Browsing by Author "Joines, William T"
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Item Open Access A 3D Active Microwave Imaging System for Breast Cancer Screening(2008-12-11) Stang, JohnA 3D microwave imaging system suitable for clinical trials has been developed. The anatomy, histology, and pathology of breast cancer were all carefully considered in the development of this system. The central component of this system is a breast imaging chamber with an integrated 3D antenna array containing 36 custom designed bowtie patch antennas that radiate efficiently into human breast tissue. 3D full-wave finite element method models of this imaging chamber, complete with full antenna geometry, have been developed using Ansoft HFSS and verified experimentally. In addition, an electronic switching system using Gallium Arsenide (GaAs) absorptive RF multiplexer chips, a custom hardware control system with a parallel port interface utilizing TTL logic, and a custom software package with graphical user interface using Java and LabVIEW have all been developed. Finally, modeling of the breast (both healthy and malignant) was done using published data of the dielectric properties of human tissue, confirming the feasibility of cancer detection using this system.
Item Open Access Accurate and Efficient Methods for the Scattering Simulation of Dielectric Objects in a Layered Medium(2019) Huang, WeifengElectromagnetic scattering in a layered medium (LM) is important for many engineering applications, including the hydrocarbon exploration. Various computational methods for tackling well logging simulations are summarized. Given their advantages and limitations, main attention is devoted to the surface integral equation (SIE) and its hybridization with the finite element method (FEM).
The thin dielectric sheet (TDS) based SIE, i.e., TDS-SIE, is introduced to the simulation of fractures. Its accuracy and efficiency are extensively demonstrated by simulating both conductive and resistive fractures. Fractures of variant apertures, conductivities, dipping angles, and extensions are also simulated and analyzed. With the aid of layered medium Green's functions (LMGFs), TDS-SIE is extended into the LM, which results in the solver entitled LM-TDS-SIE.
In order to consider the borehole effect, the well-known loop and tree basis functions are utilized to overcome low-frequency breakdown of the Poggio, Miller, Chang, Harrington, Wu, and Tsai (PMCHWT) formulation. This leads to the loop-tree (LT) enhanced PMCHWT, which can be hybridized with TDS-SIE to simulate borehole and fracture together. The resultant solver referred to as LT-TDS is further extended into the LM, which leads to the solver entitled LM-LT-TDS.
For inhomogeneous or complex structures, SIE is not suitable for their scattering simulations. It becomes advantageous to hybridize FEM with SIE in the framework of domain decomposition method (DDM), which allows independent treatment of each subdomain and nonconformal meshes between them. This hybridization can be substantially enhanced by the adoption of LMGFs and loop-tree bases, leading to the solver entitled LM-LT-DDM. In comparison with LM-LT-TDS, this solver is more powerful and able to handle more general low-frequency scattering problems in layered media.
Item Open Access Coordinated analysis of delayed sprites with high-speed images and remote electromagnetic fields(2010) Li, JingboOne of the most dramatic discoveries in solar-terrestrial physics in the past two decades is the sprite, a high altitude optical glow produced by a lightning discharge. Previous sprite studies including both theoretical modeling and remote measurements of optical emissions and associated radio emissions have revealed many important features. However, in-situ measurements, which are critical for understanding the microphysics in sprites and constraining the existing models, are almost impossible because of the sprites' small time scale (a few ms) and large spatial scale (tens of km). In this work, we infer the lightning-driven ambient electric fields by combining remote measured electromagnetic fields with numerical simulations. To accomplish this, we first extract the lightning source current from remotely measured magnetic fields with a deconvolution technique. Then we apply this current source to an existing 2-D Finite Difference Time Domain (FDTD) model to compute the electric fields at sprite altitudes. These inferred electric fields make up for the deficiency of lacking in-situ measurements. A data set collected at two observation sites in 2005 combines simultaneous measurements of sprite optical emissions and sprite-producing lightning radiated electromagnetic fields. Sprite images from a high speed camera and the measured wideband magnetic fields removed the limitations imposed by the small sprite temporal scale and allow us to precisely determine the sprite initiation time and the time delay from its parent lightning discharge. For 83 sprites analyzed, close to 50% of them are delayed for more than 10 ms after the lightning discharges and empirically defined as long-delayed sprites. Compared with short-delayed sprites, which are driven by the lightning return stroke, all these long-delayed sprites are associated with intense continuing current and large total charge moment changes. Besides that, sferic bursts and slow intensifications are frequently detected before those long-delayed sprites. These observations suggest a different initiation mechanism