Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 424

The summary of the characteristics of majority of the signals could be made
in the following way. The rise time of the initial spike is about 8-15 ms, and the
polarization of perturbations is so that the vector of electric field is predominantly
directed from South to North (Fig. 11.1 ). Within the time interval of 30-70 ms the
field decreases approximately exponentially. Powerful detonations, as a rule, are
accompanied by the shock excitation of the irregular vibrations lasted nearly 0.4 s.
These vibrations reach 20 % of the signal magnitude (for example, see Fig. 11.2 a).
On numerous occasions the initial positive spike was followed by the negative
half-wave with duration from 0.5 to 1 s as shown in Fig. 11.3 a, b with lines 1. The
power spectrum of the EMP has spikes in the frequency ranges of 2-8 and 20-
30 Hz. Besides the power spectrum tends to increase with decrease in frequency,
which are lower than 2 Hz.
One more example is the nuclear test referred as “Bilbi,” which was detonated
at the depth of 714.5 m on September 1963. This contained underground explosion
had a TNT equivalent Y D 235 kt, i.e. more than that considered above by one
order of magnitude (Zablocki 1966 ). In this case the amplitude of electric field
component, E ' , was 3.6 V/m at the distance 7.62 km to the South (along the
magnetic meridian) of the detonation epicenter point. The rise time of the initial
spike of the EMP did not exceed 15 ms and the time of the decrease down to zero
level was approximately 150 ms. Notice that the rise time of the initial spike varies
within 8-15 ms for all the tests. This value is larger than that of atmospherics, whose
typical build-up time does not exceed 5 ms.
The relaxation time, r , of the electric component of EMP as observed in the
series of experiments is shown in Fig. 11.4 as a function of TNT equivalent, Y ,of
the detonation. This empirical dependence can be approximated by the following:

r D 30Y 1=3 ms, (11.1)

where Y is measured in kt (1 kt is approximately equal to 4 10 12 J).
The magnitude of electric field variations decreases approximately inversely
proportional to the cubed distance, at least as the distance is smaller than 10 km. The
empirical dependence of the horizontal component of electric field on the epicentral
distance and TNT equivalent is given by (Zablocki 1966 ):

E D 2:2 10 2 Y 0:44 =R 3 ; V/m. (11.2)

where the distance R is measured in kilometers. The typical magnitude of electric
field can reach several tens mV/m under the detonation with TNT equivalent smaller
than 150 kt (Malik et al. 1985 ; Sweeney 1989 ).
It is usually the case that the magnetic component of the EMP by nuclear
underground explosions varies from several pT to several nT at the epicentral
distances which are no more than 10 km. For example, the magnetic measurements
during the detonation “Hardin” at the testing area in Nevada in 1987 have shown that