Half-space Environment Research Articles

Application Comparison of Different Transient Electromagnetic Devices in Water Exploration of Tunnel Advanced Geological Prediction

Detecting the possibility of water inrush in front of tunnel face is always the key of tunnel advanced geological prediction. Because of its sensitivity to groundwater, transient electromagnetic method is gradually being used in water exploration of tunnel advanced geological prediction. However, the current transient electromagnetic theory is established based on the half-space environment. In order to adapt to the detection work in the full-space environment such as tunnels, different transient electromagnetic devices have been improved. The effectiveness of these improved devices still need to be tested in actual engineering projects. In this paper, a tunnel under construction is used as the engineering background, and different transient electromagnetic devices are placed in front of the same tunnel face for detection. The detection results are compared and analyzed with the actual situation of tunnel excavation, which can provide reference for water exploration of tunnel advanced geological prediction.

High-Power Electromagnetic Pulse Effect Prediction for Vehicles Based on Convolutional Neural Network

This study presents a prediction model for high-power electromagnetic pulse (HPEMP) effects on aboveground vehicles based on convolutional neural networks (CNNs). Since a vehicle is often located aboveground and is close to the air-ground–half-space interface, the electromagnetic energy coupled into the vehicle by the ground reflected waves cannot be ignored. Consequently, the analysis of the vehicle’s HPEMP effect is a composite electromagnetic scattering problem of the half-space and the vehicles above it, which is often analyzed using different half-space numerical methods. However, traditional numerical methods are often limited by the complexity of the actual half-space models and the high computational demands of complex targets. In this study, a prediction method is proposed based on a CNN, which can analyze the electric field and energy density under different incident conditions and half-space environments. Compared with the half-space finite-difference time-domain (FDTD) method, the accuracy of the prediction results was above 98% after completing the training of the CNN network, which proves the correctness and effectiveness of the method. In summary, the CNN prediction model in this study can provide a reference for evaluating the HPEMP effect on the target over a complex half-space medium.

Substrate-mediated lattice Kerker effect in Al metasurfaces

Surface lattice resonances (SLRs) emerging in regular arrays of plasmonic nanoparticles (NPs) are known to be exceptionally sensitive to the homogeneity of the environment. It is considered necessary to have a homogeneous environment for engineering narrowband SLRs, while in a half-space environment, SLRs rapidly vanish as the contrast between the refractive indices of the substrate and superstrate increases. From this conventional wisdom, it is apparent that the delicate lattice Kerker effect emerging from SLRs and resonances on constituent NPs should be difficult to achieve in a non-homogeneous environment. Using a rigorous theoretical treatment with multipolar decomposition, we surprisingly find and explain a narrowband substrate-mediated lattice Kerker effect in two-dimensional arrays of Al nanocylinders in a half-space geometry. We propose to use this effect for sensing applications and demonstrate its broad tunability across the UV/Vis wavelength range.

An Accurate Model for the Efficient Simulation of Electromagnetic Scattering From an Object Above a Rough Surface With Infinite Extent

To accurately simulate the electromagnetic (EM) scattering from an electrically large and complex object above a rough surface, we propose a new scattering model that can effectively capture the EM scattering phenomenon including the coupling between the object, the rough surface, and the infinite environment. Specifically, the proposed scattering model consists of the object and a representative of the underlying rough surface residing in a half-space environment. To make the model consistent with the real scenario and the simulation fast yet with reasonable accuracy, we apply the hybrid shooting and bouncing rays-physical optics (SBR-PO) method to model the interaction between the object and its representative underlying rough surface, and the far-field half-space dyadic Green's functions to account for the effect of an infinite half-space environment. Compared with existing scattering models, this new scattering model can emulate the interaction between the object and the infinite half-space environment more accurately. In addition, graphics processing units (GPUs) are used to accelerate the intensive computations of the prediction process. The numerical results obtained validate the proposed method and its implementation in terms of accuracy, truncation stability, and runtime performance.

Efficient implementation of superdirective beamforming in a half-space environment

In this paper, the case considered is a planar microphone array placed in front of a wall of the room so that the microphone array plane is perpendicular to that of the wall. For this arrangement, a so-called half-space propagation model has been recently proposed, which accounts for the joint contribution of the direct path and the earliest reflection introduced by the adjacent wall. Based on this propagation model, a numerical process to estimate a model of the diffuse noise spatial coherence, which accounts for the presence of the reflecting surface, is proposed. The suggested noise covariance model is used in order to extend the superdirective beamformer in half-space, achieving notable improvements in performance in comparison to a more typical implementation that involves the spherical isotropic coherence model.

