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Titel |
New sensor for study of ULF magnetic activity |
VerfasserIn |
Andriy Marusenkov, Fedir Dudkin, Vira Pronenko, Valery Korepanov |
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 |
250032646
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Zusammenfassung |
Ultra low frequency (ULF) variations (0.001-3 Hz) of natural magnetic field are very
important for ground study of magnetospheric and ionospheric magnetohydrodynamic waves
and solar-terrestrial interaction. Besides it a lithospheric ULF magnetic activity
is recently considered as very promising candidate for application to short-time
earthquake (EQ) forecasting. Present progress in spectral analysis methods and
data processing instrumentation allows studying of signal fine structure almost in
real-time operation condition, which is of great importance, particularly, for short-time
prediction problem. Usually the lithospheric ULF EQ magnetic precursors are much
weaker than magnetospheric signals and their frequency ranges are completely
overlapped.
At present for measurement of ULF magnetic field variations the magnetometers with
fluxgate and induction sensors are used. Fluxgate sensors are very compact (pencil-shaped
form at length ~ 3 cm) with SND in ULF band about 10-500 pT/Hz0.5 (here and further a
maximum SND value relates to a lower part of frequency range).
ULF induction (or search-coil) sensors usually have comparatively large dimensions
(length 0.8-1.2 m, diameter 10-15 cm) and weight (few kilograms) but essentially lower
SND (about 0.1-200 pT/Hz0.5). At 3-component magnetic field measurement it
is necessary to provide spacing between them about 1-2 m for avoiding mutual
influence. This requirement creates problems caused by non-rigidity of 3-sensors
construction and their space instability relatively ground surface (or horizontal plane).
In addition, for such a long sensor a ratio of length/diameter is big enough, what
leads to increased sensor sensitivity to variety mechanical deformations of sensor
body.
These factors cause additional noise appearance due to induction effect in the Earth’s
magnetic field what creates heavily recognized artefacts at signal processing. Simple
calculations show that sensitivity to changing of sensor axis direction can achieve a
level about 250 pT for one second of arc. It means that for realization of SND low
limit 0.1 pT/Hz0.5 it is necessary to provide a directional stability of sensor axes
approximately at level 2 ppb (parts per billion). Listed problems are aggravated at field
works.
Therefore, the influence of mechanical instabilities on 3-component sensor construction,
and consequently on efficient noise density (mainly in frequency range below 0.1
Hz) has to be significantly decreased. The way out from these difficulties can be
found at development of fluxgate magnetic sensor of increased length and improved
SND.
The fluxgate magnetic field sensor with 10 cm length has been developed. This device has
SND in ULF band about 1-30 pT/Hz0.5 and moderate consumed power. Its additional merit is
extremely low noise density – about 3-30 pT/Hz0.5 in the most prospective EQ magnetic
precursor frequency range (0.001-0.03 Hz), which is less than SND for the best recent
search-coil sensors.
A ULF magnetometer with such a compact solid sensor unit at partial compensation of
the Earth’s magnetic field in the sensors volume allows drastic decreasing the mechanical
artefacts influence and facilitates the constructing of measuring sites for field works in remote
areas. For geophysical observatory a combination of proposed sensor unit with a laser
real-time measurement of angular axes deviation gives possibility for additional decreasing of
a sensitivity threshold in ULF band at the cost of correction factors calculation during data
processing.
This work was supported by STCU grant 4818. |
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