Daniel E. Lawrence

Electromagnetic scattering from a dielectric cylinder buried beneath a slightly rough surface

An analytical solution is presented for the electromagnetic scattering from a dielectric circular cylinder embedded in a dielectric half-space with a slightly rough interface. The solution utilizes the spectral (plane-wave) representation of the fields and accounts for all the multiple interactions between the rough interface and the buried cylinder. First-order coefficients from the small perturbation method are used for computation of the scattered fields from the rough surface. The derivation includes both TM and TE polarizations and can be easily extended for other cylindrical buried objects (e.g., cylindrical shell, metallic cylinder). Several scattering scenarios are examined utilizing the new solution for a dielectric cylinder beneath a flat, sinusoidal, and arbitrary rough surface profile. Results indicate that the scattering pattern of a buried object below a slightly rough surface differs from the flat surface case only when the surface roughness spectrum contains a limited range of spatial frequencies. Furthermore, the illuminated area of the incident wave is seen to be a critical factor in the visibility of a buried object below a rough surface.

Acoustic and electromagnetic wave interaction: analytical formulation for acousto-electromagnetic scattering behavior of a dielectric cylinder

An analytical solution for the bistatic electromagnetic (EM) scattering from an acoustically excited vibrating dielectric circular cylinder is presented. The incident acoustic wave causes a boundary deformation as well as a dielectric inhomogeneity within the dielectric cylinder. First, a perturbation method is developed to calculate the EM scattering from a slightly deformed and inhomogeneous dielectric cylinder. Then, assuming the vibration frequency is much smaller than the frequency of the incident EM wave, a closed form expression for the time-frequency response of the bistatic scattered field is obtained. The solution for acoustic scattering from an elastic cylinder is applied to give the displacement on the surface as well as the compression and dilation within the cylinder. Both the surface displacement and the variation in material density (dielectric constant fluctuation) within the cylinder contribute to the Doppler component of the EM scattered field. Results indicate that the Doppler frequencies correspond to the mechanical vibration frequencies of the cylinder and that the Doppler components only become sizeable near frequencies corresponding to the natural modes of free vibration in the cylinder. These resonances depend only on the object properties and are independent of the surrounding medium. Thus, utilizing the information in the Doppler spectrum scattered by an acoustically excited object vibrating at resonance could provide a means for buried object identification.

Tissue plasminogen activator-mediated PDGF signaling and neurovascular coupling in stroke

Summary.  The use of tissue plasminogen activator (tPA) as a thrombolytic treatment in ischemic stroke is limited largely due to concerns for hemorrhagic complications. The underlying mechanisms are still unknown, but evidence is beginning to emerge that tPA interacts with key regulators of the neurovascular unit (NVU), and that these interactions may contribute to the undesirable side effects associated with the use of tPA in ischemic stroke. Understanding these connections and tPA’s normal function within the NVU may offer new insights into future therapeutic approaches.

Acoustic and electromagnetic wave interaction: estimation of Doppler spectrum from an acoustically vibrated metallic circular cylinder

The idea of using acoustically induced Doppler spectra as a means of target detection and identification is introduced. An analytical solution for the calculation of the bistatic scattered Doppler spectrum from an acoustically excited, vibrating, metallic, circular cylinder is presented. First, the electromagnetic scattering solution of a slightly deformed circular cylinder is obtained using a perturbation method. Then, assuming the vibration frequency is much smaller than the frequency of the incident electromagnetic wave, a closed form expression for the time-frequency response of the bistatic scattered field is obtained which can be used directly for estimating the Doppler spectrum. The acoustic scattering solution for an incident acoustic plane wave upon a solid elastic cylinder is applied to give the displacement of the cylinder surface as a function of time. Results indicate that the scattered Doppler frequencies correspond to the mechanical vibration frequencies of the cylinder, and the sidelobe Doppler spectrum level is, to the first order, linearly proportional to the degree of deformation and is a function of bistatic angle. Moreover, the deformation in the cylinder, and thus the Doppler sidelobe level, only becomes sizeable near frequencies of normal modes of free vibration in the cylinder. Utilizing the information in the scattered Doppler spectrum could provide an effective means of buried object identification, where acoustic waves are used to excite the mechanical resonances of a buried object.

