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Guaranteeing the quality of the transmitting current under low switching frequency conditions is the crucial point in the helicopter transient electromagnetic (HTEM) system which affects the efficiency and exploration accuracy. HTEM requires high efficiency and low switching loss of the inverter power supply due to the facts that HTEM uses air-launched and air-received measurement methods, and the power storage capacity of the airborne transmitting system is limited. Paradoxically, low switching frequency directly affects the transmitting waveform quality and thereby affects the detection accuracy. In this study, we present a semiperiodic mirror symmetry selective harmonic elimination pulse width modulation (SHEPWM) based on the subsection control approach to balance transmitting current quality and switching loss. In the SHEPWM method, the semiperiodic mirror symmetry SHEPWM nonlinear equations are established by the time frequency domain information of the inverter output voltage and resolved by the artificial neural network (ANN) algorithm to attain switching time sequence of desired transmitting current. The simulation and experimental results verify the effectiveness of the SHEPWM subsection control strategy, which can reduce the switching loss while ensuring the current waveform quality and detection accuracy.

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.

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 [Formula: see text] pseudorandom series obtaining signals at [Formula: see text] 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.

Three-dimensional great deep distributed resistivity imaging instrument has been developed for gob detection. Poledipole array is selected for deep detection. Diagonal measurement method is adopted to reduce the number of data. To test the performance of the instrument, a Gob at Taochong in Anhui province was investigated using three-dimensional great deep distributed resistivity imaging instrument. The data acquisition was carried out using a roll-along technique for 3-D data acquisition. The inversion results are coincided with real situation. It shows that the detection depth of three-dimensional great deep distributed resistivity imaging instrument can reach 300m and the instrument can detect the presence of gob.

To improve the subsurface imaging capability of transient electromagnetic method, we developed a new instrument system named ATTEM. By removing the primary electromagnetic field from the total electromagnetic field in early stage of transient response, the ATTEM can obtain the pure secondary electromagnetic field, which related to the electrical properties of the subsurface geology. The meaningful improvements in ATTEM comprised two aspects: (1) ATTEM transmitter can record the turn-off current waveform to estimate the primary electromagnetic field. (2) ATTEM receiver adopted double ADCs sampling technique to enlarge the dynamic range of recording and improve the resolution of early transient response

The Vibroseis technique has been widely used in oil and gas exploration and engineering investigation. However traditional high-power vehicle based Vibroseis has such questions to be solved: complex structure; cumbersome; and low upper limiting sweep frequency (180 Hz). In this paper, a 5 kW electromagnetic driven Vibroseis system is introduced, the power supply transformation technique is discussed, double reversion circuits and multifold switching power amplifier are described. The problem of sweep bandwidth in hydraulic driven Vibroseis is solved by this new design.

A nonuniform sampling technique has been used to correct the sampling errors caused by an unstable timebase and interleaving. However, the nonuniform sampling method is intentionally used in this paper for a logarithmically changing signal to save storage space and reduce spike-like noise and Gaussian noise with a robust statistical method and variable rate stacking. The variable sampling rate, subsampling and robust stacking techniques are described. The advantages of the methods provided here are saving storage space, improving signal-to-noise ratio (SNR), reducing stacking time and minimizing spike-like and Gaussian noise effectively. The method provided is applied to the transient electromagnetic (TEM) measurement system design and the system SNR is improved significantly.