Scintrex CG5 used for superconducting gravimeter calibration

The scale factor accuracy of superconducting gravimeters （SG） can be largely improved by a high repetition rate of calibration experiments. At stations where the availability of absolute gravimeters is limited, carefully calibrated spring gravimeters can be used for providing the reference signal assuming the irregular drift is properly adjusted. The temporal stability of the SG scale factor is assessable by comparing the temporal variations of M2 tidal parameters observed at neighboring SG sites or from synthetic tide models. Combining these methods reduces the SG scale factor error to a few 0.1%0. The paper addresses the particular procedure required for evaluating the calibration experiments based on spring gravimeters and presents results obtained at Conrad observatory （Austria）. Comparing the M2 amplitude factor modulation helped to reveal a SG scale factor offset of about 0.2%0 due to re-installation.

Far-field coseismic gravity changes related to the 2015 MW7.8 Nepal(Gorkha)earthquake observed by superconducting gravimeters in Chinese mainland

Using data from five SGs at four stations in Chinese mainland,obvious permanent gravity changes caused by the 2015 MW7.8 Nepal(Gorkha)earthquake were detected.We analyzed the gravity effects from ground vertical deformation(VD)using co-site continuous GPS(cGPS)data collocated at the Lijiang and the Wuhan station,and hydrological effects using GLDAS models and groundwater level records.After removing these effects,SG observations before and after the earthquake revealed obvious permanent gravity changes:−3.0μGal,7.3μGal and 8.0μGal at Lhasa,Lijiang and Wuhan station,respectively.We found that the gravity changes cannot be explained by the results of dislocation theory.

Least Squares Spectral Analysis and Its Application to Superconducting Gravimeter Data Analysis

Detection of a periodic signal hidden in noise is the goal of Superconducting Gravimeter (SG) data analysis. Due to spikes, gaps, datum shrifts (offsets) and other disturbances, the traditional FFT method shows inherent limitations. Instead, the least squares spectral analysis (LSSA) has showed itself more suitable than Fourier analysis of gappy, unequally spaced and unequally weighted data series in a variety of applications in geodesy and geophysics. This paper reviews the principle of LSSA and gives a possible strategy for the analysis of time series obtained from the Canadian Superconducting Gravimeter Installation (CGSI), with gaps, offsets, unequal sampling decimation of the data and unequally weighted data points.

Comparison of noise-levels between superconducting gravimeter and gPhone gravimeter

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Accurate determination of calibration factor for tidal gravity observation of a GWR-superconducting gravimeter

It is recognized widely nowadays that the superconducting gravimeter (SG) is a kind of best relative gravimeter with high observing precision, good continuity and stability. However, it is necessary to calibrate the direct output (change in voltage) by using scale value (calibration factor) before getting the change of the real gravity field. Studies show that the accuracy of the scale value will influence the late analysis and explanation of the observations. By using absolute gravity measurements of a FG5 absolute gravimeter (AG) at Wuhan international tidal gravity fundamental station (two campaigns each for 3 days) and by using known tidal parameters at the same station, the calibration factors of the SG and their precision are studied in detail in this paper.

Calibration of the superconducting gravimeter based on a cold atom absolute gravimeter at NIM

The scale factor of a superconducting gravimeter(SG) is usually calibrated by using simultaneous and co-located gravity measurements with the FG5-type absolute gravimeter(AG). In this paper, another new kind of absolute gravimetercold atom gravimeter(CAG) is first reported to calibrate the SG. Five-day side-by-side gravity measurements have been carried out by using our CAG(NIM-AGRb-1) to calibrate the SG(iGrav-012) located at Changping Campus of the National Institute of Metrology(NIM) of China. A weighted least-squares method is applied to determine the scale factor and the result is given as(-928.01 ± 0.73) nm·s^(-2)·V^(-1) with a precision of 0.79‰. We have demonstrated that a calibration precision of 1‰ level can be achieved after 3 days of parallel observations at spring tide. The obtained calibration results are then compared with the previous calibration by FG5 X-249, which shows that the calibration precision obtained by using NIM-AGRb-1 was slightly higher than FG5 X-249 with the same time interval. The factors affecting the calibration precision are analyzed in the calibrations by means of different AGs. Finally, several calibration experiments for SG iGrav-012 are discussed. The final scale factor is estimated as(-927.58 ± 0.36) nm·s^(-2)·V^(-1) with an accuracy of 0.39‰. Our main results demonstrate that the CAGs can be used for high-precision calibrations of SGs.

