Toshihiro Higashi

Accurate estimation of atmospheric effects on gravity observations made with a superconducting gravity meter at Kyoto

Abstract Atmospheric effects on gravity observations at Kyoto were estimated by using meteorological data sets at an interval of 12 h during a 4 month period from July to October in 1993. The effects owing to the air mass near the gravity station were evaluated by numerical integrals, and those distant from the station were calculated by using spherical harmonic expansions of meteorological data. The error in the calculated atmospheric effects was of the order of 0.1 μgal at most, except for the error related to the response of the oceans near the station based on the inverted barometric loading mode. About 90% of the atmospheric effects were attributed to local atmospheric variations within 50 km of the station. The remaining effects owing to the air mass outside this zone were of the order of 1 μgal, in which different features were recognized as compared with the effects owing to the regional air mass around Kyoto. The atmospheric effects thus estimated were compared with gravity data obtained by a superconducting gravity meter at Kyoto. The residuals showed gravity changes of a few microgals, a part of which might be caused by sources such as variations of the ground water level around the station.

Status of the CLIO project

The CLIO project involves the Cryogenic Laser Interferometer Observatory (CLIO) detector complex for gravitational wave detection and the Kamioka Laser Interferometric Strainmeter for the acquisition of geophysical data. CLIO has been constructed to demonstrate the feasibility of a future project, the Large-scale Cryogenic Gravitational wave Telescope (LCGT). It will utilize the low seismic and stable environment of the Kamioka mine as well as sapphire mirrors and suspension fibres at low temperature to reduce thermal noise. We designed CLIO to have a noise level limited by the thermal noise of sapphire mirrors and sapphire suspension fibres, which vary from 3 × 10−19 m Hz−1/2 at 300 K to 2 × 10−20 m Hz−1/2 at 20 K around 100 Hz. The strainmeter has already succeeded in monitoring the Earths tidal motion with a strain sensitivity of 2 × 10−12. The seismic noise veto between these same-scale interferometers is expected to provide an effective means of data selection for the gravitational wave signal analysis, and the ground motion data obtained by the strainmeter will help to maintain the stable operation of CLIO.

Design and construction status of CLIO

Construction of CLIO (cryogenic laser interferometer observatory) with 100 m baseline length has begun in the Kamioka mine. The tunnel for CLIO has been dug and infrastructure work is now in progress. CLIO is the final step to LCGT (large scale cryogenic gravitational wave telescope) and the first practical construction of a cryogenic interferometer in the world. The objective of CLIO is to demonstrate two of three features of LCGT, which are to utilize the quietness and stable environment of the underground site and to adopt cryogenic sapphire mirrors for thermal noise reduction. Also, it is a joint project by gravitational wave and geophysics researchers. CLIO has a locked Fabry–Perot configuration equipped with ring mode cleaners and cryocoolers to cool the sapphire mirrors to 20 K. The noise level of CLIO is designed to trace the thermoelastic noise of sapphire mirrors which varies from 10−18 m Hz−1/2 at 300 K to 10−19 m Hz−1/2 at 20 K around 100 Hz. A 7 m single-arm cryogenic test facility has been built at ICRR (Institute for Cosmic Ray Research), while the 20 m room temperature interferometer is in operation at Kamioka. Technical knowledge developed by these prototypes will be leveraged to realize CLIO.

Current status of the CLIO project

CLIO (Cryogenic Laser Interferometer Observatory) is a Japanese gravitational wave detector project. One of the main purposes of CLIO is to demonstrate thermal-noise suppression by cooling mirrors for a future Japanese project, LCGT (Large-scale Cryogenic Gravitational Telescope). The CLIO site is in Kamioka mine, as is LCGT. The progress of CLIO between 2005 and 2007 (room- and cryogenic-temperature experiments) is introduced in this article. In a room-temperature experiment, we made efforts to improve the sensitivity. The current best sensitivity at 300 K is about 6 × 10-21/√Hz around 400 Hz. Below 20 Hz, the strain (not displacement) sensitivity is comparable to that of LIGO, although the baselines of CLIO are 40-times shorter (CLIO: 100m, LIGO: 4km). This is because seismic noise is extremely small in Kamioka mine. We operated the interferometer at room temperature for gravitational wave observations. We obtained 86 hours of data. In the cryogenic experiment, it was confirmed that the mirrors were sufficiently cooled (14 K). However, we found that the radiation shield ducts transferred 300K radiation into the cryostat more effectively than we had expected. We observed that noise caused by pure aluminum wires to suspend a mirror was suppressed by cooling the mirror.

Absolute Gravity Measurements in Australia and Syowa Station, Antarctica

Absolute gravity measurements have been carried out in Perth and Canberra, Australia, and at Syowa Station, Antarctica, as a part of activities of the 45th Japanese Antarctic Research Expedition during 2003–2004.

Crustal movements around the Beppu Bay area, East-Central Kyushu, Japan, observed by GPS 1996-1998

In order to detect the crustal movement in the Beppu Bay area, where the highest intra-plate strain rates in Japan is expected, precise GPS measurements have been repeatedly conducted since 1994 at one-year intervals. The result shows N-S or NNE-SSE extension taking place at a rate of more than 10 mm/yr around Beppu Bay. This is essentially consistent with the result obtained by the geodetic survey conducted over the last 100 years, and it suggests a constant rate of crustal motion over the period. The result gives a strong constraint for future modeling of the stress field recovery in the area. However, we failed to identify whether the movement occurred at any specific fault. A denser array of GPS points is needed in the area, and especially near the major faults, in order to investigate possible fault-related motion.

Repeated Measurements of Gravity with the Absolute Gravimeter FG5 #210 at Matsushiro, Japan and Comparison with the Superconducting Gravimeter T011

Absolute gravity measurements have been repeatedly made with the FG5 #210 at the superconducting gravimeter station in Matsushiro, Japan. Four out of the five experiments so far performed were successful. The data from the absolute gravimeter are used to calibrate the instrumental sensitivity and drift of the superconducting gravimeter TO 11 with good results. Drift of the absolute gravimeter is investigated at the same time. Long-term gravity changes are discussed by collocating the data from the absolute and superconducting gravimeters.

A Gravity Database of Southwest Japan: Application to Bouguer Gravity Imaging in Kyushu District, Southwest Japan

In order to investigate subsurface density structures with the highest precision, two major gravity databases and some unpublished data files, which cover Southwest Japan, are recompiled. This paper presents some examples of the detailed gravity anomaly maps in Kyushu District, southwest Japan. These maps indicate various fine subsurface structures, which may arose with tectonic movements and/or volcanic activities of this region. With an aid of geological and topographical information, we discussed relationships between characteristic patterns of the detailed gravity anomalies and subsurface structures in this region. The most conspicuous feature is an existence of a steep horizontal gradient zone of gravity anomalies running through the Aso Caldera in the ENE-WSW direction. Although the location of the westward extension of the Median Tectonic Line (MTL) in this region is still debatable, this zone seems to be the westernmost part of MTL.

Gravity and GPS Observations in the Inland, Antarctica

In the severe environment of polar regions geophysical and geodetic data are relatively sparse. Although various satellite data are recently available in polar regions, it is extremely important that surface observation data be used as ground truth.