Practical 3D AEM inversion using 2.5D modelling

2.5D electromagnetic (EM) modelling computes the response due to a 3D source from an arbitrary 2D geoelectrical model which has an infinite strike length. As such, airborne EM (AEM) data can be inverted using 2.5D modelling provided that the geoelectrical cross-section is relatively constant along a strike length that exceeds the AEM system footprint. The program ArjunAir is introduced for modelling and inversion based on a 2D finite-element method that enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries and very high resistivity contrasts. Inversion is based on an iterative Gauss-Newton method that is solved using the damped eigenparameter algorithm. Examples are presented for synthetic frequency and time-domain AEM surveys for which inversion runtimes are in the order of hours.