Estimated Green's function extracted from superconducting gravimeters and seismometers

Estimated Green’s function （EGF） between stations has been extracted from ambient seismic noise, direct surface wave and coda waves. It is also confirmed by laboratory experiments on ultrasonics and theoretical derivations assuming diffusive wave field, equi-partition of modes or random sources on an enclosed surface. This method provides a new approach to study the crust and mantle structure at regional scale, continental scale and global scale. Following the achievements with seismometer records, the records of infrasonic station, hydrophone and microphone were also used to obtain the EGFs of different wave fields. Since superconducting gravimeter is a better long period instrument than regular seismometer, EGF at longer period is expected to be obtained with the cross correlation of gravity data. In this paper, we will show the EGFs extracted by cross-correlations between the superconducting gravimeters and the seismometers. Both the travel times and dispersion curves obtained from different data types are consistent. The result shows that it is possible to retrieve the deep structure by the cross correlation of gravity data.

Anomalous Signals Prior to Wenchuan Earthquake Detected by Superconducting Gravimeter and Broadband Seismometers Records

Using 1 Hz sampling records at one superconducting gravimeter （SG） station and 11 broadband seismometer stations, we found anomalous signals prior to the 2008 Wenchuan （汶川） earthquake event. The tides are removed from the original SG records to obtain the gravity residuals. Applying the Hilbert-Huang transform （HHT） and the wavelet analysis to the SG gravity residuals leads to time-frequency spectra, which suggests that there is an anomalous signal series around 39 h prior to the event. The period and the magnitude of the anomalous signal series are about 8 s and 3×10^-8 m/s^2 （3 μGal）, respectively. In another aspect, applying HHT analysis technique to 11 records at broadband seismometer stations shows that most of them contain anomalous signals prior to the Wenchuan event, and the marginal spectra of 8 inland stations show an apparent characteristic of double peaks in anomalous days compared to the only one peak of the marginal spectra in quiet days. Preliminary investigations suggest that the anomalous signals prior to the earthquake are closely related to the low-frequency earthquake （LFE）. We concluded that the SG data as well as the broadband seismometers records might be significant information sources in detecting the anomalous signals prior to large earthquakes.

Detection of Inner Core Translational Oscillations Using Superconducting Gravimeters

An attempt has been made to search for the translational oscillations of the Earth＇s solid inner core in the gravity measurements recorded with the superconducting gravimeters （SG） from the worldwide network of the Global Geodynamics Project （GGP）. All the SG data were prepared and analyzed by the same method to remove accurately the signatures related to gravity tides, local baro- metric pressure, the Earth＇s rotation, the long-term trend and so on. We obtained the estimations of the power spectral densities of each residual series and the estimations of the product spectral densities in the subtidal band （0.162-0.285 cph） were obtained by using a multi-station stacking technique after further eliminating atmospheric effects. The inner core translation triplet was detected in the subtidal band. We find 6 groups of signal with high signal-to-noise ratio that are consistent with the characteris- tics of the triplet, and 4 groups of the results that are close to the previous studies and the differences are less than 0.92%. It implies that the groups of signatures all have the possibility to be related to the inner core translational oscillations.