of long-delayed sprites. To reveal that, we inferred the lightning-driven electric fields at the sprite initiation time and altitude. Our results show that although long-delayed sprites are mainly driven by the continuing current instead of the lightning return stroke, the electric fields required to produce those long-delayed sprites are essentially the same as fields to produce short-delayed sprites. Thus the initiation mechanism of long delayed sprite is consistent with the conventional breakdown model. Our results also revealed that the slow (5{20ms) intensifications in continuing current can significantly increase high altitude electric fields and play a major role in initiating delayed sprite. Sferic bursts, which were suggested as a direct cause of long-delayed sprites in previous studies, are linked to slow intensifications but not causal. Previous studies from remote measured low frequency radio emissions indicate that substantial electric current flows inside the sprite body. This charge motion, with unknown location and amount, is related to the detailed internal microphysics of sprite development that is in turn connected to the impact sprites have on the mesosphere. In our data, the recorded high speed images show the entire development history of sprite streamers. By assuming streamers propagate along the direction of local electric fields, we estimate the amount of electric charge in sprites. Our results show that individual bright core contains significant negative space charge between 0.01 to 0.03 C. Numerical simulations also indicate that this sprite core region is at least partial or perhaps the dominant source of the positive charge in the downward positive polarity streamers. Thus the average amount of charge in each downward streamer is at least 2 - 4 103 C. The connection between these charge regions is consistent with previous observations. The reported amount and location of the electric charge provide the initial condition and key data to constrain the existing streamer models. After initiation, sprite streamers propagate in the inhomogeneous medium from a strong field region to a weak field region. The propagation properties reflect the physics in sprite development. For the first time we measured the downward streamer propagation behaviors over the full sprite altitude extent. We found that downward streamers accelerate to a maximum velocity of 1 - 3 x 107 m/s and then immediately decelerate at an almost constant rate close to 10 10 m/s2. The deceleration processes dominant downward streamer propagation in both time and distance. Lightning driven electric fields have been inferred at streamer tip locations during their propagation. We found that most of the deceleration process occurs at a electric field less than 0.1 Ek. The results also show the dependence of sprite termination altitude on the ambient electric field. A minimum ambient electric field about 0.05 Ek is consistently observed for streamers in different sprites or at different locations in a single sprite. These streamer propagation properties as well as their connections to the ambient electric fields can be applied to further constrain the streamer models.Item Open Access Efficient Computation of Electromagnetic Waves in Hydrocarbon Exploration Using the Improved Numerical Mode Matching (NMM) Method(2016) Dai, JunwenIn this study, we developed and improved the numerical mode matching (NMM) method which has previously been shown to be a fast and robust semi-analytical solver to investigate the propagation of electromagnetic (EM) waves in an isotropic layered medium. The applicable models, such as cylindrical waveguide, optical fiber, and borehole with earth geological formation, are generally modeled as an axisymmetric structure which is an orthogonal-plano-cylindrically layered (OPCL) medium consisting of materials stratified planarly and layered concentrically in the orthogonal directions.
In this report, several important improvements have been made to extend applications of this efficient solver to the anisotropic OCPL medium. The formulas for anisotropic media with three different diagonal elements in the cylindrical coordinate system are deduced to expand its application to more general materials. The perfectly matched layer (PML) is incorporated along the radial direction as an absorbing boundary condition (ABC) to make the NMM method more accurate and efficient for wave diffusion problems in unbounded media and applicable to scattering problems with lossless media. We manipulate the weak form of Maxwell's equations and impose the correct boundary conditions at the cylindrical axis to solve the singularity problem which is ignored by all previous researchers. The spectral element method (SEM) is introduced to more efficiently compute the eigenmodes of higher accuracy with less unknowns, achieving a faster mode matching procedure between different horizontal layers. We also prove the relationship of the field between opposite mode indices for different types of excitations, which can reduce the computational time by half. The formulas for computing EM fields excited by an electric or magnetic dipole located at any position with an arbitrary orientation are deduced. And the excitation are generalized to line and surface current sources which can extend the application of NMM to the simulations of controlled source electromagnetic techniques. Numerical simulations have demonstrated the efficiency and accuracy of this method.
