Yuichi Imanishi

Verifying the precision of a new generation absolute gravimeter FG5—Comparison with superconducting gravimeters and detection of oceanic loading tide

We carried out a several day experiment at two sites to verify the precision of the FG5 absolute gravimeter through collocation with superconducting gravimeters. We found 2 to 4 microgal diurnal/semi-diurnal signals in the time series of absolute gravity measurements. These signals agreed well with theoretical ocean tide both in amplitude and in phase, demonstrating that the FG5s precision is better than 2 microgal. Furthermore, we found that the discrepancy between the FG5 measurements and the SG measurements is 1.3 to 1.5 microgal. These results verify that the FG5 is precise to 1 to 2 microgal.

Comparison of Superconducting Gravimeter and CHAMP Satellite Derived Temporal Gravity Variations

The operational Superconducting Gravimeter (SG) network can play an important role for validation of satellite-derived temporal gravity field variations. A comparison shows a quite good agreement between SG and CHAMP results within their estimated error bars. It could be proved that the SG-derived temporal gravity variations are representative for a large area within the µgal accuracy, if the local gravity effects are removed.. The long-periodic tidal waves are well determined by ground measurements, therefore they can be applied as a reference for validation. For further validation, field SG measurements should be carried out in representative areas with large gravity variations (e.g. Amazon area).

Gravity Observation by Means of a Superconducting Gravimeter at Matsushiro, Japan

Gravity observation by means of the superconducting gravimeter #11 has been started at Matsushiro Seismological Observatory, Japan Meteorological Agency. This gravimeter is the one that used to be located at Kakioka, Japan. The new gravimeter site is inside a tunnel where the environmental conditions are very stable. The gravimeter is bottom mounted to eliminate the disturbances from local ground motions. Absolute gravity measurements were carried out to calibrate the scale factor of the superconducting gravimeter. Preliminary analysis of the gravity data shows that the noise level of the gravimeter at seismic bands has been improved with its move to Matsushiro.

Free oscillations of a fluid sphere in an infinite elastic medium and long-period volcanic earthquakes

A source model of long-period volcanic earthquakes is presented. We consider that a fluid-filled spherical cavity surrounded by an infinite elastic medium is excited into resonance like the Earth’s free oscillations. The eigenequation of this system is derived in a general manner, making use of the spherical harmonic and spherical Bessel expansions. The solution is given as a complex number; its real part is the eigenfrequency and the imaginary part represents the attenuation coefficient of the oscillation. The eigenmodes are classified into five groups: (1) the compressional modes in a fluid sphere, (2) the compressional modes in a solid medium, (3) the shear modes in a solid medium, (4) the Stoneley modes, and (5) the torsional modes. We apply them to the long-period volcanic earthquake observed at Asama volcano, Japan. Estimating the characteristic frequencies and attenuation coefficients of the observed vibrations and assuming that the primary component (f = 1.73 Hz) corresponds to the fundamental translation mode of a fluid sphere as one of the compressional modes in fluid, we conclude that the resonator which is a spherical cavity of diameter 220 m filled with steam of temperature 500°C and pressure 170 atm is favorable.

Gravity variation around Shinmoe-dake volcano from February 2011 through March 2012—Results of continuous absolute gravity observation and repeated hybrid gravity measurements

We report here on continuous absolute gravity measurements made between February 2011 and March 2012 and repeated relative gravity measurements in the vicinity of Shinmoe-dake volcano, which commenced erupting in late January 2011. We find that 20 of 24 eruptive events are associated with precursory short-term gravity decreases occurring over 5–6 hours followed by quick recoveries lasting 1–2 hours. Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm. To isolate the gravity signal associated with volcanic processes, we compared gravity measurements made at 15 sites in March 2011 and again in March 2012. The gravity changes and crustal deformation observed during the one year period are well explained by 6×106 m3 inflation of a magma reservoir at a depth of 9 km and intrusion at shallower depths of a dike with dimensions of 10 km × 0.5 km × 0.5 m.

Observation of seismic core modes from a superconducting gravimeter record

Abstract There have been few reliable reports of observation of seismic core modes, in spite of their geophysical importance. In this study, we analyze low-frequency seismic signals from a superconducting gravimeter (SCG) located at the Esashi Earth Tides Station, Japan, to make observations of core modes. Spectral analysis of an SCG record from the great Macquarie Ridge earthquake, which occurred during the period of our test runs of a SCG, by means of the sompi method has yielded well-resolved signals which are most likely to be the core modes 2S2 and 6S2. Close investigation of the analysis results, however, has revealed that there may be systematic bias in their Q estimates due to some methodological problems. Our tentative conclusion is that the Q values of the core modes observed here are relatively low (probably not exceeding 1000), and also that their amplitudes are larger than theoretically predicted from a spherically symmetric Earth model. At any rate, the present results demonstrate that the SCG also has excellent potential as a long-period seismometer.

Comment on "Incessant excitation of the Earth's free oscillations" by Nawa et al.

The report of Nawa et al. (1998) that incessant excitation of the Earth’s free oscillations was discovered in the low frequency spectra of the superconducting gravimeter at the Syowa station in Antarctica is reexamined. The mean spectra for Syowa are compared with those for Matsushiro, another superconducting gravimeter station in an extremely calm circumstance, hence showing a much lower noise level than Syowa. Although the spectra for Syowa have many peaks below 3 mHz corresponding to those present in the time-frequency diagram of Nawa et al., no similar peaks are found in the spectra for Matsushiro. This result suggests that the peaks below 3 mHz in the Syowa spectra are not real signals of the Earth’s free oscillations.

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.

Combined Use of a Superconducting Gravimeter and Scintrex Gravimeters for Hydrological Correction of Precise Gravity Measurements: A Superhybrid Gravimetry

A variant of hybrid gravimetry using both a superconducting gravimeter and Scintrex gravimeters is proposed. One of the main factors limiting the accuracy of time lapse gravity measurements is the instrumental drift of spring-type gravimeters. Running the Scintrex CG-5 gravimeter in the nighttime on the same pier as the superconducting gravimeter allows us to model the long-term behavior of the former and to remove efficiently the effect of irregular drift on measured gravity. Initial tests performed at Ishigakijima, Japan, proved that accuracy of a few μGal level can be achieved with this method. This will help us precisely correct for the effect of underground water on superconducting gravimeters with 2-dimensional local gravity survey.