Progress in Defining Deeper Realistic Geology Using AEM 2.5D Joint Inversion

Including interpretations from airborne geophysical surveys to build geological models continues to improve outcomes for deriving more realistic geological representations. It is now highly practical for AEM data to be both forward modelled and inverted using 2.5D numerical approximations. In this paper we present one such code for performing these workflows: a finite element method that assumes a 2D Geological source and 3D electrical source. This is provided that the geo-electrical cross-section is relatively constant along a strike length that exceeds the AEM system footprint. Lines that exceed 30 kilometres can now be modelled and inverted on a standard laptop computer.

• The AEM inversion technique presented is usefully capable of defining detailed geometry for synclines, anticlines and deeply dipping beds.
• AEM 2.5D Inversion appears to match realistic geology trends to depths of up to 1 km and hence is cost-effective compared detailed drilling.
• Experience shows it’s best to use a half-space starting model, slightly adapted to the gross resistivity characteristics of the observed signal.
• Fully geologically-constrained inversion attempts to find the compromise between the observed geophysical signal, and the interpreted sections.
• Rock properties in the form of estimated and inferred resistivities for each formation, play an integral part in this AEM inversion workflow.