The small-loop transient electromagnetic (TEM) method has made great contributions in urban underground space detection in recent years due to its small working space. For the problems of single sampling rate and low sampling rate of traditional receivers, a dual sampling rate receiver is designed to record electromagnetic response with high accuracy. In addition, this paper employs the passive clamping technique and oscillation absorption technique to realize the falling edge linear fast shutdown of the triangle and trapezoidal wave emission currents, and to improve the excitation capability of the shallow target. In order to avoid the saturation of the early response, the primary field intensity of the center point is reduced by bucking coil. In order to solve the problem of high precision data interpretation in shallow layer, this paper adopts the method of remaining primary field removal and response splicing to splice the pure secondary field response of triangle wave and trapezoidal wave current waveform, and realizes the joint resistivity imaging of electromagnetic data. According to laboratory test results, the transmitter can output triangle wave and trapezoidal wave with adjustable parameters; the sampling rate of the receiver is 64 kHz and 256 kHz, which can effectively record the electromagnetic response and current waveform. The field experimental results exhibit that the small-loop electromagnetic detection system can effectively measure the shallow anomalies in urban underground space, which demonstrates the effectiveness of the dual waveform combined small-loop detection system for improving the accuracy of shallow exploration and promoting the development of geophysical refinement detection process.

The transient electromagnetic method (TEM) has been widely used as a geophysical exploration method in recent years. When Maxwell’s equations are discretized in the time domain by the direct solution method, the initial field is used as a substitute for the source, so the electromagnetic response of a shallow three-dimensional anomalous body cannot be calculated. Maxwell’s equations in the frequency domain are simple in form, and the current source can be directly added without calculating the initial field. However, large linear equations must be calculated. The coefficient matrix is large, and the calculation speed is slow, which limits their application. Based on Yee grids, this paper combines octree grids with the finite-difference frequency-domain (FDFD) method. Ensuring a sufficiently large computational area, octree grids are used to refine the area of anomalous bodies, while coarse grids are still used to reduce the total number of grids and improve the efficiency. In the numerical simulation, the vacancy positions are set to zero to solve the data storage problem of the coarse grids and fine grids. The binary paraboloid interpolation method is used to solve the electromagnetic field component transfer problem at the intersection of the coarse grid and fine grid. Finally, the electromagnetic response curve in the time domain is obtained by a frequency-time transformation. By comparing the calculation results of typical models, the correctness of the FDFD method based on octree grids is verified. By comparing the computational time of complex anomalous bodies for Yee grids and octree grids, it can be concluded that the efficiency of the FDFD method based on octree grids is improved to a certain extent.

The small-loop time-domain electromagnetic (TDEM) method has been widely used in urban underground space detection in recent years due to the small workspace requirements. Due to the small side length of the transmitting coil of the small-loop electromagnetic method, more difficulties have been introduced in modeling and instrument development. The traditional modeling method cannot include source calculation, and the error becomes significantly large when calculating the initial field of the small-loop, and the cross iteration of electric and magnetic fields also increases the modeling error of the small-loop. Therefore, a high-precision 3-D small-loop source TDEM modeling method need to be proposed to provide the theoretical basis for feature analysis, inversion, and instrument parameter design. In this article, the electromagnetic wave equations are adopted as the controlling equations and discretized based on the Crank–Nicolson finite-difference time-domain (CN-FDTD) method. The entire computational space, including air and ground, is subdivided into sources to support 3-D small-loop TDEM modeling for shallow anomalous body conditions. Furthermore, the iterative formulas of the electromagnetic wave equations in the complex frequency-shifted perfect match layer (CFS-PML) are derived, the reflection errors are largely suppressed, and the modeling accuracy is significantly improved. Finally, the effectiveness of the improved method is verified by homogeneous models, layered models, and complex anomaly models. In addition, analyzing the propagation characteristics of the small-loop can guide the setting of the receiver sampling rate and improve the accuracy of detection. The results show that the improved method can achieve stable, low-memory, and high-precision 3-D small-loop source TDEM modeling, which can provide theoretical support for the application of the TDEM method in urban and shallow detection.

Electromagnetic (EM) sounding methods in the time and frequency domains are widely used in polymetallic ore exploration and can measure both EM induction and induced polarization (IP) responses simultaneously. Depth inversion is a crucial factor affecting the accuracy of data interpretation. The effect of IP on EM wave propagation has been ignored in traditional studies of diffusion depth and skin depth. This article is based on the generalized effective-medium theory of the IP effect (GEMTIP) model to represent the IP effect. The frequency domain solution based on GEMTIP is obtained. To address the problem whereby the time-domain solution cannot be solved by inverse Laplace transform, rational approximation and linear programming methods are used to simulate the variation of pulse field source with time and distance. We analyze the influences of geometrical parameters and physical properties of rocks and ores with the IP effect on the depth. Finally, we consider the influence of intrinsic chargeability and improve the classic diffusion depth formula. The formulae for estimating the time-domain diffusion depth of the IP effect under certain conditions are given and verified. These methods are of great significance to better understand the relationship between the subsurface dispersive media and the propagation of EM waves.

An improved finite‐difference time‐domain (FDTD) solution is developed for three‐dimensional transient electromagnetic modeling in source‐free media. Based on Guptasarma's linear filtering method and the fast Hankel transform, the improved method selects the electric fields (E) at the first two time‐steps, and the time derivative of the magnetic induction (∂B/∂t) at the second time‐step as initial conditions. This selection can avoid approximating ∂E/∂t at initial times and improve initial accuracies. During the iterations, the electric fields and ∂B/∂t are both defined at the same integer time indices and the deriving equations are modified based on the FDTD method. Consequently, it can avoid the approximation of the electric fields at intermediate time indices and magnetic fields at integer time indices, and improve the iteration precisions. The convolutional perfectly matched layer (C‐PML) boundary condition is employed to reduce the reflection errors and the effects are verified. At last the proposed approach is checked via homogeneous half‐space models, layered models and 3D anomalous models with complex backgrounds. The results show that the improved method can reduce simulation errors twice or thrice and deepen effective exploration depth by 20%, which has verified the effectiveness of the improved method for three‐dimensional transient electromagnetic modeling, especially for long‐time and high‐resistance modeling.

The wire loop method of the transient electromagnetic (TEM) method is used to nondestructively detect conductor breaks of grounding grid. For this purpose, grounding grids serve as an underground wire loop, and the measuring points are arranged on the ground. At each measuring point, a receiving loop is employed to detect the electromagnetic response generated by transmitting the current of the transmitting loop. Conductor breaks can be diagnosed by analyzing the slices of the electromagnetic response. We study the effect of loop size and height difference through the simulation of an intact 2×2 grounding grid, confirming that it is easier to obtain the topological structure using a small transmitting loop and a small height difference. Furthermore, simulations of an intact 4×4 grounding grid and grids with different locations of conductor breaks are also conducted with a small transmitting loop. It is easy to distinguish the topological structure of the grounding grid and the locations of conductor breaks. Finally, the detection method is applied experimentally. The experimental results confirm that the proposed method is an effective technique for conductor break diagnosis.