Transmission of stress induced electric signals in dielectric media. Part III
The frequency dependence of the electric field produced by an electric current dipole lying inside or very close to a highly conductive cylinder (s), of infinite length and radius R, embedded in a significantly less conductive medium (s') is investigated. The study is extended to the case when the dipole is located inside or very close to a highly conductive layer (s), of infinite extent and width w, embedded in a significantly less conductive medium (s'). At large distances d from the source and for appreciably low frequencies, i.e., f<<fc = 1/(2pžsR(2)) for the case of the cylinder or smaller than some f0 = 1/(žsw(2)) for the case of the layer, the electric field follows mainly the properties of the outer (host) medium and hence its attenuation is governed by a skin depth deltaout corresponding to the outer medium. For higher frequencies, i.e., f>fc or f >f0, the electric field is attenuated with a skin depth significantly smaller than deltaout having as a lower limit the skin depth deltain for a full volume of conductivity s. The properties of the recordings, at remote distances, from a current dipole source emitting an exponentially decaying signal of unit initial amplitude and a relaxation time constant t is also studied. The source is located either inside a full volume of conductivity s' or inside a conductive half-space at a depth h (<< d). In the case of homogeneous medium s', a certain relaxation time t0 = žs'd(2)/4 exists due to the diffusion dynamics, leading to the following conclusions. If t << t0 the recordings have an enlarged duration of around t0 and an amplitude orders of magnitude smaller than that corresponding to the static case. If t > t0 the recordings have a duration almost equal to that of the emitted signal, and arrive after a time of around t0, and their amplitude approaches that of the static case. In the case of the current source located within a conductive half-space, the signal recorded at remote observation sites on the interface, arrives in two parts. The first part (which "diffuses" from the source to the interface and then propagates as a surface wave) has a duration significantly smaller than the second part ("solely diffusing" through the conductive medium). The study is extended to the case of a series of signals emitted at equal time-intervals. The application of the above to the earth explain the absence of co-fracture signals but do indicate, that the precursory Seismic Electric Signals reach detectable values at certain sites on the earth's surface.
Bibliogr. 21 poz.