3D EM inversion considering induced polarization effect

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 one-dimensional (1D) forward and Jacobian modelling. We have developed a three-dimensional (3D) electromagnetic forward and inversion method within the EEMverter modelling platform, incorporating IP effects. The 3D 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 modeling of any IP parameterization, ranging from the simple constant phase angle model to a full Debye decomposition. Furthermore, 3D forward modeling 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 3D EM-IP inversion, we introduce a novel 3D inversion workflow. This workflow allows for hybrid 1D-3D computations, both sequentially and spatially, enabling 3D modeling exclusively in the most significant and interesting areas of the survey. We tested the hybrid 1D-3D 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 3D modeling on large-scale datasets. It enhances interpretation reliability in regions with strong 3D 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 3D effects than the chargeability phase parameter.