determine in areas characterised by rugged terrain. We estimate the magnetic signal related to the topographic effect by: (1) filtering the magnetic data based on the wavelength band in which magnetic and topographic data show maximum coherency, (2) inverting the filtered magnetic data in order to obtain a model of the magnetic susceptibility distribution associated with the topography, and finally (3) calculating the magnetic signal response of the topography-related susceptibility model. We successfully tested this approach in the Río Blanco-Los Bronces and El Teniente porphyry copper districts (Andes of Central Chile), which are characterised by rugged topography and the presence of highly magnetised volcanic rocks. Validation is achieved by comparing the magnetic response over zones with a good geological and petrophysical knowledge. In these examples, the topography-corrected magnetic data show the distribution of geological units and susceptibility better than the non-corrected magnetic data.