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Titel |
Comparison of Superconducting and Spring Gravimeters at the Mizusawa VLBI Observatory of the National Astronomical Observatory of Japan |
VerfasserIn |
Satoshi Miura, Hiroshi Ikeda, Tae-Hee Kim, Yoshiaki Tamura |
Konferenz |
EGU General Assembly 2017
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 19 (2017) |
Datensatznummer |
250146937
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Publikation (Nr.) |
EGU/EGU2017-11009.pdf |
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Zusammenfassung |
Continuous microgravity monitoring is utilized to gain new insights into changes in the
subsurface distribution of magma and/or fluid that commonly occur beneath active volcanoes.
Rather new superconducting and spring gravimeters, iGrav#003 and gPhone#136 are
collocated with a superconducting gravimeter, TT#70 at the Mizusawa VLBI Observatory
of the National Astronomical Observatory of Japan, since the end of September,
2016 in order to evaluate those performances before field deployment planned in
2017.
Calibration of iGrav#003 was carried out by collocation with an absolute gravimeter FG5
of the Earthquake Research Institute, University of Tokyo (Okubo, 2016, personal comm.) at
a Fundamental Gravity Station in Sendai in July, 2016. Based on the scale factors of
iGrav#003 obtained by the calibration and of gPhone#136 provided by the manufacturer
(Micro-g LaCoste, Inc.), tidal analyses are performed by means of BAYTAP-G (Tamura et
al., 1991, GJI). Amplitudes and phases of each major tidal constituent mutually agree well
within ±4 % and ±3 degrees, respectively.
The instrumental drift rate of iGrav#003 is very low, about 5 micro-Gal/month, whereas
that of gPhone#136 is very high, about 500 micro-Gal/month. The high drift rate of
gPhone#136, however, is well approximated by a quadratic function at present and can be
removed. The detrended time series of gPhone#136 shows good agreement with iGrav#003
time series in the overall feature: gravity fluctuations with amplitudes of about a few
micro-Gal and with durations of a few days, which may be due to variations in the
moisture content of the topmost unsaturated sedimentary layer and the water table
height. It suggests that both instruments may capture volcanic signals associated
with pressure changes in magma chambers, dike intrusion/withdrawing, and so
on.
iGrav#003 will be installed in the Zao volcanological observatory of Tohoku University
located at about 3 km from the summit crater, and gPhone#136 will be deployed
in the Jododaira Astronomical Observatory located at about 0.5 km from Oana
crater of Azumayama volcano in the spring of 2017. Both of the volcanoes, Zao and
Azumayama show minor volcanic activity with frequent shallow earthquakes, changes in
the total magnetic force, pressure changes at depth, and so on in 2014 and 2015. |
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