Topographic Effects On Magnetic Data: Data Reduction And Application To The Southern Andes

Despite the many advances on magnetic surveying technology and semi-automated modelling and interpretation routines, topographic effects on the data are normally neglected; usually due to the assumption that magnetic data collected on a surface parallel to the ground will not suffer from terrain effects. However, on areas of substantial topographic relief like the Andes, large magnetic anomalies induced by the terrain can be common and of similar amplitude than the anomalies generated by the geological target of interest. These effects are mostly dependent on the orientation of the effective magnetic field versus the topographic slopes, and the large susceptibility contrast between air and the ground. We present a combined 3D forward and inverse modelling technique to reduce magnetic data from topographic effects. The algorithm is applied on a dataset at the Southern Andes, where topographic relief is in excess of 4000 m and the observed magnetic anomalies showed direct correlation with topography, but field mapping did not identify any major faults at the location of the magnetic lineaments. Although the computation could possibly be automated, it is recommended to apply it carefully on a case by case basis and with proper geological control.