Geoscience Reference
In-Depth Information
Chapter 7
Geomagnetic Perturbations (GMPs)
Abstract
In previous chapters we discussed the magnetospheric, ionospheric, and
atmospheric sources of the ULF electromagnetic fields. The main emphasis has
been put on studies of the global lightning activity, which results in an energy
storage inside the resonators followed by the excitation of global electromagnetic
resonances such as Schumann and IAR resonances. Furthermore, there are a
variety of other terrestrial and atmospheric causes for the generation of ULF
electromagnetic fields: ocean waves, tsunami, volcanic eruptions, earthquakes
(EQs), meteoritic falls to the atmosphere, as well as the man-made sources such as
the stray current, atmospheric and underground explosions, and so on. It is usually
the case that the large-scale ULF perturbations resulted from the magnetospheric
sources and lightning activity can be easily distinguished from the terrestrial fields
and different kinds of local noises but sometimes there are a few problems because
the terrestrial fields may be very weak.
In this chapter we focus our attention on the study of low-frequency electro-
magnetic fields associated with large-scale tectonic processes. In the course of this
text, some of the phenomena related to natural disasters will be treated in detail
and others in a more sketchy fashion. We also consider in-situ measurements and
laboratory tests of the low-frequency electromagnetic fields generated during the
deformation and fracture of solid.
We start with the problem of generation and propagation of the geomagnetic
field perturbations caused by the seismic waves travelling in the ground. These
perturbations are of special interest in applied geophysical studies because they
provide us with additional information about the depth and energy of seismic source
as well as about the conductivity and other rock parameters taken along the seismic
rays. The signals due to perturbations of the Earth's magnetic field may be useful for
the analysis and interpretation of co-seismic phenomena associated with the EQs.
A theory of these phenomena is necessary in order to study both the signal-to-
noise ratio and the electromagnetic interferences due to device vibrations caused
by seismic wave propagation.
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