Study of the Earth's free core nutation by tidal gravity data recorded with international superconducting gravimeters

By stacking high-precision tidal gravity observations obtained with superconducting gravimeters at six stations in China, Japan, Belgium, France, Germany and Finland, the local systematical discrepancies in the parameter fitting, caused by atmospheric, oceanic tidal loading and the other local environmental perturbations, are eliminated effectively. As a result, the resonance parameters of the Earth's free core nutation are accurately determined. In this study, the eigenperiod of free core nutation is given as 429.0 sidereal days, which is in agreement with those published in the previous studies. It is about 30 sidereal days less than those calculated in theoretical models (about 460 sidereal days), which confirms the real ellipticity of the fluid core of the Earth to be about 5% larger than the one expected in assumption of hydrostatic equilibrium. The quality factor (Q value) of free core nutation is given as about 9543, which, compared with those determined before based on the body tide observations, is much larger, but more close to those obtained using the VLBI observations. The complex resonance strength is also determined as (?6.10(10?4, ?0.01(10?4)(/h, which can principally describe the deformation characteristics of an anelastic mantle.     

Detection of inertial gravity oscillations in the Earth's core with a superconducting gravimeter at Brussels

As stated by Crossley and Rochester the discovery and identification of even a few undertone periods would provide valuable gross constraints on the stability profile and hence, the thermal regime of the core. Three years of continuous measurements of gravity variations with a superconducting gravimeter at Brussels offer a possibility to detect such oscillations in that part of the spectrum where tidal contributions can be eliminated. Spectral peaks of 10–15 ngal amplitude have been detected around a period of 13.9 h which were enhanced immediately after the two deep earthquakes of magnitude 7.2 which occurred during the observation interval.     

武汉九峰地震台超导重力仪观测分析研究

连续重力观测和GPS的技术结合能够监测到物质迁移和地壳垂直形变之间的量化关系.和相对重力测量以及绝对重力测量技术相比,其避免了时间分辨率和观测精度低,无法精细描述观测周期内的物质迁移过程问题.本文利用武汉九峰地震台超导重力仪SGC053超过13000 h连续重力观测数据;同址观测的绝对重力仪观测结果;气压数据;周边GPS观测结果;GRACE卫星的时变重力场;全球水储量模型等资料,采用同址观测技术、调和分析法、相关分析方法在扣除九峰地震台潮汐、气压、极移和仪器漂移的基础上,利用重力残差时间序列和GPS垂直位移研究物质迁移和地壳垂直形变之间的量化关系.结果表明:在改正连续重力观测数据的潮汐、气压、极移的影响后,不仅准确观测到2009年的夏秋两季由于水负荷引起的约(6~8)×10-8m·s-2短期的重力变化.而且在扣除2.18×10-8(m·s-2)/a仪器漂移和水负荷的影响后,验证了本地区长短趋势垂直形变和重力变化之间具有一致的负相关性规律.同时长趋势表明该地区地壳处于下沉,重力处于增大过程,增加速率约为1.79×10-8(m·s-2)/a.武汉地区重力梯度关系约为-354×10-8(m·s-2)/m.     

武汉超导重力仪观测最新结果和海潮模型研究

利用武汉台站GWR_C032超导重力仪观测资料,在对原始数据进行有效预处理的基础上作调和分析,获得反映地球内部介质特征的重力潮汐参数.基于卫星测高技术和有限元方法同时考虑验潮站数据作约束条件获得的多个全球海潮模型,利用负荷理论和数值褶积积分技术计算了重力负荷,对周日和半日频段内的重力潮汐参数实施负荷改正,提出了“负荷改正有效性”概念,研究了全球海潮模型适应性.数值结果说明,海潮改正的有效性高达91%（O1,NAO99）和92%（M2,ORI96）.基于11个海潮模型对主波（O1,K1,M2和S2）的负荷改正说明平均有效性为（86%,70%,73%和84%）,振幅因子与理论模型间的差异分别从（212%,155%,116%和080%）降到（031%,039%,034%和008%）,同时还说明利用NAO99和ORI96全球海潮模型能获得比其他模型更佳的负荷改正效果.文章还利用国际地球动力学计划网络其他7个台站的超导重力仪观测研究了全球海潮模型的适定性问题,结果说明不同模型中不同潮波具有明显的区域特点,早期构制的SCW80全球海潮模型仍可作为大地测量研究中的重要参考模型.     

