10:15 AM - 10:30 AM
[SGD01-16] Geodetic measurements and quantitative evaluation for reduced gravitational redshift uncertainty of NICt optical frequency standards -Part 3-
Keywords:optical atomic clock, gravity measurement, gravitation redshift, groundwater, GNSS, leveling
The National Institute of Information and Communications Technology (NICT) has developed a Sr optical lattice clock and optical ion clocks employing In+ and Ca+. The centimeter-level uncertainty of site elevation is the cause of 10-18-level frequency uncertainties of optical frequency standards. It is significantly important to understand frequency changes caused by solid-earth tides that often range from 10 to 20 cm in amplitude, by oceanic tidal loading, crustal deformations due to earthquakes, and ground movements with groundwater changes for the stable operation of optical atomic clocks.
NICT and partners including the Geospatial Information Authority of Japan (GSI) have been jointly conducting leveling surveys and relative gravimeter observations at NICT's headquarters in Koganei. These observations reduce the contribution of gravitational redshift to the total uncertainty of the optical lattice clock to the 10-19 level. With the support of National Institute of Polar Research (NIPR), absolute gravity measurements were performed in August 2019 and May 2022 to evaluate the effects of the 2011 off the Pacific coast of Tohoku Earthquake on postseismic crustal movement. The obtained absolute gravity change between the two periods was -43.8 μGal, which matches the trend of GNSS vertical movement obtained by GSI, which show a vertical movement of up to 31.5 mm from August 2019 to May 2022, equivalent to about -10 μGal gravity change, even though the values do not agree precisely.
We have introduced a Micro-g LaCoste's gPhoneX gravimeter for continuous gravity measurements nearby the optical clocks in the end of 2021 and have started to investigate the temporal variation of the ground water level in Koganei. We carried out evaluation of our gPhoneX precision by comparison to absolute gravity measurements by supporting Mount Fuji Research Institute (MRFI) and at Ishioka Geodetic Observing Station of GSI. Comparing gPhoneX data sets and two FG5 absolute gravity data sets of the Earthquake Research Institute (ERI) of the University of Tokyo, the precision is about 22 μGal. We are also monitoring vertical crustal movements by geodetic GNSS measurements. The groundwater levels of 42 observation wells around Tokyo are operationally monitored by the Tokyo Metropolitan Government (TMG) with one-hour temporal resolution. This data sets will help us investigate effects of groundwater variations on gravity change. In addition, we will install a soil moisture sensor in Koganei to compare ground water change and precipitation. Studying these effects close to our optical clocks will help us gain an understanding of their influence on the clock's operation.
NICT and partners including the Geospatial Information Authority of Japan (GSI) have been jointly conducting leveling surveys and relative gravimeter observations at NICT's headquarters in Koganei. These observations reduce the contribution of gravitational redshift to the total uncertainty of the optical lattice clock to the 10-19 level. With the support of National Institute of Polar Research (NIPR), absolute gravity measurements were performed in August 2019 and May 2022 to evaluate the effects of the 2011 off the Pacific coast of Tohoku Earthquake on postseismic crustal movement. The obtained absolute gravity change between the two periods was -43.8 μGal, which matches the trend of GNSS vertical movement obtained by GSI, which show a vertical movement of up to 31.5 mm from August 2019 to May 2022, equivalent to about -10 μGal gravity change, even though the values do not agree precisely.
We have introduced a Micro-g LaCoste's gPhoneX gravimeter for continuous gravity measurements nearby the optical clocks in the end of 2021 and have started to investigate the temporal variation of the ground water level in Koganei. We carried out evaluation of our gPhoneX precision by comparison to absolute gravity measurements by supporting Mount Fuji Research Institute (MRFI) and at Ishioka Geodetic Observing Station of GSI. Comparing gPhoneX data sets and two FG5 absolute gravity data sets of the Earthquake Research Institute (ERI) of the University of Tokyo, the precision is about 22 μGal. We are also monitoring vertical crustal movements by geodetic GNSS measurements. The groundwater levels of 42 observation wells around Tokyo are operationally monitored by the Tokyo Metropolitan Government (TMG) with one-hour temporal resolution. This data sets will help us investigate effects of groundwater variations on gravity change. In addition, we will install a soil moisture sensor in Koganei to compare ground water change and precipitation. Studying these effects close to our optical clocks will help us gain an understanding of their influence on the clock's operation.