Estimation of free core nutation parameters and availability of computing options

The Earth’s Free Core Nutation(FCN) causes Earth tides and forced nutation with frequencies close to the FCN that exhibit resonance effects.High-precision superconducting gravimeter(SG) and very long baseline interferometry(VLBI) provide good observation techniques for detecting the FCN parameters.However,some choices in data processing and solution procedures increase the uncertainty of the FCN parameters.In this study,we analyzed the differences and the effectiveness of weight function and ocean tide corrections in the FCN parameter detection using synthetic data,SG data from thirty-one stations,and the 10 celestial pole offset(CPO) series.The results show that significant discrepancies are caused by different computing options for a single SG station.The stacking method,which results in a variation of0.24-5 sidereal days(SDs) in the FCN period(T) and 10^(3)-10^(4) in the quality factor(Q) due to the selection of the weighting function and the ocean tide model(OTM),can effectively suppress this influence.The statistical analysis results of synthetic data shows that although different weight choices,while adjusting the proportion of diurnal tidal waves involved,do not significantly improve the accuracy of fitted FCN parameters from gravity observations.The study evaluated a series of OTMs using the loading correction efficiency.The fitting of FCN parameters can be improved by selecting the mean of appropriate OTMs based on the evaluation results.Through the estimation of the FCN parameters based on the forced nutation,it was found that the weight function P_(1) is more suitable than others,and different CPO series(after 2009) resulted in a difference of 0.4 SDs in the T and of 103 in the Q.We estimated the FCN parameters for SG(T=430.4±1.5 SDs and Q=1.52×10^(4)±2.5×10^(3)) and for VLBI(T=429.8±0.7 SDs,Q=1.88×10^(4)±2.1×10^(3)).

Analysis and comparison of the tidal gravity observations obtained with LCR-ET20 spring gravimeter

Based on the tide gravity observations recorded with LCR-ET20 spring gravimeter at Wuhan international fundamental tidal gravity station, the characteristics of ET20 and atmospheric and oceanic gravity signals are studied systematically by using international standard data pre-processing and analysis methods and by comparing the results with those obtained by superconducting gravimeter (SG) at the same station. The numerical results indicate that the identical tidal gravity parameters are the same as those with the SG, the instrument can be effectively used to record temporal change of the gravity field, though the ET20 accuracy is one order lower than that of the SG, and has the large drift induced by the spring creep character.

Frequency characteristics and far-field effect of gravity perturbation before earthquake

We used high-pass filtering and the Fourier transform to analyze tidal gravity data prior to five earthquakes from four superconducting gravity stations around the world. A stable gravitational perturbation signal is received within a few days before the earthquakes. The gravitational perturbation signal before the Wenchuan earthquake on May 12, 2008 has main frequency of 0.1–0.3 Hz, and the other four have frequency bands of 0.12-0.17 Hz and 0.06-0.085 Hz. For earthquakes in continental and oceanic plate fault zones, gravity anomalies often appear on the superconducting gravimeters away from the epicenter, whereas the stations near the epicenter record small or no anomalies. The results suggest that this kind of gravitational perturbation signals correlate with earthquake occurrence, making them potentially useful earthquake predictors. The far-field effect of the gravitational perturbation signals may reveal the interaction mechanisms of the Earth’s tectonic plates. However, owing to the uneven distribution of gravity tide stations, the results need to be further confirmed in the future.

Temporal variations in free core nutation period

Based on the nearly diurnal resonance in the tidal gravity observations, the temporal variations in period of the Earth＇s free core nutation （FCN） are investigated by using the tidal gravity observations of 18-year duration recorded continu- ously with a superconducting gravimeter （SG） at Brussels. The effects of the global oceanic tide loading and local barometric pressure on the SG observations have been removed by using eleven high-precision global digital models of oceanic tides and barometric pressure measurements recorded simultaneously at the same site. The results indicate that there exist decade-scale variations in the FCN period. The results should be further confirmed by the measurements using other space-based geodetic techniques （such as the very long baseline interferometry） and the SG observations from globally distributed stations.

A case study of detecting anomalous signals prior to the great 2010 Maule earthquake

The Hilbert-Huang transform （HHT） is used to analyze the time series from nine gravimeter （SG） stations and 22 broadband seismometers to investigate the anomalous signals superconducting prior to the great 2010 Maule earthquake. The results show that seven SG time series and 20 broadband seismometer time series have anomalous signals lasting about one to three days before the earthquake occurrence. The anomalous signals appear around the frequency bands 0.07Hz and 0.15Hz in SG records while around the frequency band 0. 13Hz -0.2Hz in seismic records, and the reason why they appear in different bands might be attributed to the intrinsic nature and different sensitivities of different kinds of instruments. Because more than 87% records have the anomalous signals prior to the earthquake, and no typhoon event is found in our chosen time window, we may conclude that the anomalous signals might be precursory signals of the great 2010 Maule event. However, we do not rule out other possible excitation sources.

