Lossy Soil Research Articles

A linear inverse scattering algorithm for realistic GPR applications

ABSTRACTThe paper deals with a microwave‐tomography‐based solution algorithm tailored for use with GPR data‐processing applications. The algorithm tackles an inverse scattering problem in the frequency domain through the use of a linear model of the electromagnetic scattering, based on the Born approximation. In particular, we evaluate the reconstruction capabilities of the linear inversion algorithm in terms of the retrievable spatial variations of the unknown contrast function, whilst considering the problem of choosing an optimal frequency measurement step, theoretically, using diffraction tomography arguments. A numerical analysis of the technique is performed by means of the singular‐value decomposition tool, which allows us to extend the theoretical results to more realistic cases involving lossy soils. Finally, we present a series of reconstructions, obtained using synthetic and experimental data, which show the performance of the method under realistic conditions.

Electromagnetic radiation from vertical dipole antennas near air-lossy soil interface?a finite-difference time-domain simulation

Radiation from vertical dipole antennas, which are located over or under the surface of lossy earth, is analysed by the finite-difference time-domain (FDTD) method in cylindrical coordinates. A novel generalized perfectly matched layer (PML) has been developed and used for the truncation of the lossy soil. In order to decrease the memory requirements and for having an accurate modelling, an efficient ‘non-uniform’ mesh generation scheme is used. The excitation is considered in the form of sine carrier modulated by Gaussian pulse (SCMGP) and in each time step, computation is limited to that part of the mesh where the radiated pulse is passing (computational window). This could considerably reduce the required CPU time. In this manner, large-scale problems can be solved and the values of radiated field at far distances (up to 500λ0 in this work) can be obtained directly by the FDTD method. The frequency-domain results are calculated from the obtained time-domain results by taking the Fourier transform. The spatial distributions of the amplitude and phase of radiated field are shown in illustrations for different types of soil and different positions of antenna. The influence of the lossy soil on dipole's admittance is also shown. Copyright © 2005 John Wiley & Sons, Ltd.

A simple detection method of buried cylindrical targets applicable to landmines

AbstractThis paper presents a contribution to the problem of detecting cylindrical targets. We first show that cylindrical targets embedded into lossy dielectric media, such as soil have natural resonant frequencies which can be calculated using simplified models. Next, we demonstrate that those resonant frequencies can be observed experimentally using an adequate field detector and could be used as the signature of the presence of landmines into lossy soils. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 80–83, 2003

A simulation model for electromagnetic transients in lightning protection systems

This paper deals with the evaluation of electromagnetic transients in a lightning protection system (LPS). A field approach is used, based on the numerical solution of a modified version of the thin-wire electric field integral equation in the frequency domain. Time profiles of interesting electromagnetic quantities are computed by using a discrete fast Fourier transform algorithm. The model takes into account coupling effects among aerial parts and ground electrodes in order to correctly estimate the quantities which can determine electromagnetic hazard inside the LPS; transient touch and step voltages can be easily evaluated also taking into account the human body presence on the soil surface. To this purpose, a crucial point is the accurate evaluation of the current distribution among the earthed branches of the LPS and this needs to correctly consider mutual electromagnetic interference among the aerial parts and the earth-termination system of the same LPS. A suitable approach to consider the lossy soil is employed. Validation of the proposed model is performed by comparing the results with those measured and computed available in technical literature. Simulation examples related to realistic LPS structures are presented and discussed to show the flexibility and the accuracy of the model in the range of practical applications inside the volume to be protected.

The multiple interaction model for nonshallow scatterers buried beneath 2-d random rough surfaces

The multiple interaction model is hybridized with the robust Steepest Descent Fast Multipole Method (SDFMM) to compute the signature of nonshallow penetrable scatterers buried beneath two-dimensional random rough surfaces. The most attractive feature of the multiple interaction model with using the SDFMM is removing the quasi-planar structure constraint for analyzing nonquasi-planar scatterers. The results show that the buried object's signature is largely due to the first interaction mechanism; however, the contribution of each additional interaction is explicitly calculated, though they may become insignificant especially for lossy background soil.

Time-domain analysis of transient electromagnetic field generated by aerial busbars above lossy soil

Based on multiconductor transmission lines (MTLs) and the antenna method, a hybrid method is proposed to analyze the transient electromagnetic fields due to the operation of switches in substations. The method is valid in either near or far field. At first, the wave process along the busbars is calculated by using the finite-difference time-domain method. Then the field is calculated by using electrical dipole theory. In order to take the effect of lossy soil into consideration with reason, the closed-form of the generalized Sommerfeld integral is calculated by using generalized pencil-of-function and an equation derived from the Sommerfeld identity, which saves much time. After a model of busbars in a 500-kV substation is set up, the transient electromagnetic field is predicated.

