Surface transient electromagnetic (TEM) data with large transmitter loops for deep mineral exploration are often complicated by the non-trivial coupling between extended sources and arbitrarily oriented geological targets. This case study reports a TEM field data set acquired across terranes with strong lateral inhomogeneity, which is responsible for the high inconsistency in TEM data patterns along the survey line, as well as for the negative TEM transients (sign reversal) at some near-central loop stations. 3D forward modeling and inversion, as maturing tools in recent years, offer unique opportunities to extract as much geological information from such data as possible. 3D forward simulations of representative synthetic models found that the phenomenon of sign reversal at some TEM stations is associated with compact conductors enclosed by the transmitter loop and receivers that are in the loop, but off the conductor–a situation that is common in large-loop TEM and can only be explained by 3D models. However, 3D inversion of the field data with a uniform subspace as the initial and reference model fails to converge, another point of evidence that 3D inversions of large-loop TEM data are more likely to be subject to stability issues. Our solution is to warm-start the inversion with the representative model in the forward simulation experiments as the initial model, so the ill-posed 3D inversion can escape from local minima. Finally, the vertical contact structure in our 3D-inversion model is verified by a resistivity cross section of the CSAMT method. Our case study demonstrates the demand and capability of 3D electromagnetic modeling and inversion for high-resolution deep mineral exploration. It also provides an easy-to-follow template for carrying out 3D interpretation for complex geology in practice.

Efficient and accurate simulation of three-dimensional gravity fields is essential for mineral exploration in complex geological settings, yet it remains computationally demanding for large-scale models and rugged topography. In this study, we propose a fast forward modeling method in a space–wavenumber mixed domain to improve computational efficiency while maintaining high accuracy. The method transforms the 3D spatial convolution into a series of independent one-dimensional vertical integrals using a two-dimensional Fourier transform, significantly reducing computational complexity and memory requirements. Accuracy is enhanced by retaining vertical integration in the spatial domain, which enables flexible discretization with fine grids near the surface and coarser grids at depth. In addition, the Gauss–FFT technique is incorporated to suppress boundary truncation effects and avoid singularities associated with zero wavenumbers. To further improve efficiency for realistic applications, a fast algorithm is developed for gravity modeling over undulating terrain by reusing intermediate computations across elevation levels. Numerical experiments demonstrate that the proposed method achieves accuracy comparable to analytical solutions while significantly reducing computational time. The method is particularly suitable for large-scale 3D gravity modeling in complex terrain conditions, showing strong potential for practical applications in mineral exploration.