卫星重力学与重力卫星研究进展

综述了地球重力场研究对揭示其运动和时变与地震之间的关系的重要性；介绍了当今国际固体地球科学与防灾研究的一个新热点——卫星重力学与重力卫星研究的进展。随着重力卫星计划的实施，地球重力场的研究也将因此产生质的变化。文章对CHAMP、GRACE和GOCE重力卫星作了介绍。     

地球重力场的时间变化

目前人们对地球物理、大气现象引起重力场变化的原因及影响还不是十分清楚.地球重力场反映了组成地球全部物质的分布.重力场的变化是由固体地球运动引起的,这种运动与太阳、月亮相互吸引(潮汐)有关.也与由气候变化引起的大气、海洋、极冰盖和地下水变化相互影响有关.这些变化影响地球转动.改变地球卫星轨道.引起海平面变化并且间接地影响全球气候.     

利用GRACE空间重力测量监测长江流域水储量的季节性变化

2002年3月成功发射的美德合作重力卫星计划GRACE(Gravity Recovery And Climate Experiment)已经开始提供阶次数达到120、时间分辨率为约1个月的地球重力场模型时变序列. GRACE的星座由两颗相距约220 km, 高度保持300~500 km, 而倾角保持约90°的近极轨卫星组成. 由于采用星载GPS和非保守力加速度计等高精度定轨技术以及高精度的星-星跟踪数据反演地球重力场, 在几百公里和更大空间尺度上, GRACE重力场的精度大大超过此前的卫星重力观测. 根据GRACE时变重力场反演的地球系统质量重新分布对固体地球物理、海洋物理、气候学以及大地测量等应用有重要的意义. 在长期时间尺度上, GRACE的结果可用于研究北极冰的变化, 并进而研究极冰融化对全球气候变化, 特别是对海平面长期变化的影响. 在季节性时间尺度上, 利用GRACE重力场的精度足以揭示平均小于1 cm的地表水变化或小于1 mbar的海底压强变化. 除了巨大的社会和经济效益外, 这些变化对了解地球系统的物质循环(主要是水循环)和能量循环有非常重要的意义. 利用2002年4月至2003年12月之间共15个月的GRACE时变重力场揭示了全球水储量的明显季节性变化, 并重点分析了中国长江流域水储量的变化. 结果表明长江流域水储量周年变化幅度可达到3.4 cm等效水高, 其最大值出现在春季和初秋. 根据GRACE时变重力场反演的水储量变化与两个目前最好的全球水文模型的符合相当好, 其差别小于1 cm等效水高. 研究表明现代空间重力测量技术在监测一些大流域的水储量变化(如长江流域)、全球水循环和气候变化上有巨大的应用潜力.     

我国大地重力学和固体潮研究进展

地球重力学是研究重力场时空分布及其物理机制的一门学科．地球重力场的空间分布通常可用于三个方面：一是空间科学和大地测量学，主要是利用地表的重力观测对在其上测得的几何量加以归算，以及给出重力场的高空赋值以修正卫星和近地飞行器的轨道；二是反演地球内部结构，主要是三维密度的不均匀分布，并对诸如地慢对流等作约束；三是勘探在时间变化方面，最主要影响来自固体潮，当然还有许多内部力源导致动力学效应．本文着重对我国大地重力研究和固体潮研究的进展作一回顾．     

基于Slepian方法和地面重力观测确定时变重力场模型: 以2011—2013年华北地区数据为例

时变重力场是研究地球系统内部物质运动和时空演化过程的有效途径.目前广泛使用的GRACE时变重力场模型受限于其空间分辨率（约400 km）,难以探测较小空间尺度的重力变化.本文首次尝试利用Slepian局部谱分析方法和多期地面重力观测确定更高空间分辨率的时变重力场模型.Slepian方法通过构建研究区域内的正交基函数,将信号能量集中在研究区域内部,是构建球面局部重力场模型的理想方法.本文根据Slepian方法的特点给出了区域重力场建模及参数优化的步骤,以我国华北地区为例,基于2011—2013多期地面观测确定了区域时变重力场模型,并与同区域由Slepian方法和GRACE卫星数据确定的重力变化进行了对比分析.结果表明：（1）贝叶斯信息量准则可作为确定Slepian展开最佳截断数的有效手段;（2）基于研究区域内现有重复测点数据,能够恢复120阶时变重力场,空间分辨率（半波长）约150 km;（3）2011—2013年间研究区域内GRACE估计结果与120阶地面结果在时空分布的显著趋势上存在较好的对应,证明了本文利用Slepian方法和地面观测所得时变重力场模型的可靠性.本文研究结果可为区域重力场建模提供新的参考,也可为华北地区水资源变化监测、构造活动分析以及地震风险性评估等研究提供高分辨率的时变重力场模型支撑.