Incessant excitation of the Earth's free oscillations: global comparison of superconducting gravimeter records

Records of superconducting gravimeters (SGs) at Canberra (Australia), Esashi (Japan), Metsähovi (Finland) and Syowa Stations (Antarctica) were analyzed to search for further evidence of background free oscillations of the Earth. Spectrograms for 1-year period and averaged power spectra for seismically quiet periods were obtained for each of the stations. Anomalous features of the oscillations observed at Syowa Station, such as an apparent seasonal variation and a high intensity at frequencies between 3 and 4 mHz, were absent at the other SG stations. Among the SG stations used in this study, the background free oscillations were detected most consistently and distinctly at Canberra, where the noise level was comparable to that at the IDA quietest station, while that at Syowa Station was close to the critical limit for detecting the oscillations. The background free oscillations provide a good reference to evaluate the noise level in the milliHertz band.     

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.     

Tri-frequency spectrum method and results for resolving the parameters of Earth's liquid core free nutation

The parameters of Earth free core nutation (FCN) are two relatively significant geophysical parameters. Sasao et al. (1980) and Wahr and Bergen (1986) provided the theoretical estimation values of FCN parameters. Gwinn, Herring and Shapiro (1987) first obtained the observational values of FCN parameters by very long base Interference (VLBI) at Cambridge University. In the same year, Neuberg and Zürn in former West Germany and Hinderer in France began to retrieve FCN parameters by the observation of gravity tides and introduced the stacking method. The other scholars who researched into the same geophysical problems by applying the data of gravity tides basically followed the stacking method. The results they reached were similar to the observational result of FCN parameters given by Neuberg et al. in 1987. But the observational results of FCN parameters gained from gravity tides were not identical with those from VLBI, mainly because of the large difference of quality of FCN. So there was not an affirmative observational result of FCN parameters since then. In this paper, The authors firstly introduce the tri-frequency spectrum method with clearly geometrical and geophysical meaning for the resolution of FCN parameters, and the observational results of FCN parameters obtained from tide data at three superconducting gravity stations were accordant with those from VLBI, which will be relatively important to arriving at a certain observational result of FCN parameters.     

利用VLBI和超导重力资料研究自由核章动周期时变特征

本文根据自由核章动在章动观测和时变重力观测中引起的与其频率相近的受迫章动项或固体潮潮波的共振特性,选取6个不同机构解算的VLBI天极偏差序列和全球超导重力仪网络提供的7个台站高精度时变重力观测资料,根据加权最小二乘方法拟合地球自由核章动参数,研究其本征周期的时间变化特征.VLBI资料获得的本征周期变化幅度在1天之内,存在十年尺度的时间变化特征,迭积多台站同时段重力资料获得的自由核章动本征周期时变特征与VLBI获得的结果相比变化幅度较大,但趋势大致符合.在此基础上,通过自由核章动的理论模型探讨了影响其本征周期的几个主要因素,结果表明FCN周期变化与电磁耦合存在相关性,核幔边界磁感应强度径向分量变化导致的电磁耦合的变化可能是造成FCN周期时变性的原因.     

Study of the Earth’s free core nutation by tidal gravity data recorded with international superconducting gravimeters

By stacking high-precision tidal gravity observations obtained with superconducting gravimeters at six stations in China, Japan, Belgium, France, Germany and Finland, the local systematical discrepancies in the parameter fitting, caused by atmospheric, oceanic tidal loading and the other local environmental perturbations, are eliminated effectively. As a result, the resonance parameters of the Earth’s free core nutation are accurately determined. In this study, the eigenperiod of free core nutation is given as 429.0 sidereal days, which is in agreement with those published in the previous studies. It is about 30 sidereal days less than those calculated in theoretical models (about 460 sidereal days), which confirms the real ellipticity of the fluid core of the Earth to be about 5% larger than the one expected in assumption of hydrostatic equilibrium. The quality factor (Q value) of free core nutation is given as about 9543, which, compared with those determined before based on the body tide observations, is much larger, but more close to those obtained using the VLBI observations. The complex resonance strength is also determined as (−6.10×10⁻⁴, −0.01 ×10⁻⁴)°/h, which can principally describe the deformation characteristics of an anelastic mantle.     

