Geoscience Reference
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estimates for amplitudes ıB x , ıB z and E y are similar to those given by Eqs. ( 7.25 )-
( 7.27 ) because in both cases the diffusion of the electromagnetic perturbations
dominates over the effect of “frozen-in” magnetic field lines.
Under the inverse requirement ı 2 k R 1 that means that the magnetic field is
practically frozen in the conducting media, all the magnetic components are of the
order of V 0 B 0 =C R while the electric field variations are as much as V 0 B 0 .
Gorbachev and Surkov ( 1987 ) have studied Earth's magnetic field perturbations
due to Rayleigh surface waves generated by linear and point acoustic sources. In
these cases the surface wave fields radiated by seismic sources can be represented by
a superposition of quasi-harmonic Rayleigh waves given by Eqs. ( 7.59 ) and ( 7.60 ).
Far away from the point source the Rayleigh wavepacket decreases in inverse
proportion to the square root of distance r. In the model of perfectly conducting
half-space the same tendency keeps for the magnitude of the GMPs.
To summarize, we recall that during the diffusion regime the amplitude of GMPs
falls off more rapidly with distance due to the electromagnetic energy absorption
and dissipation in conducting media. At a later moment the GMPs are localized
in the vicinity of seismic wave front that results in a slow decrease of amplitude
with distance. In this case the seismic and electromagnetic perturbations depend on
distance in the same manner; that is, for the primary/longitudinal wave they decrease
as r 1 whereas for the Rayleigh surface wave they vary as r 1=2 . A strong EQ and
an underground explosion are accompanied by a variety of seismic waves including
the primary, secondary/transverse, Rayleigh, and Love surface waves, which can
be detected at the distances about several thousands of kilometers. This implies
that the co-seismic GMPs caused by these large-scale tectonic phenomena may be
detectable at such distances.
It is interesting to note also that the dispersion-dissipative properties of actual
geological media resulted from viscosity, nonuniform inclusions, and so on may
have different effects on seismic and electromagnetic perturbations (Dunin and
Surkov 1992 ). In the theory, the mass velocity amplitude V 0 of seismic surface wave
propagating in dissipative media decreases with distance as r n , where index n can
vary from 0:5 to 1:75-2:25 whereas the seismic wavelength R increases as r 1=2 .
In the frequency range f (5-50) mHz we can use estimates ( 7.25 ) and ( 7.26 )
for GMPs according to which ıB max V 0 R B 0 = m and E 0 max V 0 B 0 . Whence
it follows that ıB max r nC1=2 and E 0 max r n . Thus, in the dissipative media
the magnetic perturbations ıB max can fall off slowly as compared to the seismic
ones. The interpretation we make is that the mass velocity of conducting media
determines only the local current density whereas the magnetic perturbations are an
integral effect resulted from the net action of all the currents induced in the region
with typical scale r 1=2 .
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