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The surface-to-borehole transient electromagnetic (TEM) method, which uses a rectangular loop as its source, has shown great potential in mineral exploration at greater depth. However, recent studies have demonstrated that a grounded-wire source can resolve the resistive targets and provide detection at greater depths than the loop source. Thus, we propose a surface-to-borehole TEM method using a grounded wire as a transmitter at short offsets. We conducted numerical experiments to explain the variation of different parameters and its comparable effects on the TEM transients. Similarly, a systematic comparison of the cross-sectional electric field distribution maps, borehole profiles of the time derivatives of the magnetic field, qualitatively reveal that the proposed method induces detectable responses for conductive and resistive targets at greater depths. In the present study, unlike the loop source configuration, the time derivative of an underground magnetic field in horizontal direction showed high sensitivity to resistive bodies. Furthermore, 1-D inversion was performed to provide quantitative interpretation of the borehole data. The 1-D inversion of the surface and borehole data was realized using an Occam-type inversion scheme. The results of both synthetic and field data indicate that inverting the surface or borehole data separately resulted in degraded resolution in both shallow and deep earth. Thus, we also compared the results from the inversions of surface and borehole data in order to explore the merits of joint inversion in resolving shallow and deep structures.

In northern China, thick loess strata with low resistivity are widely present and can shield the deep induced electromagnetic field signal and influence the accuracy of deep targets while electromagnetic prospecting is being performed. To achieve deep target exploration with high resolution, a new technique known as the short offset transient electromagnetic method (SOTEM) is applied in this research area. This method can be used to perform near-source configuration surveys, but shows no obvious difference between the deep magnetite deposit and low-resistivity layer in this survey area. Therefore, it cannot explore the magnetite deposit directly using the traditional magnetic component (Hz) of SOTEM. The Ex component, which can more effectively distinguish the high-resistivity surrounding rock and overlying low-resistivity layer than the Hz component, is used for exploration. First, 2D forward modeling and sensitivity analysis of the high-resistivity body are carried out to determine the different responses of the Ex and Hz components, respectively. Moreover, to better determine the interface between the surrounding rock and low-resistivity layer, 1D Occam inversion and the generalized inverse matrix inversion method are used. Furthermore, aiming at the static effect of the Ex component in field exploration, joint inversion of the Ex and Hz components is carried out for comprehensive interpretation. Finally, a field test is conducted, and the drilling results are used for verification. The research results can provide an effective geophysical model for the exploration of contact metasomatic magnetite deposits.

The grounded-wire transient electromagnetic (TEM) surveying is often performed along the equatorial direction with its observation lines paralleling to the transmitting wire with a certain transmitter–receiver distance. However, such method takes into account only the equatorial component of the electromagnetic field, and a little effort has been made on incorporating the other major component along the transmitting wire, here denoted as axial field. To obtain a comprehensive understanding of its fundamental characteristics and guide the designing of the corresponding observation system for reliable anomaly detection, this study for the first time investigates the axial electric field from three crucial aspects, including its decay curve, plane distribution, and anomaly sensitivity, through both synthetic modeling and real application to one major coal field in China. The results demonstrate a higher sensitivity to both high- and low-resistivity anomalies by the electric field in axial direction and confirm its great potentials for robust anomaly detection in the subsurface.

Traditional TEM study mainly focuses on the generation and application of the TE field using a loop or grounded-wire source; but in recent decades, lots of efforts have been made for implementation of the TM field and even the integration of the TE field with the TM one into anomaly detection in the subsurface. However, no applicable principles have been proposed for selecting the optimal electromagnetic field for various subsurface targets. The transient electromagnetic (TEM) fields generated from grounded-wire source consist of the TE-mode response (current-carrying wire), the TE–TM mode response (grounding ends) and the combined TEM-mode response (current-carrying wire and grounding ends). This study performs a comparison of TE/TE–TM/TEM fields by generating them from grounded-wire source and testing their distribution characteristics, detection depth, and sensitivity to anomalies, using both synthetic 1D model and two field surveys in China. The comparisons demonstrate that, the detection depth of the TE–TM field is smaller than those of both the TE and combined TEM fields. Meanwhile, for electric field, the TE–TM response provides a better detection than the TEM one, but with an uneven distribution. Therefore, the TE–TM electric field requires well-designed arrangements of receiving positions when applied to real projects. For the magnetic field, the TEM response has the best detection capability compared to the TE and TE–TM ones, but is least sensitive to layer thickness and resistivity, especially for an embedded layer with low resistivity.