Geoscience Reference
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zone of intensive radial fracturing is as much as several hundreds meters in length.
When the wave amplitude falls off below the rock strength, the medium behaves like
elastic one. At this stage which is referred as the seismic one, the SW transforms
into the elastic/seismic wave. If the dispersion-dissipative properties of the rock are
not important, then the amplitude of the longitudinal seismic wave decreases with
distance as r 1 . When this wave reflects from the day surface, it is split into the
primary/longitudinal (P-wave), secondary/transverse (S-wave) and surface Rayleigh
and Love waves.
In this brief overview we have omitted a few details of camouflet underground
explosions such as the generation of the unloading wave, dilatancy of the fractured
rock, dynamics of the camouflet chamber and etc.
The stress wave propagating in the ground and rocks is known to generate a
variety of low-frequency electromagnetic phenomena so that the interpretation of
the observation is often troublesome. One source of this variety is that the natural
materials and rocks are very inhomogeneous as for their rheological structure and
electrical parameters. For example, the ground conductivity strongly depends on the
humidity and porosity which vary with depth. The rock fracturing and pore collapse
caused by the SWs gives rise to the generation of the local electric fields and great
charges near the cracks and pores that can be accompanied by the local electric
breakdowns of the medium.
Now we discuss the existence of the shock polarization effect in nonuniform
media at different structural levels (Surkov 2000 ). Firstly, the microscopic move-
ments of the charged dislocations and point defects can result in the polarization
of individual monocrystals and grains. This effect is enhanced essentially in the
vicinity of the grain boundaries, microcracks, small inclusions, and pores. Secondly,
considering the macroscopic scale we note that the polarization processes are
localized in the regions of enhanced stresses; that is, near tips of large cracks and
individual blocks of fractured rock. So one may expect that there exists certain
hierarchy of relaxation times of the shock polarization. The largest values of the rise
and decay times of the shock polarization can exceed by several orders of magnitude
the same parameters observed under laboratory tests. Thus, we come to the
conclusion that the SW generates its own, as a rule, low-frequency electromagnetic
field due to the polarization of different structural units of nonuniform matter.
In what follows we assume the linear dependence between the rock polarization
… and the amplitude of pressure P m (Allison 1965 )
D ǛP m 1 exp
exp
.t/;
t
f
t
r
(11.18)
where Ǜ is empirical coefficient of proportionality, f is the rise time, r is the decay
time of the polarization, and .t/ denotes the step function.
The shape of the SW resulted from an explosion is very similar to a spherical
one. Suppose that behind of the wavefront the matter is polarized in the radial
direction. Besides the medium polarization has a weak asymmetry that can be
due to an irregular distribution of fracturing, asymmetry of the shock wavefront,
 
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