Finally, the improved numerical mode matching (NMM) method is introduced to efficiently compute the electromagnetic response of the induction tool from orthogonal transverse hydraulic fractures in open or cased boreholes in hydrocarbon exploration. The hydraulic fracture is modeled as a slim circular disk which is symmetric with respect to the borehole axis and filled with electrically conductive or magnetic proppant. The NMM solver is first validated by comparing the normalized secondary field with experimental measurements and a commercial software. Then we analyze quantitatively the induction response sensitivity of the fracture with different parameters, such as length, conductivity and permeability of the filled proppant, to evaluate the effectiveness of the induction logging tool for fracture detection and mapping. Casings with different thicknesses, conductivities and permeabilities are modeled together with the fractures in boreholes to investigate their effects for fracture detection. It reveals that the normalized secondary field will not be weakened at low frequencies, ensuring the induction tool is still applicable for fracture detection, though the attenuation of electromagnetic field through the casing is significant. A hybrid approach combining the NMM method and BCGS-FFT solver based integral equation has been proposed to efficiently simulate the open or cased borehole with tilted fractures which is a non-axisymmetric model.
Item Open Access Multiscale forward and inverse problems with the DGFD method and the deep learning method(2020) Zhang, RunrenA fast electromagnetic (EM) forward solver has been developed for the subsurface detection, with application includes producing synthetic logging data and instructing large-scale field test and inversion. A deep learning based full wave inversion method has also been developed to reconstruct the underground anomaly.
Since the gas and oil industry has very high demands for the forward modeling speed when doing inversion, the inversion model is usually simplified to a 1D or 2D problem by supposing the geometry of object invariant in two or one direction. The full 3D inversion is still a hot topic for research, which requires both fast 3D forward solver and efficient inversion method. The bottleneck for the forward solver is how to solve the large-scale linear system efficiently; the bottleneck for the inversion is how to pick the global minimum from lots of local minimums efficiently for the inverse problem.
For the forward part, the domain decomposition method (DDM) inspired discontinuous Galerkin frequency domain (DGFD) method has been extended to model the vertical open borehole resistivity measurement with structured gradient meshes; besides, the DGFD method has been extended to model the logging-while-drilling (LWD) resistivity measurement in high-angle and horizontal (HA/HZ) well and curved layers with a flipped total field/scattered field (TF/SF) mixed solver. An approximated casing model has also been proposed to accelerate the large-scale curved casing modeling with borehole-to-surface measurements.
For the inversion part, a convolutional neural network based inversion has been developed to reconstruct the lateral extent and direction of the hydraulic fracture through scattered electromagnetic field data under borehole-to-surface measurements; further, the deep transfer learning is applied in the same scenario to improve the performance of the inversion. Additionally, a fully connected neural network has been developed for the Devine field data and successfully reconstruct the shape of the hydraulic fracture with good agreement to the conventional inversion.
Item Open Access Novel Designs of End-fire Dipole Array Antennas(2017) Ma, TianyeIn this report, a novel type of dipole array antennas is discussed. A narrow-band dipole array antenna and a broad-band dipole array antenna are simulated and measured as two examples. Metallic strips and broadside I-shaped resonators are also placed for improving the bandwidth and gain. More than the bandwidth and directivity, the broad-band antenna also has undistorted radiation pattern, high front-back ratio and small size. These properties make it suitable for point-to-point wireless communication, imaging applications and wireless power transmission.
