Why every electromagnetic inversion needs a 3D forward simulation

Modern electromagnetic (EM) surveys deliver high-quality data with broad spatial coverage and high spatial density, providing an opportunity to produce higher-quality images of the earth’s subsurface than in the past. Because geologic structures are inherently 3D, the ultimate goal of an EM survey is to recover a 3D conductivity model that supports robust decision-making and interpretation. Nonetheless, practical inversion and interpretation workflows often still rely on simplifying assumptions, such as a 1D layered earth or parametric targets like plates. This paper illustrates the value of incorporating 3D forward simulation into the interpretation workflow. By running a forward simulation for the EM responses of inversion-derived models in full 3D, practitioners can assess the consistency of inversion results with the measured data and identify potential artifacts. Using two examples, namely, a dipping conductor and a porphyry-style deposit, we demonstrated how models that appear plausible under 1D assumptions fail under 3D scrutiny. Identifying areas where 1D assumptions fail can prompt additional analysis, such as iterative hypothesis testing or more focused 3D inversions. As such, 3D forward simulation should be considered an essential tool in the EM practitioner’s toolbox.