     

第8届重力学与固体潮及重力仪器讨论会简况

中国地球物理学会第8届重力学与团体潮及重力仪器学术讨论会于1996年11月10日一12日在长沙市举行．会议收到论文摘要共36篇，到会代表引人，其中近半数为年轻的科技工作者．大会宣读论文兀篇，包括：地球重力场理论研究、重力固体潮观测与理论分析，大地水准面研究，航空重力与绝对重力测量，重力勘探金矿、地震监测及超导重力仪等方面．许厚泽院士在会上作了特邀报告：《地球重力学在地球物理和地球重力学研究中的作用》，系统地论述了近年来在地球重力场随时间、空间变化及应用等方面的研究进展，介绍了代表性的最新研究成果．会上的报告…     

卫星重力梯度测量及其应用前景探讨

卫星重力梯度测量是研究空间重力梯度探测与应用的新领域,特别适于检测地球重力场的短波特征.目前它尚处在研制超灵敏张量梯度仪器和应用预研究的阶段.本文考察了卫星梯度测量的重要性和优越性.阐述了它在惯性导航和地球重力场模型精化中的作用;探讨了卫星梯度测量在重力勘探、地球动力学与地震预测研究中的多种用途;讨论了它在检测微重力环境、检验重力场延拓理论和改善飞行器轨道受力分析中的重要意义.     

卫星重力梯度测量及其应用前景探讨

卫星重力梯度测量是研究空间重力梯度探测与应用的新领域,特别适于检测地球重力场的短波特征.目前它尚处在研制超灵敏张量梯度仪器和应用预研究的阶段.本文考察了卫星梯度测量的重要性和优越性.阐述了它在惯性导航和地球重力场模型精化中的作用;探讨了卫星梯度测量在重力勘探、地球动力学与地震预测研究中的多种用途;讨论了它在检测微重力环境、检验重力场延拓理论和改善飞行器轨道受力分析中的重要意义.     

芦山M_S7.0地震震前GRACE卫星时变重力场特征研究

基于GRACE卫星时变重力数据,利用小波多尺度分解方法,分析研究了芦山MS7.0地震前重力场的时空动态演化特征.结果表明,雅安地区震前卫星时变重力场具有明显的重力变化信息.卫星时变重力场在时间域表现为,累积重力场和差分重力场存在较强的规律性动态变化,以2003—2004年均值为基值的2009—2012年的变化特征与2005—2007年的变化特征具有较好的相似性,以2009年均值为基值的2009—2012年即芦山地震前4年,芦山震区的时变重力场呈现上升→减速上升→加速上升以及上升→轻微下降→快速上升的变化特征;卫星重力场在空间域表现为,震中位置由靠近重力场正变化的中心→接近重力场正变化边缘→接近重力正变化四象限分布中心的变化特征.因此,芦山地震的重力场动态变化信息和波场分离结果符合逆冲推覆兼走滑型地震引起的区域变形特征.     

重力卫星精密星间测距系统滤波器技术指标论证

本文基于重力卫星精密星间测距测量模式,从星间测距观测值与地球重力场频谱关系的角度,建立了距离观测值关于重力位系数的敏感矩阵,分析了各阶次重力场位系数对应的敏感矩阵的频谱特性,讨论了星间测距信息中能反应地球重力场信息的有效信号频带,给出了能最大限度保留地球重力场信息的低通滤波器的通带截止频率、通带增益波纹和频率采样率技术指标设计方案,可为我国首期卫星跟踪卫星重力测量计划的主要技术指标的初步设计提供参考.     

CHARACTERISTICS OF MULTI-SCALE GRAVITY FIELD VARIATION AND SEISMOGENIC MECHANISM ANALYSIS IN THE SOUTHEASTERN TIBETAN PLATEAU