Electromagnetic scattering from vibrating penetrable objects using a general class of time-varying sheet boundary conditions

Calculation of electromagnetic (EM) scattering from vibrating penetrable cylinders of arbitrary cross-section is presented using a general class of time-varying sheet boundary conditions (SBCs) in conjunction with the method of moments (MoM). Sheet impedance and admittance expressions are first derived from the exact scattering solution for a penetrable circular cylinder with perturbed radius. Then, using the SBCs, integral equations are derived and solved numerically so that vibrating cylinders with arbitrary cross-section can be treated. Cylinder vibrations are assumed to be non-relativistic, allowing a simplified calculation of the scattered Doppler spectrum. A critical factor in the calculation of the potentially small Doppler components is that the time-varying nature of the cylinder boundary, contained within the sheet impedance and admittance expressions, can be isolated from the unperturbed terms in the scattered field. Comparison with exact and analytical perturbation solutions are presented to demonstrate the accuracy of the numerical solution.

Electromagnetic scattering from vibrating metallic objects using time-varying generalized impedance boundary conditions

In this paper, we are interested in using impedance boundary conditions ( IBCs) to model the scattering from a vibrating object which can be considered a perturbation problem where the perturbation (and the corresponding IBC) varies with time. One of the most basic objects for which a scattering solution can be obtained is an infinite PEC cylinder. Using the exact solution for a circular cylinder, the IBCs needed to model the scattering from a perturbed PEC cylinder are derived for both the TM and TE polarizations. It is shown that the standard IBC (SIBC) is accurate to first order for the TM case, but a second generalized IBC (GIBC) is needed to provide first order accuracy for the TE case. Results from a moments method (MoM) implementation of the IBCs are presented.

Electromagnetic scattering from a dielectric cylinder beneath a slightly rough surface

Over the past few decades, a significant amount of research effort has been spent toward developing a viable buried object detection scheme. There are many difficulties associated with detecting objects beneath the ground due to the existence of clutter and low RF signal penetration into moist soil. In order to better understand the phenomenology of scattering from buried objects, this paper presents an analytical solution to a canonical scattering problem involving a buried dielectric circular cylinder below a slightly rough surface with arbitrary one-dimensional surface profile. The solution is obtained by employing the spectral representation of the fields and adding successive reflections from the rough half-space boundary and scattered fields from the cylinder. First order reflection and transmission coefficients from the small perturbation method (SPM) are used for the rough surface scattering.

Elastic-wave scattering from a solid circular cylinder embedded in an elastic half-space

An analytical solution is presented for the elastic-wave scattering from an infinitely long, solid circular cylinder embedded in an elastic half-space. The solution utilizes the spectral (plane-wave) representation of the fields and accounts for all the multiple interactions between the interface and the buried cylinder. Compressional and shear waves are allowed in both the half-space medium and the cylinder. A buried line-source is used for excitation and several cases are considered to examine the interaction of the incident elastic waves with the buried cylinder. The solution presented here is derived in the frequency-domain but is efficient enough to provide useful time-domain results through Fourier transform techniques. Using an incident Gaussian pulse in the time-domain, simulation results demonstrate the wave components (compressional, shear, Rayleigh) on the surface of the half-space and show the effect of burial depth on the scattered wave displacement. The analytical solution is general and not limited to buried object detection but can be used in other areas, such as non-destructive evaluation of composite materials or dynamical effect studies of composites.

Acousto-electromagnetic interaction in the detection of buried objects

An analytical study of electromagnetic scattering from a dielectric cylinder buried beneath an acoustically excited interface is presented. The solution utilizes the spectral (plane-wave) representation of the fields and accounts for all the multiple interactions between the interface and the buried cylinder. First order coefficients from the small perturbation method are used for computation of the scattered fields from the rough surface. Results show that the illumination area should be made close to the size of the buried object of interest to reduce the interference caused by the rough surface.

Analytical formulation for acousto-electromagnetic interaction in buried landmine detection

Calculation of the electromagnetic scattering form a buried object, such as a landmine, under acoustic vibration requires a scattering solution for an object beneath an interface with acoustically-induced surface roughness. An analytical solution is presented for the electromagnetic scattering from a dielectric circular cylinder embedded in a dielectric half-space with a slightly rough interface. The solution utilizes the spectral representation of the fields and accounts for all the multiple interactions between the rough interface and the buried cylinder. First order coefficients from the small perturbation method are used for computation of the scattered fields from the rough surface. The derivation includes both TM and TE polarizations and can be easily extended for other cylindrical buried objects. Scattering scenarios are examined utilizing the new solution for a dielectric cylinder beneath both flat and arbitrary surface profiles.