Interaction of electromagnetic waves and complicated targets/environment: theory, methods, and applications}{Interaction of electromagnetic waves and complicated targets/environment: theory, methods, and applications

With the rapid development of computational electromagnetic theories and methods, it is now possible to study accurately and quantitatively the basic physical principle and rules of interaction between electromagnetic waves and complicated targets/environment through numerical methods. Such numerical modeling and theoretical prediction can lay a solid foundation and inspire innovative ideas for the research and development of new technologies in the fields of target detection and identification in a complex environment, exploration of underground resources, information acquisition in a ground-air or ocean-air environment, electromagnetic stealth design, electromagnetic countermeasures, etc. The study and application of accurate and efficient numerical modeling tools will remarkably promote the research and development of relevant technologies. In this article, we first review the research background and scientific problems in the interaction of electromagnetic waves and targets/environment. Next, we focus on the introduction of theoretical modeling and numerical methods for the composite scattering of targets in a half-space environment, the state-of-the-art development of half-space Greens function and its efficient computation, integral equation methods, and fast algorithms. Finally, we propose several key research topics and areas for the interaction of electromagnetic waves and targets/environment, including composite scattering model and numerical methods for targets in a high-contrast half space; highly efficient analysis of electrically large targets in a half space; approximation models for analyzing composite scattering; accuracy study and comparison of full-wave numerical and high-frequency or ray methods in a half space; combination of stochastic time-varying model of the rough ground/sea surface and the deterministic half-space model, rules and adaptivity (for example, the differential RCS of targets in a half space and the random scattering of rough surfaces as additive noise); numerical models of composite scattering in a half space for sensor applications; spatial selection and adaptive focusing in the low-frequency near-field detection in a layered medium; and verification, validation, and evaluation of numerical models in a large-scale electromagnetic computation.

Generalized Wideband Harmonic Imaging of Nonlinearly Loaded Scatterers

Wideband electromagnetic sensing and imaging of nonlinearly loaded scatterers is considered. Harmonic scattering theory is first presented, and then a generalized near-field, direct imaging functional is proposed for free-space and near-ground target localization within the context of forward-looking radar standoff detection exploiting sequential single-tone excitation. The developed scattering and imaging analysis framework is illustrated for point-like and extended targets through numerical experiments performed with a hybrid method-of-moments solver, in conjunction with a harmonic balance approach and an asymptotic field propagation technique. The steady-state harmonic scattering responses are examined in the time, frequency, and image domains for scatterers in free-space and half-space environments, and accurate target localization is demonstrated in all cases for each harmonic order considered.

Scattering and Imaging of Nonlinearly Loaded Antenna Structures in Half-Space Environments

The electromagnetic scattering responses of nonlinearly loaded antenna structures excited by single-tone or multi-tone incident fields are considered in the frequency domain by employing a combination of the method-of-moments and a harmonic balance technique. Subsequently, standoff detection and localization of the scatterers in the presence of a half space is demonstrated with a subspace imaging procedure by exploiting the harmonic scattering responses.

Parallelization of Half-Space MLFMA Using Adaptive Direction Partitioning Strategy

Parallelization of multilevel fast multipole algorithm for scattering analysis of targets situated in a half-space environment is presented. Due to the introduction of image sources to deal with the effects of far interface interactions, the communication traffic is nearly doubled compared to that in the free-space case. An adaptive direction partitioning strategy with improved load balance is developed. The number of plane-wave direction partitions at a given tree level varies with tree node descendants to make the algorithm suitable for both uniform and nonuniform tree levels and efficient to utilize modern CPUs with 8, 10, 12, or other numbers of cores. Both weak and strong scalability of the algorithm is investigated, and numerical results show that the algorithm is very efficient using up to 2048 CPU cores.

Optimization of the array mirror for time reversal techniques used in a half-space environment

Time reversal (TR) utilizes an array of transducers, a time reversal mirror (TRM), to locate sources. Here TR is applied to simple sources using steady-state waveforms in a numerical, point source model in a half-space environment. It is found that TR can effectively localize a simple source broadcasting a continuous wave, depending on the angular spacing. Furthermore, the angular spacing and the aperture of the TRM are the most important parameters when creating a setup of receivers for imaging a source. This work optimizes a TRM when the source's location is known within a region of certainty.

Efficient preconditioner for the finite-element boundary integral analysis of electromagnetic scattering in the half space

In this study, the finite-element boundary integral (FE-BI) method is applied to analyse the electromagnetic scattering from arbitrary objects in the half-space environment. Combined with the real image method to approximate the half-space Green's function, the half-space FE-BI can be extended for electrically large problems by the fast multipole method. An efficient preconditioner is constructed with both the finite-element matrix and the near-field part of the boundary integral equation operator for the FE-BI analysis. The numerical examples demonstrate that this preconditioner is efficient for electromagnetic scattering in the half space.

