Very Early Time Electromagnetic Research Articles

Detection of buried targets using a new enhanced very early time electromagnetic (VETEM) prototype system

In this paper, numerical simulations of a new enhanced very early time electromagnetic (VETEM) prototype system are presented, where a horizontal transmitting loop and two horizontal receiving loops are used to detect buried targets, in which three loops share the same axis and the transmitter is located at the center of receivers. In the new VETEM system, the difference of signals from two receivers is taken to eliminate strong direct-signals from the transmitter and background clutter and furthermore to obtain a better SNR for buried targets. Because strong coupling exists between the transmitter and receivers, accurate analysis of the three-loop antenna system is required, for which a loop-tree basis function method has been utilized to overcome the low-frequency breakdown problem. In the analysis of scattering problem from buried targets, a conjugate gradient (CG) method with fast Fourier transform (FFT) is applied to solve the electric field integral equation. However, the convergence of such CG-FFT algorithm is extremely slow at very low frequencies. In order to increase the convergence rate, a frequency-hopping approach has been used. Finally, the primary, coupling, reflected, and scattered magnetic fields are evaluated at receiving loops to calculate the output electric current. Numerous simulation results are given to interpret the new VETEM system. Comparing with other single-transmitter-receiver systems, the new VETEM has better SNR and ability to reduce the clutter.

3-D imaging of large scale buried structure by 1-D inversion of very early time electromagnetic (VETEM) data

A simple and efficient method for large scale three-dimensional (3D) subsurface imaging of inhomogeneous background is presented. One-dimensional (1D) multifrequency distorted Born iterative method (DBIM) is employed in the inversion. Simulation results utilizing synthetic scattering data are given. Calibration of the very early time electromagnetic (VETEM) experimental waveforms is detailed along with major problems encountered in practice and their solutions. This discussion is followed by the results of a large scale application of the method to the experimental data provided by the VETEM system of the U.S. Geological Survey. The method is shown to have a computational complexity that is promising for on-site inversion.

An addendum to "Numerical modeling of an enhanced very early time electromagnetic (VETEM) prototype system"

For original paper see Cui et al. (IEEE Antennas and Propagation Magazine, vol.42, no.2, p.17-27, 2000 April). Cui et al. proposed two numerical models to simulate an enhanced very early time electromagnetic (VETEM) prototype system, used for buried-object detection and environmental problems. In the first model, the transmitting and receiving loop antennas were accurately analyzed using the method of moments (MoM), and then conjugate gradient (CG) methods with the fast Fourier transform (FFT) were utilized to investigate the scattering from buried conducting plates. In the second model, two magnetic dipoles were used to replace the transmitter and receiver, because the working frequency for the VETEM system is usually low. Both the theory and formulation were correct, and the simulation results for the primary magnetic field and the reflected magnetic field were accurate. We have compared the simulation results for the magnetic field reflected by a wire-conductor mesh on the ground with measured data. They fit very well. However, the scattered magnetic fields in the simulation results were inaccurate, because we did not use a sufficient number of iterations in the CG-FFT algorithm when the frequency was very low.

Numerical modeling of an enhanced very early time electromagnetic (VETEM) prototype system

In this paper, two numerical models are presented to simulate an enhanced very early time electromagnetic (VETEM) prototype system, which is used for buried-object detection and environmental problems. Usually, the VETEM system contains a transmitting loop antenna and a receiving loop antenna, which run on lossy ground to detect buried objects. In the first numerical model, the loop antennas are accurately analyzed using the method of moments (MoM) for wire antennas above or buried in lossy ground. Then, the conjugate gradient (CG) methods, with the use of the fast Fourier transform (FFT) or MoM, are applied to investigate the scattering from buried objects. Reflected and scattered magnetic fields are evaluated at the receiving loop to calculate the output electric current. However, the working frequency for the VETEM system is usually low and, hence, two magnetic dipoles are used to replace the transmitter and receiver in the second numerical model. Comparing these two models, the second one is simple, but only valid for low frequency or small loops, while the first modeling is more general. In this paper, all computations are performed in the frequency domain, and the FFT is used to obtain the time-domain responses. Numerical examples show that simulation results from these two models fit very well when the frequency ranges from 10 kHz to 10 MHz, and both results are close to the measured data.