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,     

Results from 44 months of observations with a superconducting gravimeter at Moxa/Germany

Results for more than 42 months of observations with the superconducting gravimeter CD-034 at the Geodynamic Observatory Moxa are discussed. Moxa observatory is one of the newer stations within the ‘Global Geodynamics Project’ (GGP). A special feature of the gravimeter at Moxa is its dual sensor system; differences in the results obtained from the two sensor recordings are generally well within the standard deviations of the tidal analysis. One significant difference concerns the slightly different drift rates of 31 and 49.5 nm/s² per year for upper and lower sensor; both sensor drifts can be fitted by a linear function. We find that the noise levels are close to the ‘New Low Noise Model’ for the seismic-modes and are also low in the tidal bands. Due to this low noise, Moxa is a station well suited to search for small geodynamic signals. The long-period variation in the gravity residuals correlates well with the polar motion.The difference signal between the two sensor recordings has a peak-to-peak amplitude of about 6 nm/s² and shows systematic variations. Its spectrum is characterised by instrumental noise between 0.2 and 0.4 cph. The noise level of the difference and of the sum of the two residual datasets are clearly lower, respectively, higher than the noise contents of the gravity residuals themselves. This is a strong indication for the existence of broadband signals common to the two residual datasets, leading to the conjecture that the reduction of environmental effects is still not sufficient.Our results once more emphasize the necessity to correct the data for barometric pressure effects when analyzing the data for seismic modes. The reduction visibly increases the signal-to-noise ratio in the low frequencies of the mode band and helps to avoid misinterpreations of peaks. Besides the well known barometric pressure influence we can establish hydrological effects in the data which are probably caused by soil moisture and groundwater table variations as well as by batch-wise water movement within the weathering layer. As the major part of the observatory surroundings is above gravimeter level, an anticorrelation between hydrological and gravity changes is observed. In addition, it can be shown that global hydrological effects reach an order of magnitude that makes it necessary to consider these effects when analyzing long-period signals like polar motion. Vice versa these effects are large enough to be detectable in the gravity data. A first joint analysis of five datasets from the GGP network shows no indications for signals related to the Slichter triplet or core modes.     

利用泉州地震台gPhone重力仪观测资料检测地球自由振荡

gPhone重力仪低频段良好的噪声水平有利于记录地球自由振荡信息。利用泉州基准地震台gPhone重力仪原始观测数据,分析直接检测2021年7月29日美国阿拉斯加州以南海域M_S 8.1地震激发的地球自由振荡信息的可能性,结果表明,基频振型₀S₄—₀S₆₀和多个谐频振型的实测频率值与地球初步参考模型（PREM）的理论值基本一致,可见利用该台gPhone重力仪可有效检测地球自由振荡信号。文中所得结果可用于地球内部结构研究,有助于做出更精细的地球模型。     

利用张家口地震台体应变观测资料检测地球自由振荡

在保证低频自由振荡信号分辨率,又不对高频自由振荡信号产生抑制效应的前提下,利用张家口地震台体应变观测资料,采用功率谱密度估计方法,获得2011年3月11日日本9.0级大地震激发的₀S₂-₀S₇₄基频球型振荡和17个谐频球型振荡（₃S₂、₅S₃、₂S₁₀、₂S₁₂、₃S₉、₁₁S₁、₇S₁₀、₅S₁₃、₄S₁₈、₅S₁₄、₁₄S₄、₄S₂₄、₁S₃₈、₇S₁₈、₁₄S₈、₉S₁₉、₅S₃₀）,并与地球初步参考模型（PREM）的理论自由振荡周期进行对比,发现与实测振荡周期基本一致。     

