Electromagnetic Effects Resulted from Explosions - Ultra and Extremely Low Frequency Electromagnetic Fields - page 422

treaty verification of nuclear undearground tests (e.g., Latter et al. 1961 b; Gorbachev
et al. 1999a ,b). Much emphasis has been put on studies of the EMP in order to detect
any underground nuclear testing especially in the case of the so-called decoupling
of underground nuclear explosion (Zablocki 1966 ; Sweeney 1989 ). The decoupling
means that the underground explosion is realized in an evacuated volume in the
chamber of large size in order to diminish seismic effect of the explosion (Latter
et al. 1961 a; Herbst et al. 1961 ; Patterson 1966 ). Although the EMP magnitude of
the explosion with decoupling can even be greater in comparison with that of the
explosion conducting under the usual size of the explosion chamber (Gorbachev
and Semenova 2000a , b ).
Below we review experimental data and then focus on basic physical mechanisms
of this phenomenon and estimate the amplitude of ULF electromagnetic variations.
The observations have shown that the EMP of underground explosions decreases
rapidly with distance so that it is practically undetectable at the distance over
10 km from the detonation point. For instance, the electric field amplitude was
approximately 1 V/m at the epicentral distance of 6.72 km (Zablocki 1966 ). A
typical scheme of the recording electrodes arrangement under Hardtack II series
of nuclear testing is shown in Fig. 11.1 . The non-screened isolated copper wire
with length of 750 m was laid on the ground from the observation point towards
to the explosion epicenter in the East-West direction. The wire ends were linked
with the lead electrodes buried in the ground 1-3 m deep. The same length wire
was laid in the perpendicular North-South direction. Next one was put into a
hole at the depth of 30 m. This wire is ended by the lead electrode as well. The
recording sensors measured the potential difference between grounded ends of each
wire. The natural potential difference that always occurs while a pair of grounded
electrodes is connected was compensated at the inputs of the recorder by means
of a potentiometer circuit. The bandwidth of sensors was in an interval from 0
to 220 Hz. Such a system allows us to control all three components of the low-
frequency electromagnetic field.
The magnetic field perturbations were also recorded during a series of nuclear
tests in 1961. The magnetic coils with vertically directed axis were used to perform
the measurements of vertical component of the magnetic perturbation. Eight turns
of wire were winded round the frame of coils, whose size was from 7.5 to 18.6 m.
The eigenfrequency of electromagnetic vibrations of the coil was within 10-20 kHz,
and these values significantly exceed the typical frequencies of the EMP.
Magnetic antennas with horizontal axis measured the horizontal component of
magnetic field variations. The coils have an area of 2 m 2 area and 3:2 10 4 turns that
correspond to the eigenfrequency about 60 Hz. The other kind of coils are 1 m 2 area,
7 10 3 -turn loop of wire, so that the eigenfrequency is 200 Hz. A variety of ampli-
fiers and filtering schemes were used to give the maximum of signal-to-noise ratio.
The EMP signal recorded at a proving ground in Nevada in 1958 during one
of five underground explosions of the Hardtek series is depicted in Fig. 11.2 .The
depth of this explosion was 254 m, and trinitrotoluene/TNT equivalent was 19 kt
(kiloton) (Zablocki 1966 ). What draws first attention is almost complete polarization
of the electric field in the direction of azimuthal component, E ' , and this feature was
practically observed in all the tests.