Item Open Access Research and Development of Low-Profile, Small Footprint Antennas for VHF-UHF Range Applications(2012) Olaode, OlusolaEfficient, but low-profile and small-footprint antennas for VHF-UHF range applications remains an ongoing work. VHF range spans approximately 54 - 88 MHz while UHF roughly ranges from 174 - 890 MHz. The inverse relationship between the physical length and resonant frequency of an antenna, which is a measure of its operating frequency range, is well known. A direct correlation between an antenna's physical length and radiation efficiency has also been established. Therefore, a combination of these constraints complicates the design of low-frequency antennas that have small physical size but with enough radiation resistance to be an efficient radiator when connected to a source having a comparable resistance. Given the frequency bands above, their corresponding wavelengths will be: 3.4-5.5 m (VHF) and 0.3-1.7 m (UHF). The length of an antenna operating at these wavelengths would need to be electrically-small i.e. a fraction of wavelength given size constraints for applications such as defense or commercial mobile communication equipment. As a consequence, the radiation resistance of the antenna, which is a function of its radiation efficiency, is greatly reduced. In other words, the input impedance or radiation impedance (assuming negligible ohmic losses in the antenna structure) features a small resistive component and a large capacitive component, causing reflections of most of the incident power to the antenna. Highly-reactive antennas are not desired for most transmitters and receivers. Therefore, the radiation resistance of an antenna must be increased by increasing its electrical length while simultaneously maintaining a low profile and footprint. This aim can be achieved by configuring the antenna to excite a resonance at, or very close to a desired operating frequency. An approach that I will explore in this dissertation is to exploit the broadband characteristics of meander-line and helical (or "spiral") antennas typically applied in the microwave frequency range to the UHF-VHF range. I will also propose novel antenna geometries that combine spiral and meander-line properties and analyze their performance. These antennas offer significant size reductions; for example, a bowtie meander dipole antenna studied yielded a height reduction of 55% at 64 MHz relative to a half-wave dipole antenna of the same resonant frequency. In addition, I will present a set of equations developed for predicting the fundamental resonant frequency and radiation resistance of meander-line antennas.
Item Open Access Shielded Metal Waveguides with Uniform Electric Field Distributions(2010) Zhou, TaoThis research focuses on achieving uniformly distributed electric field within a metal waveguide. A rectangular waveguide centrally loaded with a dielectric product is investigated rst since rectangular waveguides are widely used and can be easily made as exposure chambers and applicators. Then a dielectric slab loaded rectangular waveguide (TEM waveguide) with a uniform electric eld distribution across its cross-section is typically introduced. Due to the limitation of the TEM waveguide,
in this research, more practical methods are explored by changing the shape of the cross-section of a rectangular waveguide. The simulation results show that the new methods increase the uniform electric eld region greatly and even lower the cutoff frequency which means that a smaller waveguide may operate at the same frequency as a larger waveguide.
Item Open Access Using Maximally-Flat and Genetic Algorithm Solutions to Achieve Wideband Radar Cloaking(2017) Samsul, WiwiThis work revolves around Jaumann wave absorber, its solutions (either obtained through maximally flat solutions and through genetic algorithm), its possible implementation for designing wave absorbing and radar cloaking systems, and measurements of ambient RF energy in four Duke buildings to gauge the feasibility of harvesting energy from RF band.
The first part of introduction provides a short recap of radar, electromagnetic wave absorbers, and its equivalence to transmission line system. The next part describes a brief summary of the history of Jaumann wave absorber, its development over the years, and the short comings of previous attempts of finding its solutions. Finally, we conclude the introduction with contributions of this work.
The second chapter presents the mathematical formulation of Jaumann absorbers which is crucial in deriving its maximally flat solution. This chapter lays the foundation of which we explore other solutions through cut-and-try and genetic algorithm.
The third chapter expands the search for solutions through automated means. We first try the simple cut-and-try method before moving on to a more sophisticated genetic algorithm. The bulk of this chapter describes the details of genetic algorithm and how it is implemented into our Jaumann wave absorber problem.
The fourth chapter provides the results and analysis obtained from the algorithms described in previous chapter. However, only samples of such results are presented in this chapter while the bulk of the results is moved to Appendix \ref{Appendix:Genetic_Algorithm_Expanded_Results_Tables} for the sake of brevity.
The fifth chapter describes our experimental works in verifying a sample result of Jaumann wave absorber and measurement of ambient RF energy in four Duke buildings. We also include the measurement results and analysis in this chapter. A brief comparison for the ambient energy measurement to other studies in other locations and environment is also included.
Finally, the sixth chapter concludes this work.