The borehole transient electromagnetic (TEM) method can be useful in deep mineral exploration to detect blind ore bodies beside or below the borehole, and is especially adapted to finding small-scale, deep, rich ore bodies. In this method a transmitting loop is deployed on the top surface of the Earth, while a receiving coil is moved down the borehole. As the borehole TEM method is limited by the borehole’s location and depth, so its exploration scope is limited. The surface to airborne TEM method is a semi-airborne TEM configuration that transmits on the surface and receives TEM response in air. The two systems are combined into one system in this study, sharing the transmission loop deployed on the surface. With this combined system, the TEM response in the borehole and in the air can be observed at the same time. This paper employs a joint interpretation method based on the equivalent filament, which is introduced to obtain more reliable geometric information for the target with both borehole and aerial TEM data. The eddy currents induced in a thin confined conductor can be represented by equivalent current filaments, and the distribution of filaments can reflect the position and geometry of the conductor. Therefore, geometric parameters of targets can be obtained by filament inversion, and the joint inversion can be more accurate with both borehole and aerial response. Numerical modeling results show that the joint inversion based on the equivalent filament results can reliably obtain the geometric parameters of the thin conductive plate embedded in half space.

The full waveform effects refer to the total effects of turn-on, steady, and turn-off durations for a transmitting-current waveform as well as its repetition number in transient electromagnetic (TEM) methods. In this study, the full waveform effects are investigated using both forward-modelling and inversion methods considering typical background noise. The forward-modelling results of homogeneous half-space models show that the magnetic induction, bz, is less affected by the background noise but more affected by the full waveform effects than the time derivative of magnetic induction ∂bz/∂t. Therefore, this study focuses on investigating the full waveform effects on bz. The inversion results for synthetic and field examples show that the inversion algorithm without considering the full waveform effects leads to over-estimated resistivities in deeper parts of the recovered models compared to the true model. Therefore, it is crucial to consider the full waveform effects when processing TEM data. Furthermore, a standard deviation factor (STDF) is estimated for model parameters of the inversion. The results show that the STDF increases as the layer depth increases for 1D layered models. This indicates that the inverted parameters are well resolved for shallower layers and moderately to poorly resolved for deeper layers.

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Heicigou gold mine is a gold polymetallic mine controlled by a northwest trending fault. The magnetic method and induced polarization results show that the gold-bearing alteration zone is distributed discontinuously in a northwest direction, but the spatial distribution characteristics of the alteration zone have not been effectively identified. Although some drilling work has been carried out in the latter, the number and the depths of the drillings have been insufficient to control the distribution of the alteration zone. According to the measured three-component transient electromagnetic anomaly characteristics, we designed corresponding models and carried out three-component forward simulation on the platform of EMIT Maxwell. We summarized the corresponding relationship between the three-component plane change characteristics and the orientation and occurrence of the low-resistivity medium subsurface. We applied this method to the Heicigou gold deposit and found that the occurrence of the ore-bearing alteration zone is northeast. This discovery solved the problem that the previous magnetic method and induced polarization method failed to solve. It corrected the geologists’ incorrect understanding of the spatial distribution of the alteration zone.