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

Introduction The superconducting gravimeters (SGs) are widely used to observe the change in temporal and spatial gravity field by many countries along the world, considering their characteristics as of the high-precision, good continuity and stability. By using the temporal variation of the tidal grav-ity obtained from the global network of SGs, the Global Geophysics Project (GGP) aims at some hot problems in geophysics and geodynamics (SUN, HSU, 1997). The original observations of the c…     

GWR超导重力仪潮汐观测标定因子的精密测定

超导重力仪被普遍认为是迄今为止观测精度高、连续性和稳定性最好的相对重力仪，但是在获得真实重力场变化之前，必须利用格值（标定因子）对这类仪器的直接输出量（电压变化）进行标定．研究表明，格值的准确度将影响到观测数据的后续分析和资料的解释，本文利用两期（每期３天）FG５绝对重力仪在武汉国际重力潮汐基准站上的同址测量资料和该台站已知潮汐参数，详细研究了超导重力仪标定因子及其精度．      

Wavelet approach to the determination of gravity tide parameters

A new approach is proposed for the determination of gravity tide parameters.Three pairs of compactly supported wavelet filters are introduced in the approach.They can efficiently extract the objective tides from the gravity observation series.The new approach guarantees a direct and precise analysis on the tidal gravity records of any sampling length.The new approach is applied to the harmonic analysis on Wuhan superconducting gravimeter records.The results clearly show the resonant effects of the Earth Nearly Diurnal Free Wobble (NDFW).     

武汉台重力潮汐长期观测结果

采用武汉台超导重力仪(SG C032)14年多的长期连续观测资料,研究了固体地球对二阶和三阶引潮力的响应特征,精密测定了重力潮汐参数,系统研究了最新的固体潮模型和海潮模型在中国大陆的有效性.采用最新的8个全球海潮模型计算了海潮负荷效应,从武汉台SG C032的观测中成功分离出63个2阶潮汐波群和15个3阶潮汐波群信号,3阶潮波涵盖了周日、半日和1/3日三个频段.重力潮汐观测的精度非常高,标准偏差达到1.116 nm·s^-2,系统反映了非流体静力平衡、非弹性地球对2阶和3阶引潮力的响应特征.结果表明,现有的武汉国际重力潮汐基准在半日频段非常精确,但在周日频段存在比较明显的偏差,需要进一步精化.对于中国大陆的大地测量观测,固体潮可以采用Dehant等考虑地球内部介质非弹性和非流体静力平衡建立的固体潮理论模型或Xu 等基于全球SG观测建立的重力潮汐全球实验模型作为参考和改正模型,海潮负荷效应应该采用Nao99作为改正模型.     

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.     

On the Accuracy of the Calibration of Superconducting Gravimeters Using Absolute and Spring Sensors: a Critical Comparison

Over the past two decades, superconducting gravimeters (SGs) have been a key tool to investigate a number of geophysical processes leading to time-variable gravity changes. As SGs are relative meters, even though they are the most sensitive and stable devices currently available, they need to be accurately calibrated. Each branch of Earth sciences that benefits from high-precision gravity monitoring demands calibration of gravity sensors to accuracy of better than 0.1%. This research deals with a calibration experiment performed at the Strasbourg (France) SG site by means of two FG5 (#206 and #211) absolute gravimeters (AGs) and new-generation spring meters (Scintrex Ltd. Autograv CG-3M and CG5 and Microg-LaCoste gPhone). Our goal is to try to use the newest generation of spring mechanical gravimeters (MGs) for calibrating SGs. We discuss the results in terms of precision and accuracy of the SG calibration by means of different metrological and methodological approaches. With the FG5 #211 we derive scale factors for the SG-GWR C026 located in Strasbourg in agreement with those routinely obtained since 1997 by means of the FG5 #206. This confirms that the estimation of the scale factors is independent of the AG sensor. From a moving-window regression analysis between the synthetic body tides and both the SG and MG gravity records we detect significant fluctuations of the SG scale factors over time due to the instability of the instrumental sensitivity of the MGs. Our main results demonstrate that, owing to the time variability of their sensitivity, the used spring meters, even if well calibrated, cannot be used as a stable reference for SGs. As a result, MGs are not suitable to replace AGs for SG calibration, and we conclude that currently the method using parallel recording with absolute gravity meters is still the most feasible calibration approach for SGs.     

