Propagation Velocity Of Electromagnetic Waves Research Articles

Analysis of Moving Bodies with a Direct Finite Difference Time Domain Method

This paper proposes an original and thorough analysis of the behavior of electromagnetic waves in the presence of moving bodies by using the finite difference time domain (FDTD) method. Movements are implemented by changing positions of the objects at each time step, through the classical FDTD time loop. This technique is suitable for non-relativistic speeds, thus for most encountered problems in antennas and propagation domain. The numerical aspects that need to be considered are studied. Then, different bodies in motion are examined: plane wave source with matching resistors, observation point, inclined partially reflecting surface (PRS), line source, and metallic cylinder illuminated by a plane wave. The results are compared with those of special relativity which are considered as the references. Some aspects of special relativity are present in the direct FDTD approach, such as the independence of the velocity of electromagnetic wave propagation with the speed of the source and Lorentz local time (with a different physical interpretation). It is shown that the amplitude of the electric field for a moving plane wave source does not increase with the speed of motion, if the impedance of the source is small. Moreover, for a moving scattering metallic wire, one can observe a phenomenon similar to shock waves.

Ground Penetrating Radar Survey of Arctic Polygonal Wedge Structures

Ground Penetrating Radar Survey of Arctic Polygonal Wedge Structures

Research on Underground Pipeline Detection Method Based on Background Matrix Subtraction and Three-Dimensional Velocity Spectrum

Ground-penetrating radar (GPR) can detect urban underground pipelines and image their spatial distribution. However, due to the interference of direct wave and ground reflected wave in radar profile, the detection accuracy of underground pipeline depth is low. In order to improve the reliability and accuracy of the interpretation of ground penetrating radar data, it is necessary to suppress the noise, and then to detect underground pipelines. Firstly, on the basis of data collection, a background matrix subtraction (BMS) method is proposed to suppress noise signals, and the Noise reduction effect is compared and analyzed by two groups of simulation data and two groups of measured data examples with the method of reducing average channel and singular decomposition. Then, a three-dimensional velocity spectrum (3DVS) method is proposed to estimate the buried depth of underground pipeline in radar profile, and the propagation velocity of electromagnetic wave in underground media is calculated by automatically scanning the hyperbolic reflection signal. The estimated velocity is used to carry out back-propagation migration (BPM) processing on the ground penetrating radar profile, and the underground pipeline is accurately detected. Finally, the underground pipeline detection method based on BMS and 3DVS is applied to a residential area in Nanjing, Jiangsu, China. The detection results show that the effect of BMS is obviously better than that of mean-reducing method and singular decomposition method, which can suppress the noise well on the basis of ensuring that the effective signal is not lost. It is helpful to identify the characteristics of target signal in ground penetrating radar profile, and improve the accuracy and reliability of data interpretation. The error between the velocity value estimated by the 3DVS method and the real value is less than 3.8%, and the buried depth error of pipeline target in the obtained data is 1.4%, which indicates that the algorithm has practical application value and can realize the accurate positioning of underground pipelines.

Wavelet Entropy-Based Method for Migration Imaging of Hidden Microcracks by Using the Optimal Wave Velocity

Exploring the shape and direction of hidden cracks in a tunnel lining structure is one of the main objectives of ground penetrating radar (GPR) map interpretation. The most important factor that restricts the migration imaging of hidden cracks is the propagation velocity of electromagnetic waves. Determining the optimal electromagnetic wave velocity is the key to truthfully restoring the actual shape of hidden cracks. To study the GPR characteristic response signals of hidden cracks, forward simulation and model experiments of different cracks were performed. Subsequently, a method to determine the optimal electromagnetic wave velocity based on the wavelet entropy theory was proposed, and the frequency wavenumber domain migration (F-K) and Kirchhoff integral migration imaging method were combined. Horizontal, S-type, and inclined hidden fractures were examined by migration imaging. The results show that the radar characteristic response images of different cracks can be simulated forward by using the finite difference time domain method to write the fracture model instruction. Based on the wavelet entropy theory, the error range between the estimated value and true value was controlled within 4%. Taking the optimal electromagnetic wave velocity as the velocity parameter of the conventional migration method can make the migration more effective and suppress the interference of echo signals so that the diffraction wave converges, and the energy is more concentrated; thus, the real fracture morphology can be restored to the greatest extent. The research results can provide technical support for the fine detection of hidden quality defects in tunnel lining structures by GPR mapping.

