L. Peters

Buried unexploded ordnance identification via complex natural resonances

A classification technique focused on the identification of buried unexploded ordnance (UXO) using complex natural resonance (CNR) signature is considered. The total least square (TLS) Pronys (1795) method is used to extract CNRs from time-domain data, Full-scale UXO computational models and the body of revolution moment method (BORMM) code is used to obtain the backscattered fields, which are then used to give the theoretical free-space CNRs. Baums (1993) transformation is used to relate a CNR in a lossy simple medium to the corresponding CNR in free space. A practical UXO classification yields an estimate of the UXO length from its CNR information. Successful UXO classification examples from actual measured data are presented.

Near-field dipole radiation dynamics through FDTD modeling

We use finite-difference time-domain (FDTD) numerical simulations to study horizontal dipole radiation mechanisms and patterns near half-space interfaces. Time snapshots illustrating propagation of wavefronts at an instance in time are included with antenna patterns to provide a visualization tool for understanding antenna radiation properties. Near-field radiation patterns are compared with far-field asymptotic solutions and the effects of electrical properties, antenna height, and observation distance are investigated through numerical simulations. Numerical simulations show excellent agreement with measured data collected over a water-filled tank. Near-field H-plane radiation patterns are broader and contain radiation maxima beyond the critical angle predicted by far-field solutions. A large amplitude E-plane radiation lobe is located directly below the antenna in all simulations, while the two large amplitude sidelobes are less distinct and occur at larger incidence angles than predicted by far-field solutions. Radiation patterns resemble far-field solutions by a distance of 10 wavelengths, except near the critical angle where H-plane radiation maxima and E-plane sidelobes occur at larger incidence angles than predicted by far-field solutions.

Slope diffraction and its application to horns

The first order geometrical theory of diffraction (GTD) predicts vanishing fields along the surface of a conducting wedge for the incident electric field polarized parallel to the diffracting edge. The slope diffraction coefficient is a valid correction term for incidence angles removed from the shadow boundary. A new slope diffraction function for the half plane is presented along with applications. This new form of slope diffraction coefficient for the half plane is valid through the shadow region. Reciprocity is invoked to find the far-fields for a source on the surface of the conducting wedge. In addition to applying the two-dimensional slope diffraction analysis to practical problems, the equivalent current concepts have been extended to include equivalent slope currents for the analysis of either finite or curved edges. This new form of the slope diffraction function has been successfully used to provide an H -plane horn pattern analysis that is considerably less tedious than previously possible with GTD. Both pure GTD solutions and hybrid solutions using conventional aperture integration for the main beam region and GTD for the far-out side and back lobes are compared with experimental results.

A standoff, focused-beam land mine radar

This paper describes a standoff, focused-beam, ground-penetrating radar (GPR) for detecting buried antipersonnel (AP) mines. The radar features a 1 to 6 GHz, pulsed stepped-frequency microwave source with hardware range gate, an ultrawide bandwidth feed antenna, and an offset parabolic reflector. A focused-beam, created by feeding the reflector from an offset position, reduces the surface clutter and improves the detection of buried mines. A processing technique based on spectral analysis (Prony method) is developed in an attempt to reduce surface clutter. Tests conducted at the Fort A.P. Hill countermine test lanes showed excellent detection capability for both metallic and nonmetallic mines.

Electromagnetic scattering by a straight thin wire

The traveling-wave energy, which multiply diffracts on a straight thin wire, is represented as a sum of terms, each with a distinct physical meaning, that can be individually examined in the time domain. Expressions for each scattering mechanism on a straight thin wire are cast in the form of four basic electromagnetic wave concepts: diffraction, attachment, launch, and reflection. Using the basic mechanisms from P.Ya. Ufimtsev (1962), each of the scattering mechanisms is included into the total scattered field for the straight thin wire. Scattering as a function of angle and frequency is then compared to the moment-method solution. These analytic expressions are then extended to a lossy wire with a simple approximate modification using the propagation velocity on the wire as derived from the Sommerfeld wave on a straight lossy wire. Both the perfectly conducting and lossy wire solutions are compared to moment-method results, and excellent agreement is found. As is common with asymptotic solutions, when the electrical length of wire is smaller than 0.2 lambda the results lose accuracy. The expressions modified to approximate the scattering for the lossy thin wire yield excellent agreement even for lossy wires where the wire radius is on the order of skin depth. >

A time domain technique for mechanism extraction

The properties of scattered fields from a structure can be better evaluated from the characteristics of the individual scatterers. Decomposition techniques can be classified either as a matrix or an integral formulation. With either formulation, aspect pattern or frequency information of a scattering center can be obtained. Emphasis will be placed on an integral (time domain) isolation extraction technique to obtain the frequency characteristics of scattering mechanisms. This technique has its origins in the time domain interpretation of scattered fields.