利用小波分析重力的长期变化

运用小波滤波方法估算Chandler和周年项的潮汐因子.本文分析了四个台站(Brussels, Boulder, Membach以及Strasbourg)的观测记录,运用合成潮方法得到重力残差后,用Daubechies小波带通滤波器滤波残差,得到256～512 d时间尺度上的序列,根据标准差最小原则确定观测极潮周年和Chandler项的周期,然后利用最小二乘法估算它们的潮汐因子,同时给出未经模型改正的周年重力.由于高阶Daubechies小波构造的滤波器具有良好的频率响应,且能压制信号中的高阶异常成分,使滤波的信号更加光滑,因此计算结果具有更小的均方差,更加可靠.     

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 me...     

利用重复测线校正海空重力仪格值及试验验证

通过分析海空重力测量系统误差的形成机理,我们发现海空重力仪格值标定误差是引起系统性测量偏差的主要因素之一.本文简要介绍了重力仪格值的标定方法,分析论证了格值标定的精度要求,提出了利用东西正反向重复测线检测校正海空重力仪格值的计算模型和补偿方法,分析讨论了该方法的校正精度及其适用条件,利用航空重力实际观测网数据对该方法的合理性和有效性进行了数值验证,证明该方法对消除海空重力测量系统性偏差具有显著作用.     

Investigation of the time variability of diurnal tides and resonant FCN period

The time variability of diurnal tides was investigated by analyzing gravity observations from global superconducting gravimeter (SG) stations with running time intervals. Through least-square and Bayesian approaches, FCN resonance parameters were estimated for each data section after obtaining the tidal parameters of mainly diurnal tidal waves. The correlation of the time variation in diurnal tidal waves and FCN period was discussed. For comparison, a similar method was used to analyze VLBI observations to study the time variability of nutation terms and FCN period. The variation trend of the FCN period totally depends on the Ψ1 wave in tidal gravity and on the retrograde annual term in nutation. We observed a similar variation trend in the FCN periods obtained from different SG stations worldwide and VLBI observations. The relation between diurnal tides and LOD variations is discussed and the possible mechanisms of the decadal variation in FCN periods were discussed.     

Time Variations In Gravity And Inferences On The Earth's Structure And Dynamics

This paper reviews how the study of the surface gravity changes is able to provide useful information on the Earth's structure and global dynamics. The spectral range which is observable with superconducting gravimeters is broad and goes from the seismic frequency band to periods longer than one year. We first investigate the seismic and sub-seismic bands with a special attention paid to the gravity detection of core modes in the liquid core and to the Slichter mode of translation of the solid inner core. In the tidal bands, we show how accurate measurements allow us to infer constraints on various phenomena such as mantle (an-)elasticity, as well as ocean and atmospheric loading. The observation of the Free Core Nutation resonance in the diurnal frequency band is reviewed and indirectly suggests an increase in the ellipticity of the core-mantle boundary with respect to its hydrostatic value. A similar resonance is also theoretically predicted in the diurnal band for the rotation of the solid inner core (Free Inner Core Nutation) but we show that its detection is much more difficult because of the small amplitude and lack of a nearby tidal frequency. Oceanic and atmospheric loading mechanisms induce gravity changes over a wide spectral range and we present some recent progress in this field. Finally, because superconducting gravimeters have high calibration stability and small long-term instrumental drift, they can easily measure longperiod gravity variations due to polar motion and hydrogeology.     