Terahertz slow-wave scalable interconnect based on back-end-of-line (BEOL) in 40 nm CMOS

This paper proposes a terahertz (THz) slow-wave scalable interconnect based on multilayered back-end-of-lines in a complementary metal-oxide-semi-conductor process. Capacitances are realized beside and below the signal line. The ground plane is slotted to significantly reduce the propagation velocity of electromagnetic waves and improve the quality factor. Compared with a conventional microstrip line, the proposed slow-wave interconnect not only realizes a slow-wave factor up to 1.96 but also achieves a quality factor higher than 20 at 0.14–0.15 THz. In addition, a one-step data processing method is proposed to directly calculate the performance metrics of interconnects by using the measured S-parameters of device-under-test and the Thru calibration kit. For millimeter-wave and THz chips, the proposed slow-wave interconnect is a promising candidate to realize on-chip passive components with increased quality factor as well as reduced footprint and loss.

Hyperbola Detection with RetinaNet and Comparison of Hyperbola Fitting Methods in GPR Data from an Archaeological Site

Hyperbolic diffractions in Ground Penetrating Radar (GPR) data are caused by a variety of subsurface objects such as pipes, stones, or archaeological artifacts. Supplementary to their location, the propagation velocity of electromagnetic waves in the subsurface can be derived. In recent years, it was shown that deep learning tools can automatically detect hyperbola in radargrams using data measured over urban infrastructure, which are relatively clear. In contrast, in this study, we used an archaeological dataset with diverse underground structures. In the first step we used the deep learning network RetinaNet to detect hyperbola automatically and achieved an average precision of 0.58. In the next step, 10 different approaches for hyperbola fitting and thus velocity determination were applied. The derived information was validated with manually determined velocities and apex points. It was shown that hyperbola extraction by using a threshold and a column connection clustering (C3) algorithm followed by simple hyperbola fitting is the best method, which had a mean velocity error of 0.021 m/ns compared to manual determination. The average 1D velocity-depth distribution derived in 10 ns intervals was in shape comparable to the manually determined one, but had a systematic shift of about 0.01 m/ns towards higher velocities.

Electromagnetic wave forward modeling of coal-gangue mixed model in top coal caving mining face

The automatic control of top coal caving is of great significance to realize intelligent coal mining. In the process of top coal caving, a coal-gangue mixed area containing coal, gangue, and air is formed at the tail beam of the hydraulic support, which has different electromagnetic parameters, different volumes, and different shapes. To explore the transmission characteristics of electromagnetic wave in coal-gangue mixed model and the influence of different gangue ratios on electromagnetic wave propagation, the coal-gangue mixed model is established based on the random medium theory. Some electromagnetic forward modeling is carried out with different coal-gangue granularities, electromagnetic parameters, and gangue ratios based on finite-difference time-domain (FDTD) and finite-integration time-domain (FITD) methods. The results show that different granularities of coal and gangue will affect the amplitude of electromagnetic wave time-domain waveform. Under the same particle size, the equivalent electromagnetic parameters in the coal-gangue mixed medium will be larger with higher gangue ratio. Furthermore, the difference of transmitted wave signals between different gangue ratios will be larger with higher electromagnetic parameters difference of the coal and gangue. For higher refractive index, the propagation velocity of electromagnetic wave in the medium and the transmitted wave amplitude will be smaller. In addition, the comparison results illustrate that the rules of electromagnetic wave propagation obtained by FDTD and FITD methods are basically the same, which verifies the correctness of the simulations in this paper. The simulation results can lay a theoretical foundation for identifying the coal-gangue mixed degree in the process of top coal caving.

