Magnetic models of crystalline terrane; accounting for the effect of topography

Igneous rocks commonly have large magnetic susceptibilities so that high topographic relief in crystalline terrane can produce significant anomalies in aeromagnetic surveys. Topographic anomalies are particularly significant in relatively undeformed volcanic terrane because young volcanic rocks generally have large natural remanent magnetizations as well as large susceptibilities. These anomalies commonly appear in aeromagnetic surveys as a complex pattern of high-amplitude, short-wavelength magnetic features that tend to obscure anomalies caused by deeper geologic sources. We have facilitated geologic interpretation of an aeromagnetic survey of the Oregon Cascade Range by calculating the magnetic field caused by a three-dimensional (3-D) topographic model. Maps of the calculated field are compared with observed aeromagnetic data both visually and with a numerical technique that produces a contour map of correlation coefficients for the model. These comparisons allow quick recognition of anomalies caused by normally or reversely magnetized topographic features and, more importantly, identification of anomalies caused by geologic features not obviously caused by the topography.