蓟县地震台重力仪观测数据质量与水文响应分析

对天津蓟县地震台2套相对重力仪2018—2019年观测数据进行潮汐分析和提取非潮汐重力残差处理,研究水文变化所引起的重力响应。结果表明,蓟县地震台2套重力仪精度较高;GS-15重力仪重力残差与水文变化间存在较好的相关性,具体表现为降水时重力减少,降水结束后重力增大,水位下降时重力减少,水位上升时重力增大,但其间都存在延迟对应关系。     

地球重力场的精细频谱结构及其应用

综述了近年内在全球地球动力学合作观测和研究网络框架下开展的重力场观测、频谱结构和应用研究方面的成果. 内容涉及精密大气、海潮负荷信号检测, 重力潮汐和自由核章动参数测定, 海潮模型和重力固体潮模型有效性检验, 重力潮汐实验模型构制, 地球球型基频和低阶震型谱峰分裂现象和地球Chandler摆动等方面. 文章还介绍了综合现代大地测量技术, 全球超导重力仪的长期、连续观测在地表水循环、同震和震后形变、地球慢形变和地壳垂直运动等方面将发挥重要作用的情况.     

LCR-ET20弹簧型潮汐重力仪观测结果的分析和比较

基于武汉国际重力潮汐基准站LCR-ET20弹簧重力仪潮汐观测资料，利用国际标准的数据预处理和分析方法，采用与GWR型超导重力仪同址观测对比方法，较系统研究了ET20仪器特征以及大气和海洋重力信号．数值结果表明，尽管ET20的观测精度要比超导型重力仪的精度低一个量级，弹簧蠕变性导致的仪器漂移十分大，但该仪器仍获得了与超导重力仪相当的潮汐参数，能有效用于重力场的时间变化观测．      

贝叶斯算法在拟合自由核章动参数中的应用

用高精度重力技术检测地球自由核章动(FCN)参数(包括周期和Q值等)的难点是资料观测信噪比低,传统的方法是使用最小二乘拟合,但是获得的FCN共振参数精度不理想.本文利用武汉国际潮汐基准站高精度超导重力仪和全球超导重力仪观测的时变重力资料,根据贝叶斯算法拟合地球自由核章动(FCN)参数.我们将贝叶斯拟合方法与传统最小二乘法实施了对比分析,研究了不同台站资料差异.讨论了潮波选择和不同海潮模型等因素对FCN参数的影响.结果表明用贝叶斯算法获得的FCN品质因子与空间大地测量VLBI结果吻合的更好,这说明贝叶斯算法可靠性高,为研究地球深内部构造参数(核幔边界粘滞系数等)提供了有效依据.     

泉州重力潮汐观测的分析

为了研究海潮与固体潮的相互作用,对近海的泉州台用三台重力仪（二台LaGoste-G型、一台GS型）为期四个半月的观测资料进行了初步分析,探讨了仪器的性能及误差来源。由分析得出: 1.由LaCoste-G型584和589重力仪观测得到的结果是一致的,三台仪器对于主要潮波O₁和M₂波的观测结果也相符较好,其互差不超过8‰。利用Schwiderski图和我国等潮图作海潮改正后的潮汐因子接近理论模型值。但由于泉州为近海台,大的余差可能反映了局部地区海潮的影响。 2.LaCoste-G型重力仪的误差主要来自灵敏度的变化,应在基本台上作定期标定。 3.GS-207重力仪的观测结果受气压的影响较大,特别是K₁和S₂波,经初步分析得出约7微伽/毫巴。     

绝对重力仪国际比对现状

当前负责重力测量的国际组织CCM-WGG和SGCAG决定今后将举办绝对重力仪区域性国际比对,并开始着手在世界范围内选择区域性国际比对点.CCM-WGG和SGCAG制定了国际比对的技术规范,讨论了绝对重力仪比对申请为关键性国际比对的议案.本文将介绍相关的国际情况以及中国计量科学研究院在这方面所做的工作.本文还将介绍2005年第七次国际绝对重力仪比对的情况和初步结果.     