The Akp-Btk value method and the results for the retrieval of the parameters of the Earth's free core nutation

The rotation of the Earth's liquid core creates the Nearly Diurnal Free Wobble (NDFW). It is one of the problems of researching the Earth's liquid core for us to retrieve the parameters of the Earth's Free Core Nutation (FCN), from the Earth's liquid core resonance of gravity tide waves on the diurnal frequency band. Since 1987, some scientists in many different countries have come to calculate the parameters of FCN by using the observational data of gravity tide waves on the diurnal frequency band. They basically followed the Stacking method, which needed five diurnal waves for the resolution. In this paper, authors introduced the Akp-Btk value method with clearly geometrical meaning as a new method, which only requires three very high signal-noise-ratio waves O1, K1 and P1 for the resolution. Authors chose the observational data of the three superconducting gravimeter stations respectively located in Cantley of Canada, Wuhan of China and Brussels of Belgium, to compute the parameters of FCN. It was the first time that the observational results of the parameters of FCN obtained from gravitational tide were in accord with the parameters of FCN gained from VLBI since 1987.     

利用超导重力数据检测日本Mw9.0地震的低频自由振荡及谱线分裂

准确估计低频自由振荡及谱线分裂是约束地球内部结构和改进地球模型的重要手段.本文利用四个不同台站的超导重力观测数据系统研究了日本M_w9.0大地震激发的低于1.5 mHz自由振荡及谱线分裂.研究结果表明:(1)选取适当的数据长度,超导重力观测数据可以检测出低于1.5 mHz除₁S₁以外的所有自由振荡;(2)除₀S₂、₀S₃、₀S₀、₂S₁、₃S₁、₁S₂和₀T₂外,重点探测出₃S₂、₀S₄和₁S₄谱线分裂的所有谱峰;(3)与PREM模型理论频率相比,₀S₀观测频率平均向右偏移0.354×10^-3mHz,说明PREM理论模型中地幔底部参数与真实地球可能存在微小偏差;(4)₃S₂的谱线分裂率r为1.485267,比PREM理论谱线分裂宽度约宽50%,表明PREM中地球内核中部介质参数可能存在一定误差,需要进一步改善.另外,quasi-₀T₂的r为1.254206,比PREM理论谱线分裂宽度约宽25%.     

利用JCZ-1T地震计LP通道数据检测斐济M W 8.2深震激发的地球自由振荡

JCZ-1T地震计LP通道数据可有效应用于地球自由振荡信号探测。利用泰安基准地震台JCZ-1T地震计LP通道120小时数据记录,获得2018年8月19日斐济M_W 8.2深源地震激发的地球自由振荡,检测到基频球型振荡₀S₆-₀S₅₈几乎所有振型、球型振荡部分零级振型和高振型以及环形振荡部分基振型和高振型,与地球初步参考模型（PREM）的理论自由振荡周期进行对比,结果表明,振荡周期观测值与PREM理论值基本一致,二者微小差值应由地球介质的横向不均匀性和各向异性所致。     

利用VP型垂直摆倾斜仪观测数据检测2011日本M_w9.0级地震激发的低频地球自由振荡

甚长周期VP型垂直摆倾斜仪对某些地球物理信号有特殊的敏感性,除了在观测小地震、慢地震等方面有优势之外,还可有效应用于低频地球自由振荡信号的探测.利用安置在武汉大学珞珈山的我国自行研制的VP型垂直摆倾斜仪在2011日本Mw9.0级大地震之后不同长度的观测数据,联合EEMD方法、自回归估计(AR)方法和bootstrap法,本文不仅检测到该地震激发的频率小于4.7 mHz的零阶球型振荡(0S2至0S38)和环型振荡(0T4至0T35)几乎所有振型以及15个谐频振型,还检测到5个其频率低于1mHz的低阶球型多线态(0S2、2S1、0S3、0S4和1S2)的部分或全部谱峰分裂现象,并给出了所有检测结果的精度评估.此外,本文分析了某些球型和环型振荡之间的耦合效应,结果表明耦合效应将显著影响地球自由振荡信号的相关参数.