Two-Dimensional Imaging of FMCW Ice Sounding Radar Data via the Modified Frequency Scaling Algorithm

In this paper, we propose a novel processing algorithm for FMCW ice-sounding radar which addresses the along-track focusing of internal reflecting horizons. It is a modified frequency scaling algorithm that can handle the two-layer medium imaging model as well as the Doppler impact of the motion within the sweep in a nadir-looking radar. The proposed algorithm’s comprehensive derivation and implementation processes are described. In the range-Doppler domain, a really effective formulation for radar signals is also presented, which takes into account refraction effects and electromagnetic wave propagation velocity changes at the interface of two different mediums. Point target simulations are carried out to demonstrate the algorithm’s performance. The proposed method is also tested on the real data acquired by the Shallow-Layers-Detection Ice Sounding Radar (SLDISR). The result shows the applicability of the proposed method for imaging the ice sheet.

Experimental Investigation of Surge Propagation Characteristics on a Substation Grounding System

With the correct choice of the arrester by voltage class and compliance with the calculated protective distance without taking into account the propagation velocity of the current wave on the grounding grid, overvoltages exceeding discharge or residual voltage may occur on the protected equipment, in particular the transformer. Thus, when calculating the installation of the arrester that protects the substation from incoming lightning surges from a transmission lines, it is necessary to take into account the propagation of the current wave on the grounding grid. The propagation velocity of electromagnetic waves in a 150 kV substations grounding grid was measured. The measured wave propagation velocities are in the range of 50–100⋅106 m/s. Thus, the obtained velocity of wave propagation on the grounding grid used in service is several times less than the speed of light. The measured value correlates well with similar experiments conducted for buried conductors located in soils with similar parameters and the results of mathematical modeling for a grounding grid having similar dimensions and mesh size.

The Speed Electromagnetic Wave Propagation in the Snow-Ice Underlying Surface

The results calculations the electromagnetic wave propagation velocity in the snow-ice cover depending on the density, the proportion liquid water content, and the propagation speeds the electromagnetic wave in dry snow, dry firn, and dry ice vary very markedly depending on the proportion liquid water content, the preferred orientation, and the shape ice and air structure are presented. The inclusions in the snow. The performed estimates the complex relative permittivity the medium that determines the speed propagation electromagnetic waves show a noticeable influence the density, the proportion liquid water content and the structure the underlying surface (snow, firn, ice), which allows identifying the layers the underlying surface in order to remotely determine the possibility landing a helicopter-type aircraft on an unprepared site with snow-ice cover. Shown, when the portion the water content in the medium is equal to zero, which is typical for negative temperatures, the speed propagation electromagnetic waves in the medium will depend on the density the medium and structure the dry ice in a small range of 1 m/μs temperature. In dry snow, vertically and horizontally elongated or spherical inclusions make a significant contribution to the change in the speed propagation the electromagnetic wave. At zero temperature, in the frequency range of 2 ... 8 GHz, the share water content in the medium, the density and structure the medium will play a determining role in the speed propagation an electromagnetic wave in the medium. The purpose this article is to determine the change ranges speed propagation electromagnetic waves in snow-ice the underlying surface depending on the density, structure, water content to restore the structure the snow and ice according to radar sensing, a more accurate determination the depth snow and thickness ice cover used in the assessment the possibility the safe landing an aircraft the helicopter type on an unprepared ground with snow-ice cover

