Conventional low-frequency electromagnetic wave (LFEM) transmitting antennas are enormous in size, which limits the application of LFEM in underground space. Rotating-magnet Based Mechanical Antennas can realize the miniaturization of low-frequency antennas, which is expected to bring new possibilities for underground electromagnetic detection or ground penetration communication. Firstly, this paper establishes a synthetic field method for solving the electromagnetic field of a rotating permanent magnet in the air-earth cross-domain case. The method equates the magnet as an orthogonal superposition of vertical and horizontal dipole sources, and the phase difference between vertical and horizontal dipole sources is 𝜋/2. Based on the half-space theory of multilayer planar media, the quasi-analytical solution of the electromagnetic field of the magnetic dipole in the case of air-earth trans dimensionality can be found by superposition. Secondly, based on the finite element numerical model, the trans-earth propagation characteristics of the electromagnetic field excited by the rotating permanent magnet across the air-earth dual domain are studied. At the same time, the radiation characteristics in the rotating plane of the permanent magnet are also studied. Again, this paper systematically investigates the influence of parameters such as earth conductivity, emission frequency, and propagation distance on the electromagnetic field propagation. Finally, the principle prototype is developed and experiments are carried out. The results show that the finite element numerical model established in this paper is correct, the experimental results are consistent with the theoretical values, and a field strength signal of 0.9 nT can be received at 20 m. The research work in this paper lays the foundation for the future use of mechanical antennas for electromagnetic detection and ground penetration communication.

We demonstrate how several tools, such as stacking, cross correlation, digital filtering, significantly attenuate the correlated and uncorrelated noise encountered during the estimation of the earth impulse response of multi-transient electromagnetic survey. This study focuses on processing of correlated cultural noise using digital recursive filtering. We also applied this to process the data observed from the MTEM singleline survey conducted over an underground hydrocarbon reservoir.

Through-the-earth communication using low-frequency electromagnetic waves offers channel stability and significant penetration capability, rendering it promising for applications in mining and underground emergency rescue. However, the bulky size of conventional low-frequency antennas poses challenges for deployment in underground environments, complicating the two-way communication. The rotating permanent magnet mechanical antenna breaks through the so-called Chu’s limit and enables low-frequency antenna miniaturization by using a motor-driven rotation of a permanent magnet to generate the electromagnetic field. This innovation shows great promise for through-the-earth communication. However, existing research on rotating permanent magnet mechanical antennas is largely focused on principle verification in ground-based environments. This paper focuses on the communication methodologies and experimental investigations in a fully underground environment. A through-the-earth communication system including the miniaturized rotating permanent magnet mechanical antenna and the magnetic field sensor was constructed. By optimizing the size of the permanent magnet, we enhance the ratio of magnetic moment to rotational inertia, leading to improved signal strength for a given motor torque. To demodulate noisy data, a modified full-wave rectified non-coherent demodulation algorithm is developed. The effects of filtering, sampling rates, coding methods and noise on signal demodulation are also discussed. This paper demonstrates the successful transmission of text messages over a distance of 20 metres in a fully underground environment, laying a theoretical and experimental foundation for the practical application of the rotating permanent magnet antennas in through-the-earth communication.