Shallow Inductive Electric Field Response Measured with Capacitive Sensors

SummaryModelling software developed and results from a prototype instrument shows that a new Capacitive Array Resistivity with Inductive Source (CARIS) method being developed has potential for detecting both conductive and resistive objects and near surface conductivity contrasts. Detecting buried resistive objects has possible application in near surface exploration. Applications could include identification and mapping of chromite and mineral containing quartz veins as well as alteration, silicification. It also shows potential for other near surface applications such as UXO, archaeology, void detection, pipe delineation, or fracture detection. A prototype 100kHz CARIS instrument has been designed and built. The prototype CARIS instrument has been tested with highly repetitive results under laboratory conditions, showing good comparison with expected results from modelling. Results have shown the ability of the system to reliably detect resistive objects within a conductive uniform half-space (salt water) environment. The CARIS system has also undergone preliminary testing in the field. Initial results from field testing show high repeatability but also high lateral variability. This appears to be due to sensitivity to near surface moisture and soil consolidation contrasts. Currently a larger second prototype operating at 5kHz is in production aimed at reducing the sensitivity to soil effects moisture and consolidation effects and increasing the depth of investigation.

Fast solution of mixed-potential time-domain integral equations for half-space environments

A fast Fourier transform-accelerated integral-equation based algorithm to efficiently analyze transient scattering from planar perfect electrically conducting objects residing above or inside a potentially lossy dielectric half-space is presented. The algorithm requires O(N/sub t/N/sub s/(logN/sub s/+log/sup 2/N/sub t/)) CPU and O(N/sub t/N/sub s/) memory resources when analyzing electromagnetic wave interactions with uniformly meshed planar structures. Here, N/sub t/ and N/sub s/ are the numbers of simulation time steps and spatial unknowns, respectively. The proposed scheme is therefore far more efficient than classical time-marching solvers, the CPU and memory requirements of which scale as O(N/sub t//sup 2/N/sub s//sup 2/) and O(N/sub t/N/sub s//sup 2/). In the proposed scheme, all pertinent time-domain half-space Green functions are (pre) computed from their frequency-domain counterparts via inverse discrete Fourier transformation. In this process, in-band aliasing is avoided through the application of a smooth and interpolatory window. Numerical results demonstrate the accuracy and efficiency of the proposed algorithm.

Application of Haar-wavelet-based multiresolution time-domain schemes to electromagnetic scattering problems

The multiresolution time-domain (MRTD) algorithm is applied to the problem of general two-dimensional electromagnetic scattering. A Haar wavelet expansion is utilized. A parallel between Haar MRTD and the classic Yee finite-difference time-domain (FDTD) algorithm is discussed, and results of simulations on canonical targets are shown for comparison. We focus on the incident-field implementation, which, in our case, consists of a pulsed plane wave. Also, we consider scattering in a half-space environment, with application to subsurface sensing. The results illustrate the advantage of the Haar MRTD method as compared with the classic FDTD, which consists of reduced memory and execution time requirements, without sacrificing accuracy.

Fast multipole method for scattering from 3-D PEC targets situated in a half-space environment

The fast multipole method (FMM) is extended to the problem of an arbitrary, three-dimensional perfect conductor situated above or below a lossy, dielectric half space. The interactions between basis and testing functions within an FMM cluster, and for nearby clusters, are handled via the rigorous dyadic Green's function, with the latter evaluated efficiently using the complex-image technique. Intercluster interactions are modeled as in the free-space FMM, with the dyadic Green's function approximated via real images and equivalent reflection coefficients; these approximations have proven to be highly accurate. Example results are presented for a large trihedral fiducial target, in free space and above a lossy, dispersive half space, with comparisons presented between the FMM and a rigorous method-of-moments (MoM) solution. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 21: 399–405, 1999.

Plane-wave expansion for arrays of arbitrarily oriented piecewise linear elements and its application in determining the impedance of a single linear antenna in a lossy half-space

An infinite array of arbitrarily oriented identical elements with arbitrary identical currents is considered. The field from this array is expanded into plane inhomogeneous waves, and the mutual impedance between the array elements and an exterior arbitrarily oriented element is derived. The formulation is particularly useful when the array is located adjacent to a dielectric interface. Numerical examples are given and the relationship to earlier formulations pointed out. It is further shown that the impedance of a single element can be obtained as the average of the scan impedance taken over the entire hemisphere (called the array scanning method (ASM)). This technique has a clear physical interpretation which greatly facilitates its uses, which include the moment method solutions of wire antennas as applied to the Sommerfeld integral. Numerical evaluation is straightforward when the dipole is in the lossy half-space, and the utility of the method is demonstrated by the presentation of results for the input impedance of dipoles in a variety of half-space environments. Solution is by Galerkin's method with a piecewise sinusoidal expansion for the current. Computer time is proportional to d^{-1} , where d is the distance of the dipole to the interface. For conducting media and low frequencies an approximation is made to reduce computation time. The moment method solution of a dipole buried at a depth as small as 1/150000 wavelength in the earth is presented.