High-resolution GPR survey for masonry wall diagnostics

Ground-penetrating Radar (GPR) is a non-invasive technique with increasing focus on civil-engineering applications. Referring to existing constructions, the most recent International and National Codes suggest a knowledge-based approach, which is mandatory before any structural assessment or design of interventions. The knowledge-based process includes experimental investigations aimed at the detailed geometrical and structural relief as well as to the evaluation of the materials mechanical properties. However, when dealing with built Cultural Heritage, destructive techniques should be limited or even forbidden. In this context, non-destructive investigations play a key role and their effectiveness should be further studied in relation to specific applications. To this end, Ground-penetrating Radar surveys were performed in the test site of the Laboratory of Applied Geophysics, the University of Salento, Lecce (Italy), with the aim of simulating various real-life practical applications. In particular, several objects of different materials and geometries were buried in the subsurface and a small building was built in the area using different construction techniques, among them, multi-leaf masonry walls, also called sack masonry walls, were constructed. The GPR method can detect both the presence of hidden bodies and different leafs of the walls with a relative efficiency depending on the field context, the dielectric properties of the host material, and the nature and size of the bodies. In this work, a test was planned to verify 900 and 2000 MHz antenna resolutions. The data acquired with the 2000 MHz antenna were used to estimate the mean electromagnetic wave propagation velocity in the sack masonry layers of known thickness and to understand if the inner core was filled with material other than air. The authors propose a high-resolution method to improve the velocity estimation using geometrical optics laws and the sign of the reflection coefficient in order to properly select the arrivals from different interfaces.

Estimation of Moisture Content in Railway Subgrade by Ground Penetrating Radar

China is strongly dependent on railway transportation, but the frost heaving of the subgrade in cold regions has seriously affected the safety and comfort of trains. Moisture content is an essential parameter in the subgrade frost heave. Non-destructive and efficient geophysical methods have great potential in measuring the moisture content of railway subgrade. In this paper, we use the common mid-point (CMP) measurement of ground penetrating radar (GPR) to estimate the propagation velocity of electromagnetic waves in a subgrade application. We establish a synthetic model to simulate the railway subgrade structure. The synthetic CMP gathers acquired from shallow and thin layers are seriously disturbed by multiple waves and refraction waves, which make the routine velocity analysis unable to provide accurate velocities. Through the analysis of numerical simulation results, it is found that the primary reflection waves, multiple waves, and refraction waves are dominant in different offset ranges of CMP gather. Therefore, we propose a solution of the optimal gather at a certain range of offset dominated by the primary reflection wave to calculate the velocity spectrum and extract the accurate velocities for the subgrade model. The relative dielectric constants of the corresponding layers are calculated after the stacking velocities are converted into the interval velocities. Then, the moisture content is obtained by the Topp formula, which expresses the relationship between dielectric constant and moisture content. Finally, we apply the optimal gather scheme and the above interpretation process to the GPR data acquired at the railway site, and we form a long moisture content profile of the railway subgrade. Compared with the polarizability measured by the induced polarization (IP) method, it is found that the regions with high moisture content correspond to polarizability anomalies with different strengths. The comparison shows the reliability of GPR results to some extent.

Evaluation of the GPR (1.2 GHz) technique in the characterization of masonry shells of the Theatro Municipal do Rio de Janeiro

Abstract The geometric characterization and evaluation of structural integrity are challenges for the preservation of historical buildings. The challenges are associated to the lack of constructive records, material diversity, access and contact restrictions, and sample extraction. Non-destructive tests are indicated in these cases. The objective of this paper is to evaluate the Ground Penetrating Radar (GPR) technique with 1.2 GHz antenna in the study of the determination of material thickness and discontinuities in the masonry shells of the Theatro Municipal do Rio de Janeiro, a heritage built in early 20th century, with high historical and cultural value. With paintings by renowned artists on their intrados, the domes and vault of brick masonry were at risk of collapse in the 1970s due to differential settlements in the supports and moisture, compromising their paintings. A strengthening intervention was carried out in 1976. Considering the masonry of bricks the main component, the electromagnetic wave propagation velocity was determined as 0,15 m/ns in one of the support walls, allowing to estimate the thicknesses of the elements ranging between 20 and 31 cm. Materials incorporated into the structure were identified: reinforcement, metal inserts and bricks. However, the frequency of the antenna did not allow the detection of damages. The application of the GPR to the characterization of materials and investigation of the integrity of the domes of the Theatro Municipal highlights the potential of this technology to study heritage and other constructions.

