The paper by D. V. Thiel and R. Mittra, IEEE Trans. Geosci. Remote Sensing, vol.35, p.1357-62 (1997) is commented upon. They take part in the controversy of staked versus unstaked sensors to measure the horizontal electric fields above the Earth's surface. Their idealized computer model is most ingenious and the results are not disputed. Nevertheless, the present author raises a few issues, in which correction and clarification may be needed.

The paper by D.V. Thiel and R. Mittra, IEEE Trans. Geosci. Remote Sensing, vol.35, p.1357-62 (1997) is commented upon. The present author deals with the basis of the expression for the effective complex area of a horizontal electric dipole (HED) over a layered conductive half-space. The second related item deals with a basic flaw in an estimate of the error when the HED is lifted relative to the air/earth interface. The present author provides an additional analysis. In both cases the HED is oriented to have a maximum response.

Using an exact numerical solution, the transient response of a two-layer Earth model is obtained for an obliquely incident transverse magnetic (TM) plane wave. It is shown that the decomposition of the general case into ray contributions is an alternative approach that yields consistent results. Several examples are given to demonstrate the sensitivity of the results to the electrical properties of the subsurface region.

It is argued that a hole in a thin dielectric sheet can be represented by a current source and a sink on opposite sides of the sheet. Such a configuration is analyzed using a quasistatic assumption for current injection from one side of the sheet. The results have possible relevance to schemes for the detection of leaks in membrane-lined impoundment ponds for hazardous liquid wastes.>

In this brief review, the interaction of antennas with the ambient medium is addressed. Beginning with a simple model, it is shown that the frequency and distance dependence is a strong function of the medium properties. A borehole experiment is described and related schemes are discussed both from the telecommunications and the remote sensing points of view.>

A special anisotropic model for the Earth subsurface is adopted in which the horizontally directed conductivities may be different from one another. Allowance is also made for contrasts in the magnetic permeability in the horizontal directions. It is shown that the surface impedance will depend on the lateral variations of the exciting field. It is also found that the dependence is more complicated than for a laterally isotropic half-space.>

An idealized slab model of a horizontal coal seam is adopted. The structure is laterally uniform, but the media within and outside the slab are allowed to be inhomogeneous in the vertical direction. A general mode equation is obtained, and simplifications are made to facilitate computation of the propagation constant of the dominant mode. Using an exponential variation of the rock conductivity, it is shown that the frequency dependence of the attenuation rate is strongest for a conductivity increasing with distance into the rock media.>

An oversimplification of the treatment of the incidence angle and of the source dependence of magnetotelluric surface impedance in a previous treatment of magnetotelluric exploration of hydrocarbons (see ibid., vol.77, no.2, p.287-317, 1989) is pointed out. The author of the original paper agrees with the comments and recognizes the paper's lack of rigor. He points out that the intent of the paper was to present a broad overview to a general audience and to focus on the utility and applications of the method.>

The commenter points out that the general solution for the electric field of a current line source of alternating current over a stratified earth of any number of layers published in the above letter (see ibid., vol.75, no.1, p.170-171, 1987) was obtained by him in a paper that he published 24 years ago. The authors acknowledge Wait's prior claim, observing that parallel research of this sort is too often encountered.>

D.J. Tylavsky et al. (see ibid., vol.74, no.9, p.1290-1292, 1986) formulate the boundary value problem of an electric line source eccentrically located in a cylindrical cavity. Their solution is consistent with the full-wave solution for a similar problem arising in electrical geophysics. The commenter claims that, in reducing their solution to a simpler form, the authors make some physical approximations which are questionable. In replying, Tylavsky et al. agree with Wait's comments that the application of a planar image model to the case of a conductor eccentrically located in a cylindrical dielectric is indeed a nontrivial problem.>

We review past research on the propagation of electromagnetic pulse signals over the surface of the earth with emphasis on analytical methods to predict waveforms. The best example is the electromagnetic field radiated from a lightning stroke that can be observed at global distances or very close in if one has sufficient motivation. In any case, the propagation channel will distort the signal wave shape because of the strong frequency dependence of the attenuation factors and the phase velocities of the modes. We begin with a flat earth model that has the virtue of simplicity particularly when displacement currents in the earth are relatively small compared with the conduction currents. Here we see that the rise time of the ideally radiated step function signal is of the order of a few microseconds at a range of 50 km over average land paths. Such a prediction is in accord with published experimental data. At greater ranges, earth curvature comes into play and we show here how various analytical and numerical methods can be used in overlapping time regions which serves as a good consistency check. Other complicated effects such as mixed land-sea paths and ionospheric influences are also reviewed.

