Journal of the Geodetic Society of Japan
Online ISSN : 2185-517X
Print ISSN : 0038-0830
ISSN-L : 0038-0830
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Volume 60, Issue 2
Displaying 1-5 of 5 articles from this issue
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ORIGINAL PAPER
  • Estimation of Geopotential Values of Mean Sea Level along Japanese Coast
    Yuki Kuroishi
    2014 Volume 60 Issue 2 Pages 59-71
    Published: November 14, 2014
    Released on J-STAGE: July 11, 2015
    DOIhttps://doi.org/10.11366/sokuchi.60.59
    JOURNAL FREE ACCESS
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    The smoothing method proposed by Kuroishi (2012) is applied to tidal records at 35 tidal stations along the Japanese coast and resulting time series of smoothed tidal heights are combined with GNSS observation at continuous GNSS sites installed adjacent to the tidal stations, yielding the three-dimensional geocentric positions of mean sea level at the tidal stations. Then a high-resolution global geopotential model is used to compute the geopotential values of MSL at the tidal stations. The average deviation of the geopotential values at 32 stations, all of which are referred uniquely to the origin of the Japanese vertical control network, from the potential of the geoid defined by IERS Conventions (2010) is -3.83±1.35 m2/s2, corresponding to +39.1±13.8 cm in height. We compare the height deviations to leveled heights of MSL and the differences between them show the mean bias of about +26.9 cm with the standard deviation of 7.3 cm. It indicates that the Japanese vertical datum is about 27 cm above the global geoid and that the two results of MSL heights are consistent at a level better than 10 cm. Comparison of mean dynamic ocean topography between the estimated MSLs at the tidal stations and an altimetry-derived global model shows some spatial features in their discrepancy that match qualitatively with tilts induced by geostrophic currents: the Kuroshio, the Tsushima Current, the Tsugaru Warm Current, and the Oyashio.
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TECHNICAL REPORT
  • The Importance of Hydrological Disturbance Corrections for Relative Gravity Data: A Case Study at Sakurajima Volcano, Southern Kyushu, Japan
    Takahito Kazama, Keigo Yamamoto, Yoichi Fukuda, Masato Iguchi
    2014 Volume 60 Issue 2 Pages 73-89
    Published: November 14, 2014
    Released on J-STAGE: July 11, 2015
    DOIhttps://doi.org/10.11366/sokuchi.60.73
    JOURNAL FREE ACCESS
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    An empirical water balance model was proposed to correct hydrological disturbances in relative gravity data measured repeatedly at Sakurajima Volcano, southern Kyushu, Japan. Gravity measurements have been conducted at Sakurajima Volcano to quantitatively monitor gravity signals due to magmatic activities, while the signals have often been obscured by the hydrological disturbances originating from precipitation, soil water infiltration, and groundwater flow. We here present the first applied results of the empirical model, in order to evaluate the reproducibility of the estimated hydrological disturbances. The hydrological disturbances were simply calculated by the product of the land water storage and the instantaneous gravity response to precipitation of 1 mm depth, which were estimated using the observed meteorological data and the digital elevation model, respectively. The calculated hydrological disturbance was consistent with the observed absolute gravity data at Harutayama Station from 2010 to 2011 within 8 µgal （1 µgal = 1×10－8 m/s2 ）, which was smaller than the typical accuracy of relative gravity measurements （～10 µgal）. In addition, after we subtracted (i.e., corrected) the calculated disturbances from the measured relative gravity data at Sakurajima Volcano, the average amplitude of the corrected gravity changes during 2007-2009 was reduced by 90% compared with that of the original gravity data. Since gravity changes have been measured using both absolute and relative gravimeters at volcanic areas these days, hydrological disturbance corrections should be applied to the relative gravity data, not only to the absolute one. By sophisticating the effects of spatiotemporal variations in precipitation, evapotranspiration, and infiltration capacity, this model will enable us to robustly monitor long-period and wide-spread gravity variations associated with volcanic activities.
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