Joint inversion of inductive and galvanic data: significant resolution improvement honouring both data types—3-D EM and 2-D DCIP

SUMMARY

Joint inversion of electrical and electromagnetic (E&EM) data can reduce the non-uniqueness of geophysical inversion by exploiting the complementary sensitivities of galvanic and inductive measurements. However, many existing approaches either rely on 1-D electromagnetic (1-D EM) forward modelling or neglect induced polarization (IP) effects, which can be limiting in the presence of polarizable targets or laterally complex structures. We present a joint inversion framework that combines 3-D electromagnetic (3-D EM) modelling with 2-D direct current resistivity and induced polarization (2-D DCIP) modelling. The framework uses a common maximum phase angle reparametrization of the Cole–Cole model for both inductive and galvanic data, together with decoupled forward and model meshes that allow each EM system to be modelled on a dedicated 3-D forward mesh while all data sets update a common 2-D inversion model. The method is evaluated using two synthetic examples and a mineral exploration field data set. The synthetic examples represent a hydrogeological setting with weak-to-moderate IP effects and a mineral exploration setting with stronger IP effects associated with a finite conductive and polarizable intrusion. For both cases, we compare EM-only inversion, DCIP-only inversion, 1-D EM/2-D DCIP joint inversion and 3-D EM/2-D DCIP joint inversion. The results show that the 3-D EM/2-D DCIP joint inversion improves the recovery of resistivity and IP structures when 3-D EM effects are significant. In the hydrogeological example, the 3-D EM kernel improves the imaging of laterally complex clay-rich structures, whereas in the mineral exploration example it reduces artefacts associated with the finite intrusion and improves the separation of distinct polarizable zones. The field application combines airborne EM and ground DCIP data acquired over Fe-Ti-V oxide mineralization in southern Portugal. The joint inversion honours both data sets and provides a single resistivity and IP model compatible with the main geological setting. Overall, the results indicate that 3-D EM/2-D DCIP joint inversion is useful for interpreting inductive and galvanic data sets across different IP regimes, particularly where finite or laterally complex polarizable structures make 1-D EM modelling inadequate.