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| Titel |
Measurement technology for seismomagnetic signals |
| VerfasserIn |
Valery Korepanov, Fedir Dudkin, Andriy Marusenkov |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032628
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| Zusammenfassung |
| Ultra low frequency (ULF) band (0.001-3 Hz) is usually used for study of natural magnetic
field variations of ionospheric and magnetospheric origin. At present this frequency range
gains in importance at monitoring of lithospheric magnetic activity in seismo-hazardous areas
for application to short-time earthquake (EQ) forecasting. A big number of publications
confirm that ULF magnetic precursors were recorded from few weeks up to few hours before
EQ.
The measurement technology of these signals has several peculiarities. First, the
lithospheric ULF EQ magnetic precursors as a rule are very weak and their frequency range
is overlapping with signals of magnetospheric or ionospheric origin. Second, for
resolution of magnetic precursors at the background of more powerful sources it is
necessary to have magnetic field sensors with wide dynamics and minimum possible
spectral noise density (SND) level. Additionally, monitoring of lithospheric activity
should be provided in close proximity to probable EQ area and almost in real-time
regime.
For the study of ULF magnetic precursors the magnetometers with search-coil (SC) and
fluxgate (FG) sensors are used. SC sensors for ULF band usually have length 0.8-1.2 m,
diameter 10-15 cm and weight few kilograms with SND 0.1-200 pT/Hz0.5 (here and further
maximum SND value relates to a lower part of frequency range). FG sensors are very
compact (pencil-shaped with length ~ 4 cm) but have greater SND in this band (about 10-500
pT/Hz0.5).
Next requirement, if to use SC, is that at 3-component magnetic field measurement it is
necessary to provide spacing between sensors about 1-2 of their length for avoiding mutual
influence between them. This requirement creates problems caused by non-rigidity of such
construction and their spatial instability relatively ground surface (or horizontal
plane). In addition, for such a long sensor a ratio of core length/diameter is big
enough, what leads to increased SC sensor sensitivity to variety of mechanical
deformations of sensor body. These factors increase the real SC SND because of induction
effect in the Earth’s magnetic field. Simple estimations show that sensitivity to
changing of sensor axis direction can achieve a level about 250 pT for one second of
arc.
To overcome majority of these problems, a specialized FG with length 10 cm has been
developed. This newly developed device has SND in ULF band about 1-30 pT/Hz0.5
and moderate consumed power. Additional merit of this sensor is extremely low
noise density in the most prospective EQ magnetic precursors frequency range
(0.001-0.03 Hz) – about 3-30 pT/Hz0.5- which is less than SND for the best recent SCs. A
ULF magnetometer with such a compact solid sensor unit at partial compensation
of the Earth’s magnetic field in the sensor volume allows drastic decreasing the
mechanical artefacts influence and facilitates the constructing of measuring sites for field
works.
As an example of SND necessity decrease the experimental data from seismo-hazardous
region of China are discussed. It is shown that high SND of magnetometers leads to
appearance of false background lithospheric signals and complicates the procedure of EQ
related signals selection.
The comparison of parameter set for FG and SC has been made and a specific design of
FG dedicated for seismogenic ULF signals measurements has been discussed.
This work is supported by STCU grant 4818. |
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