Propagation of electromagnetic pulses in a homogeneous conducting earth

SummaryA general analysis for the electromagnetic response of conducting media due to pulse excitation is presented. The treatment is based on the Laplace transform theory. First, a survey of the field is made and the limitations and scope of the previous work are pointed out. The theory of propagation of a plane wave pulse in a conducting and homogeneous medium of infinite extent is then reviewed. The form of these results enable one to evaluate the relative importance of the conductivity and the dielectric constant. It is indicated, for sufficiently large times in the transient response, that displacement currents may be safely neglected for sea water and for most geological media. Under this assumption, the waveform of the electric field in a conducting medium is illustrated for the case where the source is an electric dipole energized by a step-function current. Results are also presented for exponential and bell-shaped source functions. The pulse shape of the field components is profoundly modified as they propagate through the medium. It is suggested that this property may be utilized in measuring distances in the earth's crust. The more difficult problem of propagation in non-infinite conducting media is also considered. To account for the presence of the interface in a conducting half space, (i.e. homogeneous flat ground) a rather involved analytical expression for the transient fields is required. Certain special cases, such as a horizontal electric dipole at the interface, are illustrated by numerical results. The transient excitation of a wire loop lying on the surface of a homogeneous ground is also considered. Finally, transient coupling between pairs of parallel insulated wires grounded at their end points is treated as an extension of the earlier results.