Low-frequency Electromagnetic Waves Research Articles

ELF-EM signal processing while drilling based on human-computer interaction combined algorithm

In the electromagnetic wave measurement while drilling (EM MWD), the extra low frequency electromagnetic wave (ELF-EM) below 20Hz was usually chosen as the carrier because of its transmission characteristics in the formation. However, as the drilling depth increases, the electromagnetic wave signals received on the ground gradually weaken, becoming lower than a certain signal-to-noise ratio (SNR) and making it impossible to be decoded or transmitted. The attenuation of electromagnetic wave in the formation is definitely one of the causes, but what matters more is the influence of environment noise at the well site, especially the in-band interference noise and random noise. Targeting at the out-of-band noise, the bandpass filter, which is invalid to the in-band noise, can be used to eliminate the noise out of the carrier's main band. To cope with the question, an algorithm based on the human-computer interaction detection (HCID) was proposed in this paper that improves the SNR of ELF-EM signals, with the effective transmission distance of EM MWD increased. In this paper, the validity of the proposed HCID algorithm was verified through communication processing performance simulation and field data comparison, thus providing a reference for engineers and technicians in this field. Theoretical analysis and experimental data verification show that the combined algorithm decodes effectively under the in-band interference noise of − 80dB SNR and in-band random noise of − 17dB SNR.

Magnetosonics with Landau levels in GaAs semiconductor systems

The propagation of low frequency electromagnetic magnetosonic waves in a quantum magneto electron–hole plasma is investigated. Both of the semiconductor plasma species are magnetically quantized at equilibrium. The quantization is included through recoil effect and Landau effects on applying quantum hydrodynamic model (QHD). Analytical results of quantum hydrodynamical model are then verified numerically for GaAs plasma system.

Construction of hierarchical carbon fiber Aerogel@Hollow Co9S8 polyhedron for high-performance electromagnetic wave absorption at low-frequency

The development of low-frequency electromagnetic wave absorption materials faces considerable obstacles due to intense coupling of wave impedance and electromagnetic dissipation capacity. Herein, a novel hierarchical structure composed of 3D interconnected carbon fibers aerogel loaded with hollow cobalt (Co)-sulfide polyhedrons (CFA@HCS) is designed. Typical 3D interconnected carbon fibers derived from bamboo cellulose fiber aerogel serve as a skeleton to support the Co9S8 with a hollow structure. The electromagnetic parameters of the composite are effectively modified by combining the carbon-based skeleton with the hollow Co9S8. The coupling interaction of the electromagnetic characteristics at low frequency can be significantly improved. Accordingly, the minimum reflection loss (RLmin) of CFA@HCS with 15 wt% filler loading reaches −53.6 dB at 3.36 GHz and its effective absorption bandwidth can cover almost all the S and C bands (3.04–8 GHz). In addition, the minimum reduction of the radar cross section (RCS) reaches –22.6 dB m2 when the incidence theta is −60°. This work demonstrates that low-frequency EMW absorption could be improved by constructing hierarchical composites of carbon fiber/transition metal sulfide.

Observation of Small-amplitude Electromagnetic Cyclotron Waves in the Solar Wind

Our previous studies on low-frequency electromagnetic cyclotron waves (ECWs) with amplitudes larger than 0.1 nT in the solar wind revealed that the left-handed (LH) polarized ECWs are the dominant waves, and these waves preferentially occur in plasma conditions of high proton speed (V p ), high proton temperature (T p ), low proton density (N p ). In the present study, using magnetic field and plasma data from the Wind mission between 2005 and 2015, we perform a survey of small-amplitude ECWs with amplitudes smaller than 0.1 nT. It is revealed for the first time that the small-amplitude right-handed (RH) polarized ECWs tend to frequently occur in plasmas characterized by low V p , low T p , low N p , although the small-amplitude LH ECWs still preferentially occur in plasma conditions similar to the LH ECWs with amplitudes larger than 0.1 nT. Further investigation shows that the occurrences of small-amplitude RH ECWs and long-lasting radial interplanetary magnetic field (lrIMF) share the similar preferential plasma conditions of low T p and low N p . During lrIMF events, in particular, the occurrence rates of RH and LH ECWs are comparable, with the occurrence rate of small-amplitude RH ECWs slightly larger than that of small-amplitude LH ECWs. The generation mechanism of the small-amplitude ECWs is discussed.

