Practical 3D airborne EM inversion in complex terranes

Abstract

Many AEM inversion techniques have been applied successfully to locating isolated, highly conducting targets in uniform hosts. What happens when these targets occur in complex hosts overlain by non-uniform regolith with topographic features? In principle, a full 3D inversion is possible due to the availability of forward modelling programs capable of accounting for the full geoelectric complexity of any terrane. In practice, the inherent non-uniqueness coupled with modelling and data errors plus the requirement of substantial computation times render this approach unappealing for all but the most academic of inverters. On the other end of the complexity scale, simple imaging methods such as CDIs can fail badly for dipping targets and Born style approximations produce wrong models for moderate to high conductivity contrasts. This can be demonstrated by using accurate forward modelling algorithms to compute the EM responses of models produced by these methods. Practical AEM data interpretation requires a technique that is capable of recognising drill targets in a variety of terranes but one that doesn't require excess complexity or massive computing times. One such method, LeroiAir, is based on 3D thin sheet models in a layered, conducting host. The implicit forward model is fast, easy to set up and takes account of vortex and current gathering for any conductivity contrast. It is also capable of modelling the effects of other conductors such as faults and palaochannels. As expected, the thin-sheet inversion works well for simple isolated targets. As complexity increases, the inversion quality degrades but still yields useful drill target information in many cases.