3-D EM inversion considering induced polarization effect

SUMMARY Modelling induced polarization (IP) effects in electromagnetic (EM) data is increasingly becoming a standard tool in mineral exploration, but the industry standard is still based on 1-D forward and Jacobian modelling. We have developed a 3-D electromagnetic forward and inversion method within the EEMverter modelling platform, incorporating IP effects. The 3-D computations are performed in the frequency domain using the vector finite element method and then transformed into the time domain via Hankel transformation. This approach enables modelling of any IP parametrization, ranging from the simple constant phase angle model to a full Debye decomposition. Furthermore, 3-D forward modelling mesh and inversion mesh are built independently: an Octree forward mesh is designed for efficient spatial segmentation for single or multiple soundings, while the inversion parameters are defined on a structured model mesh, which is linked to the forward meshes via interpolation. In conjunction with the development of a full 3-D EM-IP inversion, we introduce a novel 3-D inversion workflow. This workflow allows for hybrid 1-D-3-D computations, both sequentially and spatially, enabling 3-D modelling exclusively in the most significant and interesting areas of the survey. We tested the hybrid 1-D-3-D inversion workflow using airborne electromagnetic (AEM) data acquired by Xcalibur with the HeliTEM system in the Staré Ransko area (Czech Republic), known for its gabbro-peridotite rocks hosting nickel-copper ± cobalt, platinum group element (Ni-Cu ± Co, PGE) mineralization. The results demonstrate that the hybrid inversion effectively addresses the challenges of 3-D modelling on large-scale data sets. It enhances interpretation reliability in regions with strong 3-D effects and shows a significant spatial correlation between resistivity and chargeability phase anomalies and known mineral deposits. Moreover, both synthetic and field data indicate that the resistivity parameter is more sensitive to 3-D effects than the chargeability phase parameter.