Portable near-surface frequency-domain electromagnetic (FDEM) instruments are effective tools for detecting underground electrical abnormal targets. However, due to the inductive reactance of the transmitting coil, conventional instruments cannot transmit high amplitude current at multiple frequencies at the same time in the high frequency range (>1000 Hz). This would lead to low detection efficiency and poor signal-to-noise ratio of the system and would ultimately affect the detection accuracy. In view of the above problems, a multi-frequency resonant strategy is proposed to simultaneously increase the transmitting current amplitude at multiple target frequencies. First, the multi-frequency resonance theory is analyzed and the circuit parameter solving method is improved. The validity of the multi-frequency resonance theory and the solution method is verified via circuit simulation. Further, the influence of the component parameter deviation on the circuit resonance is analyzed, and the selection principle of the resonance parameters based on the quality factor Q is presented. Considering the actual deviation, a five-frequency or seven-frequency resonant circuit may achieve a better resonance effect than the nine-frequency resonant circuit in practice. Finally, the strategy is validated with an actual circuit. The results show that the constructed five-frequency resonant circuit can effectively increase the amplitude at the target frequency. The maximum increase is 10.50 times, and the transmitting energy is 11.4 times higher. This study provides a feasible implementation method for multi-frequency resonance at five frequencies and above, which is helpful for improving the detection efficiency and signal-to-noise ratio of a portable near-surface FDEM system.

Ground-airborne frequency-domain electromagnettic (GAFEM) is a method that has the potential to detect underground anomalies at large depth ranges in areas with complex terrain. Currently, for the demand of field real-time rapid interpretation of GAFEM data, the apparent resistivity method cannot effectively recognize anomaly boundaries, and divergence of tipper vector imaging method has difficulties in transmitting source deployment. In view of above problems, a rapid imaging method of GAFEM based on space magnetic gradient anomaly (MGA) is proposed. This method enables the rapid recognition of underground electrical anomalies by GAFEM data without large computational resources. First, the characteristics of the magnetic field gradients in the homogeneous half-space model and the 3D anomaly model are analyzed, and the calculation method of MGA based on the background field gradient is proposed. Further, the ability to recognize underground electrical anomalies in transverse MGA, longitudinal MGA, and space MGA are analyzed by a forward model. Finally, an imaging process that can be applied to actual detection and the calculation method of background resistivity are proposed. The feasibility, the anti-noise and the multi-anomaly recognition of the imaging method are verified. The results show the imaging method based on space MGA can effectively recognize the underground electrical anomalies. The magnitude of the space MGA can determine the location and contour of the anomaly, and the real and imaginary parts can determine the electrical type of the anomaly. This study provides a new method for the interpretation of GAFEM data, which is helpful for the rapid field real-time detection of underground structures.

Portable near-surface frequency-domain electromagnetic (FDEM) can effectively recognize underground electrical anomalies. However, because the transmitting device of conventional portable near-surface FDEM systems is a transmitting coil and has a high operating frequency (>1000Hz), the inductive resistance of the coils results in a decrease of the transmitting current, which in turn results in a decrease of the transmitting magnetic field. It would result in low detection efficiency and poor signal-to-noise ratio of the system. Currently, several techniques can increase the transmitting current. However, these techniques have problems in that they are difficult to solve the design parameters, and the deviation of the component parameters has a large influence. In view of the above problems, a multi-coil parallel strategy (MCPS) is proposed in this paper. The MCPS proposes to optimize the conventional single transmitting coil to a multi-coil co-axial parallel (MCAP) structure. MCPS can increase the transmitting current amplitude at any of the target frequencies without increasing the power of the power supply. First, this paper analyzes the feasibility of the MCPS in practice. The analysis is mainly included in the following three aspects. 1. The feasibility of MCPS in an ideal state is analyzed. 2. The influence of mutual inductance and resonant capacitance deviations on MCPS is analyzed. 3. The performance parameter requirements of resonant capacitors are analyzed. The results of the simulation show that the MCPS can simultaneously increase the amplitude of transmitting currents at multiple target frequencies. In practice, MCPS has strong stability and suitability. Further, this paper verifies the MCPS with an actual circuit. The experimental results show that compared to the conventional single-coil transmitting system, the MCPS increases the current amplitude by 1.86 times, 1.87 times, and 2.04 times respectively at the three target frequencies, increases the transmitting magnetic field by 1.28 times, 1.5 times, and 1.75 times respectively, and improves the signal-to-noise ratio by 3.94 dB. Finally, the paper discusses the results of simulation and experiments. The methods to further improve the effectiveness of MCPS applications and future research directions are concluded.