An imaging method for Chang'e−5 Lunar Regolith Penetrating Radar

The Lunar Regolith Penetrating Radar (LRPR) is an instrument that will be carried by the Chang'e-5 (CE-5) lander to investigate lunar regolith. In this paper, a specific imaging method is developed to process LRPR data obtained in ground tests. We measure and subtract the ‘background’ from test data to reduce signal clutter, and apply the envelope velocity spectrum to estimate the propagation velocity of electromagnetic waves in regolith simulant. Then a modified pre-stack migration using Hilbert transformed signals is proposed for subsurface imaging. In this method, media absorption, spherical spread loss, and Snell's law are also taken into consideration. The modified migration clearly images the granite slates buried within 2.5m and produces cleaner results than the conventional method. The introduced algorithms prove to be robust and applicable to LRPR data.

Strong coupling of topological edge states enabling group-dispersionless slow light in magneto-optical photonic crystals

We investigate a two-way waveguide constructed by bringing close two identical magneto-optical photonic crystals which each supports a counterpropagating topological one-way edge state. We find a unique slow-light state that exhibits a very small group velocity $({n}_{g}=529.2)$, a zero group-velocity dispersion, distortionless pulse transport, and ease to be magnetically tuned within a broad frequency range, simultaneously. We show that this unique group-dispersionless slow-light state originates from the strong coupling of the two counterpropagating topological states, which causes complete exchange and transfer of energy flow between them, induces a vortexlike close loop of energy transport channel around the source, leads to complete destructive interference in the far field, and finally results in a very slow propagation velocity of electromagnetic waves. These results indicate that deeply digging into the interaction between photonic topological states can help to open up an arena in topological physics and find great potential of applications such as optical buffer memories and enhanced light-matter interaction.

High-Resolution Velocity Analysis Method Using the ℓ-1 Norm Regularized Least-Squares Method for Pavement Inspection

We propose a high-resolution velocity analysis method to estimate the electromagnetic wave propagation velocity in subsurface medium. The estimation is achieved by applying the l-1 norm regularized least-squares method to the conventional common-midpoint (CMP) velocity analysis algorithm. The proposed method can provide not only higher resolution than the conventional velocity analysis method, but can also be applied with a coarse sampling array system, such as our array ground penetrating radar YAKUMO, which returns eight CMP traces within a two meter width. The main purpose of this approach is for precise pavement inspection at shallow depths. We applied this method to both a simulated dataset and real data acquired by YAKUMO at a model airport taxiway to detect the slight velocity changes caused by millimeter-thin cracks filled with air or water within the 15 cm-thick asphalt pavement. In both cases slight velocity changes of about 0.005 m/ns can be detected, and the difference between air- and water-filled cracks can be distinguished. Also, this method is applied to a data acquired at airport taxi-way, the damaged parts are detected successfully and shows good agreement with the corning results. The results indicate that the proposed method is effective for pavement inspection, especially in the presence of thin cracks that cannot be seen directly with the reflected signal.

In brief

PAGE 524 Researchers from South Korea have developed a walking speed estimation and treadmill control algorithm for a user-driven treadmill. Data is gathered as the user walks on the treadmill wearing insole pressure sensors. The correlation between walking speed and centre of pressure was investigated to develop a feedforward scheme, ensuring speed control of the treadmill. The user-driven treadmill has applications in training, rehabilitation, sports and entertainment. PAGE 520 A microstrip antenna array (MAA) with metasurface has been designed and fabricated for ultra-wideband radar cross-section (RCS) reduction, whilst preserving its radiation characteristics. The research from China uses size-adjustable unit cells to achieve different reflection phase distributions. The proposed MAA can reduce the RCS in the ultra-wide frequency band 6.2–27.3 GHz for both x and y polarisations. Estimation of walking speed and treadmill control using data from insole pressure sensors. PAGE 538 Researchers from China mounted a dielectric lens on the topside of a silicon imaging chip to improve radiation patterns of an on-chip patch antenna. The integrated lens causes the main beam of radiation patterns to become narrower, thus achieving a finer spatial resolution for imaging applications. The technology can be applied in THz imaging. Microstrip antenna array with metasurface designed for ultra-wideband RCS. PAGE 546 A sixteenth-mode substrate integrated waveguide (SMSIW) bandpass filter loaded with complementary split-ring resonator (CSRR) is proposed by researchers from India. The SMSIW filter operates at 2.45 GHz and has a bandwidth of 8.2%. It is compact, occupying 6.25% of a conventional integrated waveguide. It is also simple, has low-loss and is easy to fabricate. Lens placed on topside of imaging chip improves radiation patterns of antenna. PAGE 568 An autofocusing imaging method for stepped frequency continuous wave ground penetrating radar (SFCW-GPR) is presented in this Letter from China. The method overcomes the uncertainty of electromagnetic wave propagation velocity in the subsurface medium, by using the sequential L0 and L2 norms minimisation technique. Compact, simple, low-loss and easy to fabricate bandpass filter. Autofocusing imaging method for SFCW-GPR.

