Electromagnetic Effects Resulted from Natural Disasters - Ultra and Extremely Low Frequency Electromagnetic Fields - page 391

where is the air mass density, C a the acoustic wave velocity, and @ t u denotes
the time-derivative of the ground surface displacement u . Considering the typical
microseism measurement with displacement amplitude about 10 4 cm and with the
period T D 6 s and taking D 1:2 kg/m 3 and C a D 340 m/s brings the pressure
variation in the atmosphere P 0:1 mPa, which is found to be 2-3 order-of-
magnitude smaller than the level of the atmospheric acoustic noise at the ground
surface.

10.1.7 A Problem of Direction Finding for the ULF

Electromagnetic Source

In spite of much progress towards the search of electromagnetic EQ precursors,
a major question of whether the ULF electromagnetic signals detected prior to a
seismic event are really associated with the tectonic activity is still an open question.
In this notation, the problem of direction finding of the underground source as well
as the problem of identifying a weak electromagnetic signal in the background of
natural ionospheric and magnetospheric noises and man-made interference are of
special interest in geophysical studies.
In order to solve this problem one comes across a number of serious difficulties
and complexities. First, the characteristic wavelength (in vacuum) in the ULF
frequency range is so large that an observer is always situated in the near zone,
i.e. in such a case the traditional radiowave methods, such as the wave time lag
measurement or miscellaneous interference schemes, are hardly applicable. Second,
a primary ULF source which is of main interest here, is seemingly located under the
ground, maybe at higher depths. In such a case the electromagnetic signals undergo
a strong dissipation and dispersion since the ULF field spreading in conductive
layers of the ground is governed by the diffusion law. The typical time scale of
the perturbations that can be related to the EQ precursor is as large as several tens
minutes or hours so that the front of the perturbations considerably extends in time.
The application of traditional technique to the ULF source is based on a network
of the ground-recording stations equipped with magnetometers in order to detect
the time lag or the phase difference between signals recorded at different points
(Kopytenko et al. 2002 ; Ismaguilov et al. 2003 ; Hayakawa et al. 2007a , 2011 ).
One of the challenges of EQ prediction is how to distinguish useful signals from
various electromagnetic noises originated from ionospheric and magnetospheric
sources. In this notation, one should pay attention to the polarization of the field
generated by ionospheric and magnetospheric sources. Considering the ground-
based observation, we note that the vertical component B z of this field should be
smaller than the horizontal one B h due to the conventional impedance boundary
conditions at the conducting ground surface. Here B h D B H C B D , where B H
and B D are both orthogonal components of the horizontal magnetic field. The
ratio of spectral densities of vertical and horizontal components of magnetic field