Finite velocity of ECG signal propagation: preliminary theory, results of a pilot experiment and consequences for medical diagnosis

A satisfactory model of the biopotentials propagating through the human body is essential for medical diagnostics, particularly for cardiovascular diseases. In our study, we develop the theory, that the propagation of biopotential of cardiac origin (ECG signal) may be treated as the propagation of low-frequency endogenous electromagnetic wave through the human body. We show that within this approach, the velocity of the ECG signal can be theoretically estimated, like for any other wave and physical medium, from the refraction index of the tissue in an appropriate frequency range. We confirm the theoretical predictions by the comparison with a direct measurement of the ECG signal propagation velocity and obtain mean velocity as low as v=1500 m/s. The results shed new light on our understanding of biopotential propagation through living tissue. This propagation depends on the frequency band of the signal and the transmittance of the tissue. This finding may improve the interpretation of the electric measurements, such as ECG and EEG when the frequency dependence of conductance and the phase shift introduced by the tissue is considered. We have shown, that the ECG propagation modifies the amplitude and phase of signal to a considerable extent. It may also improve the convergence of inverse problem in electrocardiographic imaging.

Recognition of Rebar in Ground-Penetrating Radar Data for the Second Lining of a Tunnel

Ground-penetrating radar (GPR) detects a tunnel with relatively low-frequency electromagnetic waves, which can result in poorly characterized reinforced steel bar (rebar) hyperbolas and make it challenging to recognize rebar. Thus, the rebar is located by using attenuation and reflection coefficients, and the hyperbolic feature of the rebar is reconstructed for recognition. Due to differences in the electromagnetic properties of rebar and other media in the second lining, the position of rebar in the one-dimensional time wave diagram (A-scan) is determined by using the attenuation and reflection coefficients. The rebar is recognized at the peak point in the A-scan. This peak point is in the opposite phase of the incident wave. The amplitude of these peaks is smaller than the peaks of the adjacent points. Then, these negative peak points are reconstructed on the two-dimensional scanning data (B-scan). Finally, the rebar is recognized by using the hyperbolic feature of the rebar. The method is applied in the Husa Tunnel in Yunnan Province, China. Moreover, in the mileage section YK81+506–YK81+542 of the Husa Tunnel, the hyperbolic feature of the reconstructed rebar is evident, and the rebar appears in the region of 5–7 ns in the B-scan.

In situ construction of complex spinel ferrimagnet in multi-elemental alloy for modulating natural resonance and highly efficient electromagnetic absorption

The contradiction between the demand for highly efficient low-frequency electromagnetic wave (EMW) absorption materials and the relatively poor permeability of existing absorbers has become increasingly prominent. However, the intrinsic natural resonance in magnetic material hinders the modulation and enhancement of permeability. Herein, (FeCoNiCu)xCr3−xO4 complex spinel ferrimagnets (CSFMs) are constructed in situ in multi-elemental alloy powders (MEAs) for the first time, via annealing treatment. The microstructure and composition of the CSFMs are regulated by adjusting the annealing temperature, aiming to modulate the natural resonance frequency and achieve highly efficient megahertz (MHz) electromagnetic absorption. Simultaneously, annealing treatment improves the crystal integrity, which is conducive to enhancing intergranular magnetic coupling to elevate permeability temperature stability and the Curie temperature. The minimum reflection loss value of the MEAs/CSFMs composites is less than −7 dB at 3 mm and the effective EMW absorption bandwidth maintains between 425 and 470 MHz at 5 mm from −50 to 150 °C. In addition, the MEAs/CSFMs composites also exhibit excellent oxidation resistance and corrosion resistance due to the oxide layer on the surface. This work offers a new and tunable strategy toward modulating the natural resonance frequency for efficient temperature-stable broadband EMW absorption at specific frequency bands.

