Challenges with starting models, correlation, and convergence for inversion of IP-affected TEM data: Examples from African countries

ABSTRACT Transient electromagnetics (TEM) is a geophysical method well suited for hydrogeological investigations, offering extensive spatial coverage and the ability to resolve hydrogeological units in the order of tens of meters across diverse terrains. However, in certain environments — such as sulfide mineral deposits, clay-rich materials, or permafrost — the TEM signal can be affected by induced polarization (IP), a phenomenon that introduces perturbations and limits the reliability of traditional inversion workflows. TEM data affected by IP, collected using a towed TEM system across four sub-Saharan African countries, illustrated the challenges associated with IP effects. Inversion of the TEM data using the maximum phase angle (MPA) re-parameterization revealed low convergence rates during inversion, with a strong dependence on the choice of starting model. The resulting models showed limited sensitivity to MPA parameters (ϕmax, τφ, C), in contrast to resistivity, which exhibited comparatively lower uncertainty. Integration of borehole or other ground-truthing data proved essential for reducing ambiguity in IP-affected TEM data interpretation. These findings underscored the limitations of current inversion approaches under IP conditions and highlighted the need for improved parameterizations that yielded reliable constraints on IP behavior.