As the most basic geophysical field, the earth gravity field has achieved wide attention, and its spatial anomaly characteristics and dynamic variation can provide important scientific basis for studying the internal structure and dynamics of the Earth. Based on the mobile gravity observation data of the southeastern Tibetan plateau from 2013 to 2016, the dynamic variation tendency and anomaly characteristics of the regional gravity field in different temporal resolutions are obtained before and after the Ludian and Jinggu earthquakes in the study area respectively. The method of wavelet multi-scale decomposition is used to analyze the relationships of gravity field variation with the earth movement, material density change, and strong earthquake preparation. The deep material variation, dynamic process and the mechanism of earthquake inoculation in the southeastern Tibetan plateau are further discussed. Results indicate that the gravity field variation in the source region before the Ludian and Jinggu earthquake respectively is characterized by obvious positive and negative anomalous transition zone and gradient zone that are consistent with the direction of fault tectonics, suggesting the strong crustal movement and mass migration during the earthquake incubation period. The result of wavelet multi-scale decomposition of the gravity field during the period from September 2013 to April 2014 shows that the gravity field variation at different depth and space scale in the crust and upper mantle of the southeastern Tibetan plateau is significantly correlated with seismic distribution and the location of active fault zone. This indicates that the earthquake inoculation in the study area is closely related to the fault movement and the distribution of material density in the crust and upper mantle, which may be affected by the complex deep dynamic process of the material migration in the crust and mantle. The characteristic that strong earthquakes always occur near positive and negative anomaly transition zones and gradient zones of gravity field change is preliminarily explained, based on the dynamic process of material migration in the crust and upper mantle of the southeastern Tibetan plateau. The research results of this paper have some reference value to the study on the earth movement and seismogenic mechanism.     

Integrated Global Geodetic Observing System (IGGOS)—science rationale

The International Association of Geodesy has decided to establish an Integrated Global Geodetic Observing System (IGGOS). The objective of IGGOS is to integrate in a well-defined global terrestrial reference frame the three fundamental pillars of geodesy, which are the determination of all variations of surface geometry of our planet (land, ice and ocean surfaces), of the irregularities in Earth rotation sub-divided in changes of nutation, polar motion and spin rate, and of the spatial and temporal variations of gravity and of the geoid. This integration will have to be done with a relative precision of 1 part-per-billion and be maintained stable in space and time over decades. IGGOS will quantify on a global scale surface changes, mass anomalies, mass transport and mass exchange and exchange in angular momentum in system Earth. It will be a novel and unique contribution to Earth system and Global Change research. It is intended to make IGGOS part of the Integrated Global Observing Strategy (IGOS).     

Integrated Global Geodetic Observing System (IGGOS)—science rationale

The International Association of Geodesy has decided to establish an Integrated Global Geodetic Observing System (IGGOS). The objective of IGGOS is to integrate in a well-defined global terrestrial reference frame the three fundamental pillars of geodesy, which are the determination of all variations of surface geometry of our planet (land, ice and ocean surfaces), of the irregularities in Earth rotation sub-divided in changes of nutation, polar motion and spin rate, and of the spatial and temporal variations of gravity and of the geoid. This integration will have to be done with a relative precision of 1 part-per-billion and be maintained stable in space and time over decades. IGGOS will quantify on a global scale surface changes, mass anomalies, mass transport and mass exchange and exchange in angular momentum in system Earth. It will be a novel and unique contribution to Earth system and Global Change research. It is intended to make IGGOS part of the Integrated Global Observing Strategy (IGOS).     

EGM2008地球重力模型数据在中国大陆地区的精度分析

本文介绍了5′×5 ′的EGM2008地球重力模型及其在全球的精度评价.按照地形变化规律,将中国大陆大致分为7个区域,在10 km网度上,将EGM2008地球重力模型数据与中国地面实测空间重力网格数据进行了对比.由于数据源的问题,中国大陆的模型数据精度普遍低于北美和欧洲.二种数据在地形平坦的东部地区差别较小,向西随着地形复杂程度的增加,二种数据之间的标准差从小于10 mGal增大到50多mGal.畸变点分析表明精度极低的网格点均分布在地形起伏大的地区.总体而言,5′×5′的EGM2008地球重力模型数据在中国大陆将近80％的面积上的精度可达10 mGal之内,可用于小比例尺重力编图和构造研究.在地形起伏较大的中国西部以青藏高原为例进一步比较了EGM2008重力模型和重力测点数据,结果表明在重力点分布稀疏不均匀的地区,平面网格数据难以准确表达重力场信息.由于缺少地面重力数据控制,EGM2008重力模型数据在中国西部精度较低,但模型数据依然在很大程度上提高了空间重力异常信息的丰富程度.将中国区域重力调查成果数据应用于地球模型的构建是一项有意义的工作.     

内核超速旋转及其对重力场的影响

本文研究由内核超速旋转引起的地球重力场的变化.论述了内核具有三个主要特征：椭球形状,各向异性对称轴与内核自转轴重合,内核自转轴与地球自转轴之间存在夹角并绕地球自转轴进动.内核超速旋转引起地球体系物质的重新分布,导致重力场变化.通过研究内核超速旋转的运动规律,建立了内核超速旋转导致重力场变化的模型,给出了由于内核超速旋转而引起的整个地球表面的重力变化,其中,在假定了内核超速旋转速率为1°/a的前提下,历经一年的最大重力变化量级约为0.37 μGal.     

