Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 433

where b is the characteristic time of chamber expansion. In Fig. 11.3 the data
recorded during the containing underground explosion “Bilbi” (Zablocki 1966 )are
compared with the numerical calculations which are based on the above model
and Eq. ( 11.17 ) (Ablyazov et al. 1988 ). As is seen from this figure, the theoretical
dependencies shown with lines 2 are in qualitative agreement with the observations
shown with lines 1. The vibrations followed by the initial spike can be explained
by the hydrodynamical instability of the expanding plasma. Hydrodynamic waves
excited in the plasma and products of detonation can be reflected from the walls
and center of the chamber thereby producing the modulation of the EMP signals
in amplitude and frequency (Gorbachev et al. 1999). It should be noted that in
specific events the EMP exhibits the polarization corresponding to the field of a
magnetic dipole whereas the polarization in other cases is rather close to the electric
dipole one.

11.2 Electromagnetic Effects Due to Shock Wave (SW)

and Rock Fracture

11.2.1 Electric Dipole Moment Due to Shock

Polarization of Rocks

A SW generated by the contained underground explosion gives rise to rock
polarization which in turn can serve as a possible source for the electric dipole
(Surkov 1986 ). The shock polarization effect in laboratory conditions have been
studied in any detail in Sect. 9.1 . Here we deal with large-scale polarization
phenomena under the natural situation. There are a few stages of the deformation
and rock fracture caused by an underground explosion. At first the fast expansion of
the underground chamber due to plasma impact and vaporation of the chamber walls
results in the generation of the strong SW with pressure amplitude 10 11 -10 12 Pa
(e.g., Zeldovich and Raizer 1963 ; Chadwick et al. 1964 ; Rodionov et al. 1971 ;
Baum et al. 1975 ). At this stage called as hydrodynamical one the rock strength
can be neglected, and the pressure amplitude decreases with distance as r 3 over
a length of several meters or tens meters. During this stage the pressure falls off
by 3-4 order of magnitude, and then the amplitude attenuation obeys the law r n ,
where 1<n<2. As before the shear stress will exceed the crushing strength
of the rock so that the fracturing of rocks occurs behind the SW front. Then the
crushing wave begins to decelerate and thus fails to keep up with the main shock
so that the primary wave is split into two waves. At the moment of the crushing
wave stop the radius of zone of complete fracturing reaches tens or hundreds meters
depending on the energy of explosion. The tension stresses take place in the region
between the fracturing zone and the SW front. Since these stresses exceed the
ultimate tensile strength, there develop the radial cracks in this region. Typically the