DETECTION OF BURIED AGRICULTURAL DRAINAGE PIPE WITH GEOPHYSICAL METHODS

One of the more frustrating problems confronting farmers and land improvement contractors in the Midwest United States involves locating buried agricultural drainage pipes. Enhancing the efficiency of soil water removal on land already containing a subsurface drainage system typically involves installing new drain lines between the old ones. However, before this approach can be attempted, the older drain lines need to be located. Conventional geophysical methods have the potential to provide a solution to this problem. Therefore, in order to determine a better way to detect buried drainage pipe, the abilities of four near-surface geophysical methods were investigated, including geomagnetic surveying, electromagnetic induction, resistivity, and ground penetrating radar (GPR). Of these four, only GPR proved capable of finding agricultural drainage pipe. Furthermore, GPR grid surveys were conducted in southwest, central, and northwest Ohio at 11 test plots containing subsurface drainage systems, and in regard to locating the total amount of pipe present at each site, this technology was shown to have an average effectiveness of 81% (100% of the pipe was found at six sites, 90% at one site, 75% at two sites, 50% at one site, and 0% at one site.) GPR proved, on the whole, to be successful in finding clay tile and corrugated plastic tubing drainage pipe down to depths of approximately 1 m (3 ft) within a variety of different soil materials. Consequently, although more research is certainly warranted, ground penetrating radar methods appear to have excellent potential with respect to agricultural drainage pipe detection.

Computation of Scattering from Penetrable Cylinders with Improved Numerical Efficiency

An integral-equation formulation is used to obtain numerical results for the scattered fields of a penetrable cylinder immersed in either a lossy halfspace or a lossy homogeneous medium. The cylinder is iluminated by a parallel electric line source. A set of plane waves interior to the inhomogeneity (scatterer) is used as basis functions. This results in more than an order of magnitude decrease in the computer time required to obtain numerical results for larger sized targets. Further, the integral-equation solution is extended to include the planar interface between the air and the earth. The validity of the approximate forms proposed earlier to represent the interface is reexamined.

IMPORTANT CONSIDERATIONS FOR LOCATING BURIED AGRICULTURAL DRAINAGE PIPE USING GROUND PENETRATING RADAR

Enhancing the efficiency of soil water removal on land already containing a subsurface drainage system typically involves installing new drain lines between the old ones. However, the older drainage pipes need to be located before this approach can be attempted. In ongoing research, a near-surface geophysical method, ground penetrating radar (GPR), has been successful in locating on average 72% of the total amount of drainage pipe present at 13 test plots in southwest, central, and northwest Ohio. The effective use of GPR for drainage pipe detection requires careful consideration of computer processing procedures, equipment parameters, site conditions, and field operations, all of which were thoroughly investigated in this study. Application of a signal saturation correction filter along with a spreading and exponential compensation gain function were the computer processing steps most helpful for enhancing the drainage pipe response exhibited within GPR images of the soil profile. GPR amplitude maps that show the overall subsurface drainage pipe system required additional computer processing, which included 2-D migration, signal trace enveloping, and in some cases, a high frequency noise filter and a spatial background subtraction filter. Equipment parameter test results indicate that a 250-MHz antenna frequency worked best, and that data quality is good over a range of spatial sampling intervals and signal trace stacking. In regard to the site conditions present, shallow hydrology, soil texture, and drainage pipe orientation all substantially influence the GPR response. Additionally, drainage pipe that are as small as 5 cm (2 in.) in diameter can be detected. However, the fired clay or plastic material of which the drainage pipe is comprised does not appear to have much of an impact. Finally, with respect to GPR field operations, bidirectional surveys offer the best chance for finding all the buried drainage pipe possible, and for displaying a subsurface drainage system on an amplitude map, the narrower the spacing between GPR measurement lines, the better the result. Although it is important to note that the amplitude maps generated with a wider spacing between GPR measurement lines, still provided plenty of useful data on drainage pipe location. The information supplied by this study can be employed to formulate guidelines that will enhance the potential of success for using ground penetrating radar in locating buried agricultural drainage pipe.

Radar images of penetrable targets generated from ramp profile functions

Images can be generated for penetrable targets from their scattered fields when the time dependence of the incident electromagnetic (EM) wave takes the form of a ramp function. Previous researchers have developed these concepts for conducting targets. This paper focuses attention on penetrable targets. Ramp response signatures of the targets for cases where the dielectric constant of the target is greater than and also less than that of the ambient medium are included. The latter case can be applied as a signature of antipersonnel mines. The results contained herein are based on: (1) scattering measurements in The Ohio State University ElectroScience Laboratory compact range; (2) scattering computation using an eigenfunction solution and a method of moments solution; and (3) a very limited set of measurements generated from a buried land mine using the ElectroScience Laboratory ground penetrating radar. The targets presented in this paper include metallic and dielectric spheres and actual land mines.