Transient electromagnetic method (TEM) is often used in urban underground space exploration and field geological resource detection. Inversion is the most important step in data interpretation. Because of the volume effect of the TEM, the inversion results are usually multi-solvable. To reduce the multi-solvability of inversion, the constrained inversion of TEM has been studied using the least squares method. The inversion trials were performed using two three-layer theoretical geological models and one four-layer theoretical geological model. The results show that one-dimensional least squares constrained inversion is faster and more effective than unconstrained inversion. The induced electromotive force attenuation curves of the inversion model indicate that the same attenuation curve may be used for different geological conditions. Therefore, constrained inversion using known geological information can more accurately reflect the underground geological information.

In the process of tunnel construction and urban underground space development, karst caves and cavities often cause casualties and economic losses. However, due to the influence of complex terrain and buildings on the ground, conventional geophysical detection methods cannot obtain accurate information on hidden geological bodies under buildings and in front of tunnels. The grounded-wire source transient electromagnetic method (TEM) has a relatively simple transmission and reception system and is widely used in the detection of coal mine goafs and karst caves. Therefore, we apply this method to bore-holes and propose a cross-hole grounded-wire source TEM detection method. This method involves deploying grounded-wire transmitting sources and receiving electrodes in different bore-holes to accurately detect concealed geological hazards in urban underground construction processes. In this study, we use the research methods of the TEM field diffusion mechanism of a vertical grounded-wire source, a numerical simulation of a cross-hole and grounded-wire source TEM of different electrical models and an indoor physical experiment in a strong interference environment to analyze the detection ability of a cross-hole grounded-wire source TEM. Differential processing was applied to both the numerical simulation and experimental data. The research results indicated that, owing to the influence of the ground–air boundary, extreme values of the z component of the electric field moved toward deeper regions away from the grounded-wire source. Differential processing effectively increased the detection accuracy of this method, and a higher differential order provided clearer characterization of the boundaries of the target body. Spatial averaging filtering can effectively remove the influence of random noise, enhancing the ability of the cross-hole grounded-wire source TEM to identify anomalous bodies. After difference processing, the cross-hole grounded-wire source TEM data could more precisely capture the boundaries of concealed geological bodies in urban underground spaces.

It is not uncommon for underground water supply or heating pipelines to break and leak, but such events can cause serious inconvenience to the lives of residents. The accurate and timely detection of the leak locations is the primary objective in solving pipeline rupture issues. Based on the direct current (DC) resistivity method, which provides a stable signal, convenient interpretation, and strong anti-interference ability, we adopted the pole-pole device form of the DC potential method to detect the locations of leaks in high-resistivity water supply or heating pipelines. A finite element numerical simulation method was used to study the characteristics of the leakage point potential distribution with and without metal pipeline interference. The numerical simulation results show that the pole-pole DC potential method has strong anti-interference ability and can effectively locate locations of leaks in pipelines with metal interference. The numerical simulation results were verified by physical experiments. The research results show that the pole-pole DC potential method can effectively detect the locations of leaks in high-resistivity pipelines.

Coal-bearing strata belong to sedimentary strata, and there are multiple aquifers. The accurate detection of deep aquifers is helpful to the safe mining of the working face. In order to provide guidance for the interpretation of the surface-to-underground transient electromagnetic method (SUTEM) that can be used to detect deep aquifers, we used theoretical analysis and numerical simulation methods in this study. Taking uniform half-spaces, single aquifers, and double aquifers as examples, we systematically studied the data characteristics and degree of influence of SUTEM under the influence of shallow aquifers. The results indicate the following: Under the influence of the primary field distribution, the x or y component of the induced electromotive force received by the underground receiving point has a positive and negative inflection point, which increases the difficulty of data interpretation, and the z component is easier to use for data interpretation. The influence of the aquifer on the early data of the underground receiving point is much greater than that of the ground receiving point, and the late influence is closer to the ground receiving point. The change in resistivity of the shallow aquifer has the greatest influence on the ability of each measuring point to detect the data of the deep aquifer; this influence is followed by change in thickness, and change in depth has the least influence on the detection capability of each measuring point.

To address the challenge of accurately detecting hidden water inrush hazards ahead of working faces, a cross-hole transient electromagnetic (TEM) method utilizing a grounded-wire source is proposed. The technique positions a step-current-driven grounded-wire source within a working-face borehole, while electrode arrays in adjacent boreholes measure secondary electric field responses. This configuration minimizes interference from metal supports or machines, thereby enhancing the signal-to-noise ratio of the TEM signals. A theoretical analysis based on the unstructured finite-element (FE) method is used to investigate the configuration. The collected data are processed using differential techniques, and the results confirm the method’s effectiveness in detecting anomalies. This paper investigates the response of our cross-hole method to anomalies in terms of size, resistivity contrasts, and spatial location, with anomaly boundaries quantitatively delineated via first-order differential analysis. This significantly enhances the capability of TEM detection in identifying anomalies. A comparison between our cross-hole method and the traditional roadway–borehole TEM method, using the trapped column model, demonstrates that the proposed cross-hole device more effectively locates anomalies and improves accuracy. Furthermore, this technique enables the formation of a 3D observation framework by utilizing existing boreholes, presenting promising prospects for future applications.

Generally, ground grouting is used to treat confined water areas before mining at the working face, but there is a lack of testing methods for determining the effectiveness of such a grouting treatment on the floor of ultrawide working faces. Therefore, we propose a 3D DC resistivity method for mines and apply it to the detection of the effect of grouting on the mine floor. This study took the Yongmei Xinqiao Coal Mine in Henan Province as the research object and used a combination of theoretical analysis, numerical simulation, and measured data analysis to study the effect of the 3D resistivity method on detecting the effect of grouting on the floor of an ultrawide working face in the mine. The research results indicated that compared with the 2D observation mode of same-side power supply and reception, the 3D observation mode of opposite-side power supply and reception using the tunnels on both sides of the working face was more sensitive to the response of the water-rich area 60 m below the coal seam’s floor. Regarding the model’s set-up in this article, when traditional apparent resistivity calculations were used, the apparent resistivity obtained by the 3D observation mode was opposite to the model’s setting, and accurate electrical information of anomalous bodies must be obtained through 3D inversion. The measured data showed that although the ground grouting treatment effectively reduced the water volume in the floor, the treatment’s result was affected by human factors, and the water in the floor was redistributed.