ELF-EM fields in the multi-layer spherical ‘Earth-ionosphere’ model based on WKB

Abstract With a high signal-to-noise ratio and a great depth of exploration, the wireless electromagnetic method (WEM) has wide applications in the exploration of deep mineral resources and oil and gas reservoirs. Extremely low-frequency electromagnetic (ELF) waves emitted from a horizontal antenna are used to achieve synchronous acquisition for different receivers of multi-coverage information in a global region. However, previous research based on a planar model ignored the curvature of the Earth. This work focuses on the electromagnetic fields (EM fields) in the model of a spherical ‘Earth ionosphere’ to extend the coverage of WEM. By transferring the EM fields from a vertical electric dipole (VED) as well as a vertical magnetic dipole (VMD) in the multi-layered medium of the Earth, we obtain the formulae for the EM fields emitted by a horizontal electric dipole (HED) by using a reciprocity theorem. The correctness of the proposed method is verified by comparing it with the approximate analytical formula and previous work. Based on the above results, we have studied the propagation and frequency characteristics of electromagnetic fields in a spherical waveguide consisting of the ionosphere and earth. The results show that the electromagnetic fields under the spherical model produce interference effects that are different from those of the planar model. The electromagnetic response of the layered Earth was then discussed, and its potential as an electromagnetic technique for exploring the deep Earth was demonstrated.

Speech Communication System Based on Piezoelectric Electret Mechanical Antenna

Low-frequency electromagnetic waves have strong penetration and low attenuation performance, making them widely used in long-wave communication areas such as underwater communication and earthquake prediction. Mechanical antennas are seen as a promising alternative to traditional low-frequency resonant antennas, as they offer the potential for miniaturization and low power consumption. However, existing communication systems using mechanical antennas are usually rudimentary, incompatible, and inconvenient to use. To address these problems, a novel speech communication system based on a piezoelectric electret mechanical antenna was proposed. This communication system includes a hardware system, a communication protocol for mechanical antennas, and a software system with a modular structure. This system includes hardware, a communication protocol, and a modular software system. The results of the experiments showed that this communication system provides real-time speech communication with a user-friendly design, high compatibility with different communication devices, and easy customization to meet specific requirements. This design holds great potential for applications in mineral detection, seismic research, submarine communication, industrial RF control, and other related mechanical antenna applications.

Effect of a Strong Local Atmospheric Perturbation on the Propagation of Low-Frequency Electromagnetic Waves in the Earth Ionosphere

Coefficients of reflection and transmission of electromagnetic waves in a frequency interval of 1–10 kHz are numerically calculated for the passage through a flat inhomogeneous layer of the lower ionosphere in the presence of a strong local perturbation of the atmosphere. A point source of energy is used to simulate the perturbation. When the perturbation passes through the layer of the lower ionosphere, the wave transmission coefficient sharply decreases within 1–2 min while the diagonal components of the matrix of reflectioncoefficients increase to almost unity.

Effect of a Strong Local Atmospheric Perturbation on the Propagation of Low-Frequency Electromagnetic Waves in the Earth Ionosphere

Effect of a Strong Local Atmospheric Perturbation on the Propagation of Low-Frequency Electromagnetic Waves in the Earth Ionosphere

Propagation Characteristics of Extremely Low-Frequency Electromagnetic Waves in a Uniformly Infinite Polar “Sea-Sea Ice” Half-Space

Propagation Characteristics of Extremely Low-Frequency Electromagnetic Waves in a Uniformly Infinite Polar “Sea-Sea Ice” Half-Space

Особенности характеристик КНЧ-волн в многокомпонентной ионосферной плазме

We have examined the properties of low-frequency electromagnetic waves in multicomponent ionospheric plasma in the 1–30 Hz band, using the magnetoionic theory. Complex permittivity tensor components and refractive indices of normal waves (ordinary and extraordinary) were calculated at altitudes from 80 to 750 km. The calculations show that the refractive indices are highly dependent on frequency and height. Polarization of ordinary and extraordinary waves is elliptical over the entire range of the frequencies investigated. The refractive index and the polarization of normal waves are demonstrated to tend to magnetohydrodynamic (MHD) values only at frequencies lower than 1 Hz. The group velocity vector of an extraordinary wave is not directed along the magnetic field, as follows from the MHD approximation, but it lies inside a cone within ±(5–10) degrees, depending on frequency. The group velocity vector of an ordinary wave is practically independent of the angle with the geomagnetic field as in the MHD approximation. The proposed method for calculating the characteristics of normal waves in the ionosphere can be used to study ULF wave propagation from both natural and artificial ionospheric sources, which arise under the action of powerful HF radio waves in the lower and upper ionosphere.

