Geomagnetic Perturbations (GMPs) - Ultra and Extremely Low Frequency Electromagnetic Fields - page 274

7.2 Local GMPs Due to Seismic Waves

in Conductive Ground

7.2.1 In-Situ Measurements

The seismic waves in the Earth's crust caused by EQs and explosions, the heavy
sea, tsunami can generate the local perturbations of the Earth's magnetic field. In
the case under consideration the geomagnetic field plays the role of an external
magnetic field and the ground serves as a conducting medium. The electric currents
in the ground are developed due to the conductor motion in the seismic wave. The
electric currents give rise to the geomagnetic field perturbations. In other words,
this MHD effect arises from the movement of a conducting layer of the Earth's
crust or seawater in the presence of the Earth's magnetic field. In geophysical
studies this effect is frequently termed the inductive seismomagnetic effect. One
more phenomenon termed the seismoelectric effect in moist soil will be examined
in more detail in Chap. 8 . Both of these phenomena associated with seismic waves
propagation, i.e., seismoelectric and inductive seismomagnetic effects, are referred
to as the class of co-seismic phenomena.
In this section we deal with the MHD effects, i.e., the perturbations of the Earth's
magnetic field, which are capable of explaining the electromagnetic perturbations in
the atmosphere as observed by ground-based magnetometers and antennas during
and just after the seismic wave arrival at a measurement point. The great majority
of signals routinely detected are observed far away from large-scale explosions. As
one example, let us consider the ground-based magnetic and electric measurements
made at distances 1.5-5.5 km from the explosion point during a series of industrial
explosions in Khorezmskiy region of the USSR (Anisimov et al. 1985 ). It was
found that the seismic, electric, and magnetic perturbations take place practically
simultaneously. The total duration of both seismic and electromagnetic signals
was about 7-11 s. Additionally, the frequencies of the electromagnetic and seismic
vibrations are about 1 Hz and correlate with each other. Typically, the magnitudes
of the electric field vary within 1-10 V/m while the magnetic field variations are
about several nT.
The similar ULF phenomena have been observed during underground nuclear
explosions (e.g., see the paper by Sweeney ( 1989 )). The measurements were made
at the ground-recording stations located at the distances 5 and 10 km from the
epicenter of explosion. At the first station the electromagnetic signal has been
detected 1:5 s after the explosion and it has been observed at the second station after
3 s. The amplitudes of the magnetic and electric components exceed hundreds pT
and tens V/m, respectively. These signals are usually observed at the background
of the so-called electromagnetic pulse (EMP) appearing practically simultaneously
with the explosion moment. In Chap. 11 we show that the EMP is caused by products
of nuclear detonation gamma quantum emission and so on. In this picture, the time
lag of the recorded signal for several seconds can be explained by the action of
another mechanism such as the local GMPs or seismoelectric effect because the