Noise limitations in the core-mode band of superconducting gravimeter data

In recent years, several claims of detection of weak harmonic signals in the sub-tidal band of frequencies of high-quality gravimeter data have been made. Here, we review our attempts at confirming Smylie's claim of detection of the Slichter mode of inner-core oscillation using the same four data sets he and his colleagues used. We further examine the homogeneous 2 year data sets obtained from the superconducting gravimeters sited in Strasbourg and at Cantley, Quebec. We show that the power spectra of tidal-reduced, pressure and slew-corrected gravity residuals from these data are indistinguishable from a random walk process except in narrow bands dominated by residual earth tides and by harmonics of the diurnal thermal-atmospheric tide. Such a random ‘brown’ residual could result from mechanical instabilities and electronic noise in the instruments themselves, from site-specific tectonic noise, from local hydrological variations and pier instabilities or it could derive from unaccounted for variability in the atmospheric pressure in the vicinity of the instruments. We argue that the homogeneous 2 year data sets show no evidence whatever of Slichter harmonics even though our preprocessing methods reduce ‘apparent noise levels’ in the core-mode band by almost an order magnitude in comparison with the four data sets originally employed by Smylie et al. and, in their first attempted confirmation of his claimed discovery, by Jensen et al.     

中国大陆精密重力潮汐改正模型

利用理论和实验重力固体潮模型,充分考虑全球海潮和中国近海潮汐的负荷效应,建立了中国大陆的精密重力潮汐改正模型.结果表明,采用不同的固体潮模型会对重力潮汐结果产生相对变化幅度小于0.06％的差异;在沿海地区海潮负荷的影响约为整个潮汐的4％,而中部地区约为1％,其中中国近海潮汐模型的影响约占整个海潮负荷的10%,内插或外推潮波的负荷约占海潮负荷的3％.通过比较实测的重力数据表明,本文给出的重力潮汐改正模型的精度远远优于0.5×10^-8 m·s^-2,说明了本文构建的模型的实用性,可为中国大陆高精度重力测量提供有效参考和精密的改正模型.     

Some critical factors for engineering and environmental microgravity investigations

The gravity method is one of the geophysical tools used for engineering and environmental investigations where the detection of cavities, karst phenomena, subsoil irregularities, or landfills is essential. In many cases, deep or small-scale heterogeneities generating low-amplitude anomalies have to be detected and the reliability of further interpretation requires highly accurate measurements, carefully corrected for any quantifiable disturbing effects. The purpose of this study is to investigate the factors likely to limit measurement quality and how to make improvements.Calibrations of a Scintrex gravimeter were made between French relative and absolute base stations, and the relative uncertainties on the calibration factors were estimated for these links. Ranging from 10⁻³, for calibration on an old gravity net, to 10⁻⁴, for a high amplitude absolute base line, this accuracy will be generally sufficient for microgravity surveys.Continuous gravity recordings of Scintrex gravimeters, installed at the same stable site, enabled the estimation of the stability and accuracy of the instruments and revealed that some of the time variations of g measurements, such as instrumental drift, tidal effects and seismic noise, are not entirely removed by standard processing procedures. The accuracy of corrected gravity measurements is mainly limited by inadequate corrections of tidal effects and by a poor estimation of ocean loading effects. In comparison with residual defaults in tidal corrections, instrumental and seismic noises are taken more properly into account by statistical data processing.In field operation, residual tidal effects are generally integrated into an experimental terrain drift estimated on the basis of frequent repeated measurements. A differential gravity approach, based on a fixed gravimeter reference whose recordings are used to correct measurements made with a mobile gravimeter, has also been investigated at a test site. Compared to standard processing, this method can help improve repeatability of gravity measurements.Microgravity surveys in the urban environment require effective and accurate consideration of the effects of infrastructures, nearby buildings and basements, as well as those of topography, in the vicinity of a gravity station. Correction procedures, applied at the same experimental site, where gravity points are located close to buildings, walls and basement slope, appear to have almost totally eliminated these disturbances.