Seismologic applications of GRACE time-variable gravity measurements

The Gravity Recovery and Climate Experiment (GRACE) has been measuring temporal and spatial variations of mass redistribution within the Earth system since 2002. As large earthquakes cause significant mass changes on and under the Earth’s surface, GRACE provides a new means from space to observe mass redistribution due to earthquake deformations. GRACE serves as a good complement to other earthquake measurements because of its extensive spatial coverage and being free from terrestrial restriction. During its over 10 years mission, GRACE has successfully detected seismic gravitational changes of several giant earthquakes, which include the 2004 Sumatra–Andaman earthquake, 2010 Maule (Chile) earthquake, and 2011 Tohoku-Oki (Japan) earthquake. In this review, we describe by examples how to process GRACE time-variable gravity data to retrieve seismic signals, and summarize the results of recent studies that apply GRACE observations to detect co- and post-seismic signals and constrain fault slip models and viscous lithospheric structures. We also discuss major problems and give an outlook in this field of GRACE application.     

Retrieval of Large-Scale Hydrological Signals in Africa from GRACE Time-Variable Gravity Fields

Since its launch in April 2002, the Gravity Recovery and Climate Experiment (GRACE) mission is recording the Earth’s time-variable gravity field with temporal and spatial resolutions of typically 7–30?days and a few hundreds of kilometers, allowing the monitoring of continental water storage variations from both continental and river-basin scales. We investigate here large scale hydrological variations in Africa using different GRACE spherical harmonic solutions, using different processing strategies (constrained and unconstrained solutions). We compare our GRACE estimates to different global hydrology models, with different land-surface schemes and also precipitation forcing. We validate GRACE observations through two different techniques: first by studying desert areas, providing an estimate of the precision. Then we compare GRACE recovered mass variations of main lakes to volume changes derived from radar altimetry measurements. We also study the differences between different publicly available precipitation datasets from both space measurements and ground rain gauges, and their impact on soil-moisture estimates.     

On the effect of a low viscosity asthenosphere on the temporal change of the geoid—A challenge for future gravity missions

New satellite technology to measure changes in the Earth’s gravity field gives new possibilities to detect layers of low viscosity inside the Earth. We used density models for the Earth mantle based on slab history as well as on tomography and fitted the viscosity by comparison of predicted gravity to the new CHAMP gravity model. We first confirm that the fit to the observed geoid is insensitive to the presence of a low viscosity anomaly in the upper mantle as long as the layer is thin ( 200 km) and the viscosity reduction is less than two orders of magnitude. Then we investigated the temporal change in geoid by comparing two stages of slablet sinking based on subduction history or by advection of tomography derived densities and compared the spectra of the geoid change for cases with and without a low viscosity layer, but about equal fit to the observed geoid. The presence of a low viscosity layer causes relaxation at smaller wavelength and thus leads to a spectrum with relatively stronger power in higher modes and a peak around degrees 5 and 6. Comparing the spectra to the expected degree resolution for GRACE data for a 5 years mission duration shows a weak possibility to detect changes in the Earth’s gravity field due to large scale mantle circulation, provided that other causes of geoid changes can be taken into account with sufficient accuracy. A discrimination between the two viscosity cases, however, demands a new generation of gravity field observing satellites.     

What Can be Expected from the GRACE-FO Laser Ranging Interferometer for Earth Science Applications?

The primary objective of the gravity recovery and climate experiment follow-on (GRACE-FO) satellite mission, due for launch in August 2017, is to continue the GRACE time series of global monthly gravity field models. For this, evolved versions of the GRACE microwave instrument, GPS receiver, and accelerometer will be used. A secondary objective is to demonstrate the effectiveness of a laser ranging interferometer (LRI) in improving the satellite-to-satellite tracking measurement performance. In order to investigate the expected enhancement for Earth science applications, we have performed a full-scale simulation over the nominal mission lifetime of 5 years using a realistic orbit scenario and error assumptions both for instrument and background model errors. Unfiltered differences between the synthetic input and the finally recovered time-variable monthly gravity models show notable improvements with the LRI, on a global scale, of the order of 23 %. The gain is realized for wavelengths smaller than 240 km in case of Gaussian filtering but decreases to just a few percent when anisotropic filtering is applied. This is also confirmed for some typical regional Earth science applications which show randomly distributed patterns of small improvements but also degradations when using DDK4-filtered LRI-based models. Analysis of applied error models indicates that accelerometer noise followed by ocean tide and non-tidal mass variation errors are the main contributors to the overall GRACE-FO gravity model error. Improvements in these fields are therefore necessary, besides optimized constellations, to make use of the increased LRI accuracy and to significantly improve gravity field models from next-generation gravity missions.