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

Magnetic susceptibilities measured on outcrop are often assumed to be reasonable values to use when modelling aeromagnetic data. We have undertaken two exercises to understand how useful these measurements are for magnetic modelling. Estimates of apparent magnetic susceptibility can also be derived from aeromagnetic data by mathematical transformation if certain stringent conditions are satisfied. In the first exercise comparison of these two sets of values shows that when the measured values are below 1 × 10−3 SI, there is no correlation between the measured and apparent values. Above this value, the measured and estimated values agree to within a factor of 10, so these measured values can only be used as very rough constraints. In the second exercise, a database of outcrop measured magnetic susceptibilities was used to estimate a mean value for each lithology. When the mean of the measured values was used in our forward modelling exercise, we obtained a poor fit. However, when we inserted additional layers with larger susceptibilities that are represented on the histogram of a regional compilation of that lithology, the fit was satisfactory. In some cases, these additional layers were located in areas where there is relatively large variability in the magnetic susceptibility values measured on nearby outcrops. A large database of outcrop magnetic susceptibilities proved useful for providing a reasonable initial guess and the range of values that encompass the possible heterogeneities in each rock type.