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 likelyequivalent in amplitude with the signal of geological interest. Synthetic models demonstrate that this magnetic terraineffect has the same wavelength as the topography and cannot be overcome by drape flying. The magnetic terrain effectamplifies negative and positive magnetic signals over steep valleys and ridges, respectively. These magnetic artefactsmay induce incorrect geological interpretation of magnetic features. In order to remove these spurious signals, we developa semiquantitative methodology based on 3D magnetic modelling of the topographic effect. Observed total magnetic fieldis then corrected by subtracting the synthetic field related to the topographic effect. The key element in this approach is theappropriate estimate of the magnetisation associated with the topography, which is especially difficult to determine in areascharacterised by rugged terrain. We estimate the magnetic signal related to the topographic effect by: (1) filtering themagnetic 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 associatedwith the topography, and finally (3) calculating the magnetic signal response of the topography-related susceptibilitymodel. 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 volcanicrocks. Validation is achieved by comparing the magnetic response over zones with a good geological and petrophysicalknowledge. In these examples, the topography-corrected magnetic data show the distribution of geological units andsusceptibility better than the non-corrected magnetic data.