Using the idealized model of a leaky coaxial cable in a circular tunnel to represent a leaky-feeder communication channel, we examine both pulse transmission and the bandwidth limitations of the channel. The phase and group velocities of the bifilar mode are found to be nearly independent of frequency between 1 MHz and 100 MHz, and the channel bandwidth is found to be quite large. The effect of mode excitation is also examined by treating the mutual impedance between a pair of short electric dipoles as the transfer function and the channel bandwidth is still large.

The theory of transmission in a semicircular tunnel containing two axial conductors is simplified for the case of radio frequencies. It is shown that the effect of ohmic losses in the wall can be represented as series impedances per unit length in the equivalent coupled transmission line circuits. This opens the way for a systematic calculation of the mode conversion phenomena that occur when one of the axial conductors has a discrete shunt load to the ground plane. Explicit formulas are obtained for the modal conversion coefficients. These are used to discuss the effect of a shunt load on the trolley wire on the performance of a radio frequency communication system. It is confirmed that the unloaded dedicated communication line provides for a low loss mode that is hardly affected by the shunt load on the adjacent trolley wire in the tunnel.

We present an analysis of the propagation in a uniform tunnel of semicircular cross section that contains two axial conductors. In particular, we show that, even if one of these conductors has high ohmic loss, one of the two propagating modes has relatively low attenuation. This corresponds to the situation where an unloaded dedicated wire is used to improve radio frequency communication in a mine tunnel that contains a heavily loaded trolley wire.

The braided coaxial cable is studied as a communication scheme in a mine tunnel. A simplified rectangular waveguide model is adopted for the tunnel, and the shield of the cable is assumed to behave as a single inductive transfer impedance. Specific results on the attenuation of the monofilar and bifilar (or coaxial) modes of propagation, taking into account the possible existence of a thin lossy film on the cable, are presented. In order to estimate the maximum possible range of communication, we consider the coupling factors of these modes to transmitting and receiving dipoles inside the tunnel, and we present results on these factors for various cable parameters and over a wide range of frequencies.

Summary form only given, as follows. Prof. Delogne has made some very useful and pertinent comments on the subject of this paper. An improved but, alas, a more complicated approach to the coupling losses is contained in a recent paper, "Calculated transmission loss for a leaky feeder communication system in a circular tunnel," by D.A. Hill and J.R. Wait, Radio Sci., vol. 11, pp. 315-321, Apr. 1976.

The coupling loss between a coaxial cable with a circumferential gap and a dipole antenna is analyzed. The numerical results have application to continuous-access guided communication (CAGC) and to related leaky feeder systems now being used in intramine communications. It is shown that the coupling is very strong in the neighborhood of the cable axis even when the linear distance between the dipole and the cable gap is large. This, of course, is a desirable feature of such limited-access communication systems.

The propagation constant of an electromagnetic wave on a long horizontal thin wire is determined from a modal equation for various situations: an elevated wire; a buried bare wire; a bare wire located in the air-earth interface; and a buried insulated wire. Analytical expressions are derived under the assumption that the height or depth of the wire is much less than the skin depth of the conducting earth and that the angular variation around the wire can be ignored. Although not restrictive in frequency, these conditions are satisfied for extremely low frequency (ELF) and very low frequency (VLF) applications except possibly for deeply buried wires when the effect of the interface is negligible anyway. We show that the propagation constant of a buried bare wire is significantly different from an elevated one for almost all heights of practical interest. When the bare wire is in the interface or buried in close proximity of the earth surface, we find that the square of the wave propagation constant is the average of the squares of the propagation constants in the two media. Of particular interest is the fact that a buried insulated wire has a much smaller propagation constant than a bare wire at the same location. Thus, a horizontal ELF radiating structure, such as the Sanguine antenna, could behave in much the same manner whether it is elevated or located in a buried cylindrical insulating cavity. This conclusion is strictly valid when the length of the structure is small compared with the wavelength of the propagating current in either case, and when adequate grounding of the structure can be achieved.

The death of Dr. Janis Galejs at age 49 in October 1972, was a great loss to the scientific community. His contributions to the theory of ELF electromagnetic waves have been outstanding. His most recent hook, Terrestrial Propagation of Electromagnetic Waves, published in 1972, summarized, in a masterful fashion, the numerous scientific papers on ELF propagation and radiation theory that he has published over a tragically brief career.