Can magnetic susceptibilities measured on outcrops be used for modelling (and constraining inversions of) aeromagnetic data?

Abstract

Magnetic susceptibilities measured on outcrop and drill-core samples using hand-held instruments have been shown in the literature to be useful for identifying mineralogical changes. It is not yet clear how useful these measurements are for constraining magnetic modelling and inversion. We have generated estimates of the apparent magnetic susceptibility of the ground by mathematical transformation of an aeromagnetic data and assumed that these values can be used to model the magnetic data. In the same area we have a large number of measurements on outcrop and have compared these two independent estimates. When the measured values are below 1x10-3 S.I., there is a no correlation between the measured and apparent values, interpreted to be likely due to the influence or interference from nearby or underlying magnetic sources. Hence, in this case the measured values cannot be used to constrain modelling and inversion. When the measurements are above this value there is a limited correlation, with values only agreeing to within a factor of about 10, so these values can be used as very rough constraints. The poor correlation is interpreted as due to the presence of remanent magnetization or heterogeneity of the magnetic susceptibility within the rock. A large database of outcrop measurements gives an indication of the range of the variation in magnetic susceptibility values that could be used in modelling.