Peculiarities of ULF wave characteristics in a multicomponent ionospheric plasma

We have examined the properties of low-frequency electromagnetic waves in multicomponent ionospheric plasma in the 1–30 Hz band, using the magnetoionic theory. Complex permittivity tensor components and refractive indices of normal waves (ordinary and extraordinary) were calculated at altitudes from 80 to 750 km. The calculations show that the refractive indices are highly dependent on frequency and height. Polarization of ordinary and extraordinary waves is elliptical over the entire range of the frequencies investigated. The refractive index and the polarization of normal waves are demonstrated to tend to magnetohydrodynamic (MHD) values only at frequencies lower than 1 Hz. The group velocity vector of an extraordinary wave is not directed along the magnetic field, as follows from the MHD approximation, but it lies inside a cone within ±(5–10) degrees, depending on frequency. The group velocity vector of an ordinary wave is practically independent of the angle with the geomagnetic field as in the MHD approximation. The proposed method for calculating the characteristics of normal waves in the ionosphere can be used to study ULF wave propagation from both natural and artificial ionospheric sources, which arise under the action of powerful HF radio waves in the lower and upper ionosphere.

Simulation study of alleviating the communication blackout using high-power microwave irradiating plasma sheaths

During hypersonic vehicle flight at high speed, plasma sheath on the vehicle surface will attenuate or even interrupt the communication signal, leading to the “communication blackout” problem. The vehicle probably moves a long distance during the communication blackout due to its high speed, which is a serious threat to the safety of the vehicle. This paper proposes a method to solve the communication blackout problem using high-power microwave (HPM) irradiation. The multicomponent compressible model, finite difference time domain algorithm, and multi-fluid model are used to simulate the interaction between HPM and plasma sheath. The results show that after HPM irradiation, the electromagnetic (EM) wave transmissivity of the plasma sheath will change, and the electric field (E-field) amplitude and irradiation time of HPM significantly influence the change of transmissivity. Thereafter, analyses of the changes of the collision and plasma frequencies of the plasma sheath after HPM irradiation showed the transmissivity of the plasma sheath to low-frequency EM waves is improved by optimizing E-field amplitude and irradiation time of HPM. Therefore, HPM irradiation can be performed to enhance the transmissivity of the plasma sheath to the communication signal, thus alleviating the communication blackout problem.

Annealed Covalent Organic Framework Thin Films for Exceptional Absorption of Ultrabroad Low-Frequency Electromagnetic Waves.

Different from harvesting of ultraviolet and visible lights via electronic transitions, absorption of low-frequency electromagnetic waves is sophisticated in mechanism and poor in efficiency, imposing the structural design arduous and challenging. Here, the first example of exploring covalent organic frameworks for highly efficient absorption of low-frequency electromagnetic waves is reported. Three pyrene frameworks are synthesized and annealed into porous networks, which upon mixture with paraffin are processed into thin films with tunable thickness. The films absorb ultrabroad low-frequency electromagnetic waves covering S, C, X, and Ku bands and achieve exceptional efficiency of 99.999% with a thickness of only 2.5mm and a loading content of only 20%. This result originates from a synergistic effect of conductivity, heteroatoms, and pores and outperforms the state-of-the-art polymers, carbons, and metals. This approach opens a way to electromagnetic wave absorption.