A novel CFS‐PML boundary condition for transient electromagnetic simulation using a fictitious wave domain method

AbstractIn this study, we apply fictitious wave domain (FWD) methods, based on the correspondence principle for the wave and diffusion fields, to finite difference (FD) modeling of transient electromagnetic (TEM) diffusion problems for geophysical applications. A novel complex frequency shifted perfectly matched layer (PML) boundary condition is adapted to the FWD to truncate the computational domain, with the maximum electromagnetic wave propagation velocity in the FWD used to set the absorbing parameters for the boundary layers. Using domains of varying spatial extent we demonstrate that these boundary conditions offer significant improvements over simpler PML approaches, which can result in spurious reflections and large errors in the FWD solutions, especially for low frequencies and late times. In our development, resistive air layers are directly included in the FWD, allowing simulation of TEM responses in the presence of topography, as is commonly encountered in geophysical applications. We compare responses obtained by our new FD‐FWD approach and with the spectral Lanczos decomposition method on 3‐D resistivity models of varying complexity. The comparisons demonstrate that our absorbing boundary condition in FWD for the TEM diffusion problems works well even in complex high‐contrast conductivity models.

Variable phase propagation velocity for long‐range lightning location system

AbstractThe electromagnetic wave propagation velocity at low radio frequencies is an important input parameter for lightning location systems that use time of arrival (TOA) method. This velocity is normally fixed at or near the speed of light. However, this study finds that the radio waves from two submarine communication transmitters at 20.9 kHz and 23.4 kHz exhibit phase propagation velocities that are ~0.51% slower and ~0.64% faster than the speed of light as a result of sky wave contributions and ground effects. Therefore, a novel technique with a variable phase propagation velocity is implemented for the first time in the TOA method and applied to electric field recordings with a long‐baseline lightning location system that consists of four radio receivers in western Europe. The lightning locations inferred from variable velocities improve the accuracy of locations inferred from a fixed velocity by ~0.89–1.06 km when compared to the lightning locations reported by the UK MetOffice. The normal distributions of the observed phase propagation velocities in small geographic areas are not centered at the speed of light. Consequently, representative velocities can be calculated for many small geographic areas to produce a velocity map over central France where numerous lightning discharges occurred. This map reflects the impact of sky waves and ground effects on the calculation of lightning locations as a result of the network configuration. It is concluded that the use of variable phase propagation velocities mitigates the influence of sky waves and ground effects in long‐range lightning location networks.

Real and Imaginary Properties of Epsilon-Near-Zero Materials.

From the fundamental principle of causality we show that epsilon-near-zero (ENZ) materials with a very low (asymptotically zero) intrinsic dielectric loss do necessarily possess a very low (asymptotically zero) group velocity of electromagnetic wave propagation. This leads to the loss function being singular and causes high nonradiative damping of the optical resonators and emitters (plasmonic nanoparticles, quantum dots, chromophore molecules) embedded into them or placed at their surfaces. Rough ENZ surfaces do not exhibit hot spots of local fields suggesting that surface modes are overdamped. Reflectors and waveguides also show very large losses both for realistic and idealized ENZ materials.