Wide bandwidth time-domain electromagnetic sensor for metal target classification

A portable, wide bandwidth, time-domain electromagnetic sensor system has been developed and used extensively to measure the eddy current time-decay response of a wide variety of metal targets. The sensor has demonstrated the ability to measure metal target decay times starting approximately 3 to 5 /spl mu/s after the transmitter current is turned off and target decay time constants as short as 1.4 /spl mu/s. The sensor has demonstrated the potential for detecting very low-metal content mines due to the void they create in some types of electrically lossy soils. The development of the sensor is described in detail, and time-decay data are shown for a variety of metal targets, including low metal antitank and antipersonnel mines.

Linear GPR inversion for lossy soil and a planar air-soil interface

A three-dimensional inversion scheme for fixed-offset ground penetrating radar (GPR) is derived that takes into account the loss in the soil and the planar air-soil interface. The forward model of this inversion scheme is based upon the first Born approximation and the dyadic Green function for a two-layer medium. The forward model is inverted using the Tikhonov-regularized pseudo-inverse operator. This involves two steps: filtering and backpropagation. The filtering is carried out by numerically solving Fredholm integral equations of the first kind and the backpropagation is performed using fast Fourier transforms. Numerical results are provided to illustrate the performance of the inversion scheme.

On the use of evanescent electromagnetic waves in the detection and identification of objects buried in lossy soil

In some electromagnetic (EM) systems proposed for the detection of buried objects, such as landmines, the transducers (antennas) are located very close to the surface of the Earth. The coupling of energy into the Earth is then by the near field of the transducers, or, more precisely, by evanescent waves as well as propagating waves in the spectrum for the radiation from the transducers. Evanescent waves also contribute to the coupling of the scattered field from the shallowly buried object to the transducers. In this paper, we use simple models based on a plane-wave spectral analysis to perform a preliminary examination of the role that evanescent waves can play in the detection and identification of the buried object. The degree to which features in the image of the object can be resolved is of particular interest, since the features can be used to distinguish the object from clutter (such as rocks). The effect of loss in the soil on imaging is also of interest.

A New Method of Subsurface Imaging—The LASI High Frequency Ellipticity System: Part 1. System Design and Development

A new electromagnetic (EM) imaging system has been developed that allows enhanced identification of buried hazardous waste and contaminated soil or groundwater. This system utilizes frequencies in the range of 31 kHz to 32 MHz, and as such fills an important gap between conventional low‐frequency EM induction surveys and high‐frequency ground‐penetrating radar (GPR) surveys. This capability permits interpretation of both conductivity and dielectric permittivity at depths of interest for environmental studies in lossy soils. Mapping the dielectric permittivity will allow location of certain environmental targets that are not detectable with the more commonly used conductivity (or resistivity) tools. This system will be described in a three part series: Part 1 (presented here) details the system design and development; Part 2 will describe the data processing and interpretation methods; and Part 3 will provide five case histories showing the use of this new system. The system involves measurement of the ellipticity of the magnetic field at high frequency (HF) and calculation of both conductivity and dielectric permittivity as a function of depth. The design incorporates unique transmitter and receiver coils which combine wide bandwidth and high sensitivity; it also includes a method for eliminating electric‐field interference, which is vital for accurate measurements in this frequency range. And finally, the system complies with FCC regulations for avoidance of interference with licensed radio transmissions and ANSI guidelines for human safety.

A general method for FDTD modeling of wave propagation in arbitrary frequency-dispersive media

A general formulation is presented for finite-difference time-domain (FDTD) modeling of wave propagation in arbitrary frequency-dispersive media. Two algorithmic approaches are outlined for incorporating dispersion into the FDTD time-stepping equations. The first employs a frequency-dependent complex permittivity (denoted Form-1), and the second employs a frequency-dependent complex conductivity (denoted Form-2). A Pade representation is used in Z-transform space to represent the frequency-dependent permittivity (Form-1) or conductivity (Form-2). This is a generalization over several previous methods employing either Debye, Lorentz, or Drude models. The coefficients of the Pade model may be obtained through an optimization process, leading directly to a finite-difference representation of the dispersion relation, without introducing discretization error. Stability criteria for the dispersive FDTD algorithms are given. We show that several previously developed dispersive FDTD algorithms can be cast as special cases of our more general framework. Simulation results are presented for a one-dimensional (1-D) air/muscle example considered previously in the literature and a three-dimensional (3-D) radiation problem in dispersive, lossy soil using measured soil data.

Resonances of perfectly conducting wires and bodies of revolution buried in a lossy dispersive half-space

The method of moments (MoM) is utilized to compute the complex resonant frequencies and modal currents of perfectly conducting wires and bodies of revolution buried in a lossy dispersive half space. To make such an analysis tractable computationally, the half-space Green's function is computed via the method of complex images, with appropriate modifications made to account for the complex frequencies characteristic of resonant modes. Results are presented for wires and bodies of revolution buried in lossy soil using frequency-dependent measured parameters for the complex permittivity, and we demonstrate that the resonant frequencies generally vary with target depth. In addition to presenting results, relevant issues are addressed concerning the numerical computation of buried-target resonant frequencies.