Ground-airborne frequency-domain electromagnetic (GAFDEM) rapid imaging method holds the potential for swiftly detecting underground anomalies in complex terrains at great depths. However, the current GAFDEM rapid imaging methods have not yet achieved satisfactory detection results in actual complex terrains. This is primarily due to two reasons: 1. The detection environment in complex terrain involves non-horizontal land and significant relief caused by the presence of mountains. However, current GAFDEM rapid imaging methods fail to consider the influence of terrain on imaging effectiveness. 2. In controlled-source electromagnetic detection, the design of the observation scheme directly affects the detection effectiveness of imaging methods. In actual detection, terrain constraints limit the design of observation schemes. For GAFDEM, the design of its transmitting sources is greatly influenced by terrain since these sources need to be deployed on the ground. Addressing these issues, this study analyzes the challenges faced by the Space Magnetic Gradient Anomaly (SMGA) in complex terrains, which exhibits effective detection under a horizontal ground model. Based on the analysis results, an optimization method for SMGA in complex terrains is proposed. This method achieves rapid correction of the background field in SMGA, thereby enhancing its detection efficacy in complex terrains and enabling the effective identification of underground electrical anomalies. Furthermore, performance testing of the optimized SMGA (O-SMGA) demonstrated that O-SMGA exhibits excellent applicability. Subsequently, an examination is performed on multiple parameters of the transmitting source assessing their effects on the imaging performance of O-SMGA. Based on the analysis results, design guidelines for GAFDEM transmitting sources in complex terrains applicable to O-SMGA are provided to further enhance its detection effectiveness in actual detection. Finally, a synthetic test of O-SMGA and the transmitting source design guidelines is conducted using models with actual terrain data and actual experimental results. The results indicate that these methods still exhibit excellent performance in actual detection applications.

Ground–airborne frequency-domain electromagnetic method (GAFEM) is a geophysical exploration technique capable of detecting subsurface electrical structures over large areas and at great depths in complex terrains. Tipper divergence imaging, one of the principal GAFEM interpretation approaches, offers high lateral resolution and rapid imaging speed. However, conventional tipper divergence imaging relies on magnetic field data generated by orthogonal dual-source excitation, which is difficult—or in extreme cases impossible—to deploy in complex terrains, and it also fails to delineate the vertical boundaries of anomalies effectively. In this paper, we propose a tipper apparent resistivity-united-gradient imaging method with single-source excitation. The proposed approach combines lateral and vertical spatial gradients of tipper apparent resistivity derived from a single-source excitation to achieve high-resolution detection of subsurface electrical anomalies. We first present the derivation of the theoretical formulation underpinning the proposed imaging method. We then apply this approach to GAFEM datasets generated from three‐dimensional simulation models—incorporating anomalies at varying depths, anomalies whose resistivities closely match the background, and multiple coexisting anomalies—to further analyze the method’s three‐dimensional imaging characteristics. We further assess the effect of background resistivity estimation errors and noise interference on the imaging method. Finally, field experiments validate the method’s practical applicability. Compared with conventional tipper-based techniques, our method requires only a single-source excitation, overcoming the challenge of orthogonal dual-source deployment and enhancing feasibility in field surveys; moreover, the tipper apparent resistivity-united-gradien t strategy resolves the inability of conventional divergence imaging to identify vertical anomaly boundaries, thus offering a novel pathway for precise geophysical exploration.

Apparent resistivity imaging is a typical rapid imaging method in the ground-airborne frequency-domain electromagnetic method. At present, the apparent resistivity is typically calculated by the measured magnetic field, however, this imaging method exhibits limited capability in recognizing the center of three-dimensional anomalies. Therefore, this paper proposed the calculation of apparent resistivity using magnetic field gradients. To solve the problem of random artificial anomalies that existed during the calculated process, this paper presents a hybrid least square support vector machine (LSSVM) and Northern Goshawk optimization (NGO) to establish the mapping relationship between the magnetic field gradient and apparent resistivity variation. This approach enables accurate prediction of apparent resistivity variations and effectively resolves the challenge of correcting background resistivity. Furthermore, three typical theoretical models and field examples are used to predict the apparent resistivity variation, the imaging results demonstrate that the proposed NGO-LSSVM algorithm is a feasible and efficient tool for predicting the apparent resistivity variation with high accuracy. This study provides a novel and efficient imaging method, which facilitates the application of ground-airborne frequency-domain electromagnetics for high-resolution detection requirements, such as mineral exploration.