Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 444

The observed low frequency electric field is believed to be due to the ionization of
gas and explosion products as well as from the electrification of fractured rock and
soil fragments. In the theory by Adushkin et al. ( 1990 ) the gas-dust cloud is modeled
by a uniformly charged column filled by detonation products. At first, the negatively
charged ground fragments are assumed to be located at the top of the cloud. The
kinematic characteristics of the model were chosen in such a way to fit the numerical
calculation of the dust cloud evolution with the filming of surface explosion. The
ground fragments fall with gravity acceleration, while the precipitation of the small
dust particles is described by the following equation:

d v
dt D g

V V g
C

q E
m ;

(11.31)

where g is the free fall acceleration, V is the speed of descent of the dust particles,
V g is gas velocity, is the air viscosity, and m, q, and a stand for average mass,
charge, and size of particles, respectively. In the case of laminar flow the relaxation
time is given by D m=.6a/.Here E is the sum of geoelectric field and self-
consistent field generated by the charged particles in the air and by induced charges
on the ground surface. Assuming for the moment that the electric field is close to
the breakdown threshold in the air, that is E D 32 kV=m, and taking the numerical
parameters a D 20-50m, q D 100e (e is elementary charge), we obtain that
qE=m 0:7-1:8 m=s 2

g. In practice this means that the electric field can be
neglected as compared to the gravity.
Figure 11.9 displays the results of numerical calculation shown with dotted line
1 and the experimental observation of vertical electric field generated by a cratering
explosion with HE mass 23.8 g (line 2). To fit the numerical and experimental data,
the maximal charge of the dust cloud and the height of the cloud lift are estimated as
1.84 C and 4.2 m, respectively. In making the plot of E z the following parameters
were used D 1:7 10 5 Pa s, a D 48m. It should be noted that the shape of
initial half-wave essentially depends on the charge distribution in the ground dome
and in the gas-dust cloud. The sharp spike in the beginning of the second half-wave
(dotted line 1) is based on the assumption that all the rock fragments begin to fall
simultaneously. As is seen from Fig. 11.9 , the experimental data is qualitatively
consistent with the simple model presented above. So, the quasi-static electric field
caused by the excavating explosion is most likely to be due to the motion of electric
charges located in the ground dome and gas-dust cloud.
One more example of transient electric fields detected during a powerful surface
explosion with mass 500 t is displayed in Fig. 11.10 (Soloviev and Surkov 2000 ).
The seismic wave arrival at the ground-based station has not a visible effect on the
electric field, and the reason may be that the signal was below the sensor sensitivity.
The vertical electric field at the distance 1.5 km (line a) reaches a peak value about
2.5 kV/m at the moment t D 40 s. The numerical calculation shown with line c
is based on the simple model of gas-dust cloud which consists of the spherically
symmetric charge q 1 situated at the altitude h 1 over the ground and the uniformly
charged column with total charge q 2 and altitude h 2 . The best fit of the calculated