Objective The observational data derived using the surface-borehole transient electromagnetic (TEM) method tend to feature complex curve morphologies, a problem posing significant challenges to data processing and interpretation. This study aims to ascertain the mechanisms behind the generation of such complex responses using forward modeling.

Methods Using the vector finite element-based spectral method and the Cole-Cole model, this study developed a 3D forward modeling technique incorporating the induced polarization (IP) effect. By constructing a typical theoretical model of low-resistivity veined orebodies, it systematically analyzed the electromotive force (EMF) responses excited by transmitter loops at different orientations.

Results and Conclusions The results indicate that the induced EMF curves from surface-borehole TEM responses were characterized by complex morphologies, with multiple sign reversals commonly observed. These characteristics were jointly controlled by the relative locations between transmitter loops and survey points, the resistivity contrast between veined orebodies and surrounding rocks, and the IP effect. The sign reversals in the late delay stage arose primarily from the individual or combined effects of the IP effect and the resistivity contrast between veined orebodies and surrounding rocks, representing a significant indicator for identifying concealed veined orebodies. The results of this study provide a theoretical basis for the mechanistic analysis and geological interpretation of complex TEM responses.