ARTIFICIAL INTELLIGENCE IN DETECTING ATHEROSCLEROSIS PLAQUES AND ELECTROMAGNETIC THERAPY TO ALTER CHOLESTEROL LEVEL

Introduction: Artificial Intelligence (AI) application in healthcare fields include matching patient symptoms to appropriate physician, helped to diagnose patients, determine patient’s prognosis, drug discovery, translating languages, and organize data. AI technology used in analysis and diagnosing coronary atherosclerotic plaques has increased annually. There are studies suggested that extremely low-frequency electromagnetic fields exposure can affect lipid metabolism. Purpose: To use applied AI in a device that can both identifying atherosclerosis in blood vessel and treat with low-frequency electromagnetic wave. Method: Experiments carried out using a device to detect and treat blockage of blood vessels by emitting electromagnetic signals. This tool is an AI breakthrough that not only includes a diagnostic function, but can also perform electromagnetic therapy. Result: Previous studies mentioned that AI can be applied to identify atherosclerosis. There are some studies that states low-frequency electromagnetic waves can alter HDL and cholesterol levels in blood. Larger studies are needed to combine both function in identify atherosclerosis plaques and electromagnetic therapy. Conclusion: Rapidly developing AI technology can be applied to identify atherosclerosis plaque and treat them. Further studies are required to provide definitive proof of electromagnetic wave effects on the removal of cholesterol from plaques using thermal effects or raising HDL concentrations in blood.

Phthalocyanine-mediated interfacial self-assembly of magnetic graphene nanocomposites toward low-frequency electromagnetic wave absorption

Exploring multi-component composite is promising for advanced electromagnetic wave (EMW) absorption materials, yet challenged by balancing low-frequency absorption and cost-effective preparation mode. Herein, a series of dielectric-magnetic nanocomposites are fabricated as EMW absorbers via facile phthalocyanine-mediated interfacial self-assembly approach. Phthalocyanine could act as a functional reagent to modify magnetic Fe3O4 nanosphere and guarantee the self-assembly with reduced graphene oxide nanosheet simultaneously. Owing to the flexible self-assembly, the as-synthesized ternary nanocomposite exhibits highly tunable EMW absorption performance toward low-frequency absorption. The minimum reflection loss reaches −49.0 dB (99.998 % wave absorption) simultaneously at a low frequency of 5.4 GHz and a high frequency of 8.3 GHz, which displays superb absorption efficiency in both the C band and X band. The phthalocyanine-mediated dielectric-magnetic synergy endows the nanocomposite with enhanced impedance matching and multiple dissipation pathways. This study demonstrates a convenient strategy for the rational design of low-frequency EMW absorbers with high-efficiency absorption.

Location Accuracy Improvement of Long-range Lightning Detection Network In China by Compensating Ground Wave Propagation Delay

Very low frequency (VLF) electromagnetic waves distort along the long propagation path, and that causes the arrival time of the signals measured by the long-range lightning system to be delayed. In this paper, based on the propagation correction method by compensating the peak time delay of the ground wave, the location accuracy of the long-range lightning detection network in China is greatly improved. The improvement of the relative location accuracy and location offsets are evaluated by comparing with the Advanced Direction Time Lightning Detection System (ADTD) datasets. It shows that the mean relative accuracy is improved from 7.74 km to 4.32 km, and the median relative accuracy is improved from 7.28 km to 2.46 km. The mean westwards offset of the total lightning location data drops from 2.05 km to 0.93 km, and the mean southwards offset drops from 1.19 km to 0.63 km. In addition, it is found that the location accuracy will be greatly improved if the observation site affected by the terrain is removed. The mean relative location accuracy is further improved to 4.11 km and the median to 2.32 km.

Experimental Research on a Pipeline Through-wall Communication System Based on Extremely Low Frequency

The traditional fixed-point operation robot of natural gas pipeline uses cable as the medium to realise the throughwall communication. However, with the increase of working distance, problems such as increasing cable weight or friction force with the pipeline wall will prevent the work from continuing and have a high potential risk. In order to solve this problem, this paper puts forward an overall design of the through-wall communication using the extremely low-frequency (ELF) electromagnetic wave with strong penetration ability. Firstly, a model of the solenoid transmitting coil is established using the Maxwell finite element simulation module to simulate the distribution of the extremely lowfrequency electromagnetic field. Then, the influence of different factors such as environment and medium on magnetic field intensity are analysed. With STM32F405 as the main control chip and AD9959 as the transmitting signal source, the hardware circuit of the communication system is built. At the same time, an experimental test of the ELF communication system is carried out and the experimental results show that the designed through-wall communication system can achieve good communication inside and outside the tube. The communication distance along the radial direction of the tube can reach 140 cm, which can meet the communication needs of fixed-point operation of the robot in the tube.