Determination of the Earth’s free core nutation parameters by using tidal gravity data

The Earth＇s free core nutation （FCN） is a retrograde eigenrnode which is attributed to the interaction between the solid mantle and the liquid core of the rotational elliptical Earth. This mode appears as an eigenmode of nearly diurnal free wobble （NDFW） in a terrestrial reference frame with a period of about one day （XU et al, 2001）. Therefore, the NDFW will lead to an obvious resonance enhancement in the diurnal tidal gravity observations, especially those of the tidal waves with frequencies closed to its eigenfrequency such as P1, K1, ψ1 and Ф1. The FCN resonance parameters can be retrieved accurately by high-precision tidal gravity observations, especially those recorded with the superconducting gravimeters （SG）. The Global Geodynamics Project （GGP） organized by IUGG took it as an important content for determining the FCN resonance parameters by using gravity data. However, the results are affected by many factors such as station location, background noise, the selection of the tide-generating potential tables, ocean tide models, data processing techniques and so on. In our study, the FCN parameters will be retrieved by using the SG observations at Wuhan, and the effects of the choices of various tide-generating potential tables, oceanic models and weight functions on the estimation of the FCN parameters will be discussed in detail,

Modeling of Atmospheric Gravity Effects for High-Precision Observations

Temporal variations of atmospheric density distribution induce changes in the gravitational air mass attraction at a specific observation site. Additionally, the load of the atmospheric masses deforms the Earth’s crust and the sea surface. Variations in the local gravity acceleration and atmospheric pressure are known to be corrected with an admittance of about 3 nm/s2 per hPa as a standard factor, which is in accordance with the IAG Resolution No. 9, 1983. A more accurate admittance factor for a gravity station is varying with time and depends on the total global mass distribution within the atmosphere. The Institut für Erdmessung (IfE) performed absolute gravity observations in the Fennoscandian land uplift area nearly every year from 2003 to 2008. The objective is to ensure a reduction with 3 nm/s2 accuracy. Therefore, atmospheric gravity changes are modeled using globally distributed ECMWF data. The attraction effect from the local zone around the gravity station is calculated with ECMWF 3D weather data describing different pressure levels up to a height of 50 km. To model the regional and global attraction, and all deformation components the Green’s functions method and surface ECMWF 2D weather data are used. For the annually performed absolute gravimetry determinations, this approach improved the reductions by 8 nm/s2 (-19 nm/s2 to +4 nm/s2). The gravity modeling was verified using superconducting gravimeter data at station Membach inBelgiumimproving the residuals by about 15%.

Constraining the focal mechanism of the Lushan earthquake with observations of the Earth's free oscillations

The amplitudes of the Earth＇s free oscillations have a close relationship to earthquake focal mechanisms. Focal mechanisms of large earthquakes can be well analyzed and constrained with observations of long period free oscillations. Although the 2013 Lushan earthquake was only moderately sized, observable spherical normal modes were excited and clearly observed by a su- perconductive gravimeter and a broadband seismometer. We compare observed free oscillations with synthetic normal modes corresponding to four different focal mechanisms for the Lushan earthquake. The results show that source parameters can be analyzed and constrained by spherical normal modes in a 2.3-5 mHz frequency band. The scalar seismic moment M~ has a major influence on the amplitudes of free oscillations; additionally, the strike, dip, rake and depth of the hypocenter have mi- nor influences. We found that the synthetic modes corresponding to the focal mechanism determined by the Global Centroid Moment Tensor show agreement to the observed modes, suggesting that earthquake magnitudes predicted in this way can readily reflect the total energy released by the earthquake. The scalar seismic moment obtained by far-field body wave inver- sion is significantly underestimated. Focal mechanism solutions can be improved by joint inversion of far- and near-field data.