A Tipper Apparent Resistivity-United-Gradient Imaging Method with Single-Source Excitation for GAFEM

Ground-airborne frequency-domain electromagnetic method (GAFEM) is a geophysical exploration technique capable of detecting subsurface electrical structures over large areas and at great depths in complex terrains. Tipper divergence imaging, one of the principal GAFEM interpretation approaches, offers high lateral resolution and rapid imaging speed. However, conventional tipper divergence imaging relies on magnetic field data generated by orthogonal dual-source excitation, which is difficult—or in extreme cases impossible—to deploy in complex terrains, and it also fails to delineate the vertical boundaries of anomalies effectively. In this paper, we propose a tipper apparent resistivity-united-gradient imaging method with single-source excitation. The proposed approach combines lateral and vertical spatial gradients of tipper apparent resistivity derived from a single-source excitation to achieve high-resolution detection of subsurface electrical anomalies. We first present the derivation of the theoretical formulation underpinning the proposed imaging method. We then apply this approach to GAFEM datasets generated from three‐dimensional simulation models—incorporating anomalies at varying depths, anomalies whose resistivities closely match the background, and multiple coexisting anomalies—to further analyze the method’s three‐dimensional imaging characteristics. We further assess the effect of background resistivity estimation errors and noise interference on the imaging method. Finally, field experiments validate the method’s practical applicability. Compared with conventional tipper-based techniques, our method requires only a single-source excitation, overcoming the challenge of orthogonal dual-source deployment and enhancing feasibility in field surveys; moreover, the united-gradient strategy resolves the inability of conventional divergence imaging to identify vertical anomaly boundaries, thus offering a novel pathway for precise geophysical exploration.