For surveys in areas with difficult access, ground-based electromagnetic (EM) systems are difficult to operate, time-consuming, and expensive, whereas airborne EM (AEM) systems are unsafe and uneconomic for small survey areas. We have developed a new frequency-domain airborne electromagnetic (FAEM) system called the ground-airborne frequency-domain EM (GAFEM) system to achieve rapid data acquisition in complex terrains with high safety and low cost. The system consists of a grounded electrical source and an airborne receiver carried by an unmanned rotorcraft. The grounded electrical source transmits a [ ] pseudorandom series obtaining signals at [ ] frequencies in one flight, which can maintain the resolution without reducing efficiency. The airborne receiver based on the unmanned rotorcraft makes the system flexible, convenient, and economical to operate in areas with difficult access. The system can conduct surveys in far- and non-far-field regions and can obtain the apparent resistivity to present a quick overview of the subsurface structure. To demonstrate the GAFEM system’s effectiveness and practical detection potential, we have performed two field surveys that revealed the shallow structure with tunnels and the deep structure at source-receiver distances of 3 and 6 km, respectively. The GAFEM system effectively revealed the apparent-resistivity difference with a sufficient investigation depth at large source-receiver distances. Thus, the system can be extended to various important applications for rapid surveys or deep investigations in areas with difficult access.

The ground–airborne frequency-domain electromagnetic (GAFDEM) survey is a mixture of ground and airborne electromagnetic methods with a source on the ground and a magnetic field receiver in the air. The technique probes different depths of the earth by transmitting waveforms at different frequencies. Therefore, multiple closely spaced frequencies are needed when surveying a specific target depth in detail. However, in the existing GAFDEM systems using a square wave or a 2n pseudorandom wave, the ground source needs to transmit more sets of waveforms with different fundamental frequencies in more flights to improve the vertical resolution for the same observed line, which will seriously reduce the exploration efficiency. In this paper, we design a depth-focused waveform to solve this problem based on a new full-cycle asymmetrical selective harmonic elimination pulse width modulation (SHEPWM) method. In the SHEPWM method, we obtain a desired spectrum by directly controlling the switching angles of a binary sequence with an artificial neural network algorithm. In our study, we first provide the basic theory and method for waveform design. Then, we realize the depth-focused waveform and pseudorandom waveform via simulation. Next, we employ the depth-focused waveform into practice and compare it with the 2n pseudorandom wave in a GAFDEM field survey. The test shows that the survey result with a depth-focused waveform has a higher vertical resolution and efficiency.

The towed frequency-domain electromagnetic (TFEM) detection system is an effective tool for detecting underground anomalous targets. To improve detection efficiency and measurement accuracy for specific depth targets, the depth-focused waveform with multiple probing frequencies closely spaced in the spectrum was introduced. However, the transmitting coil impedance severely suppresses the transmission energy of the excitation signal, which will reduce the transmission efficiency and signal-to-noise ratio (SNR). To solve this problem, a novel optimization method for the traditional depth-focused transmitting system is proposed to simultaneously improve the transmission energy of multiple probing frequencies in the effective band. First, the traditional depth-focused waveform is optimized with the objectives of matching the desired spectrum and maximizing the transmission energy by the nonlinear programming approach, which provides closer spectral energy between the probing frequencies and less undesired harmonic pollution. Then, we design the corresponding broadband resonant compensator based on an improved LC series resonant circuit to simultaneously enhance multiple spectral amplitudes in the effective band. Further, the enhancement effect of compensator parameters on the depth-focused waveforms is analyzed, and the design principle of compensator parameters is provided. Finally, the feasibility of the proposed optimization method is verified by simulation and laboratory experiments and the actual limitation of the novel scheme is discussed in detail.

Aiming at the frequency-domain electromagnetic method (FEM) detection system, a depth-focused transmitting scheme based on multi-frequency sinusoidal pulse width modulation (MFSPWM) method is proposed to solve the problems of low exploration efficiency in conventional single-frequency transmitting scheme and low vertical resolution in 2n multi-frequency pseudo-random (MFPR) transmitting scheme. Depending on the target depth range and required measurement accuracy, a depth-focused waveform is designed to focus the excitation field energy on a narrow frequency band. In this article, the principle of MFSPWM method is theoretically analyzed and the expression of depth-focused waveform is derived by Fourier analysis. Then, the parameter optimization of the main frequencies is performed for the desired spectrum. Further, simulations and experimental tests were carried out to verify the feasibility and effectiveness of the proposed scheme. The results show that the proposed scheme has better spectral features with multiple closely spaced frequencies and less undesired harmonics, which ensures high vertical resolution for the specific depth target. After parameter optimization, the peak-to-peak value of the current waveform was reduced by about 40% and the effective signal of the high-frequency component was increased by about 20%. The forward simulation and field test results show that the proposed scheme has higher detection accuracy and interference resistance for the specific depth target compared to the MFPR scheme.

The open‐loop gain of capacitive power transfer (CPT) circuit, especially for applications like wireless charging of laptop or keyboard, is sensitive to the changes of coupling capacitance, which makes the regulating of output voltage and output power difficult. Hybrid coupler has been proved to be able to combine the advantages of both inductive power transfer (IPT) and CPT. In this study, a hybrid coupler is designed with the aim of keeping a stable open‐loop gain when the coupling capacitance changes. The structure as well as the operation principles of the hybrid coupler is explained firstly. Then, the open‐loop gains of CPT, IPT and hybrid wireless power transfer (HWPT) are derived and followed by a simplified design guideline of the whole HWPT system using the proposed hybrid coupler. Finally, the proposed coupler is verified with a 40 W, 6.78 MHz experimental prototype.

A distributed phase measurement method of frequency-domain electromagnetic detection is proposed. The theoretical analysis of the previous literature shows that in electromagnetic detection applications, phase as an independent measurement parameter can reduce the diversity of solution, and it also has the advantage of greater steepness than the amplitude. Currently, there are few literature dealing with the phase in the practical application of audio electromagnetic detection, because it is difficult to obtain the phase from the separated devices. A synchronization recording system is designed based on the most widely used GPS and fiducial timestamp (FIDTS) technology. The phase errors caused by FIDTS, hardware circuit noise, and FFT spectrum leakage are analyzed. In addition, the synchronization data lost by GPS can be supplemented by interpolation. To prove the effectiveness of the method, a test platform was built in the laboratory. System test results show that the synchronization error between any two systems is less than 48 ns, and the phase error is less than 0.173° at 10 kHz. The experimental result of abnormal loop shows that the phase is more accurate than the amplitude.