3-D Inversion of Loop-Source Semiairborne Transient Electromagnetic Data Incorporating Topographic Effects

Abstract

Semiairborne transient electromagnetics (SATEMs) is an emerging geophysical method for mineral exploration, hydrogeological investigations, and engineering applications owing to its high resolution for shallow targets and strong sensitivity to conductive bodies. However, the inherent nonuniqueness of conventional 1-D inversion, together with response distortions induced by topography, can significantly degrade the accuracy and resolution of loop-source SATEM imaging. To address these issues, we develop a 3-D forward and inversion framework that explicitly incorporates topographic effects for SATEM data interpretation. The forward solver uses an unstructured vector finite-element (FE) formulation coupled with an unconditionally stable, second-order backward Euler (SBE) time-stepping scheme, enabling high-fidelity simulations overrugged surfaces. The inversion is accelerated using a limited-memory BFGS algorithm, substantially reducing the computational cost and memory demands associated with sensitivity calculations. Synthetic experiments demonstrate that the proposed approach accurately recovers the geometry and resistivity of subsurface anomalies, and that incorporating topography markedly improves boundary delineation. We further assess the method using two geologically realistic benchmark models: a geological–geophysical model from the Eagle Mountain Mine (USA) and a borehole-constrained model from Voisey’s Bay (Canada).