Richard G. Plumb

A matched-filter-based reverse-time migration algorithm for ground-penetrating radar data

Ground-penetrating radar (GPR) is a remote sensing technique used to obtain information on subsurface features from data collected over the surface. The process of collecting data may be viewed as mapping from the object space to an image space. Since most GPRs use broad beam width antennas, the energy reflected from a buried structure is recorded over a large lateral aperture in the image spare, migration algorithms are used to reconstruct an accurate scattering map by refocusing the recorded scattering events to their true spatial locations through a backpropagation process. The goal of this paper is to present a pair of finite-difference time-domain (FDTD) reverse-time migration algorithms for GPR data processing. Linear inverse scattering theory is used to develop a matched-filter response for the GPR problem. The reverse-time migration algorithms, developed for both bistatic and monostatic antenna configurations, are implemented via FDTD in the object space. Several examples are presented.

An FDTD near- to far-zone transformation for scatterers buried in stratified grounds

The finite-difference time-domain (EDTD) technique is being used with increasing frequency for modeling the scattering characteristics of buried objects. The FDTD has, for some time, been able to model the near-zone scattered fields of buried objects due to near-zone sources. This is adequate for modeling the scattered returns of ground-based ground-penetrating radar, but not for airborne radar. This paper describes an FDTD-compatible technique whereby far-zone scattered fields of objects buried in a stratified ground can be calculated. This technique uses the equivalence principle to model a buried object in terms of equivalent electric and magnetic currents. The fields radiated by these currents in the presence of a stratified ground are then calculated using the reciprocity theorem and the well-known field equations for plane waves in a stratified media. Numerical results are presented that show excellent agreement between this technique and both analytical and numerical results.

Electromagnetic imaging of underground targets using constrained optimization

Several high-frequency electromagnetic techniques have been used in recent years to detect and identify buried objects. Post-processing of the collected data is performed in many of these techniques to obtain high-quality images of buried targets. Accurate reconstructions of the targets constitutive parameters can be obtained by casting the imaging problem in terms of an inverse electromagnetic scattering problem. A number of techniques have been put forth recently to invert the electromagnetic data to obtain such images. The authors use a frequency-domain Born iterative method to reconstruct images of shallow targets. The Born iterative technique requires successive solutions to a forward scattering problem followed by an inverse scattering problem at each iteration step. They use a finite-difference time-domain (FDTD) algorithm to solve the forward scattering problem and constrained optimization for the inverse problem. Two-dimensional simulated data for several canonical objects buried in the ground are obtained using the FDTD technique. The same FDTD code is also used in calculating the Greens function required for solving the constrained optimization problem. Lossy, inhomogeneous ground models are used in several simulations to illustrate the use of this technique for practical situations. The inversion process can be used to reconstruct images for many realistic dielectric contrasts for which a linear Born approximation fails. Moreover, it is also shown that a small number of measurements results in accurate reconstructions with this technique. Use of multiple frequencies is also investigated. >

FDTD modeling of scatterers in stratified media

The FDTD technique is well suited for calculating the fields scattered by buried objects when the sources are close enough to the air/ground interface so that they can be incorporated into the solution space. Difficulties arise, however, when the sources are far from the interface since the total fields in the solution space are not all outgoing waves. Using well-known formulas for the fields transmitted and reflected by stratified media, this paper discusses a method whereby the fields scattered by a buried object can be easily calculated by the FDTD technique when the incident field is a plane wave. >

The use of ground-penetrating radar with a cooperative target

A cooperative target (CT) is proposed to enhance the ground-penetrating radar (GPR) signal-to-clutter ratio (SCR) for buried man-made targets. Applications include tagging high-value buried structures and monitoring microtunneling equipment. Results are presented for a time-domain CT, a dipole antenna connected to an unterminated delay line. By using several independent time-domain CTs, strategically arrayed about a target, the rotational aspect of the target can also be obtained. Finally, harmonic generation is demonstrated as a technique for a frequency-domain CT.

A waveform-range performance diagram for ground-penetrating radar

The growing professional interest in ground-penetrating radar (GPR) is driving the need to formulate methods for characterizing the performance capabilities of GPR systems. Because a number of different factors, such as antenna ringing, noise, radio frequency interference and receiver dynamic range, contribute to limit a GPRs capability, no single performance measure gives an adequate system characterization. Instead capability can best be described by indicating the boundary of marginal performance for the various parameters that affect performance in the form a waveform-range performance diagram. The various limiting processes are used to define the performance boundaries. Within the performance boundary the radar has a capability, and outside the boundary the radar has no capability. Performance measures such as system performance, loop gain, and system dynamic range are interpreted using the waveform-range performance diagram.

A hybrid formulation for the probe-to-patch attachment-mode current for rectangular microstrip antennas

In this paper, we present a hybrid representation for the attachment-mode current existing on the suirface of a coaxially fed rectangular microstrip patch antenna. The hybrid representation consists of a one- or two-term residue anid an eigenfunction series which gives the attachment-mode curirent at all points on the patch surface, including the probe-to-patch junction. It is numerically demonstrated that the residue and eigenfunction series blend smoothly in the close vicinity of the Stokes region which includes the probe-to-patch junction. The Stokles region is a very narrow region over which the attachment-mode current changes rapidly. The residue and eigenfunction series are used outside and within this Stokes region, respectively. This hybrid representation can be used in either spectral- or space-domain techniques for a full-wave solution to the microstrip antenna problem. The residue series is obtained by using an equivalent contour integral representation of the infinite eigenfunction series, which subsequently reduces to the Watson transform under the assumption of a slightly lossy substrate. At or near the Stokes regions the residue series is inadequate, as it cannot model the rapidly varying junction currents there. Examination of the residue series shows that the attachment mode is a superposition of exponentially attenuated, leaky, surface-traveling modes. This hybrid representation is expected to provide the optimum trade-off between speed and accuracy for computationis involving electrically large finite arrays.

Ground-penetrating radar antenna modeling

Detecting subsurface objects by using ground-penetrating radar (GPR) has received considerable interest. In order to interpret radar signals from buried objects, one must have the ability to model a large range of objects, grounds and radar antennas, theoretically or numerically, so that a real GPR system can be simulated. Many investigations have been done for modeling objects (scatterers) and grounds, but few have involved realistic antennas. This paper presents a technique to model real GPR antennas located above a ground in which an object is buried. Numerical results are presented to verify this technique.

Modeling far field patterns of cylindrical arrays for rapidly deployable radio networks (RDRN) using the NEC-basic scattering code

The E/sub /spl theta// patterns in the /spl theta/=90/spl deg/ plane of an array of /spl lambda//2 electric dipoles in presence of a circular cylinder are studied using the NEC-basic scattering code (NECBSC2). The results indicate that acceptable antenna patterns can be obtained by symmetrically weighting the amplitude excitation of dipoles with respect to the center element. The numerical technique for obtaining these patterns, and the effects of creeping waves in the azimuth (/spl theta/=90/spl deg/) plane, are discussed.

Modeling baluns with the method of moments

When using the method of moments (MoM) to model large arrays with large numbers of balanced feeds, the feed baluns can always be incorporated into the analysis by combining the multiport admittance matrix for the array with the admittance matrices of the baluns. This technique is straightforward but requires that the MoM equations be solved for a large number of right-hand sides. The paper shows how the effects of perfect baluns can be incorporated directly into the MoM equations, requiring only one right-hand side. This technique yields the exact results with a significant savings in computing resources. >